THE prison service has confirmed an incapacitating spray was used on inmates for the first time during a tense 14-hour siege at the new jail in Peterhead.
PAVA, said to be significantly more potent than CS gas and used to stop pirates boarding cargo ships, has been available since 2007.
May's incident at HMP Grampian, in which nine prisoners were sprayed, was the first time it had been used in a Scottish jail.
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More than 40 inmates were transferred to other prisons after the siege, which lasted for 14 hours and caused about £150,000 of damage.
The trouble flared just days after prison staff wearing protective gear were involved in a two-hour stand-off with inmates in the same area of the jail.
It was one of at least three violent incidents at the £140 million prison since it opened in early March.
The Scottish Prison Service confirmed the incident after Liberal Democrat MSP Alison McInnes raised the issue in parliament.
She said: "The SPS has had the power to use incapacitant spray since 2007 and I am anxious to understand why it was used on this occasion. I am also concerned that deploying such powerful sprays in a confined space could affect people who were not being targeted, including police and prison staff.
"The SPS must ensure local staff have the resources, knowledge and experience they need to be able to resolve difficult situations before they go from bad to worse."