Michael Benn, 37, was one of a team of three maintenance workers removing sludge and debris from part of the cooling tower in August 2007.
A HSE investigation found he was working in poorly lit conditions inside the tower, and had gone into an enclosed culvert to check the depth of water in the sump.
Colin Mew HSE principal inspector said colleagues working nearby heard Benn’s shouts, but when they got to the sump he had disappeared from view.
His employer, Epsco pleaded guilty to health and safety offences at Mold Crown Court and was fined £35,000 and ordered to pay costs of £120,000.
Mew said: “Epsco Ltd would have known Benn or one of his colleagues would need to approach the sump in the course of their work.
“The inherent risk of working in this manner should have been obvious to any diligent employer. The enclosed area where this work was taking place was poorly lit, noisy and conditions were wet and slippery.
“The cost of providing barriers or other measures to prevent this incident and the time and effort involved would have been minimal.”