Experience of surveillance schemes adopted for magnox steel reactor pressure vessels

Abstract Surveillance or monitoring schemes are recognised to be an important part of any strategy to demonstrate that reactor pressure vessels used in civil nuclear power stations are operated within a safe regime. In the paper the authors describe the experience obtained from the surveillance schemes adopted for the UK's magnox nuclear power stations that were constructed with C–Mn steel reactor pressure vessels. These power stations were constructed in the late 1950s and 1960s and the last ceased generating in 2006. During the lifetime of the fleet with steel pressure vessels, there were developments in testing, observed changes in properties and understanding of 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 an input to fracture mechanics based assessment of the steel vessels. Over the operating life, a series of challenges emerged that were successfully addressed, including both hardening and non-hardening embrittlement, the latter due to impurity phosphorous segregation in weld metal and contributions from thermal nuclear embrittlement. These challenges led to the adoption of sophisticated statistical techniques to assess changes in embrittlement properties of the most critical construction material – submerged arc weld metal. A large scale sampling and testing programme of submerged arc weld metal removed from a decommissioned reactor pressure vessel 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.