The effect B impurity and Xe intrusion in the graphite moderator on void reactivity feedback of thermal MSR

Abstract

Thermal Molten Salt Reactor (MSR) massively utilizes graphite as both moderator and reflector. The molten fuel circulates around the primary loop and directly contacts with the graphite. Gaseous fission products, such as neutron poison Xe-135, are normally separated out from the core. However, deposition of Xe-135 in graphite could take place through direct Xe-135 intrusion into graphite pores, or salt intrusion that eventually undergoes fission in the graphite and leaves the Xe-135 inside. Another possible way of strong neutron absorber deposition in the graphite is boron impurity. Beside reducing the core excess reactivity, the existence of strong neutron absorbers in graphite could alter the core reactivity feedbacks. Thus, this study is meant to analyse the effects of Xe-135 and natural boron deposition in the graphite to the core reactivity feedbacks. The evaluations were done by monte Carlo calculations on their effects on void coefficient, fuel, and moderator temperature reactivity feedbacks. The introduction of void into under-moderated thermal MSR core generally produces positive void reactivity feedback. The deposition of strong neutron absorbers in the graphite decreases the void reactivity coefficient, yet still positive. Despite of core excess reactivity loss, the existence of strong neutron absorber in the graphite is favourable in safety point of view.