Practical application of international recommendations and safety standards in the systematic planning and implementation of remediation of sites or areas with residual radioactive material

T L Yankovich; M Roberts; J Brown; Y Mori; G A Williams; F Charalambous; S Pepin

doi:10.1088/1361-6498/ac6a87

1. Introduction

Human activities that involve the utilization of radiation or radioactive material can give rise to radiation exposure to people (workers, the public, patients) and to wildlife (flora, fauna) in the environment (European Commission et al 2014). Consistent with a graded approach, the stringency and extent of control measures, including regulatory oversight, for such exposure, need to be commensurate with the risks.

In some situations, there is a loss of control of radioactive material and a corresponding requirement to make decisions regarding the types of actions that may or may not need to be taken to (re)gain control of the material, depending on the circumstances (ICRP 2007, European Commission et al 2014). This can lead to an existing exposure situation, which is defined as 'a situation of exposure that already exists when a decision on the need for control needs to be taken' (IAEA 2019). Existing exposure situations include cases where there is exposure due to:

Areas of high natural background radiation (e.g. areas with naturally high radon levels), which is amenable to control (in cases where natural background radiation is not amenable to control, the site or area where it occurs is typically excluded from regulatory control; European Commission et al 2014);
Sites or areas with residual radioactive material originating from past activities that were never subject to regulatory control, or that were not subject to control in accordance with current standards; and
Sites or areas with residual radioactive material from a nuclear or radiological emergency after an emergency has been declared to be ended (European Commission et al 2014, IAEA 2022).

Given the diverse range of sites or areas contaminated with residual radioactive material and the often high uncertainties relating to such situations, for example, due to a lack of records and/or monitoring data, a lack of past oversight, and/or the unique site-specific conditions or prevailing circumstances due to the event or activities leading to the existing exposure situation, it is not possible to provide prescriptive guidance that would be relevant across all such situations (Yankovich et al 2014a, IAEA 2022). Instead, due to the uniqueness of such situations, it is necessary to plan and implement remediation applying a stepwise, multi-phased process with adequate flexibility that can be applied to address a given site or area.

1.1. Objectives

The objectives of the current paper are:

To provide an overview of the international system of radiological protection as it applies to remediation of sites or areas with residual radioactive material and the application of the remediation process to address such situations, based on updated IAEA recommendations (e.g. European Commission et al 2014, IAEA 2022);
To describe the key aspects of new IAEA safety standards on remediation and the application of the process of remediation to address such situations; and
To demonstrate how to systematically apply the remediation process by use of a step-wise approach through case studies.

2. Overview of the international system of radiological protection

The fundamental safety objective is 'to protect people and the environment from harmful effects of ionizing radiation' and this must be achieved 'without unduly limiting the operation of facilities or the conduct of activities that give rise to radiation risks' (IAEA 2006). In doing so and in accordance with Principle 7 of the IAEA Safety Fundamentals (para. 3.18 of SF − 1, IAEA 2006):

'People and the environment, present and future, must be protected against radiation risks. Radiation risks may transcend national borders and may persist for long periods of time. The possible consequences, now and in the future, of current actions have to be taken into account in judging the adequacy of measures to control radiation risks.'

Furthermore, as stated in Principle 10 of the IAEA Safety Fundamentals (IAEA 2006):

'Protective actions to reduce existing or unregulated radiation risks must be justified and optimized.'

Consistent with these fundamental principles, the international system of radiological protection consists of three general principles (ICRP 2007, European Commission et al 2014):

(a)
Justification;
(b)
Optimization of protection and safety; and
(c)
Limitation of exposure.

Justification is 'the process of determining for ... an existing exposure situation whether a proposed protective action or remedial action is likely, overall, to be beneficial; that is, whether the expected benefits to individuals and to society (including the reduction in radiation detriment) from introducing or continuing the protective action or remedial action outweigh the cost of such action and any harm or damage caused by the action' (European Commission et al 2014, IAEA 2019). This definition highlights the need to weigh out the benefits of remediation relative to harm or damage from radiation exposure, also taking account of other impacts or stressors (e.g. physical, chemical, biological, environmental).

Optimization of protection and safety is 'the process of determining what level of protection and safety would result in the magnitude of individual doses, the number of individuals (workers and members of the public) subject to exposure and the likelihood of exposure being "as low as reasonably achievable, economic and social factors being taken into account" (ALARA)' (European Commission et al 2014, IAEA 2019). Through this process, beneficial versus detrimental impacts are weighed out and evaluated to determine viable options to address an existing exposure situation, taking account of radiological risk, along with economic, social and other factors. In the case of the remediation of a historic site affected by past activities, for example, it might be necessary to evaluate the benefits of relocation of radioactive material to a different site relative to the potential occupational health and safety risks related to transport of the material, the concerns of interested parties (also called 'stakeholders') living in the area, and the associated environmental and monetary costs. Based on such an evaluation, it might be determined that the material should remain on site and contained in an engineered structure. In this case, ongoing long-term oversight of the remediated area with appropriate controls on access and land use would be necessary to ensure the future integrity of the on-site disposal structure.

Limitation of exposure involves the setting of criteria and ensuring that measures are taken to prevent exposure from exceeding set criteria. Throughout the lifetime of remediation, different types of criteria are needed (see section 4), for example, to ensure and verify protection over the entire remediation process, as well as to determine that a given remedial action or set of remedial actions are performing as anticipated and to confirm that one phase of remediation has been completed and that it is time to initiate the next phase (see section 3).

The international system of protection must, therefore, be applied in accordance with these principles to ensure there is net benefit to individuals and society (including current and future generations), and to the environment upon which we rely, with an objective of achieving pragmatic and balanced outcomes, while taking account of all relevant factors (European Commission et al 2014, IAEA 2022). In doing so, there is a need to evaluate exposure to ionizing radiation, along with other stressors, and the corresponding risks, to facilitate optimization of protection and safety and prioritization of hazards and risks in decision-making. This should be done applying a systematic, stepwise process, as described in section 3 that follows.

