Session: 04-05: SMRs, Advanced Reactors and Fusion

Paper Number: 134672

134672 - Feasibility Study of Variable Spectrum Molten Salt Reactor by Adjusting Fuel Salt Compositions for the Lif-Bef2-Zrf4-Uf4 System 

Abstract: 

Nuclear energy has received renewed attention for the reduction of greenhouse gas emissions, while the disposal of high-level radioactive waste is the remaining issue of the utilization of nuclear energy. The partitioning and transmutation technology has been investigated to reduce the burden of radioactive waste disposal by transmuting long-lived and highly radiotoxic nuclides. In contrast, many concepts of advanced reactor systems have been proposed these days, without considering the back-end issues.

A Molten Salt Reactor (MSR) is considered an important option for the future nuclear energy system, considering the issues of radioactive waste, due to the employment of liquid salt and the possibility of online reprocessing. The employment of liquid salt enables to obtain various compositions of fuel salt in the range of an operation temperature and chemical properties. By the adjustment of the fuel salt composition in combination with a moderator, an MSR has the potential to produce a variable neutron energy spectrum. In principle, effective transmutation is achievable by providing an “appropriate“ neutron energy spectrum for each nuclide taking into account its cross-section properties. An MSR has the capability to achieve effective transmutation in a single nuclear reactor system by introducing the liquid salt containing the specific nuclides for the transmutation generated from the process of online reprocessing into the dedicated flow channel designed with an “appropriate” neutron energy spectrum.

On the basis of these advantageous characteristics of MSR, we propose the Variable Spectrum Molten Salt Reactor (VSMSR). The present study investigates the feasibility of VSMSR from the viewpoint of criticality and the variability of a neutron energy spectrum by adjusting the fuel salt compositions. The LiF-BeF₂-ZrF₄-UF₄ molten salt system is selected to consider the past successful operation of the Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory in the 1960s and a wealth of reported thermodynamic properties of the molten salt. The candidate fuel salt compositions are selected on the basis of the ternary phase diagrams of LiF-BeF₂-UF₄, LiF-ZrF₄-UF₄, LiF-BeF₂-ZrF₄, and BeF₂-ZrF₄-UF₄, at the operation temperature of 600 °C. For example, the UF₄ and ZrF₄ molar fractions are determined lower than 35 mol% and 52.5 mol%, respectively. The density of candidate fuel salt compositions is analyzed through molecular dynamics simulations. The neutronics analyses are performed on the basis of the candidate fuel salt compositions and the calculated density for the infinite medium with the limitation of the ²³⁵U enrichment to less than 20 wt%. By the comparison with the k-infinity of 1.099 obtained from the fast spectrum fluoride MSR (MSFR) with the Li-ThF₄-²³³UF₄ (77.5-19.985-2.515 mol%), the fuel salts satisfy the k-infinity when the UF₄ content of the 20 wt% ²³⁵U enrichment is more than 10 mol%, and the requirement of the ²³⁵U enrichment is not less than 15 wt% to achieve the target k-infinity. Also, calculated neutron energy spectra show that the thermal neutrons obtained with LiF-BeF₂-UF₄ (55-35-10 mol%) are more than 600 times larger than those obtained with LiF-UF₄ (65-35 mol%). It is concluded that the neutron energy spectrum is variable by adjusting the fuel salt compositions for the LiF-BeF₂-ZrF₄-UF₄ system in conjunction with satisfying the target k-infinity.

Presenting Author: Naoto Aizawa Tohoku University

Presenting Author Biography: Dr. Naoto Aizawa earned a Ph.D of engineering from Tohoku University in 2013.
Since April 2013, he has been working as an Assistant Professor at Tohoku University.
His major is transmutation engineering and reactor physics.
His interest is in the design of advanced reactor systems such as an accelerator driven system and a molten salt reactor.

Authors:

Naoto Aizawa Tohoku University
Hiroki Shishido Tohoku University
Koji Fujikura Tohoku University