A PROBABILISTIC APPROACH TO ASSESSING AGR CORE LIFE

The current ACR core safety cases are based on two limits, 20% brick e ffective weight loss (equivalent to a peak value of 40%) and failure o f any component (e.g a key/keyway or core restraint). The former has b een adopted because of the paucity of experimental observations on AGR graphite strength from test reactors beyond that peak value. It has n o substantiation in terms of structural integrity of the core and is t herefore clearly arbitrary. The latter is a structural feature, but th e consequences for the overall serviceability of the core have not bee n addressed. It is now reasonably clear that the failure of a graphite component does not imply a loss of core functionality. It is, therefo re, more appropriate to develop a safety case methodology which consid ers the impact of the irradiation-induced changes in the graphite core components upon the requirement for certain essential operational and safety features (e.g. control rod insertion, maintenance of cooling, insertion/withdrawal of fuel). Nuclear Electric has been developing su ch a methodology based upon the probabilistic analysis of core ageing and component failure with a computer code CORSET. The code examines t he distortion of the core under various operating conditions, allowing for local discontinuities due to component failure. These distortions are repeatedly assessed allowing for statistical variations in compon ent dimensions and properties, thus constructing a probabilistic distr ibution against which the required operational and safety features can be confirmed In this way, it is intended to construct a safety case f or continued operation beyond the first failure of a graphite componen t. AEA have carried out a large programme of work aimed at verifying a nd validating certain constituent parts of this new methodology. The n ew methodology meant that there was a requirement for statistical data on material properties and strength in particular. First, the availab le data was reviewed and a database encompassing all relevant graphite thermo-mechanical properties was produced. The graphite component fai lure criterion was then reviewed and a new approach was developed know n as the fractional remanent strength. The fractional remanent strengt h criterion required critical stress values for failure of components under different modes of failure relevant to the AGR cores (e.g key/ke yway loading and pinching, internal stress). A series of experiments, both slice and full size brick geometries, were conducted to determine the failure statistics and the response of core components in a varie ty of situations (e.g cracked bricks, simulated internal stress and pi nched key/keyways). Finally, the effects of material variability were included in finite element calculations leading to a probabilistic dis tribution of remanent strength at various times through reactor life.