The Magnox life extension story19 June 2013
Harry Steven, Magnox Ltd engineering director, explains why one UK Magnox reactor was life-extended three times in as many years, and hints at a final life extension of Wylfa 1. Interview by Will Dalrymple
NEI: Was there ever an expected lifetime for Magnox units?
HS: Absolutely; Magnox had a 25-year design life. It was obvious that they were so robustly designed and built, they would have significant margins that go well beyond life. In about 2000-2001, the lifetimes and closure plans were announced for all the remaining stations: Dungeness A, Sizewell A, Oldbury and Wylfa. Those dates were to fit in with the government's project to finish reprocessing at Sellafield by 2020 (the OsPar committment). And they fitted in with the periodic safety review (PSR) birthday, at least for most of them. Magnox fuel production ceased in about 2006-7; that was a decision made based on this closure programme. We operated Dungeness A and Sizewell A up to the end of 2006, as per the plan. In about 2006/07 it became clear that reprocessing at Sellafield was behind and there was an opportunity to extend Oldbury and Wylfa.
NEI: Tell us about the Oldbury extension.
HS: There were several steps involved in extending Oldbury. In the first step, the safety case was extended at Oldbury from April 2008 to March 2010, a two-year step. We argued this on the basis that the miles on the clock wouldn't exceed the original plan, because we'd had outages. [Margins included] the fatigue and creep life, and the running hours in hand, and in terms also of the in-core radiation of graphite. We made an updated graphite safety case and a case for revision of the PSR, and we argued to the regulators that we were within the original operating hours. That was our first extension.
Then a second extension went from April 2010 to June 2011, for both reactors. This was the one and only time we extended the period of the graphite safety case. We did probabilistic assessment to back up the deterministic case that we had already done [circa 2007], and put in place a new fuel integrity monitoring system. We had one already, a burst can system, but we put in a second fuel failure monitoring system to reduce risks further. On that basis, ONR [the regulator] approved it to June 2011.
At that point, we ran out of fuel. The last extension for reactor 1 only was in July 2011 to end February 2012, and for that, we used inter-reactor transfer. There is no new fuel, so we used the shutdown reactor as a donor, and took out low-irradiated fuel and replaced the high-burnup fuel in the operating reactor.
Altogether those three steps generated 7.4 TWh of electricity, and provided £350 million of income for the government. [It owns the reactors; operator EnergySolutions, as Magnox Ltd, also got a bonus].
NEI: What about Wylfa?
HS: Wylfa we did in one step. It was different than the others. Wylfa was 30 years old in 2004, and I project-managed the PSR in 2004 to justify an extra 10 years. The closure date was announced to be 2010; we planned to only generate for six more years. When we came up towards 2010, and realized that the reprocessing schedule at Sellafield had slipped, that we didn't need to defuel, and that we had four years left in the 10-year window for a PSR, we decided to do it in one go. In any case the graphite case has got much larger margins [than at Oldbury], but we did know that there was not enough fuel to reach 2014, and that we would have to use inter-reactor transfer again. However, DECC [the government] chose not to announce the extension in one step but in two: 2010-12, and 2012-14, apparently for political reasons.
Most of the previous Magnox reactors went for 40 years. The real difference between Oldbury and Wylfa and the others was the pressure vessel; they had concrete, like the AGRs; the others had steel pressure vessels. We would have faced some reluctance from the regulator to go beyond 40 years because of the steel pressure vessel. The concrete is immensely robust.
NEI: Without fuel being limiting, would you have carried on with the Magnox reactors?
HS: We always said that we would operate the plants as long as it was safe and economic to do so. Given unlimited supplies of fuel, and supposing just for a moment no timescale for reprocessing fuel, to operate them for five or 10 years would require doing a PSR and investing heavily in the plant. In 2004 when I did the last 10-year PSR for Wylfa, we spent £60 million upgrading the plant. In the previous PSR we spent more than £100 million. There is a fine tradition in the UK of doing 10-year PSRs, which do assess the reactors against modern standards. We have upgraded against seismic and wind load. But it is very expensive, and I think it wouldn't have been economical to do another 10-year operational period at Wylfa, and definitely not at Oldbury; they are only 200 MW units, which is small.
NEI: Can we have an update on Wylfa 1?
HS: We are very confident we can get to 30 September 2014, which is the PSR date, and inter-reactor transfer is going well. There is the possibility of going beyond 2014 at Wylfa for only a limited period because, even with inter-reactor transfer, we are running out of low-irradiated fuel. We have been taking channels of low-irradiated fuel, and we will have used all of them. The final fuel optimization that we can do is to harvest fuel element 8 from top of the donor reactor, which, due to axial form factor, is all low-radiation. We are planning a modest increase after 2014, which is not permissioned yet. The longest it would be is 15 months beyond September 2014, or December 2015.
NEI: What about graphite?
HS: We have an ongoing graphite monitoring programme. We sample the reactor every year, and review the safety case every year, and confirm that the samples are consistent with it. The safety case runs to 2014, and there are good margins; an extra 15 months wouldn't require a new programme of work.
NEI: And the boilers?
HS: The boilers at Wylfa were always a concern. But the problem is under control. When we get tube leaks, we have to plug them off, and lose some surface area, from the outside. The boilers were oversurfaced by about 30%, and so far we have only used 5% of that. So there is still a margin.