3. Stepwise process for remediation planning and implementation

As discussed in section 1, a wide range of prevailing circumstances and site-specific conditions are inherent to sites or areas contaminated with residual radioactive material worldwide. As a result, for a given situation and set of conditions, in order to determine whether or not there is potential for significant adverse effects, it is necessary to undertake an assessment of exposure against which relevant criteria (see section 4) will then need to be compared (IAEA 2018a, 2018b, 2022).

Over the lifetime of remediation, monitoring needs to be performed to verify that the actual exposure is consistent with the anticipated exposure (as defined and approved during remediation planning), for example, to evaluate remediation effectiveness before and after implementation of specific remedial actions or the overall remediation (IAEA 2005). In this way, monitoring data should be compared to relevant criteria that indicate the effectiveness of the implementation of remediation plans, while ensuring there is not a loss of control of radioactive material relating to remedial actions and there are no significant impacts to people or the environment.

In accordance with the IAEA safety standards (GSG-15; IAEA 2022), the phases in the lifetime of remediation (figure 1) are as follows:

(a)
Preliminary evaluation;
(b)
Detailed evaluation;
(c)
Planning of remediation;
(d)
Implementation and verification monitoring; and
(e)
Post-remediation management.

3.1. Preliminary evaluation

The first step of the remediation process involves undertaking a relatively conservative, screening-level assessment (i.e. a 'preliminary evaluation') to estimate exposure and to quickly identify and 'screen out' non-issues (i.e. those that were on the initial list that have been determined not to contribute significant risk, based on the initial conservative screening). By removing these non-issues from further consideration, focus can then be placed on actual impacts, as well as those that might cause significant detrimental effects (e.g. IAEA 2001, 2022, US-DOE 2004, Brown et al 2008, Yankovich et al 2014b) (figure 1).

This involves conducting a desk-based study that makes use of available data and other relevant information to estimate exposure, and as necessary, collecting additional information and taking measurements as part of targeted field studies. The preliminary evaluation involves use of conservative assumptions to estimate exposure, which is then compared to an appropriate screening criterion (see section 4).

If the estimated exposure exceeds the screening criterion, it can then be concluded that significant adverse effects related to the current pre-remediation 'baseline' conditions might occur and a more detailed evaluation (section 3.2) needs to be undertaken in order to ascertain whether or not this is the case.

3.2. Detailed evaluation

In cases for which it is determined, based on the results of a preliminary assessment (section 3.1), that significant adverse effects might occur if remediation is not undertaken, it is necessary to conduct a 'detailed evaluation' (Phase 2 of the remediation process; see figure 1). The objective of detailed evaluation is to determine whether or not remediation 'is' necessary on the basis of a more detailed and site-specific assessment. In doing so, an assessment of exposure is undertaken and the estimated exposure is then compared to the reference level (i.e. the level above which it is not appropriate to plan to allow exposures to occur and below which optimization of protection and safety would continue to be implemented (IAEA 2019), as described in section 2) to determine whether the reference level is exceeded.

In cases where the reference level is exceeded (compared to the estimated exposure), indicating the possibility of significant adverse effects, it can be concluded that remediation is justified (IAEA 2022). In such cases, in order to reduce radiation exposure, planning of remediation can be initiated (section 3.3) and remediation can subsequently be implemented (section 3.4), whereby 'actions may be applied to the contamination itself (the source) or to the exposure pathways to humans', recognizing that complete removal of the contamination is generally not implied (IAEA 2019).

3.3. Planning of remediation

In cases where it is determined that remediation is justified based on the outcomes of preliminary evaluation (section 3.1) and detailed evaluation (section 3.2), planning of remediation can be initiated (Phase 3 of the remediation process; see figure 1). Remediation planning involves identification and evaluation of possible remedial actions⁴ and other protective actions⁵ to be taken, as relevant, depending on the situation and the prevailing circumstances. In particular, an important part of identification of feasible remedial options in remediation planning is the estimation of the projected dose prior to remediation (i.e. 'the dose that would be expected to be received if planned protective actions were not taken'; IAEA 2019) relative to the residual dose after remediation (i.e. 'the dose expected to be incurred after protective actions have been terminated (or after a decision has been taken not to take protective actions)'; IAEA 2019) (IAEA 2022). Through such comparison, appropriate remedial options can be identified and selected applying the principle of optimization of protection and safety to minimize radiation exposure, while also taking into account other prevailing circumstances and factors, such as technical feasibility, socioeconomics, expectations of interested parties and others, as relevant (Yankovich et al 2014a). The information gained through this process should be documented in the remediation plan (IAEA 2022). For example, the remediation plan could include the projected doses prior to remediation, the proposed remedial actions and other protective actions to be undertaken, the residual doses after implementation of each remedial action and the overall remediation that are anticipated, the anticipated effectiveness of remediation, relevant criteria (see section 4), the time-line and other relevant information.

Once developed, the remediation plan should then be submitted to the regulatory body or other relevant authority for approval and if/once approved, should be used as a roadmap in the implementation of remediation (section 3.4).

3.4. Implementation and verification monitoring

Implementation of the approved remediation plan is undertaken in Phase 4 of the remediation process, i.e. Implementation and verification monitoring (see figure 1). Over the course of implementation of the remediation plan, source, environmental, and as appropriate, individual monitoring needs to be undertaken to verify that remediation in carried out in accordance with the approved remediation plan. This includes verification that approved remedial actions are meeting relevant criteria indicating their effectiveness (see section 4) and that exposure is not exceeding approved criteria indicating that adequate protection of people and the environment is being achieved.

As the end of the implementation of remediation is approaching, the conditions on the site or in the area where remedial actions and other protective actions are being undertaken (including activity concentrations in environmental media in the receiving environment, radiation exposure, residual dose and other criteria) need to be compared to relevant criteria, as documented in the approved remediation plan. This is to verify that the approved end state conditions and the overriding end state criterion have been achieved (see section 4 below). Once this has been done, it becomes possible to terminate regulatory control for the site or area that has been remediated and to enter the post-remediation management phase of the remediation process (section 3.5; figure 1).

