Neutronic performance of 208Pb-Bi eutectic-cooled fast reactor with uranium nitride fuel (enriched 15N)

Fast reactors (FRs) require more uranium enrichment than LWRs and fuel reprocessing. The solution proposed to address these issues is to use the modified CANDLE burnup scheme on the reactor core. The use of uranium nitride (UN) as fuel was proposed because the UN fuel has several advantages, such as high melting point, high heavy metal density, and large thermal conductivity. However, using UN as a fuel can produce the radioactive isotope 14C via the reaction 14N (n, p)14C. Therefore, in this study, a neutronic analysis was performed in a modified CANDLE fast reactor employing UN and UN (enriched 15N) as fuel using SRAC. The modified CANDLE fast reactor fuelled with UN (99% 15N) has a higher keff than the UN-fuelled reactor with similar parameters. Similarly, the atomic density of fissile nuclides and power density are higher because the 15N isotope has a smaller neutron capture cross-section than the 14N isotope.


Introduction
Fast reactors (FRs) were predicted to be operational between 2020 and 2030.Moreover, by 2050, about 1,500 GWe will be installed [1].This long-term scenario will provide an expanded preference regarding the design of nuclear power plants (NPP) that will be built in Indonesia in the future to support net zero emissions (NZE) in 2060.It was scheduled to analyze the reactor design criteria in 2024.The schedule is stated in the Ministry of Energy and Mineral Resources of the Republic of Indonesia (KESDM RI) Strategic Plan 2020-2024 [2,3].
NPP is one of the energy sources that will contribute to the roadmap of Indonesia towards NZE 2060 because it is one of the pollution-free alternative energy sources.Therefore, it has no contribution to global warming.In addition, nuclear power plants are also economical compared to fossil fuels [4].
However, fast reactors with open-fuel cycles require higher uranium enrichment than Light Water Reactors (LWR).The considerable enrichment of uranium-235 isotopes leads to the inefficient use of natural uranium.Meanwhile, fast reactors with closed fuel cycles require reprocessing [5,6].Those are challenges for developing countries such as Indonesia because the enrichment and reprocessing of fuel are expensive and sensitive [7].
One of the efforts to improve uranium utilization is to use burning schemes that can consume natural uranium directly as fuel.CANDLE (Constant Axial shape of Neutron flux, nuclides densities, and power shape During Life of Energy production) [8,9], Modified CANDLE [7,[10][11][12][13]. and Traveling Wave Reactor (TWR) [14][15][16] are three burnup designs that can use natural uranium as fuel input directly.
This study analyzes the Modified CANDLE (MCANDLE) burnup scheme for a fast reactor with 208 Pb-Bi-eutectic coolant and uranium nitride (enriched 15 N) fuel.It is a simple modification of the CANDLE burnup scheme by introducing a discrete region division in the radial direction.This burnup strategy resembles the Traveling Wave Reactor (TWR) design of Terrapower.
The core is divided into six regions with an equal volume in this burnup strategy.This burnup scheme only requires natural uranium as fuel input without enrichment.Fig. 1 depicts the reactor core divided according to the burnup scheme [13,17,18].In general, the fuel used in fast reactors is uranium dioxide (UO2).The fast reactors that employ nitride fuel (UN) include BREST-OD-300 and WH-LFR.It is due to the UN fuel having several advantages, such as high melting point, high heavy metal density, and large thermal conductivity [19][20][21].The use of UN fuel can also extend the fuel cycle in the core, resulting in lower electricity costs [22].
However, the use of UN as a fuel can produce the radioactive isotope 14 C through the reaction 14 N(n,p) 14 C [23,24].Nitrogen (N) has two stable isotopes, 14 N, which has about 99.6% abundance in nature, and the rest, 15 N.The utilization of UN fuel with enriched 15 N isotopes was offered as a solution to the 14 C production problem [25].In addition, the 15 N isotope has a lower neutron absorption crosssection than the 14 N isotope, which is advantageous in terms of neutron economy [26,27].Therefore, in this study, a comparison of the neutronic performance between the Modified CANDLE reactor with UN fuel and UN(enriched 15 N) fuel will be carried out.

