Modelling of clad-to-coolant heat transfer for RIA applications

Clad-to-coolant heat transfer during a Reactivity Initiated Accident (RIA) is of major importance because the clad temperature strongly influences the rod mechanical resistance against failure. The PATRICIA experimental program has been set up in order to investigate the clad-to-coolant heat transfer under fast transients. In this program, an In-conel tube centered in an annular channel was heated up by direct Joule effect. Both PWR Hot Zero Power conditions (15 MPa, 553 K-280°C-, 4 m/s) and pool conditions similar to those of the RIA experiments performed in the NSRR reactor (atmospheric pressure, room temperature, stagnant water) have been simulated. The heating rates were representative of the clad heating induced by the fuel during a RIA transient. With respect to the steady-state conditions, an increase of the Critical Heat Flux (CHF) has been observed. Boiling curves for PWR Hot Zero Power conditions have been implemented in the SCANAIR code. Further experimental results are needed to derive a reliable model in pool conditions for the simulation of NSRR experiments. Calculations of a 4-cycle UO2 rod submitted to a 30 ms power pulse in PWR conditions show that the zirconia layer has a dominant influence on the Departure from Nucleate Boiling (DNB). Once the DNB is reached, clad temperatures of the order of 1,300 K are computed and the film boiling phase lasts several seconds. © 2007 Taylor & Francis Group, LLC