3.5. Post-remediation management

The final phase in the lifetime of remediation (i.e. Phase 5) is post-remediation management. Post-remediation management is as important as each of the other phases, and its planning should be initiated at the commencement of planning of the remediation itself.

During this phase, any post-remediation control measures, as defined in the approved remediation plan, need to be put into place. In doing so, depending on the approved end state and end use of the site or area, which have been detailed in the remediation plan, in many cases, restrictions need to be put into place, as appropriate, and monitoring and surveillance needs to continue, as necessary, depending on the site-specific conditions. Throughout post-remediation management, exposure and corresponding residual doses need to be compared to relevant criteria, as described in section 4 that follows.

Crucially important aspects of effective post-remediation management are adequate collection and maintenance of all records (including those pertaining to the nature and extent of contamination; the decisions made before, during and after remediation; and information on verification of the results of remedial actions) and continuation of regular communication and consultation with interested parties.

4. Establishment of criteria in support of evaluation and decision-making

An important aspect of evaluating the effectiveness of remediation, as well as whether any anticipated adverse effects might be significant in comparison to actual radiation exposure (and residual dose), is the establishment of relevant criteria. Such criteria need to be fit-for-purpose and will differ in different phases of the lifetime of remediation (see figure 1). As discussed in earlier sections, depending on site- and facility-specific factors, a diverse range of conditions can occur in existing exposure situations. As a result, it is necessary to identify and set radiological criteria that are relevant to the situation and conditions on a case-by-case basis, taking account of prevailing circumstances (e.g. technical feasibility, socioeconomic factors) and applying the principles of justification, optimization of protection and safety, and a graded approach (see section 2). In doing so, given the uniqueness of existing exposure situations, it is necessary to allow adequate flexibility in setting criteria, including the reference level, which may result in agreed long-term criteria that are less stringent than the 'firm' dose limit and dose constraints that are established in a planned exposure situation (European Commission et al 2014).

A reference level is 'the level of dose, risk or activity concentration above which it is not appropriate to plan to allow exposures to occur and below which optimization of protection and safety would continue to be implemented' (IAEA 2019). Once the reference level has been set, it is then used as a starting point for optimization of protection and safety to minimize radiological doses to the extent possible. In an existing exposure situation, the reference level is to be set within a range of 1–20 mSv a⁻¹ (ICRP 2007, European Commission et al 2014), depending upon the prevailing circumstances (e.g. nature of exposure; feasibility of preventing or reducing exposure; experience; expectations of interested parties, also called stakeholders; resource availability of resources) (European Commission et al 2014). In addition to the reference level, for practicality and efficiency, corresponding derived criteria that are easily measurable might also be established (IAEA 2022). For example, this could include a 'derived reference level', which is defined as a 'numerical value expressed in an operational or measurable quantity, corresponding to the reference level set in dose' (ICRP 2014).

4.1. Application of criteria in justification

The reference level represents the maximum dose that is considered acceptable in an existing exposure situation and falls within the range of 1–20 mSv, or other corresponding quantity (European Commission et al 2014). Above the reference level, it is not appropriate to plan to allow exposures to occur, and below this level, optimization of protection and safety continues to be implemented (IAEA 2019).

As discussed earlier, for sites or areas contaminated with residual radioactive material, the process of remediation⁶ should be undertaken in accordance with the five phases discussed in section 3 above (as depicted in figure 1) (IAEA 2022). In the preliminary evaluation phase (section 3.1), for screening purposes, a relatively conservative screening criterion should be set and approved by the regulatory body or other relevant authority. This screening criterion should fall within the range of reference levels of 1–20 mSv a⁻¹, as defined in international recommendations for existing exposure situations (ICRP 2007, European Commission et al 2014). For example, as a starting point, the lower bound of the reference level range relevant to existing exposure situations (1 mSv a⁻¹) might be set as a screening criterion against which to compare during the preliminary evaluation phase of remediation. The projected dose prior to remediation should then be compared to the approved screening criterion to determine whether or not remediation 'might' be justified.

In cases where the projected dose prior to remediation is determined to exceed the approved screening criterion, it can be concluded that remediation might be justified and detailed evaluation (section 3.2) should be undertaken to confirm whether or not remediation is, in fact, justified. As part of the detailed evaluation phase, the reference level should be established in consultation with interested parties, taking account of the prevailing circumstances. The reference level that is determined through this process will then need to be approved by the regulatory body or other relevant authority. The projected dose prior to remediation should then be compared to the approved reference level to determine if remediation is justified; if the reference level is exceeded, remediation should be deemed justified.

4.2. Application of criteria in optimization of protection and safety

An early step in the planning phase of remediation (section 3.3), involves taking a decision on the desired end state to be achieved as a result of remediation of a site or area. The end state is defined as 'the final status of a site or area at the end of activities for decommissioning and/or remediation, including approval of the radiological and physical conditions of the site and remaining structures' (IAEA 2019, 2022). To achieve the desired end state, a corresponding overriding end state criterion (e.g. an acceptable annual risk factor which will equate to an annual dose) will need to be established, for example, on the basis of the current and future use intended for the land and its resources, resources availability, and other factors (IAEA 2022). This end state criterion is defined as 'a set of conditions that need to be met to verify that remediation has been completed and the defined end state has been achieved' (IAEA 2022). Ultimately, achievement of the end state will facilitate the release of all or part of a site or area from regulatory control, setting restrictions, as appropriate, and subsequent post-remediation management (section 3.5; IAEA 2022). By definition, the end state criterion will be more stringent than the reference level (since, by definition, the reference level is the level 'above which it is not appropriate to plan to allow exposures to occur and below which optimization of protection and safety would continue to be implemented'; IAEA 2019).

