Neutronic and Proliferation Resistance Analysis of Small Modular Pressurized Water Reactor with Various Fuel Types using SRAC 2006

One strategy for emission reduction in Indonesia is the dedieselization program, replacing diesel power plants with renewables. Pressurized Water Reactor (PWR) type Small Modular Reactor (SMR) is also a consideration. This study aims to compare fuel performance in SMRs. Four fuel types with varying fissile and fertile nuclides but equal fissile mass percentages were assessed: U235- U238, U235-Th232, U233-U238, and U233-Th232, using PIJ SRAC 2006 and JENDL 4.0. Parameters include neutron multiplication factor, conversion ratio, and isotope production. Results indicated that U233-U238 has a highest initial neutron multiplication factor, 1.55. Substituting uranium-238 with thorium-232 boosts neutron flux and conversion, reducing nuclear proliferation risk. While initial neutron multiplication may not be highest, thorium-232 can produce uranium-233 with high initial neutron multiplication factor. This research aids fuel selection for Indonesia SMR dedieselization program.


Introduction
Following the Paris Agreement, each country is obligated to ratify and register its Nationally Determined Contribution (NDC) to decrease greenhouse gas (GHG) emissions.The emission reduction objectives are defined in each country's NDC for 2020-2030.Indonesia has set a target of reducing GHG emissions by 29% through domestic efforts and 41% with international assistance in its NDC.In 2021, Indonesia raised these targets to 31.89% and 43.20%, respectively, intending to reach Net Zero Emissions by 2060 [1].
The dedieselization program is one strategy for lowering GHG emissions.The dedieselization program entails replacing existing diesel power plants or constructing new ones using alternative energy technologies like solar energy, wind energy, hydropower, and biomass [2].As a result, there is a demand for base-load power plants with low GHG emissions that may be used in remote places to replace diesel power plants.Small Modular Reactor (SMR) nuclear power reactors are one option to consider for Indonesia's dedieselization program.Small Modular Reactors (SMRs) are defined by the International Atomic Energy Agency (IAEA) as a new-generation reactor meant to be tiny and compact, with excellent mobility and requiring relatively less energy to generate power.SMR designs differ; however they typically produce less than 300 megawatts electric (MWe) [3][4][5].Small modular reactors (SMRs) can be created using generation 3 reactor technology, such as light water reactors (LWRs), and generation 4 reactors, considered more advanced reactor types.Previous analyses included liquid metal fast breeder reactors (LMFBR) using sodium (Na) and Pb/Bi coolants, molten salt reactor (MSR) types, and high-temperature gas-cooled reactors [6][7][8][9][10][11][12].The usage of Small Modular Reactors (SMRs) provides various benefits to Indonesia.Because of their modest size may be installed in remote regions and do not require big land footprints.Furthermore, SMRs have more extended reactor operating lifetimes.These considerations make SMRs viable for supplying power to Indonesia's remote and inaccessible regions.
The Integral Pressurized Water Reactor (iPWR), a form of Pressurized Water Reactor (PWR) typically powered by uranium dioxide (UO2), is one of the SMRs currently under development [13].Several systems are merged into a single "integral" unit in this reactor, incorporating the core reactor, coolant, and safety systems into a single design.The fundamental benefit of the Integral PWR reactor is its simplicity of design, which decreases the number of distinct components and systems, lowering the risk of damage and failure.This improves safety by lowering the likelihood of radioactive leakage and allowing for more effective safety systems.Integral PWRs use nuclear fuel more efficiently, resulting in less radioactive waste [14].
In recent years, there has been a surge in interest in researching alternate fuel sources to improve the performance and safety of SMRs.One strategy to accomplish this is to produce fuels with lower manufacturing costs, higher power density, and higher safety margins that are also easily convertible to SMRs with minor adjustments [15][16][17][18][19]. Thorium-232, a fertile fuel capable of producing Uranium-233 as fissile fuel, is one of the alternative fuels being studied for SMRs.To produce fissile fuel, thorium-232 fuel requires fissile material.As a result, transitional fuel such as (U235-Th232)O 2 is frequently used to make uranium-233, which is then used in the thorium fuel cycle.
Previous research has examined the performance of thorium fuel, which shows certain advantages over uranium fuel in terms of better fuel conversion capabilities due to the superiorities of uranium-233 fissile material over others.Furthermore, Thorium fuel has improved negative void reactivity and burnup performance.Another part of the examination is the issue of nuclear non-proliferation, where Thorium outperforms uranium.Some symbiotic system fuel schemes based on thorium-based fuel usage included water-cooled reactor types and fast breeder reactors [20][21][22][23][24][25][26].
This study will compare the performance of fissile and fertile materials in iPWR (Integral Pressurized Water Reactor) fuel pins.This study will combine fissile and fertile materials such as uranium-233, uranium-235, uranium-238, and thorium-232 into four fuel combinations with a constant fissile proportion.The infinite neutron multiplication factor, conversion ratio, and inventory of isotope production in the fuel are among the characteristics to be examined.

