Impact of the MSBR concept technology on long-lived radio-toxicity and proliferation resistance

The MSBR (Molten Salt Breeder Reactor) was an industrial project designed at the beginning of the seventies at Oak Ridge National Laboratory and based on Thorium. Just before, the MSRE worked very well during four years with molten fuel. The MSBR system, where a maximum breeding was wanted, included a graphite moderated core with the circulation of a 71.7%LiF-16%BeF2-12%ThF4-0.3%UF4 salt and a pyro-chemical reprocessing unit. To obtain a maximum breeding ratio, Protactinium was extracted and stored allowing decay out of the neutron flux. This required the entire salt volume to be reprocessed in ten days, the gaseous fission products and Minor Actinides being extracted continuously by helium bubbling and pyro-chemical methods. The doubling time was evaluated to around 25 years. The project has since been re-evaluated especially within the frame of the EURATOM concerted Action MOST. To have an acceptable global reactivity feedback coefficient, studies have shown various possibilities based on core geometry, neutron moderation ratio and salt composition. When requiring only a breeding ratio of one, it is possible to avoid continuous reprocessing and to strongly simplify it. These various options will be discussed. The detailed inventory will be given showing clearly the interest of the Thorium Molten Salt Reactor where the production of Americium and Curium is a factor of one hundred lower that for the U-Pu RNR. The amount of Uranium 232 which is always produced in the Thorium cycle will be calculated as well as its decay rate since its decay chain eventually results in a 2.6 MeV γ –ray which may be used to detect and hence control the U233 fuel movements. As the U233 has to be produced in other reactors (PWR, RNR or other MSR), special cares have to be taken and will be discussed