Research on molten salt fast reactors with inherent safety - No need to insert control rods even in transients or accidents -
- Anomalies that would damage the core of a conventional nuclear reactor are assumed for a molten salt fast reactor without control rods.
- Reactor power shifts to decay heat power by automatically tripping the fuel pump.
- The basic characteristics do not change greatly regardless of the types of molten salts and neutron spectra.
- Molten salt fast reactors have unique characteristics of transitioning to a safe condition in the event of any abnormalities.
For a chloride molten salt fast reactor system without control rods, the system code RELAP5-3D and the CFD code FLUENT which was combined with a computer program that incorporates the nuclear point-kinetics equations have been used to compute hypothetical events. Analyses were conducted for several events which would cause a core damaged of conventional reactors if safety rods were not inserted. As a result, for the event of a fuel pump failure, it is safe to do nothing. Even in an accident of a large reactivity insertion that would damage the fuel in a conventional nuclear reactor, it was analytically clarified that the reactor shifts to a safe condition by tripping the fuel pumps automatically. It has also been analytically clarified that the reactor system that has transitioned to decay heat is safely cooled by the convection of air without using power.
Molten salt reactors (MSRs) have attracted attention since the Fukushima accident because they are safer than other nuclear reactor systems, and competition for development is underway overseas. Light water reactors and other reactors are protected from major accidents by using engineered safety features. On the other hand, since MSRs have inherent safety characteristics that allow them to transfer to a safe condition, it is predicted that there will be no transition to a severe accident even if human errors or external anomalies are added to the reactor. The competition for development is conducted mainly among venture companies led by TerraPower, and few details on safety have been reported. Therefore, it is not easy to judge the degree of safety of MSR. Although the Japanese government and electric power companies said that "a severe accident would not occur in our country,” recognition that nuclear reactors are extremely dangerous has taken hold among Japanese people because of the Fukushima accident. However, national laboratories in the United States, France, China, and South Korea have begun supporting research with large budgets together with ventures. In Japan, researches on MSRs has traditionally been avoided among nuclear researchers, and even engineers with extensive experience in nuclear reactor design consider that a molten salt reactor is a reactor on a paper. If it is not safe, the United States and France, which have suspended research and development on sodium-cooled fast reactors, will not fund research on MSRs.In Japan, many studies have been conducted and funds have been invested to improve the use of light water reactors whose fuel runs out in about 100 years, and to enhance the safety of fast reactors of the same type as the sodium-cooled fast breeder reactor ‘Monju’. Although these nuclear reactors will become safer than ever, they are highly unlikely to be accepted by local residents because the possibility of severe accidents is not zero. However, when considering the future of energy in Japan, it is essential to obtain energy from fission reactors that do not emit carbon dioxide. This is true not only in Japan, but also in other countries. Therefore, it is necessary to make them aware that there are safer nuclear reactors, such as the molten salt fast reactor (MSFR), which is the subject of research, and to provide ideas for them to consider it as one of the options. In addition, in order to realize a molten salt fast reactor, it is necessary to integrate all the fields of chemistry, physics, and engineering, and it is a reactor type in which universities can play a major role.
Up to now, the author has proposed a method for analyzing MSRs and validated that the method is correct using data from the molten salt reactor experiment (MSRE) constructed and operated at Oak Ridge National Laboratory in the United States in the 1960s. Using this method, analysis was performed assuming transient events and accident events that are always severe accidents in conventional nuclear reactors. The MSFR to be analyzed is a chloride composition fast reactor without control rods and has a thermal output of 700 MWt. This reactor is a modular type, of which power is adjusted by the number of heat exchangers, and if research is done for one reactor output, it will be possible to design from a small size of 50 MWt to a large power of over 2000 MWt.
The lack of control rods makes most people question whether nuclear reactors are really safe. However, as a result of the analysis, the control rods and rapid fuel drain are unnecessary even in the event that the reactivity greatly exceeds prompt criticality. This event leads a severe accident for a most nuclear reactor. Experiments have shown that if this accident were to occur in a conventional light water reactor or sodium-cooled reactor, a large amount of mechanical energy would be generated in the core and the fuel would be damaged. The magnitude of the reactivity assessed far exceeds what would be added in the case of a molten salt reactor, but it is important to assess this for safety reasons. In the above accident, it has been clarified by the analysis that the fuel pump was tripped by the interlock at the stage when the reactor power increased, and the core temperature rose slightly, adding a large negative reactivity and canceling out the initial added reactivity. As for other events, such as an event in which the fuel pump cannot be operated due to a station blackout, it is evaluated that the nuclear reactor itself transfers to a safe state without doing anything.
In the case of the Fukushima accident which resulted in a severe accident, the nuclear reactor was safely shut down due to a large earthquake, but since it required a large amount of power to supply cooling water and to operate equipment removing the subsequent decay heat, it was unable to perform its functions. In the case of the reactor subjected to this research, it is evaluated whether the reactor can be cooled by natural forces using air that can be supplied without power, and it has become clear that the temperature of the reactor can be safely controlled by operating the decay heat removal system. The adopted air cooler has already been implemented in "Monju" and has been evaluated for its performance.
Fig. 1. Schematic of proposed heat transfer system for molten salt fast reactor.
Fig. 2. Schematic of heat transport system analysis system in system code and CFD code analysis system.
Fig. 3. Reactor power and long-term fuel temperature behavior when a large reactivity as much as 3$ is applied to the reactor and the fuel pump is tripped by an interlock（DHRS: Decay Heat Removal System）
- Journal :
- Journal of Nuclear Science and Technology, (2022), 2131647, 1-27.
- Title :
- Neutronics and thermal-hydraulics coupling analyses on transient and accident behaviors of molten chloride salt fast reactor
- Authors :
- Hiroyasu MOCHIZUKI
- Affiliation :
- Laboratory for Zero-Carbon Energy, Institute of Innovative Research, Tokyo Institute of Technology