Inspector Nick Hardwick praises Parc Prison in Bridgend
A jail run by private security contractor G4S has been praised by the chief inspector of prisons.
Parc prison, in Bridgend, had delivered good or reasonably good services in all areas, Nick Hardwick said in a report.
However, there are too few sex offender treatment programmes and too many young adults in violent incidents.
The prison - one of the biggest in the country - holds more than 1,300 inmates, and the UK government recently announced plans to add another 387.
Chief inspector of prisons Nick Hardwick said the expansion should not put the prison's recent successes at risk.
It comes after reports of trouble at another G4S prison - Oakwood, near Wolverhampton.
Inmates allegedly barricaded the entrance to a wing of the prison during a nine-hour disturbance earlier this month.
In his report on Parc, Mr Hardwick said young adults were over-represented in violent incidents and there were far too few treatment programmes for sex offenders.
He added though that the level of violence was low and praised "outstanding" work with prisoners' families.
"No one should underestimate the difficulties of running a large prison. Other large prisons have not been so successful," he said.
"The success of HMP Parc will not be easy to emulate and it is important to ensure Parc's own future expansion does not place its current success at risk.
"There are areas that need improvement but overall this was a very positive inspection."
UK government ministers are yet to decide whether a new 2,100-inmate super-prison in Wrexham will be privately run.