Methodology
SRAC 2006 [27] with JENDL-4.0 [28]was used in this study to simulate and model a single fuel pin cell integrated PWR.PIJ module was utilized in SRAC 2006.The PIJ module calculated neutronics at the fuel cell level using the Collision Probability Method (CPM).IGT = 4 was used in the fuel pin modeling, implying a square cell method divided into concentric circles to produce numerous zones [29].Then, changes in fuel types would be carried out using four combinations of fissile and fertile elements, namely U-235 + U-238, U-235 + Th-232, U-233 + U-238, and U-233 + Th-232, as specified in section 2.1.

Design Specification
For this study, a 160 MWt thermal power iPWR module, uranium dioxide (UO2) fuel, and a 2-year operational cycle with developed by NuScale Power was selected.In

Profile of Spectrum and Macroscopic Cross-Section
This section will compare the four fuel types neutron energy spectrum and cross-section, which is important for analyzing the neutronic parameters.Figure 2 shows the neutron spectrum at the beginning of the cycle (BOC), and Figure 3 shows the neutron spectrum at the end of the cycle (EOC).It is because that in fact the neutrons generated during the fission processes are neutrons with higher energy (fast neutrons) [30].According to Figure 2, the U235-U238 fuel type exhibits the largest peak flux within the thermal energy range, followed by U235-Th232.The reason for this discrepancy lies in the thermalization capability of each fuel in this case.Consequently, in this case, the neutron flux within the thermal energy range is larger for uranium-235 in contrast to uranium-233.
In Figure 3, the U235-Th232 fuel type achieves the highest peak flux in the thermal energy range, followed by U233-Th232 this neutron is critically used to conduct the fission reaction in thermal reactor.Furthermore, as shown in Figures 4 and 5, which depict the absorption and capture macroscopic crosssections, the fuel containing fertile material thorium-232 has a greater macroscopic cross-section than uranium-238 in thermal energy range.As a result, using thorium-232 as fertile material yields uranium-233, resulting in a larger fissile inventory in the fuel than using uranium-238, even though uranium-238 can also produce plutonium-239.7 depicts the Infinite Neutron Multiplication Factor and burn-up for the four types of fuel.In accordance with reactor specifications, the burn-up for these four fuel types approaches the same value at EOC, roughly 12 GWd/ton for one operating cycle.In Figure 7, it can be shown that the U233-U238 fuel has the highest initial k inf , which is 1.55.Fuel containing fissile uranium-233 has a higher k inf than fuel containing uranium-235.This is due to the fact that, despite the fact that the fission macroscopic cross-section in Figure 6 shows no significant difference, the total amount of neutrons produced in each fission reaction (υ) in all energy range of uranium-233 is greater than uranium-235.Conversion ratio shows ability to produce fissile material in fuel.The U233-Th232 fuel type exhibits the highest conversion ratio, averaging at roughly 0.482.Fuels containing thorium-232 as a fertile material exhibit a superior conversion ratio in comparison to fuels containing uranium-238 as a fertile material.This is congruent with Figures 4 and 5, which show that fuels containing the fertile substance thorium-232 have greater absorption and capture macroscopic cross-sections than uranium-238.

Figure 9. Fission Inventory Ratio
Figure 9 displays the fission inventory ratio (FIR) for the four different fuels.Fission inventory ratio is FIR is the ratio of the total fissile inventory at a specific time to the initial fissile inventory.In this case, the considered fissile materials are U-233, U-235, Pu-239, and Pu-241.It can be observed that in the beginning, fuel with U-238 as a fertile material has a larger FIR.But, in the EOC, it can be observed that fuel with Th-232 as a fertile material has a larger FIR.It shows that fuel with Th-232 produced more fissile material at the time of operation than fuel with U-238.This is congruent with Figures 8, which show that fuels containing the fertile substance thorium-232 has greater conversion ratio than uranium-238.

