Six Ways Fukushima is Not Chernobyl

March 18, 2011 — The crisis at Japan’s Fukushima Daiichi has already been dubbed the worst nuclear accident since Chernobyl, and the situation there continues to worsen.

But along with references to the “ch-word,” as one nonproliferation expert put it, experts have been quick to provide reasons why the Daiichi crisis will not be “the next Chernobyl.”

Experts have noted several key differences in the design of the reactors in question, as well as in the government’s reaction to the crisis:

1. Chernobyl’s reactor had no containment structure.

The RBMK reactor at Chernobyl “was regarded as the workhorse of Soviet atomic energy, thrifty and reliable — and safe enough to be built without an expensive containment building that would prevent the release of radiation in the event of a serious accident,” The Guardian’s Adam Higginbotham noted.

As a result, when a reactor exploded on April 26, 1986, the radioactive material inside went straight into the atmosphere.

Fukushima’s reactors are surrounded by steel-and-concrete containment structures. However, as the New York Times reported Tuesday, the General Electric Mark 1 reactors at Fukushima have “a comparatively smaller and less expensive containment structure” that has drawn criticism from American regulators. In a 1972 memo, a safety official suggested that the design presented serious risks and should be discontinued. One primary concern, the Times reported, was that in an incident of cooling failure — the kind Fukushima’s reactors are now undergoing — the containment structures might burst, releasing the radioactive material they are supposed to keep in check.

At least one of Fukushima’s reactors — No. 2 — seems to have cracked, and has been releasing radioactive stream. The seriousness of this breach is still unclear, with a Japanese government official maintaining on Wednesday that the damage to the containment structure may not be severe.

2. Chernobyl’s reactors had several design flaws that made the crisis harder to control. Most crucially, their cooling system had a “positive void coefficient,” which means that as coolant water is lost or turns into steam, the reaction speeds up and becomes more intense, creating a vicious feedback loop.

Shan Nair, a nuclear safety expert who spent 20 years analyzing the consequences of Loss of Coolant Accidents like the one at Fukushima, discussed this factor on TIME’s Econcentric blog. Nair was a member of a panel that advised the European Commission on how to respond to Chernobyl. As he explained:

[Fukushima] can’t be Chernobyl because the Boiling Water Reactors (BWRs) at Fukushima are designed differently than the High Power Channel-type Reactor (RBMK) reactor at Chernobyl. The RBMK was designed so that the hotter the core gets the greater the reactivity — so you have a situation where you are in a vicious cycle and a race to an explosion. [Fukushima’s] BWRs are designed in such a way that the hotter it gets the less radioactive the core gets so there is a self-shutdown type of mechanism. But the problem is that before you can get to a safe level you might have a complete meltdown. I believe that’s what they are battling against now in Japan.

3. The carbon in Chernobyl’s reactor fueled a fire that spewed radioactive material further into the atmosphere. Fukushima’s reactors do not contain carbon, which means that the contamination from an explosion would remain more localized.

Dr. Colin Brown, director of engineering for the UK-based Institution of Mechanical Engineers, described another of the Chernobyl reaction’s design flaws in a post on the Institution’s website explaining why it was “unlikely” that Fukushima “will turn into the next great Chernobyl with radiation spread over a big area.” He wrote:

The reason why radiation was disseminated so widely from Chernobyl with such devastating effects was a carbon [graphite] fire. Some 1,200 tonnes of carbon were in the reactor at Chernobyl and this caused the fire which projected radioactive material up into the upper atmosphere causing it to be carried across most of Europe. There is no carbon in the reactors at Fukushima, and this means that even if a large amount of radioactive material were to leak from the plant, it would only affect the local area.

Britain’s Chief Scientific Officer, Sir John Beddington, made a similar point about the localized nature of an explosion in a speech about Fukushima on Tuesday:

In this reasonable worst case you get an explosion. You get some radioactive material going up to about 500 metres up into the air. Now, that’s really serious, but it’s serious again for the local area. It’s not serious for elsewhere even if you get a combination of that explosion it would only have nuclear material going in to the air up to about 500 metres…And to give you a flavour for that, when Chernobyl had a massive fire at the graphite core, material was going up not just 500 metres but to 30,000 feet [about 9144 metres]. It was lasting not for the odd hour or so but lasted months, and that was putting nuclear radioactive material up into the upper atmosphere for a very long period of time. But even in the case of Chernobyl, the exclusion zone that they had was about 30 kilometres. And in that exclusion zone, outside that, there is no evidence whatsoever to indicate people had problems from the radiation.

One of the most pressing worries about Fukushima is that radiation might be spewed into the atmosphere not from reactors themselves, but from spent fuel rods exposed to the air once the pool of water protecting them boils away. According to the Los Angeles Times, U.S. officials believe one of the spent fuel pools has been breached, potentially exposing 130 tons of uranium.

4. Unlike Chernobyl, however, a meltdown at Daiichi could end up contaminating the water table.

One troubling possibility that has received little attention is that a reactor meltdown could send radioactive material downwards until it reaches the water table, which could contaminate both water supply and crops. Discussing Daiichi on TIME’s Ecocentric blog, Nair, the nuclear safety expert, noted:

If the entire fuel has melted the odds are it will go straight through the pressure vessel and therefore through the ground until it gets to the water table. Then it will cool down, but the problem is that the water table will start leaching actinides and fission products from the melted glob of fuel into the environment. So you will end up with some radioactive contamination of water supplies and ultimately crops and other products. That’s a major problem because radioactive particles are much more dangerous when digested — they cause internal irradiation of organs with resulting increased cancer risks…The severity of the water table risk depends on the local topography — it depends on the depth of the water table, which itself moves up and down. I would imagine the water table is quite close to the surface right now because of all the flooding, which is not good.

At Chernobyl, fears that the radioactive material from the exploded reactor would reach the water table prompted a massive two-part project: first, to use liquid nitrogen to freeze the ground beneath the exploded reactor, and secondly, to build a shielding structure beneath the reactor. Although the effort exposed many miners to intense radiation, it was ultimately unnecessary.