A core meltdown at the Fukushima nuclear plant is unlikely to result in a Chernobyl-like disaster, but its impact will still be significant, say experts.
Despite a second explosion at the site, Japanese authorities are playing down the danger of a core meltdown in one of the reactors.
Most experts are comparing the situation to the 1979 Three Mile Island accident, which resulted in radiation affecting an area 16 kilometres around the reactor.
Professor Aidan Byrne, director of the College of Mathematical and Physical Sciences at the Australian National University in Canberra, says Three Mile Island failed because of lack of cooling water.
"This one's a slightly different scenario," he says, "because they're able to put cooling water onto it, so it might not progress to that scale."
When a reactor is running normally the fuel rods are surrounded by coolant, and lowered or raised to regulate the number of nuclear reactions, known as fission.
"In a meltdown you lose the coolant and the fuel rods get hot enough to actually physically melt, which is a couple of thousand degrees Celsius," he says.
"The Japanese reactors [also] have a containment vessel around them, and the hope here is that as with Three Mile Island, if there is a meltdown then it will be within that containment vessel."
Some reports are comparing the situation at Fukushima with the Chernobyl disaster of 1986.
In that accident, nuclear fission ran unchecked and the reactor lacked the multi-barrier containment strategy that exists in the 40-year-old Fukushima plant. Byrne says, unlike Chernobyl, Fukushima's nuclear reactors have been shut down, and control rods have been inserted stopping the fission chain reaction.
The main concern is if the reactor is not cooled down, the radioisotopes in the fuel rods and decay-heat will heat up the container.
According to Japanese authorities, seawater mixed with boric acid is being used to cool the reactor down, which should reduce the risk of a fission reaction restarting.
Currently the reactors are releasing small amounts of xenon-137 and iodine-131, which have a half life of 3.8-minutes and eight days respectively. But experts are more concerned by the release of cesium-137, which has a 30-year half-life.
Byrne says there's still a chance the Japanese reactors could go into meltdown.
"It's not impossible, because they have problems with the coolant. The Three Mile Island accident which is the previous one similar to this certainly did have a core meltdown. So it could happen in one or maybe even two of these [reactors]," he says.
"The anxiety here is that it ruptures explosively and projects stuff into the atmosphere ... but even then it would not be of the magnitude of the Chernobyl reactor."
Byrne says it isn't possible to speculate on how bad the health effects would be, because it would depend on the amount of radiation released and where the prevailing winds took the particles.
But he says as long as efforts to cool the reactors continue, the likelihood of a complete meltdown will decrease.