Small modular reactors (SMRs) and floating nuclear power plants (FNPPs) are currently under research and development in many countries. These technologies have several advantages such as low initial investment, high safety, and flexibility in construction, operation and application. China is developing many SMR technologies for both land-based and sea-based, for instance, ACP100, ACPR50S, DHR400, HTR-PM, etc. and plans to build FNPPs to be deployed in the disputed waters like the East Sea. Then the implementation of studies on SMR and FNPP technologies are very necessary for Vietnam, which contributes to offering solutions to the energy security as well as the power supply for economic activities on the seas and islands throughout the country. Moreover, understanding China’s SMR and FNPP technologies helps Vietnam to prepare for any unusual situations in the East Sea in the future. From the above points, a team of researchers of Nuclear Energy Center of the Institute for Nuclear Science and Technology decided to carry out a project entitled: “Study on technology and safety of floating nuclear power plants using small modular reactors”, one of the first projects to conduct research on SMR and FNPP technologies in Vietnam.
In this project, an overview of the SMR and FNPP technologies was presented. Several reactors of FNPPs have been developed based on land-based versions of reactors (e.g. ACPR50S of China – Figure 1) or the versions of icebreakers (e.g. KLT-40S and RITM-200 of Russia) in order to save the licensing application time because of their proven technologies. In addition, there are two others new SMRs, i.e. NuScale and OFNP-300, under research and in the process of obtaining operation licences issued by the US. It can be seen that the above reactors have been improved from the traditional pressurized water reactors (PWRs) with many proven technologies in combination with advanced safety technologies. Therefore, this development trend can speed up the research, licensing, construction and operation processes of SMRs.
For FNPPs, their safety and security aspects need to be taken into consideration due to the following features: mobility, operating at sea, and affecting territory-related issues in maritime areas. Thus, safety design for FNPPs have to mention the effects of waves on thermal-hydraulic characteristics of reactors in a normal operation condition and also in an accident. Nevertheless, the integrated design of primary circuit in the reactor vessel and the operation feature of FNPPs help to prevent problems such as large break LOCA, core melting. Regarding the security of FNPPs, the coordination of many nations is necessary to ensure the security and safety of the FNPPs in the international maritime zones. At the same time, the security issues need to be investigated and discussed in more detail since safety and security risks can cause impacts on the international marine environment.
Regarding the legal regulations for FNPPs, international legal documents, i.e. the documents issued by the International Atomic Energy Agency (IAEA) have not been developed fully and systematically because FNPPs remain new and unpopular. In Vietnam, documents on safety and security of SMRs and FNPPs have not been issued. Due to complicated disputes in the East Sea and between neighboring countries, the development of relevant legal documents is absolutely necessary. The formulation and promulgation of these documents should refer to the documents of the countries in which SMR and FNPP technologies are deployed, such as Russia, the US, and China. Simultaneously, they must be based on the existing documents of the IAEA and IMO (International Maritime Organization) with the help of experts from the IAEA and countries mentioned above.
After the comprehensive overview of SMRs and FNPPs, the project team performed preliminary calculations of several neutronic characteristics and thermal-hydraulic safety analysis of these technologies in order to build human capacity for technology research, calculation and design for SMRs. The reactor core design and neutronic characteristics calculations were based on a reference ACPR50S reactor that is developing for FNPP in China. Evolutionary simulated annealing (ESA) method was applied to design a small 200 MWt reactor core. Figure 2 and Figure 3 show the optimal core fuel loading pattern (LP) as well as the changes of effective multiplication factor (keff) and relative power peaking factor (PPF) during the operation. The results show that the reactor can operate in about 900 days with a capacity of 200MW of heat and ensure the safety of the PPF (PPF < 1.5). This optimal core can be used to implement safety analysis and source term calculations for FNPPs using SMRs in future studies.
Thermal-hydraulic calculations were performed for the NuScale reactor because the technology has almost all data for modelling in the final safety analysis reports (FSAR). This is the first time the team has modelled a reactor using natural circulation. The steady-state was simulated by RELAP5. The results showed a good agreement with the FSAR, i.e. the values of the variation in important parameters ranged from 0.0 to 5.9%. After the verification with steady-state calculations, the RELAP5 was used to simulate the control rod ejection accident of the NuScale. Figure 4 and Figure 5 illustrated that RELAP5 could simulate the accident qualitatively.
Quantitatively, the simulation results were consistent with the FSAR at the early stage of the accident. The variation values of the parameters of coolant mass flow rate in the primary loop and the average temperature of the coolant in the core were under 10%. However, at the later stage, the simulation had higher variation values, perhaps resulting from the differences in calculation tools (i.e. NRELAP and RELAP5) and the limitation on information of the passive residual heat system. Therefore, it is necessary to conduct more studies on this problem.
In summary, besides giving the overview of SMR and FNPP technologies in the world as well as the safety and security issues concerning FNPPs, the team implemented preliminary studies on the physical characteristics and thermal-hydraulic analysis of two SMR technologies for FNPPs. In Vietnam, these studies have contributed to building human capacity for technology research, safety analysis, calculation and design for SMRs deployed in FNPPs.
Tran Viet Phu, INST
