HMP Dartmoor has been forced to close off dozens of cells and transfer nearly 200 inmates after radioactive gas was detected in the prison.
An elevated presence of radon – an odourless and colourless gas – was recently identified in the men’s prison in Devon.
The relocation numbers have now come to light.
Between November and February, 184 cells were shut, with official figures suggesting its total safe capacity is 640.
In the same period until February, 194 prisoners were moved out of the jail because of the radon contamination, it is understood.
A Prison Service spokesperson said: “A number of prisoners have been relocated as a precautionary measure after routine testing revealed higher than normal levels of radon.”
The shifting of inmates was a “temporary measure” amid work to “permanently reduce” radon levels in the Category C establishment to ensure no safety issues to staff and prisoners on site.
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Radon is formed by the radioactive decay of the small amounts of uranium that occur naturally in all rocks and soils, and can only be detected using special instruments.
The radioactive elements can be inhaled, and continues to decay within the body, emitting radiation. It is responsible for around 1,000 lung cancer deaths a year.
While the gas is everywhere, the levels are low as is the risk it poses to health – however, it is thought to occur more often in areas with high concentrations of granite, such as Dartmoor.
Labour, which revealed the figures in a series of questions to ministers, said it was “shocking that the government’s mismanagement of the prison estate means they are having to reduce the number of prison places during a capacity crisis”.
Ruth Cadbury, shadow prisons minister, pointed to radon being detected in 2020 but the Ministry of Justice said measures “weren’t put in place until 2022”.
She questioned the delay, adding: “After 14 years in power and a dozen prison ministers, our prisons are out of control and the government need to get a grip.”
Justice minister Edward Argar said the Prison Service was “continually investing in the prison estate to ensure that prisons remain safe, decent and secure”.
Mr Argar said the increased levels of radon detected in 2020 were in “subterranean areas adjacent to the kitchen and workshops”.
He added: “Temporary mitigations have been in place at Dartmoor in those areas since 2022 following advice from specialist contractors, pending permanent mitigations that are near completion.
“Further specialist advice has been commissioned following the recent identification of elevated radon levels in some accommodation areas of the prison.”
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Meanwhile, a report in December by Dartmoor’s independent monitoring board highlighted the prison was understaffed and overcrowded, and arguably “not fit for purpose”.
It added: “It is only in this year that some actions have been taken while further monitoring is undertaken to inform future decisions.”
