A Mechanistic Model to Predict Fuel Channel Failure in the Event of Pressure Tube Overheating

Abstract

Under normal operating conditions a pressure tube (PT) in the CANDU reactor is insulated from its outer calandria tube (CT) by a CO₂ gas annulus. If the primary loop coolant flow is compromised the PT can overheat and, if still pressurized, balloon into contact with the CT. At this point the moderator acts as an emergency heat sink. If the heat transferred from the CT to the moderator exceeds the critical heat flux (CHF) the CT can overheat, begin to strain due to the contact pressure, and eventually cause fuel channel failure. A mechanistic model is presented that describes ballooning contact of the PT and CT, the resulting thermal contact conductance, heat flux to the moderator, and, if CHF is exceeded, the development of film boiling and potential CT strain. The goal is to create a software package that predicts fuel channel failure during a pressure tube overheat event.