A Methodology to Assess Transient Contact Heat Transfer Coefficient using Contact Boiling Test Data

Abstract

One of the passive safety features present in CANDU® reactors is the heavy-water moderator surrounding the fuel channels. During some postulated loss-of-coolant accidents (LOCAs), pressure tubes (PTs) will deform radially into contact with associated calandria tubes (CTs). The contacted tubes form a radial heat removal path that is additional to the heat removal capability of the heat transport system. The stored heat and the decay heat from the fuel channel are transferred through the CT to the moderator, which acts as a heat sink. To demonstrate the effectiveness of the moderator as a heat sink and to ensure the integrity of the fuel channel, the transient contact heat transfer coefficient (CHTC) between the PT and CT after PT/CT contact is needed for safety analysis.

Since the early 1980s, several series of experiments have been conducted at AECL Whiteshell Laboratories to assess contact boiling heat transfer that occurs when a pressurized PT deforms through a CO₂ annulus gas gap into contact with a CT in a tank of heated water. Such contact boiling tests, in which full-scale PT and CT sections were used, provide sufficient data (i.e., test section temperatures, applied heater power and internal pressure) to infer the transient PT/CT CHTC based on the principles of heat transfer.

A methodology has been developed to assess the transient PT/CT CHTC. The simulation model implements a negative feedback method by comparing contact boiling test data with the corresponding transient simulation results. The effectiveness and applicability of such a CHTC simulation model has been demonstrated with several test cases. The results show that the model has the capability to capture CHTC transient characteristics from the input data including the rapid changes in PT and CT temperatures upon initial contact and associated rapid changes in contact heat transfer coefficients.

The PT/CT CHTC is an important parameter for determining fuel channel behaviour, which changes significantly during the course of a LOCA. With the methodology developed here the PT/CT CHTC transient can be determined using contact boiling test data to meet the needs of safety analysis.