RICHLAND, Wash. (AP) - A decades-long project to decrease risks at nine nuclear reactors along the Columbia River by "cocooning" them has passed the halfway point, officials said Thursday.

Hanford's H Reactor, which operated from 1949-1965, is the fifth former Hanford nuclear reservation plutonium-making reactor to be cocooned.

The Reactor Interim Safe Storage Closure project - called cocooning - involves demolishing nonradioactive portions of the reactor buildings down to the four-foot-thick concrete shield walls around the reactor cores.

All openings in the core building are sealed and a new roof is constructed. Temperature and moisture sensors remotely monitor conditions inside the "cocoon."

Demolition of the H Reactor's auxiliary buildings began in December 2001.

The nine plutonium reactors operated along the Columbia River from 1945-1986.

The 586-square-mile nuclear reservation created as part of the top-secret Manhattan Project to build the atomic bomb. Today it is the nation's most contaminated nuclear site, with cleanup expected to continue until 2035.

C Reactor was the first to be cocooned in 1998. The DR, F and D reactors came next. K West and K East reactors are scheduled to be demolished by 2011 and the N Reactor is scheduled to be cocooned in 2012.

A decision on whether to cocoon the B Reactor, the first operating reactor at Hanford, is pending while authorities consider whether to preserve it as an interpretive center.