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Conference Article: A method for assessing the effects of graphite property variability on core structural integrity criteria

Judge, R.C.B. (AEA Reactor Services, Risley (United Kingdom). Gas Cooled Reactor Assessments Dept.)

Abstract

The Reserve Strength Factor (RSF) is the factor of safety used in assessments of Advanced Gas-cooled Reactor core integrity. Values of RSF currently used are based on engineering judgement. These values should, in principle, be related to variability due to inhomogeneity of graphite, systematic error in the calculation route and uncertainties in property measurement. A probabilistic approach, bringing the safety assessment of the graphite core into line with other reactor components, is described in this paper. A probability distribution for the RSF may be obtained by repeatedly following the full calculation route. In each iteration, appropriate values for the key parameters will be obtained using the Latin Hypercube sampling technique. The number of key parameters will be kept to a minimum by first using the sensitivity calculations to eliminate the less important ones. Material properties have generally been obtained from tests on small specimens. The variability in these properties would, intuitively, be expected to be greater than that found in the brick (on which the RSF calculations are based). In the case of non-destructive properties these size effects may be treated by applying the central limit theorem. Core brick strength will be obtained from a series of on-going experiments. The statistical distribution of brick strength will be assumed to follow a Weibull function fitted to small specimen data. Work is currently underway, applying this proposed methodology to an Advanced Gas-cooled Reactor core.

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key words: gas cooled reactors; graphite; iterative methods; mechanical properties; probabilistic estimation; reactor components; reactor cores; safety analysis; sensitivity analysis; calculation methods; carbon
Reference:
Specialists' meeting on the status of graphite development for gas cooled reactors. Tokai, Ibaraki (Japan). 9-12 Sep 1991
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--690, pp:78-84