In 2011, only 43 percent of the prisons' security units were in use, according to national broadcaster Sveriges Radio (SR).
The Swedish Prison and Probation Service says that this is intentional and that they strategically avoid filling the units, as prisoners are often dangerous and cannot be placed together.
“We've said that roughly 50 percent is normal for these kinds of sections,” said Christer Isaksson, the service's head of security, to SR.
“Many of the prisoners have strong personalities and cannot be placed together just any which way, because of the conflicts or destructive constellations that might form."
But Sweden's minister for justice Beatrice Ask is concerned that the costs are too high.
“Our concern is naturally that the costs are very high. But I think it's important to say this: we cannot question the need for secure units in Swedish prisons. We have serious crime, and people who constitute a danger, both to themselves and others,” said Beatrice Ask to SR.
The three security units were built following four spectacular escapes from Swedish prisons in 2004, and they cost 750 million kronor to build.