- Criticality Safety Analysis on Degraded Reactor Core

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  During a severe nuclear reactor accident, the difference between the eutectic formation temperature of boron carbide (B₄C) and the control rod cladding (~1150°C) and the melting temperature of the fuel rods (~2800°C) causes the B₄C to relocate to the lower head of the reactor core earlier than the fuel rods. In addition to this relocation, neutron-absorbing fission products—such as xenon (Xe), cesium (Cs), and others—are also released during a severe accident. 

  According to the Severe Accident Management Guidelines (SAMG), alternative water injection is implemented to mitigate or prevent the progression of a severe accident. However, this action can lead to positive reactivity insertion, as the injected water slows down fast neutrons to thermal energies, which can sustain chain reactions in the reactor core. Furthermore, the release of fission products exacerbates the positive reactivity insertion by reducing the negative reactivity within the core. If the resulting positive reactivity is sufficient to sustain criticality in the degraded reactor core, recriticality may occur during the severe accident. 

 The potential for recriticality and subcritical boron concentrations is analyzed during the relocation of fuel rods within the assembly—referred to as the late phase of a severe accident—through the coupling of a severe accident analysis code with a whole-core Monte Carlo analysis code. 

 Through this analysis, we can assess the potential for recriticality during the progression of a severe accident and propose an optimized SAMG strategy to minimize that potential. 

- Design and Safety Analysis on the Molten Salt Reactors (MSRs)

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 Among these, molten salt reactors (MSRs) are emerging as a leading SMR design due to their ability to operate at high temperatures and low pressures, as well as their potential for simplified system designs—favorable characteristics for achieving small and modular reactors. 

 Unlike conventional light water reactors, MSRs do not use water as a coolant and are distinguished by the dissolution of fission products in molten salt during normal operation, rather than containment within nuclear fuel cladding. 

  The molten salt containing nuclear fuel circulates within the reactor to allow for the removal and separate storage of fission products. For shutdown, the reactor employs a strategy of discharging the nuclear fuel into a drain tank located outside the core.

  We are conducting the design and analysis of a molten salt reactor core to assess its feasibility for various applications, such as nuclear-powered ships and the transmutation of spent fuel from conventional light water reactors. Additionally, we aim to ensure the safety of reactivity control in the reactor core, which may contribute to the development of a regulatory framework for non-water-cooled reactors. 

- Development of Fission Product Behavior Analysis Code

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  One of the most serious nuclear accidents occurred at three of the Fukushima Daiichi reactors in Japan following the devastating 2011 Sendai earthquake and tsunami. This event highlighted the critical importance of analyzing and improving the safety of light water reactors (LWRs). As a result, significant attention is now being devoted to the analysis of severe accidents using conventional computational tools, as well as to improving these tools by incorporating state-of-the-art experimental data on phenomena that occur during such accidents. 

  Among the various phenomena associated with severe accidents, the behavior of fission products has received significant attention, as the consequences—such as radiation hazards and impacts on public health—are largely determined by the amount of fission products released into the environment. This quantity is referred to as the source term. Sustained international efforts have been made to develop guidelines for severe accident management based on source terms, as well as to minimize their magnitude during such events.  

  Among the various phenomena associated with severe accidents, the behavior of fission products has received significant attention, as the consequences—such as radiation hazards and impacts on public health—are largely determined by the amount of fission products released into the environment. This quantity is referred to as the source term. Sustained international efforts have been made to develop guidelines for severe accident management based on source terms, as well as to minimize their magnitude during such events.

  For these guidelines to be practical and effective, they must be supported by accurate information regarding the protection of public health and the environment from radiation hazards. This information can be provided through the best-estimate evaluation of the source term. To achieve such accurate estimations, it is essential to precisely analyze the behavior of fission products. 

   We are developing I-COSTA (In-Containment Source Term Analysis) and AnCheBi (Analysis of Chemical Behaviors of Iodine) to analyze the behavior of fission products, particularly in terms of their physical behavior, such as aerosol dynamics. In parallel, we are also studying the chemical behavior of iodine, as it exhibits high chemical reactivity and its various chemical forms significantly influence the effective dose received by the human body. 

- Development of source term analysis methodology for water-cooled small modular reactors (WC-SMRs)

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 The current technical basis for the source term calculation in regulatory review (TID-14844) has utilized very conservative assumptions that all of the nuclides (xenon and krypton) and 50% of the iodine in the core inventory would be released to the open air immediately following an accident.

 On the other hand, the technical report NUREG-1465, which is the basis for the U.S. Nuclear Regulatory Commission's Alternative Radiological Source Term (RG-1.183; U.S. NRC, 2023), and the Phebus experiment (Gregoire and Haste, 2013; Simmondi-Teisseire et al, 2013) show that the radiation source term can vary depending on reactor design characteristics.

 In order to make a nuclear reactor more useful to our society, it is necessary to develop a best-estimation technology of the source term that can consider not only the amount of radioactive material produced inside the core but also the chemical form of the radioactive material reflecting the design characteristics of the SMR.

 We are trying to develop the best-estimation methodology on the source term considering the physical and chemical composition of the fission product based on detailed analysis of the reactor core depletion and chemical equilibrium calculations, fission product transportation in WC-SMRs, and decontamination of fission products in a containment.