The exercise, with a command centre in Poundbury, Dorchester, will test their response to the scenario of a radioactive leak from a submarine with a damaged nuclear reactor core.

BBC: Nuclear submarine accident exercise in Portland

The operation came a day after workers plugged a highly radioactive water leak from a concrete pit connected to the number two reactor, using sodium silicate - known as ''water glass''.

BBC: Japan quake: Nitrogen pumped into nuclear reactor

The power plant has been shut down since this winter, when a small amount of radioactive gas escaped from a steam generator during a water leak.

CNN: SHARE THIS

High levels of cesium 134 and 137 were confirmed at the point of the leak roughly 300 yards from the ocean, but no detectable amounts of radioactive contamination have been found in the sea water.

WSJ: New Leak at Japan Reactor Threatens Ocean

Shut-down reactors include two at the beleaguered San Onofre nuclear power plant in southern California, which hasn't produced electricity since January 2012, when a tiny radiation leak led to the discovery of damage to hundreds of tubes that carry radioactive water.

NPR: Nuclear Chief: US Plants Safer After Japan Crisis

The twin-domed plant between San Diego and Los Angeles has not produced electricity since January 2012, when a tiny radiation leak led to the discovery of unusual damage to hundreds of tubes that carry radioactive water.

NPR: Calif. Reactors Might Be Retired If Restart Nixed

The seaside plant between Los Angeles and San Diego has been shut down since January 2012, after a small radiation leak led to the discovery of unusual damage to hundreds of tubes that carry radioactive water.

NPR: San Onofre Submits Draft Linked To Restart Bid