DOUNREAY was last night facing calls for criminal prosecution and a full independent inquiry after a breach in safety which anti-nuclear protesters claimed could have caused the worst accident in the plant's 40-year history.

Management at the Caithness complex yesterday denied the breach could have caused a nuclear reaction, but confirmed that the main electricity supply and the back-up supply to the Fuel Cycle Area, where the most dangerous nuclear work is undertaken, were both disrupted at 9pm on Thursday and were not restored until 1pm yesterday.

A contractor working on a digger, digging out ducts for BT had cut through the main cable, disrupting the power supply. The ventilation system ceased operating and all 300 non-essential personnel were evacuated.

The area includes the facility where the controversial cargo of nuclear fuel from Georgia is currently being stored and plant D1203, where the bulk of it will be processed at the end of this month.

It also embraces the near 40-year-old D1204 plant where 1200 rods of spent nuclear fuel from Australia would be reprocessed under a contract Dounreay is hoping to sign.

Its director, Dr Roy Nelson, issued a statement yesterday insisting that the situation was under control. He said: ''The system which monitors discharges from the plants is powered by the separate battery-backed supplies which continues to operate, providing further confidence that there are no abnormal discharges.

''The plants have been powered down and are in safe and stable state. Non-essential personnel are not entering the area until power supplies are restored. The rest of the site is unaffected.

''Urgent work is now being undertaken to rectify the problem. The Nuclear Installations Inspectorate (NII) and the Scottish Environment Protection Agency (Sepa) have been informed.''

Both the SNP and Greenpeace International claimed the episode had served only to display a terrifying weakness in the plant. Roseanna Cunningham, the SNP spokeswoman on the environment, was quick to resume the Dounreay debate her party had joined with the Government two weeks ago over the Georgian fuel.

''This shows up a fundamental design flaw at Dounreay, where the main power supply is on the same ring main as the back-up generator.

''What needs to be asked is: how on earth did the UK Atomic Energy Authority (Dounreay's operators) get away with such negligence till now, and why did the NII not pick up such a basic safety risk years ago?

''UKAEA have now had to shut down D1203, the only plant which has not until now failed safety inspections. This means that there is now no way they can make fuel targets out of the Georgian waste for the production of medical isotopes and, as I have said all along, Dounreay is just not suitable for dealing with such material.

''What will it take to shut Dounreay down? Last week, Sepa served an order on UKAEA for grossly underestimating the radioactive discharges, yesterday it admitted to subjecting its own workers to excessive radiation, and today we find that the power system is so flawed that in one move it can lose control of all its processing and reprocessing plants. This is surely the last straw.

''Every plant at Dounreay, including D1203, should now stay closed until a full independent inquiry is carried out. In the light of recent disclosures, and faced

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with such overwhelming evidence, I am convinced that an inquiry would have no hesitation in putting an end to Dounreay's operations permanently.''

The Convener of Scotland Against Nuclear Dumping, Mrs Lorraine Mann, said: ''This may well be the most serious breach of safety at Dounreay yet. They completely lost control in what is one of the potentially most dangerous parts of the whole Dounreay plant. They could do absolutely nothing.

''We have an incompetent management running a geriatric plant, dealing with some of the most hazardous materials known to man. Dounreay's management must be prosecuted for their total loss of control of this area.

''This incident comes less than a fortnight after the House of Commons was assured by Junior Minister Doug Henderson that Dounreay was a modern facility well placed to handle safely the Highly Enriched Uranium from Georgia. What is it going to take for people to wake up to what is going on there?''

Kevin Dunion, director of Friends of the Earth Scotland, said: ''It is simply incredible that an area so critical to operations at Dounreay can be left without power so easily.

''Fortunately for everyone, Dounreay is not allowed to do any reprocessing work at the moment, otherwise the consequences could have been the ultimate in nuclear disasters. As it is, all operations at the site should be suspended until a full and independent inquiry into this latest incident has been completed.

''This is probably the most serious accident at Dounreay since the waste shaft explosion of 1977.''

A spokesman for the NII, which reports to the Department of Trade and Industry and not the Scottish Office, said that three inspectors were on their way to Dounreay and one was an electrical engineer.

''There certainly will be a thorough investigation. We do not know yet whether indeed there was a design flaw or whether there was a coincidental failure which prevented the back up system from working. That will be part of the investigation.

''We don't think at present there would be a greatly raised risk of something going wrong, but we will only know once our investigations have been completed.''

The Shadow Minister of the Board of Trade, Mr John Redwood, said he would seek Government assurances on the safety of Dounreay.

Mr Redwood said: ''I will be tabling questions on Monday to find out from the Government the details of the incident at Dounreay.''