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:
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.
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.
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.
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.
5. Much of the public health impact of Chernobyl was the result of the Soviet government's attempt to cover up the crisis, rather than moving quickly to inform and protect the public.
In Japan, the government evacuated the 20 kilometers, or 12 miles, surrounding the Fukushima plant fairly quickly, and have continued to upgrade the warnings to citizens in the vicinity (although, according to the United States government, not quickly enough).
That didn't happen at Chernobyl. In the sunny April morning after the explosion, the residents of the nearby town of Pripyat were left to go about their business. As the Guardian has noted, children went to school, an outdoor wedding was celebrated, and sunbathers went out to enjoy the good weather, as the plume from the exploded reactor continued to fill the air with radioactive particles.
One of the plant's employees, who had been away on business, returned home to find his wife outside in the garden, where she was paying no attention to the small pieces of graphite that had landed "on the petals of her wild strawberry plants." Before long, the sunbathers began to experience strange cases of nausea and vomiting. The town would not be evacuated until the next day. And it was only after heightened levels of radioactivity set off alarms at a nuclear plant in Sweden that the Soviet government finally admitted publicly that something had gone wrong.
The delay and denial had serious implications, including an epidemic of thyroid cancer among about 6,000 people exposed to radiation as children.
As the New York Times noted, this epidemic "would probably not have happened if people had been told to stop drinking locally produced milk, which was by far the most important source of radiation."
(Russia distributed iodine tablets, as has Japan. But as we reported on Monday, these offer little protection against ingesting contaminating food or milk.)
6. Emergency workers at Chernobyl took few precautions, and may not have been fully informed about the risks they were taking.
The "Fukushima 50" who stayed at the plant on Tuesday and Wednesday to keep containment efforts underway have been facing serious risks. But they have been taking precautions, the Times reported , including breathing through respirators, wearing full-body jumpsuits, and limiting their exposure time.
At Chernobyl, the Guardian wrote:
[The firefighters] had had no protective clothing, or dosimetric equipment to measure radiation levels; the blazing radioactive debris fused with the molten bitumen, and when they had put the fires out with water from their hoses, they picked up chunks of it in their hands and kicked it away with their feet.... This heroic but utterly futile action took them closer to a lethal source of radiation than even the victims of Hiroshima...When they died two weeks later in Hospital No 6, Zakharov heard that the radiation had been so intense the colour of Vladimir Pravik's eyes had turned from brown to blue; Nikolai Titenok sustained such severe internal radiation burns there were blisters on his heart. Their bodies were so radioactive they were buried in coffins made of lead, the lids welded shut.
Lois Beckett writes for the Nieman Journalism Lab, the SF Weekly, and the East Bay Express. This story originally appeared on ProPublica.com, and was reprinted here with permission.