EIRE: WYLFA REPORT - RPII RESPONSE

- Rapport -

The RPII (Radiological Protection Institute of Ireland) has responded to queries from the Celtic League following the publication by the Institute on its visit to the Wylfa Nuclear Power Plant. In its response the RPII say while they cannot categorically rule out an accident at the plant " in the event of a

Cathal Ó Luain pour Celtic League le 22/11/07 7:54

The RPII (Radiological Protection Institute of Ireland) has responded to queries from the Celtic League following the publication by the Institute on its visit to the Wylfa Nuclear Power Plant.

In its response the RPII say while they cannot categorically rule out an accident at the plant " in the event of a release to the environment, the primary impact on Ireland would be economic".

The RPII also seem to down play concerns voiced by the Celtic League and others about the continued operation of the plant long after its original operational life-span of 25 years.

The full text of the RPII letter is set out below and the Celtic League are grateful to the Institute for providing a comprehensive response to the points we raised.

"Radiological Protection Institute of Ireland

l2th November 2007.

Mr J B Moffatt, Director of Information, Celtic League, 11 Cleiy Rhennee, Kirk Michael, Isle of Man 1M6 1HT.

Dear Mr Moffatt

RPII Report on visit to Wylfa Nuclear Power Plant

I refer to your letter of 20th October 2007 to Dr Ann McGarry in relation to the RPII's report on its visit to the Wylfa Nuclear Power Plant in October 2006. I have now compiled the necessary technical information to respond to you as follows.

'Cracks in welds where superheated steam tailpipes from each boiler penetrate the steel liner of the reinforced concrete reactor pressure vessel.'

The Nuclear Safety Directorate (NSD - formerly the Nuclear Installations Inspectorate) has advised us that there is no information to suggest cracks in superheater tailpipes. In the event of through wall defectiveness, reactor gas monitoring would indicate an increase in moisture levels and boiler leak investigations would be initiated.

As referred to in the RPII report, defects were found in superheater header welds. It was concluded that these defects, which were detected through the use of more sensitive techniques than were available when the welding was performed, were formed during the welding process rather than during the lifetime of the plant. The NSD is satisfied that the fitting of physical restraints to all 32 superheater header penetrations would prevent ejection of the penetration from the wall of the reactor vessel in the event of a weld failure and would thus limit the steam pressure to a tolerable level. During its visit to the Wylfa site the RPII team was shown the penetration restraints and noted their massive construction.

'Corrosion of the internal steelwork of the reactors, particularly the core restraint garter'

The NSD has advised us that the operating temperature of the core restraints is below that at which significant steel oxidation will occur and potentially degredate the function of the restraint.

'Radiolytic oxidation (corrosion) or loss of volume of the graphite core'

As in the case of other graphite moderated reactors, some degradation of the graphite core, which has led to a reduction in its efficiency to moderate the neutron flux, has occurred. As discussed in the RPII report, at Wylfa this has been compensated for by the use of some slightly enriched uranium in the fuel. The NSD has advised us that the extent of reactor core ageing through graphite weight loss is well within safety case parameters and that the issue is routinely considered within an annual review of the graphite core safety case which provides the basis for the biennial inspection and materials properties test programmes completed on each reactor. In addition, routine sampling of reactor coolant gas temperatures provides an early means of detection in the unlikely event of a significant crack developing within a graphite brick.

I should also point out that some of control rods incorporate articulated joints to ensure they can still penetrate the core if the control rod channels should become blocked due to distortion of the graphite. This is also referred to in the RPlI report.

'Projected life of the Wylfa nuclear power plant'

While it was originally envisaged that Wylfa would have an operating life time of 20-25 years, this was not a requirement imposed by the NSD for safety reasons.

The conditions attached to each NSD nuclear site licence does, however, include a requirement to "make and implement adequate arrangements for the periodic and systematic review and reassessment of safety cases". These were formerly referred to as Long Term Safety Reviews (LTSRs). They are now called Periodic Safety Reviews (PSRs) and have to completed and cleared by NIl on a ten-yearly cycle.

The NSD reported on the outcome of its assessment of the Wylfa LTSR in 1995. It concluded that, subject to satisfactory completion of work programmes and continuing satisfactory results from inspections and demonstrations of safety, it expected that the station should be able to operate until each reactor is at least 33 years old. In 2003 the licensee (Magnox Electric) presented a PSR to the NSD. On 4 October 2004 NIl announced that "on the basis of its assessment, the safety improvements Magnox Electric has made at Wylfa and a follow-up programme, Nil has concluded that there is an adequate basis to allow continued operation of the reactors up to the company's declared end of generation in 2010 and subsequent final defuelling of the reactors, subject to satisfactory performance."

The RPII cannot categorically state that there will not be an accident at Wylfa between now and the date of its final closure and defuelling. However, the accident scenarios published in the RPII report indicate that, in the event of a release to the environment, the primary impact on Ireland would be economic. Taking account of the results of the reviews referred to above and, in particular, the additional safety features that have been incorporated in the reactors during the course of their working life, the RPII believes that the risk of a component, whose performance is critical to safety, failing within the remaining life of the reactors to be very small. In coming to this conclusion cognizance has also been taken of the fact that the concrete pressure vessel is of massive construction and, unlike the earlier and now closed Magnox reactors, including those at Calder Hall and Chapelcross, critical components such as carbon dioxide transfer pipes and steam generators are contained within the concrete structure. As I am sure you are aware, the RPII did have concerns about these earlier reactors and welcomed their closure.

Thank you for contacting the RPII in relation to your concerns.

Yours sincerely

P.A. Colgan Director of Advisory Services"

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