Design study of a 450MW thermal Modified CANDLE fast reactor using helium gas as a coolant

The Modified CANDLE (MCANDLE) burnup scheme divides the reactor core into several regions in an axial or radial direction with equal volume. It works like the CANDLE burnup scheme except that, in the CANDLE burnup scheme, the reactor core is divided into three main regions in the axial direction. Namely, spent fuel, burning region and fresh fuel region. In this study, the design of a modular modified CANDLE fast reactor using helium gas as a coolant has been performed. One of the important roles of a modified CANDLE fast reactor is that it can utilize natural uranium as fuel without the need for enrichment or reprocessing. This type of reactor can be used, including in developing countries, without the nuclear proliferation problem. Using helium gas as a coolant gives hope for a fast reactor due to its properties, such as its less neutron absorption, less radioactive, and as helium is an inert gas, prevents chemical reactions with other materials. The study was performed on a rector with a thermal power of 450MWth. The active core was divided into ten regions with equal volume in radial directions. The refueling scheme has been optimized every ten years of burnup to obtain a good reactor design. In the beginning, the fuel was put in the first region after ten years of burnup was moved to the second region, after another ten years of burnup was moved to the third region and so on until the tenth region where the fuel gets out. The neutronic calculation has been performed using the SRAC (Standard Reactor Analysis Code system). The collision probability method (PIJ) was employed for cell burnup calculation, and CITATION was used for the reactor core design with JENDL 4.0 as the nuclear data library.


Introduction
The world is gradually facing a severe energy crisis, with an ever-increasing demand for energy overstepping its supply [1].However, energy is important in supporting the development of a nation, such as manufacturing, construction, agriculture, mining, building industry, education, health, transport, tourism, etc.A crisis can develop due to industrial actions like union-organized strikes and government embargoes.During the period of COVID-era restrictions and the Russia-Ukraine conflict in early 2022, gasoline prices have risen [2].The countries should think about the energy source which can utilize a long life to support the development of citizens.Fossil energy can be proposed as a main part of energy needs.Even if, fossil energy has problems, such as limited resources, air pollution that harms our health and global warming generated by carbon dioxide emissions [3].One of the effective strategies to minimize global carbon dioxide emissions is to replace fossil fuels with a clean source of energy.Nuclear energy can be a candidate, since nuclear power plant (NPP) generates continuously energy and unveil 100% [4].However, Nuclear materials are expensive and produce radioactive waste, which needs to be carefully managed to avoid negative outcomes for citizens and the environment.The dangerous side of nuclear power has been well-examined in Chornobyl in Russia and Fukushima in Japan nuclear disasters [5].Several innovative reactor concepts have been proposed to reduce the problems regarding the safety and high cost of current power reactors [3],[6].Among them, Sekimoto et al elaborated the CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During the Life of Energy production) reactor and Zaki Su'ud elaborated on the Modified CANDLE (MCANDLE) reactor [7].Those types of reactors can utilize natural uranium for fresh fuel without needing enrichment or reprocessing [8], [9], [10].As a result, by employing these types of nuclear reactors optimal nuclear energy usage, including in developing countries, can be easily obtained without the risk of nuclear proliferation.In a CANDLE reactor, the active core is divided into three regions: namely, spent fuel, burning region, and fresh fuel [6]. Figure 1 shows the CANDLE burnup strategy (a) and Modified CANDLE strategy in the axial direction (b).When the CANDLE burnup strategy is used, the burning region moves along the axis of the core at a speed proportional to the power output without changing the spatial distributions of the nuclide densities, neutron flux, and power density [11].A modified CANDLE reactor works like a CANDLE reactor but the active core is subdivided into several regions with equal volume in axial or radial directions.CANDLE burnup and Modified CANDLE burnup reactors have economic advantages such as saving the cost of the uranium enrichment process because only uranium resources are needed as fuel, to eliminate the problem of nuclear proliferation because uranium enrichment or reprocessing is unnecessary [6], [12].This study's main objective is to design a 450 MW thermal modular modified CANDLE fast reactor using helium gas as a coolant.In this work, a feasibility design study of a 450 MWth modular modified CANDLE fast reactor which can utilize natural uranium as fuel, has been conducted.The reactor core was subdivided into ten regions in the radial direction and the refueling process was every ten years.Initially, the fuel was put in the first region after ten years of burnup was shifted to the second region and after ten years again was shifted into the third region and so on until in the tenth region where the fuel got out.Stainless steel HT-9, UN and helium gas were employed as a cladding material, fuel and coolant respectively.The neutronic calculation has been performed using the SRAC (Standard Reactor Analysis Code system).The collision probability method (PIJ) was employed for cell burnup calculation, and CITATION was used for the reactor core design with JENDL 4.0 as the nuclear data library.Helium became a promising coolant for modular modified CANDLE fast reactors due to its properties such as its less neutron absorption tendency, less radioactive and prevention of chemical reactions with other materials because it is an inert gas [13].Considering the properties of helium like low neutron absorption and inertness, the reactor cooled by helium does not require an intermediate heat transport loop, special cleanup and decontamination system, fire protection and special cell liners [14].