4.2.1. Definition of final end state and the post-remediation end state criterion and interim end point criteria

During remediation planning, in addition to identifying the desired end state, it is also necessary to determine relevant end points (equivalent to 'milestones') to be achieved at different times over the course of implementation of remediation, as detailed in the approved remediation plan. It is also necessary to define the end state criterion for the overall remediation, and the corresponding intermediate end point criteria within the environmental impact assessment (EIA) for the remediation (IAEA 2022), along with the estimated projected dose prior to remediation and the residual dose after completion of individual remedial actions and the overall of remediation. The expected dose reduction, represented as the difference between the projected dose before remediation and the residual dose after remediation, should be documented in the EIA and should be later used to verify that the remediation is being implemented in accordance with the remediation plan, and ultimately, that it has been completed (i.e. the desired end state has been achieved).

A specific end point can be utilized to indicate completion of an individual remedial action or group of related remedial actions. For each end point, it will be necessary to establish an end point criterion, which is 'typically the level of contamination beyond which further decontamination or remediation is considered unnecessary' (IAEA 2019). The end point is 'often calculated on the basis of a level of dose or risk that is considered acceptable' (IAEA 2019, 2022).

4.2.2. Application of interim end point criteria and the final end point criterion in decision-making and verification over the lifetime of remediation

As discussed in section 3, remediation is a systematic, step-wise process consisting of 'Preliminary evaluation', 'Detailed evaluation', 'Planning', 'Implementation and verification monitoring', and 'Post-remediation management' (see figure 1). As a result, end point criteria can be set to indicate when a given phase within the remediation process has been completed and the next phase should begin (IAEA 2022). In addition, during implementation and verification monitoring, end point criteria can serve as benchmarks against which to evaluate the progress and effectiveness of remediation against the approved remediation plan (see section 3.4). Such comparisons need to be documented, for example, to serve as a record of site or area conditions and to respond to future questions that may arise regarding how decisions had been made over the course of remediation (e.g. when updating the remediation plan, when establishing controls and/or setting restrictions or when terminating regulatory control of remediated sites or areas).

In cases where a comparison of the residual dose following remediation to the end point criterion for a given remedial action or set of remedial actions indicates that the remediation is not being implemented as planned, it will be necessary to review and update, as appropriate, the remediation plan, and to conduct an assessment to verify that any changes to the remediation plan will lead to the planned end state for remediation, as specified in the EIA or other related regulatory requirements or conditions for the remediation (IAEA 2022). The iterative nature of the remediation process provides flexibility to counter uncertainty, for example, in cases where unanticipated results or conditions are revealed during remediation.

Once remediation has been completed, surveys of the site or area will need to be undertaken, and a dose assessment conducted (based on monitoring data and past surveys), to verify that the end state criterion has been met. This verification includes a comparison of the residual dose after remediation against the dose that had been specified in the EIA prior to initiation of remediation and approved as part of the conditions to be achieved to meet the end state criterion. Once it has been verified that the end state criterion has been met, the post-remediation management phase of remediation can then be initiated (see section 3.5).

5. Practical application of the international system of radiological protection

As described in section 2, an international system of radiological protection has been established for protection of people and the environment from harmful effects of ionizing radiation now and in the future. Integral to this system of protection is the assessment and comparison of projected dose prior to remediation relative to residual dose after remediation, and the setting of fit-for-purpose criteria that are appropriate to the situation and the prevailing circumstances. Such fit-for-purpose criteria include the end state criterion, which is applied to demonstrate that the final end state has been achieved for a remediated site or area (e.g. based on intended land-use), and that end point criteria, which are used to indicate the completion of specific actions or sets of actions over the course of the remediation process, have been met. From a practical perspective, such criteria need to be scientifically defensible and measurable, and should be set with consideration of the situation and the prevailing circumstances within a systematic, structured, step-wise process for remediation (as depicted figure 1) (IAEA 2022).

Application of the remediation process presented here provides a framework to justify decisions to proceed with remediation or not, depending on the situation, and to weigh up different factors in identifying appropriate (or 'fit-for-purpose') remedial options as part of optimization of protection and safety.

5.1. Application of the remediation process in international model validation programs

Working Group 1 (WG1) of the IAEA's Modelling and Data for Radiological Impact Assessments I (MODARIA I) program and WG1 of the MODARIA II program that followed have focused on the systematic application of decision-aiding tools in support of decision-making in remediation⁷ . Consistent with figure 1, in doing so, a robust, systematic, stepwise approach is applied in support of radiation decisions for remediation in different types of sites or areas contaminated with residual radioactive material. In this paper, the application of the remediation process depicted in figure 1 is demonstrated for remediation of areas on the Maralinga former nuclear weapons testing site in Australia (see section 5.2.1), and the Taparura Project, which involved the remediation of a site in Tunisia that was affected by past activities relating to phosphate processing (see section 5.2.2). IAEA (2022) provides additional examples of applying this phased remediation process (i.e. for planning and implementation of remediation following the Chernobyl and Fukushima Daiichi accidents in the Ukraine and Japan, respectively).

5.2. Application of remediation process in case studies

During the development of IAEA General Safety Guide GSG-15 on Remediation Strategy and Process for Areas Affected by Past Activities or Events (IAEA 2022) and the work of WG1 of the IAEA's MODARIA I and II international model validation programs, decision-aiding approaches and tools were evaluated and systematically applied for a number of case studies (e.g. see Pepin et al and Kontić et al, this Special Issue). In this paper, two cases (Maralinga and Taparura) are presented for which remediation decisions were made consistent with the systematic remediation process recommended in GSG-15 (IAEA 2022). The key assumptions and decisions for each of these case studies are presented in the sections that follow.

5.2.1. Maralinga case study

Between 1952 and 1957, 12 atmospheric nuclear weapons tests (referred to as 'major trials') were conducted by the United Kingdom at three locations in Australia including seven on the former Maralinga nuclear test site in South Australia, a site that is now under civilian control (Crouch et al 2009, IAEA 2022). The remaining major trials were conducted at the Monte Bello Islands off Western Australia (three tests) and at Emu in South Australia, north of the Maralinga site (two tests). The seven major trials that were conducted at Maralinga in 1956 and 1957 ranged in size from 1 to 27 kT, resulting in deposition of fallout and neutron activation products in the immediate vicinity of the 'ground zeros' on the Maralinga site. In addition, 600 'minor trials' were conducted on the site, resulting in dispersal in the local environment of long-lived radionuclides (including natural and depleted U, Am and 24 kg of Pu). It was the presence of this highly radioactive material, ranging in size from inhalable dust to collectible fragments, that necessitated the large-scale remediation of the Maralinga lands.

