A nuclear leak, which could have caused a major disaster, was only averted by a chance decision to wash some dirty clothes, according to a newly obtained official report.

On the morning of Sunday 7 January 2007, one of the contractors working on decommissioning the Sizewell A nuclear power station on the Suffolk coast was in the laundry room when he noticed cooling water leaking on to the floor from the pond that holds the reactor's highly radioactive spent nuclear fuel.

As much as 40,000 gallons of radioactive water spilled out of a 15ft long split in a pipe, some leaking into the North Sea. The pond water level had dropped by more than a foot (330mm) – yet none of the sophisticated alarms in the plant sounded in the main control room.

By the time of the next scheduled safety patrol, the pond level would have dipped far enough to expose the nuclear fuel rods – potentially causing them to overheat and catch fire sending a plume of radioactive contamination along the coastline.

The HM Nuclear Installation Inspectorate's report of the incident, obtained under the Freedom of Information Act, said: "The pond could have been drained (it takes about 10 hours) before the required plant tour by an operator had taken place. In this worst-case scenario, if the exposed irradiated fuel caught fire it would result in an airborne off-site release."

It concluded: "NII believes that there was significant risk that operators and even members of the public could have been harmed if there had not been fortunate and appropriate intervention of a contractor who just happened to be in the right plant area when things went wrong."

But the NII said the operator Magnox Electric responded quickly once the leak had been discovered. It said: "The only good action noted was that when the contractor raised the alarm the operators appeared to react very well and avoided a significant event from becoming a very serious off-site event."

The NII report was obtained by the independent nuclear consultant John Large as part of a dossier he compiled for the local Shutdown Sizewell Campaign. The owners, now Magnox South, say it had little effect on the environment and represented a fraction of the site's annual discharge limit.

Magnox South said it took the incident extremely seriously and co-operated fully with both the NII and the Environment Agency.