The importance of nuclear fuel cycle simulation has grown in recent years as some countries have faced a transitional period of rebuilding a future that uses nuclear energy in response to drastic changes in the environment related to energy. This dynamic transition is happening throughout the world following the Paris Agreement on climate change in 2015.
The nuclear fuel cycle promoted in the Global North is an extremely complex system that combines multiple processes including nuclear reactors, reprocessing, vitrification, storage, transportation, and disposal. This makes it necessary to precisely analyse nuclear energy scenarios by taking into account uncertainties in social trends and cross-sectoral perspectives in nuclear energy R&D and policy making. In addition, in order to achieve sustainable nuclear energy utilisation and advanced nuclear energy systems, radioactive waste needs to be managed. The amount and properties of radioactive waste depend on upstream conditions in the nuclear fuel cycle such as the nuclear reactor operation, reprocessing, and storage [1,2]. Integrated nuclear fuel cycle simulation spanning from the front- to the back-end is therefore required in order to design advanced nuclear energy systems and perform quantitative analysis of future scenarios.
About 30 codes for simulating the nuclear fuel cycle have previously been developed by various research institutes, and some international benchmark studies have been conducted in the last decade [3–6]. It can be said that each of the codes excels at tracking actinide nuclides and performing nuclear fuel cycle simulation, particularly in relation to the front-end and reactor. However, the conditions of back-end simulation are considered to be limited using these codes because they do not track the necessary nuclides to analyse the diverse and variable scenarios. Although FAMILY-21 [7] and NMB3.0 [8] were developed to simulate the Japanese nuclear fuel cycle, they are specialised for mass balance analysis of actinide nuclides for mainly using fast breeder reactors (FBR) and the Accelerator-driven Nuclear Transmutation System (ADS). Therefore, nuclear fuel cycle codes that flexibly simulates not only the front-end but also the back-end have not been developed.
In order to realise an integrated nuclear fuel cycle simulator that covers all stages from mining to nuclide migration after disposal, the Tokyo Institute of Technology (Tokyo Tech) and the Japan Atomic Energy Agency (JAEA) began jointly developing Nuclear Material Balance version 4.0 (NMB4.0) in 2019. NMB4.0 is designed to provide flexibility and an all-in-one packaged nuclear fuel cycle simulation for users as open code. Various ideas have been implemented to improve user-friendliness. The main technical features of NMB4.0 are as follows:
- 179 nuclides, which were selected to enable nuclear fuel cycle simulation including not only the front-end but also the back-end, are tracked.
- The Okamura explicit method (OEM) is implemented for depletion calculations including short half-life nuclides with very low calculation cost and sufficient accuracy.
- The amount of waste and footprint of the geological repository, which are related to back-end simulations, are simulated using multiple approaches from waste production treatment to thermal analysis of the geological repository.
- The accuracy of depletion calculations agrees with ORIGEN [9], and the back-end simulation can be calculated with an accuracy of within 1% of ORIGEN and COMSOL Multiphysics® (COMSOL) [10].
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