Failure Analysis of the Jet Pumps Riser in a Boiling Water Reactor-5

摘要

The arrangement of a riser coupled with two jet pumps is an important element in the Reactor Recirculation Core system of a Boiling Water Reactor. Its operational objective is to force the flow of water through the core for load variation and for safety is to keep the core flooded. In order to avoid thermal stresses, they are supported in a flexible way. In this case, the material expansion does not introduce stresses. The riser is only fixed at the upper zone welded at the riser brace. An important concern happens when the assembly vibrates under a torsional mode around its longitudinal axis. Under these conditions, helical cracks can be initiated at the riser brace weld. In this paper, a methodology for the evaluation of the structural integrity of the cracked riser is presented. It is considered that failure can take place in a brittle or in a ductile manner. For its evaluation, such cracks are projected along the axial and circumferential axis and evaluated in both conditions. In the first case, Fracture Mechanics are used. The ductile failure is evaluated applying the Collapse Limit Load analysis. The allowable crack length was determined when the flow through the core varied between 95% and 107%. These analyses were carried out when one or two recirculation circuits were in operation. In order to demonstrate the application of these results, three cases were analyzed. The results were evaluated with the Failure Assessment Diagram R6.