A leak at the Sellafield nuclear reprocessing plant in Cumbria spewed more than 20 tonnes of uranium and 160kg of plutonium onto a floor.
The Thorpe plant handles spent nuclear fuel
Plant operator British Nuclear Group, said the radioactive material was contained in 18,200 gallons of nitric acid, which leaked from a pipe.
An inquiry is continuing into the 18 April accident, details of which emerged on Wednesday.
The company said there were no injuries and no release of radioactive material.
The plant was shut down for a time.
The incident has been classified as a level 3 accident by the International Nuclear Event Scale (INES) because of the 83 cubic metres (18,257 gallons) of acid released in the incident.
INES measurements listed the 1986 Chernobyl disaster as a level 7 incident and Three Mile Island in the United States in 1979 as level 5.
The accident happened when a pipe - just a few centimetres wide - fractured, sending nitric acid onto the floor of a concrete-lined cell in Sellafield's Thorpe reprocessing complex. The cells, which are 60 metres long and 20 metres high are not accessible to staff.
The acid is used to break up solid spent nuclear fuel as part of the reprocessing procedure.
A spokesman said there was no risk to employees or the local community.
The discovery was made after a camera inspection of the feed clarification cell, which is contained within 2 metre thick concrete walls which are lined with stainless steel.
The company said the last time a level three incident occurred in the UK was at Sellafield in 1992.
'Repair of replacement'
A spokesman said: "The situation within Thorp remains safe and stable.
"The final phase of recovery of the liquor from the cell floor is planned to commence before the end of this week.
"It is anticipated that this will take approximately four weeks, however this duration will only be confirmed once recovery has actually started.
"A project group has agreed a programme of work with a number of options being considered including working around the affected tank, repair or replacement of the failed components, subject to a suitable safety case being made.
"A preliminary investigation by the Nuclear Installations Inspectorate has started and is expected to continue over the next couple of weeks.
"The incident has been reassessed as a level 3 on the International Nuclear Event Scale (INES) purely on the basis of the quantities of material released to the secondary containment."