Jiji Tsushin's article has no information as to who the Police or the Coast Guard think would be the supposed "terrorists" who would attack Fukushima I Nuclear Power Plant.
Jiji Tsushin (5/11/2013):
Police and Coast Guard held joint exercise against terrorism on nuclear plant at Fukushima II (Daini), with special force staging "gun battle"
On May 11, Police and Japan Coast Guard held a joint exercise on the assumption that terrorists were attacking Fukushima I Nuclear Power Plant. Vulnerability of Fukushima I Nuclear Plant was exposed in the March 11, 2011 disaster, and work toward decommissioning is on-going today in a situation that is still unstable. The exercise was open to the public, as there are those who have expressed doubt about the security system. A special force for counter-terrorism also participated in a mock gun battle.
The exercise was carried out at Fukushima II (Daini) Nuclear Power Plant, about 10 kilometers south of Fukushima I (Daiichi) Nuclear Power plant. About 80 policemen from the Fukushima Prefectural Police Firearms Countermeasure Unit which is stationed at the nuclear plant and from the Chiba Prefectural Police Special Force "SAT" (Special Assault Team), and about 70 officers of Japan Coast Guard including the Coast Guard Counter-Terrorism Unit participated. The joint exercise against terrorist attack on Fukushima I Nuke Plant was the first after the March 11, 2011 disaster.
Jiji has a photo of Coast Guard officers with machine guns, dressed in black-and-yellow radiation protection gear and looking like a bee or fly, having subdued the "terrorists" on board a ship:
Let's see... To disable the plant and stop the cooling of the reactors and Spent Fuel Pools, all you need would be to release a boat-load of rats and mice and wait... Or simply wait until all those huge steel tanks that are not welded start to leak, in about 3 years. Or just continue to let TEPCO do the work and wait.