Significance of Warm Prestress to Crack Initiation during Thermal Shock.

摘要

During a loss of coolant accident (LOCA) followed by operation of the emergency core cooling system, the inside wall of a nuclear pressure vessel is subjected to high thermal stresses that can cause extension of a preexisting flaw. The crack-tip stress intensity factor K(I) may achieve its maximum value early in the transient, but the critical level for crack initiation K(Ic) may not be reached until minutes later, when the loading has decreased from its peak. This phenomemon, termed warm prestress (WPS), can preclude crack extension when K(I) equals or exceeds K(IC). In an experimental program, employing three-point bend specimens, it was demonstrated that warm prestress acts to elevate the 'effective' K(Ic) of the material at the crack tip. This elevation is not uniform; it depends on the WPS level, the degree of unloading, and the increment between the temperature of WPS and the failure temperature. However, WPS can result in an effective elevation in K(Ic) to the WPS level, so that shallow cracks are arrested and deeper ones not extended. The experiments also demonstrated that failure never occurs during the unloading portion of the simulated LOCA path. Factoring WPS into an elastic analysis of crack extension during an LOCA results in predicted crack extension of greatly reduced proportions such that complete penetration of the wall does not occur. The experiments show that the phenomenon of warm prestress should be considered in predictions of reactor vessel integrity, and that this phenomenon may form a key element upon which to base assurance of vessel integrity during an LOCA. (Author)