Reactor description
Table 1 presents the reactor design parameters.The pancake-type core was selected to minimize the axial load from severe heat flow [15], [16].Figure 2 shows the reactor core design (c) and the fuel cell design (d).It employs natural uranium and enriched nitride ( 15 N) as fuel.HT-9 was selected for the cladding material due to its high resistance to irradiation and outstanding thermal conductivity [17].Helium was used as a coolant due to its less neutron absorption, less radioactive, and its prevention of chemical reactions with other materials [13].Lead (Pb) is employed as reflector material due to that gas fast reactor has a high neutron leakage out of the reactor core [18], So that the reflector, which has a low absorption cross-section, high reflection coefficient, high elastic scattering, limited swelling under neutron irradiation and oxidation resistance is needed.The fuel cell has cylindrical geometry with 0.542cm, 0.585cm, 0.7000cm and 1.4cm fuel pellet radius, cladding radius, coolant radius and fuel pin pitch diameter respectively.The core has 155cm and 120 cm radial width and axial width respectively.The fuel volume fraction, cladding volume and coolant volume fraction are 65% 10% and 25% respectively.Figure 3 shows the radial fuel shuffling strategy and the arrow indicates the refueling process from one region to another region.

Calculation method
The calculation method was performed using SRAC (Standard Reactor Analysis Code Systems) [19], [20].Detailed information on SRAC can be obtained in the reference.The calculation in this study was conducted in two steps: fuel cell neutronic and the reactor core.Fuel cell neutronic calculations calculate fuel cell burnup for 100 years and the data are collected every two years of burnup.The fuel cell burnup calculation was performed by using the collision probability method (PIJ) based on the latest SRAC nuclear data library JENDL-4.0 (Japanese Evaluated Nuclear Data Library) [21].
The power density level in each region was assumed and used to perform cell burnup calculations.We have gotten macroscopic cross-sections for every step burnup.The results were employed to solve the multigroup diffusion equation in a 2D cylinder (R-Z) geometry of the active core using the CITATION module of the SRAC.Fuel cell neutronic calculation gives several parameters such as burnup level, infinite multiplication factors (k -infinite), integral conversion ratio (Intel.C.R.), atomic density of natural uranium (U-238) and atomic density of plutonium (Pu-239).The reactor core calculation gives the effective multiplication factor (K-effective) and relative power density distribution.

Results and discussion
In this work, the main parameters used, are presented in Table 1.The power is 450MWth, and the volume fraction of 65%,10% and 25% are fuel, cladding and coolant, respectively.Figure 4 presents the K-effective changes during burnup.From the graph, we see that the K-effective increases monotonously.K-effective is greater than unity during burnup time.The criticality condition is at the beginning of the cycle (BOC), and the initial margin is high enough.Figure 5 shows the relative power density at the BOC and the end of cycle (EOC).It is shown that the distribution will move toward the higher Plutonium fuel region to a lower Plutonium region.At the EOC, the power peaking factor is small.Based on the power density overview, we can see that half of the core part is a breeding region where the accumulation of natural uranium is intensive.Other regions are active regions where most of the power density is produced.Peak power appears about 50cm from the top of the core at BOC owing to the region containing fuel with a higher burnup around the top of the core.However, due to the atomic density of Pu-239 as a fissile material, the EOC reactor's peak power appears at approximately 60 cm from the top of the core.The relative power density is zero from 160cm to 230cm because it is the reflector region.When compared to BOC, the peaking factor in EOC reduced, implying that the power distribution became more flat.Figure 6 illustrates the burnup level.It shows that during the first ten years of burnup, the burnup level increases sharply because the first region is positioned near the tenth region.After ten years of burnup, the burnup level increases slowly compared to the first ten years till about fifty-two years of burnup.After that overall burnup level increases significantly until the hundred years of burnup, and the burnup level at the EOC is about 327GWd/ton.
Figure 7 shows the K-infinite and Integral C.R change during burnup time.It shows that the Kinfinite increases sharply in the first ten years because the first region is located near the tenth region so that the natural uranium fuel receives sufficient neutron beam from the tenth region to transform its natural uranium(U-238) into Plutonium.Ten years later, the K-infinite increases notably until about 72 years of burnup.This is because the fuel moves into the most active regions.After 72 years of burnup, the K-infinite becomes flat until about 76 years of burnup.After 76 years of burnup, K-infinite reduces because of the large accumulation of Plutonium (Pu-239) and the reduction significantly of natural uranium (U-238).In the BOC, the integral conversion ratio value decreases significantly because of the accumulation of plutonium (Pu-239) and the diminishing of natural uranium (U-238).After ten years of burnup, the conversion ratio value decreases with a smaller changing rate until about 70 years of burnup time and a much smaller change until the end of the cycle (EOC).This is also because of the reduction of natural uranium (U-238) and the accumulation of plutonium(Pu-239).shown that the neutron flux level at the beginning of the cycle (first ten years) is higher than that of the period of 10 to 50 years of burnup because, during the first ten years (beginning of the cycle) of burnup, fresh fuel is put in the most active reactor core region.The flux level reaches the maximum value at 82 years of burnup and then decreases due to the reduction of fresh fuel (natural uranium).At the beginning of cycle (BOC), the energy released /fission increases sharply because the first region is positioned near the tenth region (most active).The energy released /fission increases slowly until about 36 years of burnup.After 36 years of burnup, the energy released/fission increases more until the EOC.This is due to the accumulation of plutonium and the reduction of natural uranium.
Figure 9 shows the atomic density of natural uranium (U-238) and atomic density of enriched uranium (U-235) in fuel pellet changes during burnup.We see that the U-238 decreases slowly during the beginning of the cycle.This is because of the accumulation of Plutonium (Pu-239) and the diminishing of natural uranium.After ten years of burnup, the atomic density of U-239 decreasing rate accelerated until about 80 years of burnup because of the positioning of the fuels in mostly active regions, after 80 years decreasing rate accelerates until the end.At the beginning of cycle (first ten years), the atomic density of enriched uranium (U-235) decreases sharply.This is because the U-235 is burnt speedily because of the location of the first region near the tenth region.After ten years of burnup, until about 28 years of burnup, U-235 atomic density decreases due to that, its burning process is slower than that of the first ten years.After 28 years of burnup, U-235 atomic density is gradually reduced because its burning rate gradually increases until about 82 years of burnup.After that, U-235 atomic density diminished until the end of the cycle.Figure 10 presents the atomic density of Plutonium (Pu-239) and the atomic density of Pu-242 changes during burnup time.At the beginning of the cycle (first ten years) of burnup the atomic density of Pu-239 increases very fast compared to the period of 10 to 32 of burnup.This is because the first region is near the tenth region.In the period of 10 to 32, the atomic density of Plutonium Pu-239 is slowly increasing and then sharply increases until it reaches the peak of about 74 years of burnup.After this year of burnup, the atomic density of Plutonium (Pu-239) decreased because of the significant reduction of natural uranium (U-238) atomic density.The atomic density of Pu-242 is relatively the same until about a half-life of reactor design.This is because the first region is located near the most active parts.After 54 years of burnup, Pu-242 atomic density increases sharply until the EOC.

