The release occurred on 31 July after a grab weighing 130 pounds fell 40 feet from a fuelling machine, landing on top of a fuel channel in the Magnox reactor. Operators responded by tripping and depressurising the reactor.
Air sampling showed that radioactivity levels in the reactor coolant, carbon dioxide, were abnormally high, and a decision was taken to vent the gas to atmosphere over a two-hour period. A second release was carried out several hours later after radioactivity levels in the coolant were found still to be elevated.
It was only after this operation was completed that Nuclear Electric discovered that the gas release had breached a radioactivity emission limit. The site's authorisation under the Radioactive Substances Act 1993 permits a weekly release of 65 gigabecquerels (GBq) of sulphur 35, but the release in the week of the incident totalled 141GBq.
Nuclear Electric was charged by HM Inspectorate of Pollution with two offences under the 1993 Act. One was a breach of the authorised limit for sulphur 35. The second was a failure to use the "best practicable means" (BPM) to minimise releases to the atmosphere - another requirement of its authorisation.
The BPM requirement was breached on several counts, Amlwch magistrates heard on 4 March. The reactor was blown down without an assessment of the potential consequences. The results of the gas analyses were communicated inadequately to operators, who then failed to interpret them correctly. As a result, the release was carried out by staff who did not know the wind direction or consider the potential environmental effects.
HMIP also argued that the company had failed to learn the lessons of an incident in 1990. This had indicated that the release could have been reduced substantially by cooling the reactor slowly at full pressure and isolating two items of equipment which acted as sources of sulphur 35 when the pressure was reduced.
Before the hearing on 4 March, Nuclear Electric attempted to plea bargain its way out of the BPM charge by promising to plead guilty to the breach of the authorised limit for sulphur 35. The move was rejected by HMIP, and at the hearing the company pleaded guilty of both offences. Fines of £16,000 for breaching the limit and £18,000 for failing to use BPM - both close to the statutory maximum of £20,000 available to magistrates - were imposed, and the court also awarded HMIP its full costs of £20,170.
HMIP believes that the outcome has vindicated the use of dual conditions - numerical release limits and the general requirement to use BPM - in authorisations for nuclear plant.
Both were inserted for the first time in Wylfa's authorisation in 1992, some 20 years after the station came into operation, and without this change the current prosecution would have been difficult, if not impossible, to bring.