A bayesian analysis of the influence of neutron irradiation on embrittlement in ferritic submerged arc weld metal
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I. INTRODUCTION
THE reactor pressure vessels at six of the United Kingdom commercial Magnox Power Stations currently in operation have been manufactured from ferritic steels. Magnox pressure vessels are 20 m diameter spheres containing graphite moderator and carbon-dioxide-gas coolant and, typically, operate at a pressure of 1.8 MPa and a temperature up to 340 8C. The pressure vessels have been fabricated from 75 to 100 mm thick carbon-manganese steel plates and forgings joined together either by manual metal-arc or submergedarc welds. The ductile to brittle transition temperature of these steels changes as a result of neutron irradiation and temperature during service. These changes of transition temperature are monitored by a surveillance scheme which employs Charpy impact energy specimens. It was found that the submerged-arc-weld metal exhibits the largest changes of Charpy impact properties. In the surveillance scheme for submerged-arc-weld metal, a large data base is available for irradiation at 190 8C and 198 8C. The range of neutron doses and temperatures for which data are available does not cover the full range experienced by the reactor pressure vessels. To assess the effect of irradiation temperature and neutron doses beyond the surveillance range, some specimens were subjected to accelerated irradiation in commercial Magnox reactors at Chapelcross over the temperature range from 186 8C to 292 8C. Prior to accelerated irradiation, the specimens had been subjected to neutron irradiation under surveillance R. MOSKOVIC, Senior Researcher, is with BNFL Magnox Generation, Gloucestershire, GL13 9PB United Kingdom. C. JORDINSON, Research Student, is with the Institute of Astronomy, Cambridge University, Cambridge, CB3 0HA United Kingdom. D.A. STEPHENS, Lecturer, is with the Department of Mathematics, Imperial College, London, SW7 2BZ United Kingdom. A.F.M. SMITH, Principal, is with Queen Mary and Westfield College, London, E14NS United Kingdom. Manuscript submitted December 22, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
conditions at either 190 8C or 198 8C in the commercial Trawsfynydd reactors. The analysis of the data base available for the surveillance conditions has been presented in References 1 and 2. The Charpy impact energy properties have been analyzed as a function of neutron dose using Markov chain Monte Carlo (MCMC) sampling-based techniques. The work described in this report presents analysis of a combined data base which includes data obtained from the surveillance exposures performed in the Trawsfynydd reactors and reirradiation exposures carried out in the Chapelcross reactors. II. MATHEMATICAL MODEL FOR CHARPYIMPACT-ENERGY DATA A. The Burr Distribution Function Charpy impact energy measured as a function of temperature follows a sigmoidal curve which can be modeled by a Burr distribution function[2] given by F(Ti) 5
1
1
F
T 2 T0 1 1 exp 2 i j
G2
n
[1]
where i indexes the measured Charpy impact energy data, Ti is the test temperature, n determines the symmetry of the curve, j is
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