Finally in this section, two examples of life extension proposals are cited for illustrative purposes.

2.8.6.1 Magnox Stations. In the UK, safety cases were assembled to extend the operating life of the Magnox reactors up to 40 years (Twidale, 1999). The original design life of these plants was 20-25 years. These plants were commissioned in the 1960s and 70s

Table 2.11. Issues for Magnox plant for extended operation

Degradation due to ageing — ageing of civil structures

Hot gas release — evaluation of gas ducts’ failures on post-trip cooling

Seismic events — evaluation of risk, plant integrity and ALARP principle

Extreme wind — engineering assessments of RPV and cooling pond foundations

Extreme flood — assessment of groundwater level extreme changes

Design codes and standards changes — structural modifications where required

Twidale (1999).

and many are still in operation today. The last station, Wylfa, is not due to shutdown until 2010.

Issues of concern in the cases covered both hardware and software, together with plant and system reliabilities and key performance parameters such as load and temperature histories (Bolton, 1996; Table 2.11). The hardware concerns covered the ageing of any materials that might result in loss of structural integrity and how these could be inspected and also electrical hardware. Software issues concerned the demonstration of safety margins and system reliabilities in terms of economic performance were also evaluated.

As a consequence, a programme of long term safety reviews (LTSRs) was instigated in the early 1990s to be followed up with PSRs at least every 10 years up until the end of each station’s life. These PSRs in some cases require more frequent inspection of some key components than every 10 years.

The scope of the Magnox PSRs covered re-assessment of the remaining safety margins in the plant, using probabilistic methods, where possible. Issues considered included: ageing degradation of the structures, fault loadings, external hazards (e. g. seismic, wind, flood, etc.) and changes to design codes and standards. Where structural ageing had occurred, modi­fications or repairs were carried out to ensure structural integrity was maintained. Studies were carried out to confirm the plant’s safety level for fault loadings and external hazards.

In particular, for Magnox plants, ageing degradation could occur due to one of a number of processes including carbonation, chloride attack (for coastal plants), and thermal and movement effects. In general though, the Magnox structures have remained in good condition. Loadings on the structures under low probability pipework failures outside the vessel have been assessed; in addition post-trip cooling of the reactor has also been demonstrated. Seismic qualification was performed taking advantage of more modern analytical ‘finite element’ methods than were available in the original design phase. Similarly, engineering assessments of the reactor vessel and cooling pond foundations were carried out using more up-to-date mathematical modelling and taking advantage of better understanding of the soil mechanics.