The analysis of bearing-pad t pressure-tube contact heat transfer experiments

An experimental program was established to study the influence of hot bearing pads on temperature transients of a ballooning pressure tube under postulated loss-of-coolant accident conditions in a CANDU. Twelve experiments were conducted in steam and Ar/O2 environments. Measured temperature and power transients from the experimental program were used with the mulitpurpose code CATHENA Mod-3.4/Rev 7 to infer the contact heat transfer coefficient between the bearing pad and the pressure tube. The empirically derived contact conductances are of special interest in the safety analysis of CANDU reactors. The contact heat transfer coefficients reached a maximum when the pressure-tube temperature was between 400 and 500 deg C beneath the bearing pads. The maximum value of contact heat transfer coefficient, calculated from experiments conducted in Ar/O2 environments, ranged from 1.1 to 1.5 kW/(m2 · K). The calculated contact heat transfer coefficients decreased to a negligible value (< 0.1 kW/(m2 · K)) in all experiments once the pressure-tube temperature exceed ∼ 630 deg C. This drop in heat transfer coefficient was due to localized deformation of the pressure tube beneath the bearing pads caused by the localized hot spot and the internal pressure-tube pressure. This deformation resulted in deteriorating conformity, between the pressure tube and the bearing pad, as evidenced by a decrease in the pressure-tube heatup rate and a coincident increase in the bearing-pad heatup rate. This paper summarizes the modeling methodology and analysis of results for the large-scale Integrated Bearing-Pad to Pressure-Tube Contact Heat Transfer experimental program. The work described in this paper was largely funded by the CANDU Owners Group (COG). (author)