The steel pressure vessel Magnox reactors were designed in the 1950s and were made from C–Mn plate steel and forgings welded together using a mixture of submerged-arc and manual metal arc weld metals. Each reactor contained surveillance capsules with specimens of plate steel, forgings, and the different weld metals to monitor the effects of irradiation on the Charpy impact and tensile properties. Canisters were withdrawn over the operating life and measurements were taken. During the lifetime of the fleet, there were developments in testing, observed changes in properties, and understanding of the radiation damage process that challenged the safety cases to support the operation of the stations. At the time the reactors were designed, the concept of fracture toughness was only beginning to be investigated, yet during the lifetime of the stations, fracture toughness testing was successfully adopted as standard practice as an input to fracture mechanics based assessment of the steel vessels. Both hardening and non-hardening embrittlements, the latter due to impurity phosphorus segregation in weld metal, were successfully addressed. At a relatively late stage the contribution of “thermal neutrons” to embrittlement was identified as being significant and was successfully incorporated into the assessment process. This led to the adoption of sophisticated statistical techniques to assess changes in properties of the most critical construction material—submerged-arc weld metal. A large scale sampling and testing programme of submerged-arc weld metal from a decommissioned reactor validated the assessment process. As a result of successfully addressing these and other challenges when the last two steel pressure vessel stations closed in December 2006, they had achieved lifetimes of nearly 40 years.
Wootton, MR., Moskovic, R., Bolton, CJ., & Flewitt, PEJ. (2010). Magnox Steel Reactor Pressure Vessel Monitoring Schemes—An Overview. Journal of ASTM International, 7. https://doi.org/10.1520/JAI101948