Reactor Material and Design
In this study, the reactor core has a diameter larger than its height, commonly called a pancake core.Therefore, the axial load caused by the high heat flow is reduced.[12].The cladding material is High-Cr martensitic steel, HT9, then high-dose neutron exposure can be resisted [28,29].As a coolant, we selected 208 Pb-bi eutectic to improve reactor performance [18,30].It is because, among all the Pb isotopes, the 208 Pb isotope has the lowest neutron capture cross-section.Furthermore, it can also harden the neutron spectrum [18,23,31].Another advantage of this coolant material is that it has a higher level of safety than natural lead, especially since the margin between melting and boiling points is larger [13].Other parameters are from the results of previous optimal calculations.Table 1 depicts the design parameters for reactors.Fig. 2 illustrates the fuel cell design, and Fig. 3 illustrates the reactor core in the axial direction.The reactor core is divided into six equal-volume regions to facilitate radial refueling every 15 years.After 15 years of burnup, the core center loads fresh fuel (natural UN) and transports it to the sixth region.Then, as shown in Fig. 4, it gradually moved toward the second region before being removed from the core.This scheme places natural uranium as an inner blanket in the core, which is expected that reactivity will decrease at first and breeding will increase.The placement of natural uranium in the core is also known as "modified inward fuel shuffling."It has the potential to reduce reactivity.As a result, natural uranium is located next to the fuel with a high burnup level.The goal is to compensate for the reduction in reactivity during the operation time [13,32].

Method of Calculation
SRAC (Standard Reactor Analysis Code System) was used to perform neutronic calculations.SRAC is a calculation program developed by the JAEA (Japan Atomic Energy Agency) [33].The fuel pin calculations obtain the macroscopic cross-section at each burn-up step.Then, the results were applied to solve the multigroup diffusion equation and determine the effective multiplication factor (keff) and core power density.The nuclear data library, JENDL 4.0 (Japanese Evaluated Nuclear Data Library), was used for this calculation [34].

The effective multiplication factor (keff)
Fig. 6 shows a comparison of keff values for a modified CANDLE fast reactor fueled with UN and UN (99% 15 N) for similar parameter values.The UN (99% 15 N)-fueled reactor has a higher keff value than the UN-fueled reactor.Reactors with UN fuel have subcritical keff values in the first three years.Meanwhile, reactors with UN (99% 15 N) fuel are critical for up to 15 years, with an average reactivity of about 5% ∆k/k.It is because the 15 N isotope has a lower neutron absorption cross-section value than the 14 N isotope in almost all energy ranges, as shown in Fig. 7. Therefore, UN-fueled reactors with enriched 15 N isotopes have higher keff values than the UN-fueled reactors.

The enhancement of the criticality of UN-fueled reactors
Based on the results shown in Fig. 6, the UN-fueled reactor is subcritical at the beginning of the operation.Therefore, efforts are needed to increase its keff value; hence, the neutronic performance can be compared with UN (99% 15 N)-fueled reactors.One of the methods used in this research is to increase the volume fraction of the fuel.Fig. 8 shows keff values for the UN-fueled reactor with several variations in fuel volume fraction.The minimum fuel volume fraction required by the fast reactor with the UNfueled Modified CANDLE scheme to reach critical conditions is 67%.The maximum reactivity is only 0.6% ∆k/k.

Reactivity of reactors
Fig. 8 shows that the modified CANDLE fast reactor with UN-fueled fuel schemes requires an increase in fuel volume fraction to 67% for the reactor to have a keff value greater than one.In order to obtain an UN-fueled reactor design with a keff greater than one, the fuel volume fraction in the modified CANDLE fast reactor with UN (99% 15 N) fuel was also increased to 67%.Fig. 9 shows the reactivity comparison between the two reactors.The reactivity of the modified CANDLE fast reactor with UN fuel is lower than 1% ∆k/k for 15 years.Meanwhile, the modified CANDLE fast reactor with UN fuel (99% 15 N) has a reactivity higher than 5% ∆k/k.

Figure 9.
The reactivity over 15 years with a similar fuel volume fraction of 67% for both reactors Fig. 10 shows the atomic density changes of the three fissile nuclides ( 235 U + 239 Pu + 241 Pu) for both reactors in each region.Fig. 10 shows that the amount of fissile nuclides in the UN(99% 15 N)-fueled Modified CANDLE fast reactor is higher in almost all regions.The 15 N isotope has a much lower neutron capture cross-section than 14 N. Therefore, reactors with UN (enriched 15 N) as fuel will have a higher neutron population.These additional neutrons contribute to the overall neutron population, increasing the likelihood of fission events and resulting in a higher fissile yield compared to UN.Therefore, the UN-fueled (99% 15 N) modified CANDLE fast reactor has a higher keff value than the UN-fueled reactor.

Power density of reactors
Fig. 11 shows the power density for the Modified CANDLE fast reactor with UN and UN (99% 15 N) fuel.The power density for both reactors decreased in the EOC.The higher power density at BOC is due mostly to the larger concentration of fissile isotopes in fresh fuel.The depletion of fissile material and the accumulation of neutron-absorbing fission products as the reactor works contribute to a fall in power density by the time it reaches the EOC.The power density of the UN (99% 15 N)-fueled MCANDLE fast reactor is slightly higher than the UN-fueled MCANDLE fast reactor.It is because the neutron capture cross-section for the 15 N isotope is lower than that of the 14 N isotope; hence the neutron flux is higher.

