The proper use of thermal expansion coefficients in finite element calculations

Thermal expansion coefficients (TEC) play a crucial role for the calculation of thermal stresses in many safety assessment procedures since thermal stresses are the main loads, i.e. in thermal shocks caused by an emergent fast cool-down of a reactor pressure vessel (RPV), which can reach high amount and may lead to a break-down of the structure. Therefore the safety of the RPV has to be approved by thermal shock calculations. Such calculations were made e.g. within round robin bench mark calculations organized by the OECD Committee on Safety of Nuclear Installations (CSNI) and others. The main tool employed for such calculations is the finite element method (FEM). However, in spite of this widely employed tool, the misuse of TEC may lead to misinterpretations of the results and finally to non-conservative results. This paper focuses on the proper use of the TEC and stresses some pitfalls in the numerical calculation of thermal stresses. The relevance of the problem is illustrated by means of a thermal shock calculation of a western type RPV.

► The difference between mean and instantaneous thermal expansion coefficients is shown.
► Mean expansion coefficients must be transformed to the correct reference temperature.
► A safety assessment of an RPV exposed to pressurized thermal shocks is shown.
► We show how non-conservative results of a PTS analysis can be avoided.