Weapons testing activities on the Maralinga site were suspended in 1964, pending a final decision regarding future use of the site and remedial actions were taken (e.g. ploughing to mix contamination with clean soil, thereby reducing average activity concentrations); this had been considered to be the 'final remediation' by the UK and the Australian governments at the time. However, many thousands of visible plutonium-contaminated fragments (e.g. wire, rusty steel plate, lead, pieces of a grey metal alloy of low density, bitumen and yellow Bakelite) continued to be found on the surface outside of the ploughed area, for example, close to the firing pads, even 30 years later.

The Aboriginal peoples who owned the land at Maralinga wanted to resume its possession and full use; however, despite past remediation activities on the land, a scientific survey conducted in May 1984 revealed the presence of large numbers of particles and fragments containing high levels of uranium, plutonium and americium.

The nature of the material that was discovered and the potential impacts were discussed with local Aboriginal peoples and a remediation process, consistent with figure 1, was initiated. Table 1 provides a summary of the decisions made and the actions undertaken during each phase of remediation. Additional information on the Maralinga case study is provided in IAEA (2022).

Table 1. Example of the application of a systematic, stepwise approach to remediation taken at the Maralinga nuclear weapons testing site in Australia (consistent with figure 1).

	Phase of remediation	Description of key context and decisions
1, 2	Preliminary evaluation	Large-scale site surveys: Characterization of the nature of the hazard (types of radionuclides, external dose rates, inhalation risks, presence of contaminated particles) and of contamination (particle size distributions, dust loadings, radionuclide solubilities and chemical properties); Mapping of spatial extent of contamination through aerial and ground-based surveys; Determination of radionuclide uptake by plants and animals and environmental effects. Conclusion: Major radiological hazard at Maralinga was inhalation of plutonium.
2	Detailed evaluation	Selection of Reference Level: Radiation protection criteria were set, in consultation with Aboriginal traditional landowners: Dose criteria for exposures due to inhalation and due to external radiation; A practical rationale for the removal of contaminated particles. Outcomes: Agreed criterion for 'rehabilitation' was set based on 1990 radiation protection system and dose assessment: After remediation, annual risk of fatal cancer following the inhalation and ingestion of contaminated soil was not to exceed 1 in 10 000 by the 50th year of exposure (equivalent to an annual dose of 5 mSv for 50 years from birth) (Department of Primary Industries and Energy 1990; Department of Education, Science and Training 2002). Agreed pragmatic criteria for removal of radioactive particles and fragments (noting that ²⁴¹Am was a readily-measurable field indicator of Pu activities): Removal of all particles and fragments with ²⁴¹Am activity exceeding 100 kBq; Surface density of remaining particles with ²⁴¹Am activity exceeding 20 kBq was not to exceed 1 particle per 10 m². Based on dose assessments with conservative assumptions, a 'remediation action level' of equivalent to a dose of 5 mSv a⁻¹ for exposure from soil contamination that was not to be exceeded was set and used to guide day-to-day remedial actions (e.g. for inhalation of dust and 100% occupancy, this equated to 3 kBq m⁻² of ²⁴¹Am; by limiting occupancy to that typical of hunting activities at a particular location, this equated 40 kBq m⁻² of ²⁴¹Am).
3	Planning of remediation	Identification and evaluation of remedial options: Formation of Technical Assistance Group (TAG) to propose and estimate costs for a range of remedial options Outcomes: Agreement by Australian government, in consultation with traditional Aboriginal landowners (Maralinga Tjarutja), South Australian and UK governments, on possible remedial options for further consideration (condensed from a list of 9 main options and 26 sub-options that was compiled by TAG) (Department Of Primary Industries And Energy 1990): Option 1: Fencing of 120 km² contaminated area with a high cyclone-mesh fence (13 million Australian dollars); Option 2: Burial of all contaminated soil collected from a 120 km² area (650 million Australian dollars); Option 3: Combination of parts of Options 1 and 2; Option 4: In-situ vitrification or pressure grouting of the Pu-contaminated waste buried in shallow pits, to isolate from the biosphere (20–30 million Australian dollars).
		Development of remediation strategy and plan: A remediation strategy and plan were drafted and consultations held with regulators and landowners for their agreement; following implementation, their verification and signoff was then required. A 120 km² 'non-residential area' was established covering areas where expected annual inhalation dose was estimated to exceed the 5 mSv a⁻¹ 'remediation action level', based on conservative assumptions. Transitory activities (e.g. hunting, travel) were permitted within this 'non-residential area' and routine use was discouraged (e.g. by removal of some defined access routes, revegetation of some areas, and improvement of routes bypassing this area to encourage their use). Removal of contaminated soil, particles and contaminated debris from areas with >40 kBq m^{−2 241}Am activities (averaged over 1 ha). Planning remedial actions (e.g. soil removal) to minimize the likelihood of future remediation. In-situ vitrification (ISV) of half the material in legacy disposal pits, and excavation and on-site burial of the other half in a purpose-built trench with a clean cover (the ISV process was halted mid-operations following safety concerns, and the fall-back option of burial was adopted). Funding and financing: Financing arrangements were negotiated and agreed, including compensation to the Aboriginal peoples.
4	Implementation and verification monitoring	Application of remediation criteria in implementation of remediation: Remediation boundaries were set based on agreed remediation criteria (described above): Relevant radionuclides were measured to set boundaries for soil removal (EG&G 1988); Contaminated particles and fragments with >100 kBq ²⁴¹Am activity and surface densities of particles with >20 kBq ²⁴¹Am activity were detected systematically over manageable-sized 'Lots' by vehicle-borne radiation detectors and removed. Remedial actions: Zoning of soil removal areas into individual 3–4 ha areas ('Lots'); Surface soil removal by use of heavy-duty scrapers; Soil and contaminated fragment collection and placement in excavated disposal trench. Assessment of remediation effectiveness: Verification monitoring was conducted on remediated areas (Cooper et al 2000), including: Use of hand-held monitoring equipment to search for highly localized contamination (including contaminated fragments) and contamination removal, as necessary, based on agreed criteria; Land surveys by use of high-resolution gamma spectrometry on ∼35 m resolution square grids. Verifying that criteria for release of land from regulatory control had been met involved comparing measured data to remediation criteria. Upon verification that criteria were met, a 'clearance certificate' was issued by the regulatory body (similar to confirmation that the end state criterion was met; see section 4). Post-remediation dose assessment: A comprehensive post-remediation dose assessment was conducted to verify that the remediated area was safe (in accordance with the agreed dose criteria) and to evaluate whether further remediation could be beneficial.
		Outcomes: It was verified that residual doses in remediated areas fell well below agreed dose criteria for all envisaged land uses. It was determined that restriction on occupancy within the non-residential area of 'restricted land-use' was purely precautionary (with few exceptions, doses to permanent occupants in all but a few areas fell ≪1 mSv a⁻¹). Restrictions were established in the area of the new burial trenches to discourage intrusion. Access restrictions were established in the plume deposition areas adjacent to areas of soil removal to reduce the probability of contact with undiscovered contaminated particles or fragments. The revised ICRP human respiratory tract model (International Commission On Radiological Protection 1994) was used to generate more realistic dose estimates; estimated inhalation doses from Pu and Am decreased significantly. The combination of applying the revised ICRP dosimetry model (International Commission On Radiological Protection 1994) and the more effective than anticipated remediation led to a maximum estimated inhalation dose of 3.6 mSv a⁻¹; it was, therefore, concluded that it was not strictly necessary to designate a restricted area to meet the 5 mSv a⁻¹ regulatory criterion for the inhalation exposure pathway. It was estimated that inhalation doses to casual visitors (e.g. tourists, geological prospectors, surveyors) occasionally visiting the site without engaging in activities causing abnormal resuspension of dust or large-scale disturbance of soil would fall ≪1 mSv. Under calm conditions, negligible doses are estimated. Under temporary exposure to high dust loading, e.g. during severe dust storms, negligible doses are estimated. Based on a future, updated dose assessment, further consideration will be given to decreasing (limiting) the size of the restricted area to only cover the burial trenches and the inner plume deposition areas.
5	Post-remediation management	Possible changes in future public exposure pathways were identified: Potential for exposure to Pu over many hundreds of years due to long radioactive half-lives; Weathering and transport (e.g. by saltation and resuspension) of surface contamination; Pu migration deeper into soil without off-site migration, leading to lower inhalation doses; Changes in lifestyle (e.g. adoption of a less traditional lifestyle by Aboriginal peoples), leading to much lower inhalation doses from dust (e.g. due to concrete and tarmac roads, living in modern houses). Ongoing monitoring of radionuclide levels and other relevant parameters over time and updating dose assessments for Aboriginal landowners and visitors. Maintaining ongoing regular communication and consultation by regulators and radiation safety experts with the Indigenous Aboriginal landowners is essential, especially on future decisions regarding land use and removal of restrictions, as appropriate, with time.

