Critical Channel Power Evaluation Methodologies at Atomic Energy ot Canada Limited

Abstract

The methodologies considered in the computation of the thermal margin in CANDU reactors are presented. In CANDU reactors, 12 or 13 fuel bundles (0.5 m long and containing 37 elements each) in pressurized horizontal channels are cooled by heavy water at a pressure of 10 MPa and an average mass flux of 7 Mg.s-1.m-2. The thermal limit of the fuel channel is defined by the critical channel power which is evaluated using critical heat flux correlations derived from full scale experimental data. The data base as well as some experimental observations of the beat transfer behaviour of a prototype fuel string are described in the paper. Por the thermal limit, fuel dryout defined as the first occurrence of temperature rise anywhere in the bundle is chosen at present as the criterion, However, in the full scale experiments the sheath temperature of the 37-element fuel bundle at dryout was observed to be moderate. The experimental results suggested that the present criterion is very conservative and operation beyond dryout under accident conditions would cause no safety concern, thereby allowing an increase in the thermal limits as used to determine regional overpower trip protection settings. For example, we estimate that if a sheath temperature of 375 degrees C


was taken as the criterion, the fuel channel thermal limit could be raised by up to 5%, depending on the channel heat flux and the flow conditions. At AECL, more realistic thermal limit criteria are being studied. These alternative criteria are discussed in the paper.