Operations at Sellafield’s B205 Magnox reprocessing plant had to be put on hold in April this year because gaseous discharges from the site’s Fuel Handling Plant (FHP), where Magnox fuel is ‘de-canned’, had risen perilously close to Sellafield’s site discharge limit. B205 was closed for several weeks.
FHP is the facility at Sellafield that receives incoming transports of both Magnox and AGR fuel from UK nuclear power stations. Its operations include the dismantling of the AGR fuel assemblies, the de-canning of the Magnox fuel elements (the removal of the cladding surrounding the fuel rods) and the subsequent pond storage of both fuels – prior to their reprocessing in B205 or THORP.
The rogue discharges from FHP, which have had the knock-on effect of temporarily halting Magnox reprocessing, are of Antimony-125 (Sb-125), a beta radiation emitter with a radioactive half life of 2.75 years. The release of the Sb-125 is directly associated with the process of de-canning the higher burn-up Magnox fuels.
Whilst other operating plant at Sellafield also discharge Sb-125 into the air, FHP discharges ‘the lion’s share’ of the site discharge limit for Sb-125. For several months in 2007 discharges exceeded the Quarterly Notification Level and in November 2008 its discharge exceeded Sellafield’s internal trigger level. Up to last year the annual site limit for Sb-125 stood at 2.3 GBq, but was then raised to 6.9 GBq because of the increased discharge from FHP. Anticipating further increases in discharge of Sb-125, as more higher burn-up Magnox fuel is de-canned, Sellafield Ltd applied to the Environment Agency (EA) in October 2008 for an increase to the site discharge limit of Sb-125 from 6.9 GBq to 11.5 GBq.
EA’s assessment of the proposed increase in limit forms part of its review of the Radioactive Substances Act 1993 (RSA93) authorisation, but because the Sb-125 application relates to a site discharge limit, the request to increase the limit must be put out to public consultation. This is projected to be launched by the EA this Autumn and to run for 2 months. Additionally, approval for the increased limit has to be secured from the European Commission under the Euratom Treaty Article 37. Such approval may not be forthcoming until next year.
The problem of increasing levels of Sb-125 from FHP has been known for several years and is specific to the UK as other international reprocessors have no experience of reprocessing Magnox fuel. According to the 2006 Radioactivity in Food and the Environment report (RIFE 12), Sb-125 is ‘a relatively low impact radioanuclide’ and that some localised increases in levels had been detected in air, grass and deposition samples on the Sellafield site. No discernable impact in local milk had been detected through the RIFE monitoring programme.