Evaluation of the remediation of the Maralinga site has confirmed the uniqueness of each existing exposure situation, as defined by the prevailing circumstances (IAEA 2022). In the case of Maralinga, the prevailing circumstances included:

Local environmental and sociological aspects;

The relevant radionuclides and the corresponding exposure pathways and doses, and the ease with which they could be detected;
Site-specific conditions (e.g. availability of applicable technologies, resource availability, socioeconomic factors, political factors); and

Influence of site-specific factors on cost-benefit analysis (e.g. land value, disposal options and costs).

In the era when the nuclear weapons testing was conducted (1950s and 1960s) and initial remediation was undertaken, the Maralinga lands were not valued by governments or society, in general, and decisions were taken on the basis that the area was considered uninhabitable and without consultation with the Indigenous Aboriginal landowners. As a result, decisions were made, such as the indiscriminate and undocumented burial of material in shallow pits and ploughing to dilute surface contamination, which created significant challenges 40 years later when it was decided to undertake additional remediation and to return the now highly valued land back to its original Aboriginal owners.

Due to the complexity of the situation and prevailing circumstances, implementation of a systematic, stepwise remediation process with manageable steps, consistent with figure 1, was beneficial and contributed to successful outcomes, including (IAEA 2022):

Establishment of a clear and defensible reference level;
Establishment of pragmatic remediation criteria to guide remedial actions (i.e. derived criteria for soil contamination levels that could be linked to dose; criteria for contaminated particles, which were based on a statistical assessment of the probability of finding, and then being exposed to, the radiation from particular size ranges of particles and fragments);
Systematic characterization to quantify mean contamination levels over realistic areas; and
Demarcation of the total contaminated area into smaller areas, which could then be treated individually.

This ultimately led to the exceedance of expectations regarding remediation effectiveness, and the satisfaction of the traditional landowners (the Maralinga Tjarutja), who regained sovereign possession and use of the land, which was previously highly restricted.

5.2.2. Taparura case study

To demonstrate the applicability of the remediation process depicted in figure 1 to diverse situations, this process has also been applied to a phosphate processing site in Tunisia, where past activities were not carried out in accordance with current standards (IAEA 2013). Unlike the Maralinga case study, where radioactive material was dispersed over a relatively wider area, the Taparura case study provides an example of a situation where there is residual radioactive material related to a NORM site (i.e. covering a smaller area).

