Despite the obvious risk of danger, the Number One reactor at the atomic station, operated by Nuclear Electric, was not switched off for nine hours after it was realised that a metal grab - part of a crane device used to refuel uranium cylinders - was missing.
It was claimed that officials had their "brains in neutral" and that they had in effect put money before safety for they appeared anxious that they should not face penalties from the National Grid by closing one of their two reactors down.
The chief of Britain's nuclear inspectorate, Sam Harbison, said he viewed the actions of Nuclear Electric during the incident as "a matter of grave concern". Dr Harbison said in a statement read at Mold Crown Court that, in his view, the events were potentially the most serious in the UK during his time as chief inspector: "I am particularly concerned about the blatant failure of Nuclear Electric's safety culture."
Despite the fact that the metal grab was missing, the operation of the reactor was allowed to continue, a "severe violation" of safety policies.
Failure to act promptly to prevent a possible fuel channel blockage accident by immediately shutting the reactor down meant that, should something else have occured, there could have been a serious release of radioactive material.
"In my opinion, it is irrelevant to argue with the benefit of hindsight about the likelihood and potential scale of the release that might have occurred," Dr Harbison said.
"What is important is that the operators were prepared to continue to operate the reactor for several hours without being able to know the exact coolant flow conditions in the core, running some chance that the fuel could be over-heating.
"In any event, if any action can be readily taken to avoid risk, it should be taken," he said.
"I believe that throughout these events, the operators failed to adequately grasp the safety implications. There may also have been concern with commercial considerations associated with having to shut the reactor down."
The company, which admits four charges brought under the Health and Safety at Work Act, will be sentenced today by the judge, Mr Justice Morland.
Hugh Carlisle, QC, prosecuting for the Health and Safety Executive, said that the metal grab had fallen into a fuelling channel in the reactor but there was a reluctance by station officials to assume the worst.
The incident happened in 1993 - the year the station received a Royal Society for the Prevention of Accidents' Gold Award for its safety record.
The metal grab was discovered missing at 7.20 pm on 31 July. From about 9pm, when checks revealed the missing metal must have fallen into the reactor, the only thing to do was to switch it off, said Mr Carisle. That was not done until 3.50 am.
There was a "marked reluctance" to close the reactor down but to have continued with a rogue element inside a reactor was "indefensible", said Mr Carlisle. "I have to say it was a classic example of brains in neutral," he said. "That is hardly an ideal condition to run a power station." He said staff seemed to have taken the view that because nothing was going wrong, they were entitled to assume everything was all right.
The risk of a blockage in a fuelling channel was overwhelming and self- evidence and it was suprising that there had not been a melt-down.