Professor of Physics at Surrey University, Paddy Regan said a disaster on the scale of Chernobyl in the 1980s was highly unlikely
A second explosion has hit the nuclear plant in Japan that was damaged in Friday's earthquake, but officials said it had resisted the blast.
TV footage showed smoke rising from Fukushima Daiichi's reactor 3, two days after an explosion hit reactor 1.
The latest blast, said to have been caused by a hydrogen build-up, injured 11 people, one of them seriously.
Officials said the reactor core was still intact, and that radiation levels were below legal limits.
But the US military, which has been helping the relief effort, said it had moved away from the area after one of its aircraft carriers detected low-level radiation about 100 miles (160km) offshore.
Technicians have been battling to cool three reactors at the Fukushima Daiichi plant since Friday, when the quake and tsunami combined to knock out the cooling system.
The government said an operation pumping seawater into the reactors to help lower the temperature was still going on despite the explosion.
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At the scene
We headed towards where the tsunami hit land, close to the little village of Higashiro. We had to pick our way through a sea of mud.
What should have been a road was covered in broken branches, a squashed tractor and lots of electricity cables that had been brought down. The destruction goes on and on.
The seashore was in the distance behind a row of trees. Here the waves toppled houses; they lie at crazy angles. Trees have been smashed into the buildings. A motorcycle lies twisted and bent.
Inside the houses, the furniture has been turned to matchsticks, possessions tossed everywhere, and on a few walls are portraits with the faces of those who once lived here, now stained by the waters which filled everything.
In other developments:
- Two thousand bodies have been found on the shores of Miyagi prefecture, Japanese media are reporting
- The government announced it was pumping 15 trillion yen ($182bn; £113bn) into the economy to prop up the markets - which slumped on opening
- Prime Minister Naoto Kan postponed planned rolling powercuts, saying they may not be needed if householders could conserve energy
The BBC's Rachel Harvey in the port town of Minami Sanriku says everything has been flattened until about 2km inland.
It looks unlikely that many survivors will be found, she adds.
Japanese police have so far confirmed 1,597 deaths, but the final toll is expected to be much higher.
Tens of thousands of people have been evacuated from the area around Fukushima Daiichi plant.
At least 22 people are now said to be undergoing treatment for radiation exposure.
Government spokesman Yukio Edano said there was a low possibility of radioactive contamination from Monday's explosion. He said the reactor's containment vessel had resisted the explosion.