Residual Stresses in Clad Nuclear Pressure Vessels and Their Interaction with Thermal and Mechanical Load

Abstract

Nuclear reactor pressure vessels are clad on their internal surface to provide resistance to corrosion. However, cladding also introduces residual stresses. The residual stresses combine with mechanical stress caused by internal pressure and thermal stress caused by rapid cooling known as thermal shock. The combination of stress from different sources may pose a threat to structural integrity. This work characterises the residual stress in reactor pressure vessel steel clad with nickel-alloy, and investigates how residual stresses interact with thermal and mechanical stresses.
Residual stress measurements were made on two steel plates clad with nickel-alloy using mechanical strain relaxation methods and synchrotron X-ray diffraction. One plate was as-welded, the other was post-weld heat-treated. The post-weld heat-treated plate was subjected to thermal shock and the residual stress was measured again afterwards. The cladding yielded during thermal shock which caused the residual stresses to redistribute. The results demonstrate that yielding should be accounted for when combining thermal and residual stresses in structural integrity analysis of thermal shock.
Measurements of stress were also made in the clad plate under transient conditions during thermal shock using in-situ synchrotron X-ray diffraction. It was demonstrated that the most severe thermal and residual stresses occurred during thermal shock, rather than before or after.
A new experimental technique was developed which represents residual stress in fracture specimens using fixed-displacement four-point bending, whilst allowing simultaneous application of mechanical stress. The technique was used to study the role of residual stress in failure of pressure vessel steel and high strength aluminium alloy. The results demonstrate that the initial level of residual stress has no effect on the fracture load if the material is sufficiently tough.

Date of Award	23 Jan 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	David Smith (Supervisor), Martyn J Pavier (Supervisor) & Mahmoud Mostafavi (Supervisor)