Life management of reactor pressure vessels under pressurized thermal shock loading: deterministic procedure and application to Western and Eastern type of reactors

Abstract

The integrity of the reactor pressure vessel (RPV) has to be maintained throughout the plant life and it is also one of the main considerations regarding the plant life extension. Adequate approaches to the RPV integrity assessment provides a basis for plant-safe operation and for timely implementation of preventive and corrective measures if necessary. A substantial part of the RPV integrity assessment is related to the pressurized thermal shock (PTS) analysis. The safety of the RPV during such a severe loading condition has to be proven in the design stage and during life of the component. Irradiation programs allow for an adequate description of the material response and by means of analytical and numerical methods the structural analysis can be performed. The PTS-analysis of the RPV is a multidisciplinary effort and involves among others detailed thermal hydraulic analysis and structural analysis including fracture mechanic assessment. Operational load situations during accident conditions are dominated by pressure and temperature transients during an emergency core cooling event following a loss of coolant accident relevant for PTS. Cold water is injected into the cold legs and falling into the downcomer of the RPV. Complex thermal hydraulic situations occur and the fluid temperatures in the downcomer as well as the RPV wall-to-fluid heat transfer coefficient are highly non-linear. To determine the spatial fluid temperature and wall-to-fluid heat transfer coefficient distribution in the downcomer and the respective thermal stresses with adequate accuracy, fluid–fluid mixing scenarios are taken into account. Plume cooling during a loss of coolant accident of the RPV leads to a locally increased axial stress distribution in the plume region compared to the axial symmetric cooling. The higher axial stresses make the circumferential crack more relevant for the safety assessment. The transient temperatures and stresses in the RPV-wall are calculated by means of 3D finite element (FE) calculations and are performed for given transients. The safety assessment deals with a safety margin against the material toughness curve or the determination of the allowable brittle to ductile transition temperature. Using the global thermal hydraulic parameters calculated with adequate system codes, the fluid temperature and RPV-wall-to-fluid heat transfer coefficient distribution in the downcomer has been calculated with the Siemens code KWU-MIX. In the following fracture mechanics calculation, two flaw types have been considered, axial and circumferential cracks. By means of 3D FE calculations the sub-surface flaws were investigated and from the results of the fracture parameters safety margins could be derived. In this report the Siemens procedure is applied as an example to an Eastern plant WWER-1000 type, but it has been successfully carried out for different types of RPV also: Western 3 and 4-loop plants (German and French type) where the results are reported in [Proc. ASME PVP Conf., (1999), Boston] [7] and [NUREG/CR-6651 (ORNL/TM-1999/231), Oak Ridge National Laboratory, (1999)] [5]. Although all considered pressure vessels are different in their construction: the French type is constructed with a thermal shield between RPV wall and internals, the Eastern type has two rings of injection nozzles, which leads to a complex thermal hydraulic situation the general procedure could be applied. Following results have been obtained: –plume cooling leads to increased axial stresses in the plume region –therefore circumferential flaws are more severe than axial flaws –safety margins for axial flaws are higher than for circumferential flaws.