Significance of warm prestress to crack initiation during thermal shock

Abstract

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 pre-existing flaw. During this event 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 at which time the loading has decreased from its peak. It is shown that this phenomenon, termed warm prestress (WPS), can preclude crack extension when K I equals or exceeds K Ic. NRL has conducted an experimental study, employing three-point bend specimens, to investigate the potential for elevation in K Ic by WPS, and to translate the significance of this behavior into structural terms in the sense of minimizing crack extension in a nuclear vessel during a LOCA. From the experiments it is concluded that the mechanisms associated with WPS act to elevate the K Ic of the material at the crack tip and that this fact can greatly minimize crack extension that would have been predicted theoretically without consideration of WPS. The experiments demonstrated that failure never occurs during the unloading portion of the simulated LOCA path. This finding is of major significance to the integrity of a vessel. For example, with relatively deep cracks there is a combination of conditions wherein initiation ordinarily would be predicted as K Ic is reached along a decreasing K I path. For this set of conditions the present research studies have shown that the WPS phenomenon will preclude all such crack initiation. In terms of margin of safety against fracture it is shown that the elevation in K Ic caused by WPS is not uniform but depends upon the WPS level, the degree of unloading, and the increment between the temperature of WPS and the failure temperature. For LOCA conditions, however, it is concluded that WPS can result in an effective elevation in K Ic up to the WPS level assuming, of course, that metallurgically the material is capable of exhibiting this level of toughness. In terms of structural significance it is clear that WPS by itself cannot prevent the initiation of shallow cracks. Specifically, for a reference calculational vessel under the worst combination of conditions including a long, axial flaw and severe radiation embrittlement it is shown that a shallow crack can extend to a relative depth of 0.34 of the wall thickness; further crack extension is prevented by WPS. However, cracks having initial depths greater than 0.2 of the wall are prevented, by WPS, from extending any amount. Finally, it was observed that an elastic analysis of crack extension during a LOCA has predicted nearly complete penetration of the wall without consideration of WPS. Factoring WPS into the same analysis results in predicted crack extension of greatly reduced proportions such that complete penetration of the wall does not occur. Thus, WPS may form a key element upon which to base the assurance of vessel integrity during a LOCA.