No abstract

Institute of Mathematics of the Czech Academy of Sciences provides access to digitized documents strictly for personal use. Each copy of any part of this document must contain these Terms of use. This document has been digitized, optimized for electronic delivery and stamped with digital signature within the project DML-CZ: The Czech Digital Mathematics Library http://dml.cz 41 (1996) APPLICATIONS OF MATHEMATICS No. 4, 241-267 HOW TO RECOVER THE GRADIENT OF LINEAR ELEMENTS

Reactor internals of VVER reactors are heavy components made of austenitic steel 08Kh18N10T. As they are close to the reactor core, they receive very high dose during their operation, which causes significant embrittlement. Their brittle fracture initiation has to be excluded during all operating conditions including emergency conditions to maintain its functionality and to avoid mechanical damage of fuel assemblies. During the emergency event the reactor internals are loaded by highly non-uniform temperature field due to thermal shock and also by dynamical loading due to non uniform pressure load. The temperature loading is relatively slow (critical time is about 100 s after beginning of the emergency event), while the mechanical loading is very fast (critical time is about 0.1 s after beginning of emergency event). The worst emergency event for both types of loading is LB LOCA. Due to this fact, both loading types can be assessed separately. This paper deals with the temperature loading. The procedure for fracture mechanical assessment of VVER reactor internals is given in “Guidelines for Integrity and Lifetime Assessment of Components and Piping in VVER Nuclear Power Plants (IAEA-NULIFE-VERLIFE)”, Appendix C “Integrity and Lifetime Assessment Procedure of RPV Internals in VVER NPP’s During Operation”. The procedure prescribes postulated cracks in the components and allowable value of J-integral in dependency on dose and temperature. The assessment of brittle fracture initiation is based on comparison of J-integral calculated for postulated crack for the assessed emergency regime with its allowable value. In the paper, brief description of both the VVER reactor internals design and methodology of the assessment are presented, and also some examples of the results are shown.

Warm prestressing is widely acknowledged as being able to enhance material toughness, especially in steels that exhibit lower shelf cleavage fracture. The enhancement in toughness has a significant impact on the integrity of pressure vessels, particularly during severe loading conditions, such as pressurised thermal shock. In this paper, we undertake detailed statistical analyses of experimental data provided via a comprehensive programme of fracture tests at UJV (Ústav jaderného výzkumu Řež a.s.). A warm prestressing model, developed by Chell, is used to predict the change in toughness probabilistically, using Monte-Carlo methods to predict the distribution in toughness following different warm prestressing cycles. The results obtained from this model are also compared to predictions made by the Wallin approach. Experimental data was generated, at UJV for WWER 440 RPV steel, using small single-edge-notched bend SEN(B) specimens (or pre-cracked Charpy) across a range of different fracture temperatures, warm pre-stress temperatures, and levels of preload, in both as-received and irradiated conditions. In this paper, experimental data obtained only from tests on unirradiated specimens were statistically treated. A three parameter Weibull distribution was used to map the scatter observed in the virgin toughness. The statistical significance of increase in apparent fracture toughness due to warm prestressing was evaluated using the Mann-Whitney test. It was further shown by Monte-Carlo simulations that the Chell and Wallin models provide slightly conservative predictions of the resulting fracture toughness. Both, the experimentally measured and predicted values of the resulting fracture toughness, depend on the specific tests conditions, especially on the level of preload.

Institute of Mathematics of the Czech Academy of Sciences provides access to digitized documents strictly for personal use. Each copy of any part of this document must contain these Terms of use.