Q+A: How does Fukushima differ from Chernobyl?

Workers operate a modified Putzmeister 70Z, the world's largest concrete pump mounted on a truck, to pump contaminated water to the No.4 reactor at the Fukushima Daiichi Nuclear Power plant, April 12, 2011.

TOKYO (Reuters) - Japan on Tuesday raised the severity level of its nuclear crisis to put it on par with the 1986 Chernobyl accident, the world's worst nuclear power disaster.

But for all their criticism of how Tokyo Electric Power Co and Japan's government are handling the crisis at the Fukushima Daiichi nuclear power plant, experts agree with them on one point: Fukushima is not another Chernobyl.

"Fukushima has its own unique risks, but comparing it to Chernobyl is going too far. Fukushima is unlikely to have the kind of impact on the health of people in neighboring countries, the way Chernobyl did," said nuclear specialist Kenji Sumita at Osaka University.

Here are the main points of how the two accidents differ.

ARE THE TWO DESIGNS THE SAME?

Unit 4 at Chernobyl was a water-cooled and graphite-moderated reactor -- a combination that can and did yield a runaway chain reaction. A series of gross errors and misjudgment by operators resulted in an explosion and fire that catapulted radioactivity into the upper atmosphere.

The resulting release of radiation has been compared to 10 times that released by the 1945 U.S. nuclear bomb attack on the Japanese city of Hiroshima.

The boiling water reactors at Fukushima do not have a combustible graphite core. The nuclear fuel in reactors No. 1, No. 2 and No. 3 was allowed to melt at least partially, but operators have since succeeded in cooling both the reactors and the spent fuel pools and no chain reaction is happening now.

As long as cooling operations continue and Japan can prepare tanks fast enough to store the contamination overflow, Japan can still hope to buy time to figure out how to bring the reactors to a cold shutdown.

HOW DO THE CONTAINMENT STRUCTURES DIFFER?

Chernobyl had no containment structure and nothing stopped the trajectory of radioactive materials into the air.

Fukushima's reactors are built on granite foundations and are surrounded by steel and concrete structures. The reactor vessels and containment structures, as well as some of the pipes leading from the reactors, are likely to have been damaged by the March 11 tsunami and recurring earthquakes. But with radiation levels now down to a sliver of what they were at the peak, experts say that the structures are still holding.

Chernobyl contaminated an area as far as 500 km (300 miles) from the plant, and an area spanning 30 km (18 miles) around the plant is still an exclusion zone and uninhabited.

HAVE THERE BEEN FALLOUT-LINKED DEATHS IN JAPAN?

At Fukushima, there have been no deaths so far due to radiation. Eight people have been injured. More deadly have been the 9.0 magnitude quake that hit on March 11 and the aftershocks that have rocked the site while workers tried to bring the plant under control. Two have died and three have been critically injured.

At Chernobyl, the initial explosion resulted in the death of two workers. Twenty-eight of the firemen and emergency clean-up workers died in the first three months after the explosion from acute radiation sickness and one died of cardiac arrest.

FLOW OF INFORMATION VERSUS COVER UP

Bungling, yes. Disorganized, incoherent and sometimes contradictory, yes. But it is difficult to accuse Japanese officials or TEPCO of intentionally covering up information, with round-the-clock updates and a steady stream of data.

Chernobyl was initially covered up by the secretive Soviet state, which remained silent for two days. But authorities, obliged by huge radiation releases throughout Europe, gradually disclosed details of the accident, showing unprecedented Soviet-era openness.

DOES FUKUSHIMA POSE A GREATER RISK IF IT ALL GOES WRONG?

It's not over yet. One month since the March 11 earthquake and tsunami, workers still have to inject water into the reactors, creating more contaminated water that is hampering the restoration of power to pumps to cool the reactors and bring them to a cold shutdown.

The situation led a frustrated and demoralized TEPCO spokesman to say that the total fallout could exceed that of Chernobyl. Fukushima involves loss of control at four reactors and potentially more radioactive material, that could continue to seep, leak or burst into the environment.

Officials have said that if power cannot be restored to the cooling pumps, there are other measures, such as air cooling, and that in a worst-case scenario, they could try water entombment in the reactors whose containment structures are sound.

(Created by Mayumi Negishi; Editing by David Chance and Ron Popeski)