Recursively coupled thermal and mechanical FEM-analysis of lower plenum creep failure experiments H.-G. Willschu ¨tz a, * , E. Altstadt a , B.R. Sehgal b,1 , F.-P. Weiss a a Forschungszentrum Rossendorf, Institute of Safety Research, PO Box 51 01 19, D-01314 Dresden, Germany b Royal Institute of Technology, Division of Nuclear Power Safety, Drottning Kristinas va ¨ g 33A, S-10044 Stockholm, Sweden Received 20 July 2005; accepted 30 August 2005 Available online 16 November 2005 Abstract Postulating an unlikely core melt down accident for a light water reactor (LWR), the possible failure mode of the reactor pressure vessel (RPV) and its failure time have to be investigated for a determination of the load conditions for subsequent containment analyses. Worldwide several experiments have been performed in this field accompanied with material properties evaluation, theoretical, and numerical work. At the Institute of Safety Research of the FZR a finite element model (FEM) has been developed simulating the thermal processes and the viscoplastic behaviour of the vessel wall. An advanced model for creep and material damage has been established and has been val- idated using experimental data. The thermal and the mechanical calculations are sequentially and recursively coupled. The model is capa- ble of evaluating fracture time and fracture position of a vessel with an internally heated melt pool. The model was applied to pre- and post-test calculations for the FOREVER test series representing the lower head RPV of a pres- surised water reactor (PWR) in the geometrical scale of 1:10. These experiments were performed at the Royal Institute of Technology in Stockholm. In this paper the differences between the results of a simple coupled and a recursive coupled FE-simulation are highlighted. Due to the thermal expansion at the beginning and the accumulating creep strain later on the shape of the melt pool and of the vessel wall are changing. Despite of the fact that these relative small geometrical changes take place relatively slowly over time, the effect on the temperature field is rather significant concerning the mechanical material behaviour and the resulting failure time. Assuming the same loading conditions, the change in the predicted failure time between the simple and the recursive coupled model is in the order of mag- nitude of the total failure time of the simple model. The comparison with results from the FOREVER-experiments shows that the recur- sive coupled model is closer to reality than the one-way coupled model. Ó 2005 Elsevier Ltd. All rights reserved. 1. Introduction Several scenarios – with extreme low probability – are existing for a severe accident with relocation of corium into the lower plenum. In general, the ungoverned station black out (outage of all electrically driven emergency cooling sys- tems) is considered as the initiating event. The core cannot be cooled sufficiently and is destroyed. The subsequent course of the accident depends amongst others on the fol- lowing questions (Bu ¨scher et al., 1998):  Is all the cooling water in the RPV evaporated (e.g., as a consequence of an SBLOCA) or does a water pool remain in the lower plenum?  Will the core be re-flooded?  Can the system pressure be decreased? The following facts depend from the answers to above questions:  the amount and composition of the melt, which is relo- cated to the lower plenum; 0306-4549/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.anucene.2005.08.006 * Corresponding author. Tel.: +49 351 260 3431; fax: +49 351 260 13431. E-mail addresses: h.g.willschuetz@fz-rossendorf.de (H.-G. Willschu ¨ tz), bsehgal@safety.sci.kth.se (B.R. Sehgal). 1 Tel.: +46 8 790 9252; fax: +46 8 790 9197. www.elsevier.com/locate/anucene Annals of Nuclear Energy 33 (2006) 126–148 annals of NUCLEAR ENERGY