High radiation prevented police water cannon on Thursday evening from cooling a troubled nuclear reactor that was in danger of melting down in north-eastern Japan, a news report said.
The water canon tried to spray the overheated pool at reactor 3 at Fukushima nuclear power plant I where spent fuel is stored, but the water fell short, public broadcaster NHK reported.
Cooling systems at the reactors failed in Friday's magnitude-9.0 earthquake and devastating tsunami, DPA reported.
Eleven military firefighting vehicles would also douse the reactor later in the day and more vehicles were on the way, Japanese Defence Minister Toshimi Kitazawa said.
Military helicopters dumped a total of 30 tons of water over the reactor earlier in the day. But that did not change the level of radiation around the reactor.