Thermal behaviour of CANDU fuel channel under steam flow conditions: An alternative solution.

Abstract

An alternative scheme is proposed to guarantee the safety behaviour of CANDU fuel channels under steam flow conditions. The proposed scheme is based on increasing the annulus gas gap effective thermal conductance during and after the accident. Two transient models (lumped and one-dimensional) are developed to predict the temperature of both pressure tube and calandria tube under the proposed scheme. The results show that the pressure tube temperature increases up to a maximum value after which it drops again. The pressure tube maximum temperature, at the channel vertical midplane, was found to be below 700 degrees C, 500 degrees C, and 430 degrees C when the gap conductivity is 3.0, 4.5, and 6.0 W/m.K respectively. Calandria tube temperature is maintained at 110 degrees C maximum. Therefore, the pressure tube calandria tube contact is eliminated and a "Coolable Fuel Geometry" is guaranteed. The moderator subcooling requirements are therefore relaxed and the moderator heat load is fully recovered to the first stage of feedwater heating system.