Physics Analysis on the NRU Core for an Accident Scenario of a Loop Pressure Tube Crack

Abstract

The Nuclear Research Universal (NRU) reactor loops are high temperature, high pressure test facilities, designed for power reactor fuel development and materials testing within the core of the NRU reactor. The loops allow test material to be subject to neutron flux, temperature and pressure conditions typical of a power reactor. This paper describes the physics analysis on the NRU core for an accident scenario of a loop pressure tube crack with a concurrent liner tube failure. After the crack has occurred, thermal-hydraulic analysis predicts the formation of a steam bubble of 50 cm radius in the D2O moderator/coolant around the loop test section. The steam displaces the D2O moderator and has a negative reactivity effect. This negative reactivity effect is large enough to overcome the positive loop void reactivity such that the reactor is shut down and reactor safety is not compromised. The paper also describes the sensitivity of steam bubble densities on the reactivity effect and presents results for subsequent reductions of fluxes and channel powers around the loop site.