"The penalty, bearing in mind the public's concern, has to be very substantial and has at least to sting. Certainly at present I am of the view that it really must be a six-figure sum," Mr Justice Morland told the court in Mold, Clwyd, before retiring overnight to consider the sentence.
In a two-day hearing, Nuclear Electric had pleaded guilty to four charges under the Health and Safety at Work Act relating to an accident at Wylfa - one of Britain's first-generation Magnox reactors, which will not be privatised when the rest of the industry is sold.
In the incident two years ago a crane grab broke off and fell 40ft into the reactor, causing damage and obstructing a small part of the flow of coolant gas through the structure. But nine hours passed before control- room staff shut the reactor down.
Officials were accused by the Health and Safety Executive, which brought the prosecution, of having their "brains in neutral" and putting money before safety - it was claimed that Nuclear Electric wanted to avoid any interruption of power generation. The court heard a tape recording in which staff in the control room laughed as the incident developed.
Robert Owen QC, defending Nuclear Electric, told the court yesterday that the company had failed to stipulate in its station operating instructions that the reactor should be shut down if it was suspected an item with the potential of blocking a channel had been released into the core.
"The company were at fault in the manner in which their staff were instructed to apply this operating rule," he said.
But he denied that there were any commercial reasons for the delay. And Mr Owen condemned press reports which suggested the reactor could have suffered a melt-down - a criticism which was accepted by the prosecution.
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