Conclusion
In this study, the design of a modular modified CANDLE fast reactor using helium gas as a coolant has been performed.The study was conducted on a reactor with a power of 450MWt.The dimensions of the core design are 155cm and 120cm, the radial width and axial width, respectively.The fuel volume fraction, cladding volume and coolant volume fraction are 65%, 10% and 25%, respectively.The reactor core has been subdivided into ten regions in the radial direction with a recharging process every ten years of burnup.Natural uranium has been utilized as fresh fuel without being enriched or reprocessed.The utilization of natural uranium as fuel makes a modified CANDLE fast reactor to be easily utilized in developing countries without the problem of the cost of enriched uranium.Due to the properties of helium gas such as the prevention of chemical reactions with other materials, less likely to be radioactive and its low neutron absorption properties make it a good coolant for modular modified CANDLE fast reactors.

Figure 1 .
Figure 1.CANDLE burnup strategy (a) and Modified CANDLE in axial direction (b).

Figure 2 .
Figure 2. The reactor core design (c) and fuel cell design (d).

Figure 7 .
Figure 7. Infinite multiplication factor (right) and integral conversion ratio (left) change during time.

10th 8 Figure 8
Figure8shows neutron flux level and energy released /fission change during burnup time.It is shown that the neutron flux level at the beginning of the cycle (first ten years) is higher than that of the period of 10 to 50 years of burnup because, during the first ten years (beginning of the cycle) of burnup, fresh fuel is put in the most active reactor core region.The flux level reaches the maximum value at 82 years of burnup and then decreases due to the reduction of fresh fuel (natural uranium).At the beginning of cycle (BOC), the energy released /fission increases sharply because the first region is positioned near the tenth region (most active).The energy released /fission increases slowly until about 36 years of burnup.After 36 years of burnup, the energy released/fission increases more until the EOC.This is due to the accumulation of plutonium and the reduction of natural uranium.Figure9shows the atomic density of natural uranium (U-238) and atomic density of enriched uranium (U-235) in fuel pellet changes during burnup.We see that the U-238 decreases slowly during the beginning of the cycle.This is because of the accumulation of Plutonium (Pu-239) and the diminishing of natural uranium.After ten years of burnup, the atomic density of U-239 decreasing rate accelerated until about 80 years of burnup because of the positioning of the fuels in mostly active regions, after 80 years decreasing rate accelerates until the end.At the beginning of cycle (first ten years), the atomic density of enriched uranium (U-235) decreases sharply.This is because the U-235 is burnt speedily because of the location of the first region near the tenth region.After ten years of burnup, until about 28 years of burnup, U-235 atomic density decreases due to that, its burning process is slower than that of the first ten years.After 28 years of burnup, U-235 atomic density is gradually reduced because its burning rate gradually increases until about 82 years of burnup.After that, U-235 atomic density diminished until the end of the cycle.

Table 1 .
Design parameters