Transient Cooling Process of Fuel Rod in Reactivity Initiated Accident

Abstract

Out-of-pile experiments were performed with Zircaloy-4 rods in subcooled water envi-ronment to study the basic phenomena occurring in the transient cooling process undergone by a fuel rod during a reactivity-initiated accident (RIA) affecting a light water reactor (LWR). The experimental results show that the cooling process of the fuel rod during an RIA can be divided into three phases separated by the quenching temperature T_q and the rewetting temperature T_r.
It is also noted from the experimental results that with increasing degree of subcool-ing, T_q tends to rise to levels far exceeding the maximum liquid superheat temperature of water ; T_r, on the other hand, is little affected by the cooling water temperature, and remains close to that of the maximum superheat temperature.
Numerical calculations indicate conclusively that radial heat transfer to coolant water is the dominant factor that governs the transient cooling process in an RIA affecting the cold start-up of a BWR, rather than the axial heat conduction through rod which is con-sidered to be the basic mechanism of cooling that governs the reflooding process during a LOCA.