The Taparura phosphate plant, located on the harbour of Sfax, Tunisia, operated between 1962 and 1991, releasing phosphogypsum and other residues directly onto beaches and in shallow offshore areas. During operations, a large phosphogypsum stack (i.e. aboveground pile), covering an area of approximately 50 ha, with a height of up to 8 m above sea level, accumulated near the harbour and the town (Société d'études et d'aménagement des cotes Nord de la Ville de Sfax 2002). Surrounding the phosphogypsum stack was a crusty phosphogypsum layer with a depth of up to 3 m and covering a 90 ha area. As a result of the contamination from the Taparura facility and the phosphogypsum stack, it was necessary to prevent access to the beaches and to prohibit swimming (Société d'études et d'aménagement des cotes Nord de la Ville de Sfax 2002). This led to limitations regarding develop adjacent to the facility and economic growth (e.g. tourism).

To address the environmental issues associated with the site, in 1985, a decision was taken by relevant authorities to remediate and redevelop the zone that was affected by the phosphogypsum releases in the areas adjacent to the Taparura site to allow use of the beach by local citizens (Goudier and Choura 2013). The actions undertaken and decisions made in remediation of the Taparura site are presented in table 2, applying the remediation process depicted in figure 1.

Table 2. Example of the application of a systematic, stepwise approach to remediation taken at the Taparura phosphate plant in Sfax, Tunisia (consistent with figure 1).

	Phase of remediation	Description of key context and decisions
1	Preliminary evaluation	Preliminary evaluation revealed the presence of heavy metals (e.g. Cd, As, Ni) and radionuclides from the U decay chain, and to a lesser extent, from the Th decay chain in the phosphogypsum residue. A ²²⁶Ra activity concentration of ∼0.4 Bq g⁻¹ dry mass was measured in the residue. Public exposure pathways included inhalation of radon gas released from the phosphogypsum stack and radionuclides migrating into groundwater and surface waters (e.g. the sea) (Van der Heijde et al 1988, Haridasan et al 2001). Radionuclides (e.g. ²¹⁰Po) were significantly elevated in water, sediments and biota. Public exposure was estimated to range from 2 to 10 μSv a⁻¹ (typical) and up to 150 μSv a⁻¹ (maximum), excluding exposure due to radon emanation from phosphogypsum (the latter of which can be significant in cases where there are dwellings on soil with phosphogypsum contamination, with no preventive measures against radon ingress).
2	Detailed evaluation	Detailed evaluation involved conducting several comprehensive studies to characterize the spatial extent of the contaminated area (on land, in shallow sea adjacent to the phosphogypsum stack) and estimate the amount of material for removal. The first phase of remediation of the Taparura site, to gain control of contamination sources, was initiated in 1997, following a decision of the Tunisian government (Centre national de radioprotection 2009). Gamma surveys, dust monitoring and measurement of indoor and outdoor radon were conducted, as part of radiological characterization. Selection of Reference Level: A dose criterion for public exposure (which is equivalent to the end state criterion; see section 4) of <1 mSv a⁻¹ (above background) was set by the Tunisian Radiation Protection Centre for future use of the remediated Taparura site.
3	Planning of remediation	Identification and evaluation of remedial options: Based on the preliminary and detailed evaluation, it was determined that remediation was justified. It is important to note that the main justification for remediation of the Taparura site was not the radiological impact, which had been demonstrated to be moderate. Instead, there was a need to control non-radioactive contaminants, such as heavy metals, with an overarching goal of ecological restoration and urban and economic development. However, the planning and implementation of the remediation plan enabled optimization of protection and safety, taking account of radiological and non-radiological impacts and factors. Possible remedial options and technologies were evaluated based on characterization of the extent and degree of contamination, and an appropriate remedial option was identified based on the following criteria: Minimization of the size of the contaminated area through removal of significant quantities of contaminated material from the land and sea; Long-term physical stability of the confinement area; Minimization of contaminant transport from the confinement area to surrounding groundwater. Development of the remediation plan: A remediation plan was developed, which identified remedial options that had been selected: Excavation of contaminated material, dewatering of excavated material and disposal on the original phosphogypsum stack; Reshaping the slopes of the original stack, installation of a vertical barrier around its perimeter to stabilize a confined area, installation of a cover layer of uncontaminated sand, and placement of a layer of clean topsoil Based on design studies, it was determined an uncontaminated cover with a minimum thickness of 30 cm was needed to effectively attenuate gamma radiation and radon gas emanation from the phosphogypsum stack.
		Funding and financing: An autonomous government company (Société d'Etudes et d'Aménagement des Côtes Nord de la Ville de Sfax) was responsible for implementation of remediation of the Taparura site under the supervision of the Ministry of Housing. Remediation cost was €75 million and was financed through taxes, loans from the European Investment Bank, and loans from Belgian and French financial institutions. Gaining approval: Competent Tunisian authorities (Ministry of Equipment, Housing and Territory Planning, the Ministry of Environment and Sustainable Development, the National Agency of Protection of the Environment, and the Agency of Protection and Development of the Littoral) reviewed and approved the preliminary and detailed evaluations and the proposed remedial option (Turki-Abdelhedi and Ghorbel-Zourai, 2012).
4	Implementation and verification monitoring	Implementation of remediation involved: Reconfiguration of the phosphogypsum stack from a rectangular to a circular, terraced structure (stack diameter ∼ 0.9 km). Construction of a 12 m deep barrier, embedded in bentonite, with a concrete foundation, around the phosphogypsum stack to prevent lateral transport of contaminated water from the stack to adjacent groundwater. Excavation of ∼1.7 × 10⁶ m³ of various materials and incorporation into the stack (including 7.87 × 10⁵ m³ of material from the excavation and reconfiguration of the stack from a rectangle to a circle; 4.65 × 10⁵ m³ of phosphogypsum dredged from the sea). Excavation and relocation of ∼8 × 10⁶ m³ of material (uncontaminated sand) for backfilling of remediated areas and creation of new beaches. Assessment of remediation effectiveness: To verify the effectiveness of remediation and gain approval for termination of regulatory control (Centre national de radioprotection, 2008; Société d'études et d'aménagement des cotes Nord de la Ville de Sfax, 2009), several radiological surveys, along with monitoring, were conducted after remediation, including: Gamma dose rate surveys following backfilling of remediated areas; Radon measurement campaign; Groundwater and stack seepage water monitoring (stack seepage water was highly acidic prior to remediation and was neutral after remediation).
5	Post-remediation management	As part of a larger urban and economic redevelopment project, in addition to remediation of the Taparura site, restoration activities were planned to recreate beach facilities near the town and to reclaim additional land from the sea in support of urban expansion and tourism (Société d'études et d'aménagement des cotes Nord de la Ville de Sfax, 2002; Turki-Abdelhedi and Ghorbel-Zourai, 2012; IAEA 2019) Post-remediation activities were undertaken, including: Groundwater monitoring downgradient of the stack and automated management of groundwater levels, involving pumping water out when the water level height approaches 10 cm below that of the groundwater outside the water barrier; Collection and pumping of seepage water downgradient of the stack; Collection and redirection of surface runoff into flood control channels that discharge to the harbour; Maintenance of stack cover and of the discharge channel (Société d'études et d'aménagement des cotes Nord de la Ville de Sfax, 2002); Establishment of restrictions prohibiting construction of buildings on the remediated stack, unless preventive measures are incorporated to prevent radon ingress (Société d'études et d'aménagement des cotes Nord de la Ville de Sfax, 2002).

