Analysing heat transfer between used nuclear fuel bundles in spent fuel pools using COMSOL Multiphysics

An abstract of the technical document presented:
The 19th Pacific Basin Nuclear Conference (PBNC 2014)
Hyatt Regency Hotel, Vancouver, British Columbia, Canada
August 24–28, 2014.

Prepared by:
C.J Krasnaj and W. Grant
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

Spent CANDU fuel is stored in large monolithically constructed, reinforced concrete in-ground irradiated fuel bays (Spent fuel pools). Spent nuclear fuel spends 7 to 10 years under water which provides both radiological shielding and cooling until the heat generated from its radioactive decays are sufficiently low to allow it to be transferred to dry storage.

Although the fuel remains below water during it entire residency in the fuel bay, under a beyond design basis accident, significant loss of water in the bay could result in the exposure of bundles to air. Exposed bundles will experience a reduced capability of heat removal which will lead to an increase in bundle temperature. As long as the temperature of the fuel sheath does not transgress the threshold for runaway oxidation, a zirconium fire will be prevented and the short term integrity of the fuel bundle remains safe.

This work focuses on the study of radiative heat transfer between used nuclear fuel bundles subjected to an air environment while located in the spent fuel pool. Without cooling water to remove the internal heat generated in the spent fuel, radiative heat transfer becomes an important heat transfer mechanism. The purpose of this work is to examine how different arrangements of bundles with varying decay powers in a fuel tray could influence temperature increases of neighbouring bundles in the tray. A 3-D finite element model using COMSOL multiphysics is developed to determine the amount of heat exchanged between nearby bundles and the induced temperature change they would receive.