The operators of two nuclear facilities have each been fined £2 million over radioactive spillages.
The United Kingdom Atomic Energy Authority was penalised over an incident at Dounreay in Caithness.
And BNG Sellafield was fined for a radioactive leak at its Thorp reprocessing plant in Cumbria.
The fines were imposed by the Nuclear Decommission Authority (NDA) and will be deducted from the fee the quango pays to the operators.
In September last year 266 litres of hazardous, dissolved spent fuel spilled on to a laboratory floor at Dounreay's cementation plant.
The liquid, which is kept in underground tanks, was being pumped to the plant where it is mixed with cement then stored in 500-litre drums.
No employees were injured or exposed to radiation during the scare, but it led to the plant being temporarily closed.
The penalty is thought to be the biggest suffered after a safety breach at Dounreay.
The radioactive leak at Sellafield's Thorp reprocessing plant in May 2005 involved enough toxic material to half fill an Olympic-size swimming pool.
No one was injured after the plutonium and uranium fuel dissolved in concentrated nitric acid seeped through a fractured pipe but the plant had to be shut for several months.
The fines are detailed in the NDA's annual review for 2005/06.
Its report says: "As a consequence of failings that led to incidents at Thorp and Dounreay, the NDA has made a fee deduction of £2 million from both BNG Sellafield Ltd and UKAEA respectively."
A spokesman for the UKAEA said the Dounreay accident was unfortunate but no employees had been harmed.
She added: "It shouldn't have happened, but the plant was designed to protect the workforce and the environment in case something like this did happen.
"There was no danger to any of our employees and the necessary steps were taken."
In a separate development, the operators of Dounreay could face legal action over the release of radioactive particles.
The Scottish Environment Protection Agency (Sepa) said it had submitted reports to prosecutors about the facility.
Sepa is now waiting to see if legal proceedings will be brought against the UKAEA.
Dounreay, a former experimental reactor establishment, was shut in 1994 and is earmarked for a £2.9 billion decommissioning by 2033.
More than 1,000 radioactive particles, fragments of spent uranium fuel rods about the size of a grain of sand, have been found on beaches and the sea bed around the facility.
Sepa said it had submitted reports to prosecutors in February this year and November 2004.