Through evaluation of the Taparura case study, radiation protection as only one aspect within a broader, more encompassing environmental impact assessment was highlighted, resulting in the need to consider all relevant hazards and impacts (physical, radiological, chemical, biological), along with the prevailing circumstances (e.g. social, economic), as part of remediation planning and implementation (Yankovich et al 2014a, IAEA 2022; also, see Pepin et al 2021, this Special Issue). In the case of the Taparura site, remediation of contaminated areas was only the first phase of a more extensive project involving urban and economic development.

6. Summary and conclusions

The issue of sites and areas contaminated with residual radioactive material covers a wide and diverse set of conditions and prevailing circumstances, making it highly unlikely that any two such situations will be exactly the same. This is further highlighted when considering that a combination of scientific, technical, social, environmental, economic, and political factors can drive remediation decisions to varying extents, depending on the prevailing circumstances (Yankovich et al 2014a, Kontić et al 2021, this Special Issue). As a result of this complexity, in order to effectively progress in remediation of contaminated lands and in support of decision-making, it is necessary to take a systematic, stepwise approach in evaluating possible options in a given case.

Application of the international system of radiological protection (e.g. optimization of protection and safety), making use of a process with defined steps (e.g. as depicted in figure 1; also see IAEA 2022) can, therefore, serve as a useful tool to evaluate and balance different factors and their relative importance in the overall protection and safety of people and the environment in support of remediation planning and implementation.

WG1 of MODARIA I and II have focused on evaluation of approaches and tools in support of remediation decisions in existing exposure situations. In doing so, actual case studies have been systematically considered, applying consistent conceptual frameworks to facilitate comparison and to identify examples of good practice and key international lessons beyond the background variability that exists between different cases for which remediation is being considered.

The current paper summarizes the international system of radiological protection and current IAEA recommendations on remediation of sites and areas contaminated to residual radioactive material, based on the work undertaken on this topic in MODARIA I and II, and on discussions during the development and finalization of the updated IAEA Safety Guide GSG-15 on Remediation Strategy and Process for Areas Affected by Past Activities or Events (IAEA 2022). In particular, GSG-15 (IAEA 2022) presents a stepwise framework that sub-divides the remediation process into a series of manageable steps (see figure 1). This framework is applied to two quite different case studies, a former nuclear weapons test site (Maralinga, Australia) and a historic phosphate site which had not been operated in accordance with current standards (Taparura site, Sfax, Tunisia).

In each case, despite the uniqueness of the situation and the prevailing circumstances between these two sites, by following a stepwise remediation process, it was possible to systematically compile information in support of decision-making and achieving effective remediation outcomes.

Practical application of international recommendations and safety standards in the systematic planning and implementation of remediation of sites or areas with residual radioactive material

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Dates

Abstract

1. Introduction

1.1. Objectives

2. Overview of the international system of radiological protection

3. Stepwise process for remediation planning and implementation

3.1. Preliminary evaluation

3.2. Detailed evaluation

3.3. Planning of remediation

3.4. Implementation and verification monitoring

3.5. Post-remediation management

4. Establishment of criteria in support of evaluation and decision-making

4.1. Application of criteria in justification

4.2. Application of criteria in optimization of protection and safety

4.2.1. Definition of final end state and the post-remediation end state criterion and interim end point criteria

4.2.2. Application of interim end point criteria and the final end point criterion in decision-making and verification over the lifetime of remediation

5. Practical application of the international system of radiological protection

5.1. Application of the remediation process in international model validation programs

5.2. Application of remediation process in case studies

5.2.1. Maralinga case study

5.2.2. Taparura case study

6. Summary and conclusions

Footnotes

Practical application of international recommendations and safety standards in the systematic planning and implementation of remediation of sites or areas with residual radioactive material

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Share this article

Author e-mails

Author affiliations

Author notes

ORCID iDs

Dates

Abstract

1. Introduction

1.1. Objectives

2. Overview of the international system of radiological protection

3. Stepwise process for remediation planning and implementation

3.1. Preliminary evaluation

3.2. Detailed evaluation

3.3. Planning of remediation

3.4. Implementation and verification monitoring

3.5. Post-remediation management

4. Establishment of criteria in support of evaluation and decision-making

4.1. Application of criteria in justification

4.2. Application of criteria in optimization of protection and safety

4.2.1. Definition of final end state and the post-remediation end state criterion and interim end point criteria

4.2.2. Application of interim end point criteria and the final end point criterion in decision-making and verification over the lifetime of remediation

5. Practical application of the international system of radiological protection

5.1. Application of the remediation process in international model validation programs

5.2. Application of remediation process in case studies

5.2.1. Maralinga case study

5.2.2. Taparura case study

6. Summary and conclusions

Footnotes