Isotopic Production of Uranium and Plutonium
Figures 10 and 11 present an examination of fuel composition and actinide generation for Uranium and Plutonium isotopes.The analysis of actinide composition and fuel composition is a compelling subject for comparative evaluation across various fuel cycles, particularly at the end of cycle (EOC).This analysis is crucial for understanding the significant build-up of actinides and its implications for addressing proliferation resistance concerns.The selection of isotopic plutonium and uranium compositions is based on the significance of certain isotopes for assessing fissile composition and their possible usage in explosive devices, particularly in relation to proliferation concerns.Furthermore, certain isotope compositions are utilised in the process of denaturing materials for proliferation purposes.This is mostly due to their significant characteristics such as high decay heat, high spontaneous fission neutron emission, and intense radiation exposure.These properties pose considerable technical challenges when it comes to the construction of explosive devices.Figure 10 depicts the end-of-cycle (EOC) generation of uranium isotopes in each fuel.Fuel U233-U238 and U233-Th232 have the most uranium-233 inventory, followed by U235-Th232.Uranium-233 is anticipated to be produced from converted Th-232 by capturing neutrons by thorium-232 into thorium-233, beta decay process into protactinium-233, and the second beta decay process into uranium-233.As a result, U235-Th232 fuel produces uranium-233.Meanwhile, due to the existing composition of uranium-235 composition as fresh fuel for enriched uranium loading and capturing neutrons by uranium-233 into uranium-234 and then one more capture neutron by uranium-234 into uranium-235, uranium-235 production can be estimated to be higher for U235-U238 than U235-Th232, followed by U233-U238 and U233-Th232.As a fissile substance, uranium-233 has the potential to increase proliferation within the reactor core.However, the level of proliferation can be lowered by using uranium-232 to denature uranium-233, which is a strong gamma emitter.Uranium-232 is an isotope that can be used to resist proliferation in fuel; however, due to the intense gamma radiation, extracting uranium-233 as a nuclear weapon will be more difficult.
Figure 11 illustrates that fuel compositions U235-U238 and U233-U238 provide substantial quantities of plutonium at the end of the cycle (EOC).In contrast, the isotopic pairs U235-Th232 and U233-Th232 also produce plutonium but in relatively limited quantities.The estimation of plutonium production is based on the conversion of uranium-238 to plutonium-239 and other isotopes of plutonium.Plutonium-239 exhibits the greatest composition due to its position in the nuclide chain, which begins with the neutron capture of uranium-238, resulting in uranium-239.Subsequent beta decay processes lead to the formation of neptunium-239, followed by further beta decay, resulting in the production of plutonium-239.The utilization of thorium as a fertile material in fuel production results in a significantly reduced production of plutonium due to the longer actinide chain mechanism compared to the uranium cycle.This longer mechanism involves two more steps in the production of plutonium isotopes when compared to the uranium cycle.Hence, it is possible to attain a significantly reduced concentration of plutonium, but the production of protactinium and uranium isotopes is comparatively elevated.According to this perspective, the utilization of fuel type or the implementation of a complete thorium cycle demonstrates superior resistance to the proliferation of plutonium compared to alternative fuel types.In the context of plutonium composition, the utilization of denatured plutonium isotopes, specifically plutonium-238 and plutonium-240, might effectively mitigate the proliferation potential.The plutonium isotopes, specifically plutonium-238 and plutonium-240, possess notable characteristics such as significant decay heat and spontaneous fission neutron parameters, which make them potential proliferators.

Conclusions
The purpose of this research is to compare the performance of fissile and fertile materials in iPWR (Integral Pressurised Water Reactor) fuel pins.In relation to performance, the U233-U238 fuel exhibits the greatest initial k-inf value of 1.55, followed by U233-Th232, U235-U238, and U235-Th232 fuels with initial k-inf values of 1.50, 1.35, and 1.25, respectively.The reason for this phenomenon can be attributed to the greater quantity of neutrons generated per fission event in uranium-233 as compared to uranium-235.This benefit can lead to extended periods of operation.The fuel combination of U233-Th232 exhibits the highest conversion ratio, with an average value of 0.482.Subsequently, the U235-Th232 fuel combination follows suit.Because, in comparison to uranium-238, thorium-232 has higher absorption and capture probability.A reactor that has a high conversion ratio can offer sustainability benefits.
In regards to the issue of proliferation, it is seen that U233-Th232 fuel exhibits the least generation of plutonium isotopes while concurrently displaying the largest generation of uranium isotopes.Nevertheless, the extent of proliferation can be diminished through the use of uranium-232 for the purpose of denaturing uranium-233, a highly potent gamma emitter.Uranium-232, an isotope, possesses the capability to impede the proliferation of fuel.However, the extraction of uranium-233 as a nuclear weapon is expected to encounter heightened challenges owing to the substantial gamma radiation levels associated with it.
Overall, this study provides useful insights into the benefits and drawbacks of four types of fuel in a 160 MWt integrated PWR.The substitution of uranium-238 with thorium-232 as the fertile material yields enhanced neutron flux, conversion ratio, and less nuclear proliferation.When considering the initial neutron multiplication factor, it is seen that fuels containing thorium-232 may not exhibit the largest initial neutron multiplication.However, it is noteworthy that thorium-232 has the capability to generate uranium-233 with a comparatively elevated initial neutron multiplication.

Figure 2 .
Figure 2. Neutron Spectrum at BOC Figure 3. Neutron Spectrum at EOC

Figure 7 .
Figure 7. K inf and Burn-up Different Fuel Types

Table 1 ,
the reactor's specific 10th Asian Physics Symposium (APS 2023) Journal of Physics: Conference Series 2734 (2024) 012057parameters are listed.The fuel pellet, helium gap, Areva M5 cladding, and light water coolant were all separated into zones on the fuel pin.Figure1depicts the modeling and size of the fuel pin.

Table 2
contains the fuel combinations and their specifications.

Table 2 .
Fuel Type and Specifications