Reduction of UN (99% 15 N)-fueled reactor reactivity
Fig. 9 shows that the UN(99% 15 N) -fueled modified CANDLE fast reactor has a reactivity of more than 5% ∆k/k.Therefore, in this study, the reactivity of the reactor was lowered to about 1% ∆k/k by reducing the fuel volume fraction.Fig. 12 shows keff for various fuel volume fractions of the UN (99% 15 N)-fueled reactor.It is shown that the minimum fuel volume fraction required to be critical is 54%, with reactivity below 1% ∆k/k over 15 years of operation.The fuel volume fraction required by the UN (99% 15 N)-fueled reactor is much smaller than the UN-fueled reactor.13 shows the keff values of the UN-fueled reactor with variations in 15 N isotope enrichment.For similar parameters, the minimum 15 N isotope enrichment required for the reactor to be critical is 10%.However, in a reactor core with nitride fuel, 14 C production is twice as large as in a reactor with oxide fuel.Therefore, 15 N isotope enrichment is required to reduce 14 C production.According to the study, the percentage of 15 N enrichment must be around 99% for 14 C production to be equivalent to that in reactors with oxide fuel [35].

Conclusion
The neutronic performance of the 208 Pb-Bi eutectic-cooled fast reactor with uranium nitride fuel (enriched 15 N) has been evaluated.The neutronic performance was compared with UN (Nat N).
The modified CANDLE fast reactor fueled with UN (99% 15 N) has a higher keff than the UN-fueled reactor with similar parameters.The UN (99% 15 N)-fueled reactor with a fuel volume fraction of 65% will obtain an initial keff of greater than one.Meanwhile, the UN-fueled modified CANDLE fast reactor has a keff of less than unity.It is because the 15 N isotope has a lower neutron capture cross-section than the 14 N isotope.
The minimum fuel volume fraction required by a Modified CANDLE fast reactor fueled with UN to critical is 67%.The maximum reactivity is only 0.6% ∆k/k.Meanwhile, the UN-fueled (99% 15 N) modified CANDLE fast reactor has a reactivity of more than 5% ∆k/k.It is because, in general, the modified CANDLE reactor with UN (99% 15 N) fuel has a higher atomic density of fission nuclides and a higher burnup level than the UN-fueled modified CANDLE reactor.Similarly, the power density of the UN(99% 15 N)-fueled modified CANDLE fast reactor is slightly higher than that of the UN-fueled reactor because the neutron capture cross-section of the 15 N isotope is lower than that of the 14 N isotope in almost all energy ranges.
Meanwhile, the minimum fuel volume fraction required by a fast reactor with the Modified CANDLE scheme fueled with UN (99% 15 N) to be critical is 54%, with a reactivity below 1%∆k/k during 15 years of operation.The modified CANDLE fast reactor with a fuel volume fraction of 65% requires about 10% 15 N enrichment to be critical with a reactivity of less than 1% ∆k/k.However, enrichment of the 15 N isotope to 99% is required to be similar to 14 C production when using oxide fuels.

Figure 1 .
Figure 1.The core divided in the radial direction according to the MCANDLE burnup scheme

Figure 4 .
Figure 4.The refueling scheme Fig. 5 depicts the description of the fuel.The first fuel (A) is unenriched natural uranium.The B fuel is natural uranium with a burnup time of 15 years.The image depicts the remaining compositions in detail.

Figure 5 .
Figure 5.The fuel description

Figure 6 .
Figure 6.The keff values for a modified CANDLE fast reactor core fueled with UN and UN (99%

Figure 8 .
Figure 8.The keff values for the UN-fueled reactor core with several variations in fuel volume fraction.

Figure 10 .
Figure 10.The atomic density of fissile nuclide in all regions for both reactors after 15 years of burnup

Figure 11 .
Figure 11.The power density with a similar fuel volume fraction of 67%.

Figure 12 .
Figure 12.The keff for various fuel volume fractions of UN(99%15N) -fueled reactor core Fig.13shows the keff values of the UN-fueled reactor with variations in 15 N isotope enrichment.For similar parameters, the minimum15 N isotope enrichment required for the reactor to be critical is 10%.However, in a reactor core with nitride fuel,14 C production is twice as large as in a reactor with oxide fuel.Therefore,15 N isotope enrichment is required to reduce 14 C production.According to the study, the percentage of15 N enrichment must be around 99% for14 C production to be equivalent to that in reactors with oxide fuel[35].

Figure 13 .
Figure 13.The keff values of the UN-fueled reactor core with variations in 15 N isotope enrichment

Table 1 .
Design parameters for reactors