Fifty of Rhode Island’s most dangerous inmates, many with ties to violent street gangs, would be shipped to out-of-state prisons for four years under Gov. Gina Raimondo’s plan to rebuild the High Security Center at the Adult Correctional Institutions.

Another three dozen high-security inmates would be moved to lower-security units at the ACI in Cranston if the plan to rebuild the “supermax” unit is approved by lawmakers.

Corrections officials this week described the $60-million project as a way to modernize the 38-year-old prison that’s costing taxpayers significantly more to run each year, despite housing a declining number of inmates.

“We have an extremely high cost per inmate in our high security — it is almost three times as high as similar facilities in Massachusetts,” said Jonathan Womer, director of the Office of Management and Budget. “The reason ours is so expensive is the high number of staff to man such a facility, due to the design and age. It is just a very unsustainable path for us to keep going on.”

Raimondo’s budget plan wants to borrow $45 million — without seeking voter approval — to rebuild the “supermax” and spend $15 million in state capital project reserves to rebuild the high-security unit.

Exactly what shape and size the rebuilt prison will take is being kept under wraps until a $250,000 consultant’s study is released next month.

The current building can house up to 166 prisoners but is currently only a little more than half full, with 85 inmates. Of the current prisoners there, 54 are considered a security risk, 40 have gang ties, seven are receiving mental-health treatment and four are classified as escape risks, according to a report from House fiscal staff.

“Being built in 1981, you have to remember what the philosophy and tenor of the criminal justice system was at that time. It was very tough on crime, lock ’em up and throw away the key kind of criminal justice philosophy,” state Corrections Director Patricia Coyne-Fague told the House Finance Committee Tuesday. “As corrections has evolved, we’ve realized keeping people in cells 23 hours a day is not the way to go. So this facility was not built with today’s philosophy in mind, because it has little programing space, which we know is necessary.”

Another problem with the current design, state officials say, is that extra officers are required to keep proper watch on what’s going on.

“The design doesn’t provide the sight lines needed for supervision, particularly in areas such as back rooms and showers, which under the federal Prison Rape Elimination Act puts more requirements on us in terms of the types of sight lines and supervision that needs to take place,” Coyne-Fague said.

In 2016, it cost the state $11.8 million to incarcerate 96 people in the high-security unit. This year, it’s expected to cost $14.2 million to lock up 11 fewer inmates. In addition to the inefficient design of the high-security unit, a string of raises for correctional officers starting in 2017 has bumped up the budget.

Overtime is also an issue. Rhode Island is one of the few states that allow prison guards to work “quads,” or four consecutive shifts, which accumulate large overtime payments, Coyne-Fague said. A new class of 70 correctional officer recruits is expected to help reduce overtime.

Compared with the $165,000 cost per inmate at the ACI’s high-security center, Massachusetts spends $65,000 per inmate at comparable facilities.

It’s common for states to swap prisoners — 43 Rhode Island inmates are currently behind bars in other states and 35 out-of-state inmates are here — with the sending state paying per inmate if it is not a one-for-one swap.

Raimondo’s budget assumes paying other states $3.3 million, based on Massachusetts’ per-inmate rate, to house high-security inmates while the renovation is happening. But that is more than offset by reduced overtime spending. The budget projects $5.1 million in net savings next year and $6.8 million in 2021. (That doesn’t include the $4.3 million in annual debt repayments on the new unit.)

Department of Administration spokeswoman Brenna McCabe said the state hopes to begin rebuilding the high-security unit in October.