The new, raised assessment of the Fukushima Daiichi nuclear accident puts it on par with Chernobyl in terms of potential consequences, but it is obvious to anyone that there is a difference between Japan’s ongoing crisis and the 1986 explosion in the Ukraine. Summed up in a single word, that difference is “containment.”

Steel and cement vessels surround each of the reactor cores at Fukushima Daiichi, as is the case in all but a few of the 443 atomic power units operating around the world today.

But there are a few remaining nuclear plants still operating, all early Soviet design, that do not have primary containment vessels. And one of them happens to be located in a major seismic hazard zone.

Our story, “Is Armenia’s Nuclear Plant the World’s Most Dangerous?,” looks at the renewed concerns at the Metsamor plant on the Turkish border since Japan’s crisis, and why the unique geopolitical isolation of Armenia has left the former Soviet republic with few other good energy options.

Even while Armenia’s reactors were nearing completion in the late 1970s, the nuclear industry had recognized the importance of containment vessels. This is most evident in the lengths that Finland went to ensure the safety of its two reactors of the same design—VVER-440s—that were being built at the same time. The Loviisa power reactors, opened in 1977 and in 1980, were the result of a collaborative effort unique for that Cold War era. The Russian design was modified to include a  containment structure licensed from Westinghouse in the United States, a merger of technologies dubbed “Eastinghouse.”

It is interesting to note that Japan has long been the world’s premier forger of the steel containment vessels for nuclear reactors. Although capacity has recently come on line in Russia and China, Japan Steel Works in Hokkaido has dominated the market because it is large enough to produce containment vessels in a single piece without welds, to ensure greater safety.

Of course, uncertainty about the long-term staying power of the steel vessels amid Fukushima’s crisis certainly is the reason that Japan today upped its alert to a Level 7 “major accident” on the internationally recognized scale. And it is why Tokyo Electric Power Company now says radiation releases from Fukushima could, in time, surpass the contamination from Chernobyl.

Nevertheless, the fact that containment was in place surely saved Japan from repeating the immediate aftermath of Chernobyl, the worker deaths and sickness and the high radiation release to the surrounding countryside and its unprepared population.

Except for Armenia’s plant, all remaining reactors without containment vessels are in Russia. An overview of improvements that have been made over the years at old Soviet plants is archived here in a 2003 report by the former International Nuclear Safety Program of the U.S. Department of Energy. But since its publication, several containment-free plants have been closed in Lithuiania, Slovakia, and Bulgaria.

Those still in operation include four that share the VVER (water-cooled, water-moderated) design of Armenia’s reactor: two at Kola and two at Novovoronezh.  And there are 11 that are graphite-moderated (similar to the Chernobyl design): four at Kursk, four at Leningrad, and three at Smolensk.

In an interview by telephone from Moscow, Andrey Ozharovskiy , a nuclear power specialist for the environmental group, Bellona, says the 2009 annual report of Russia’s  own nuclear regulatory agency, Rostekhnadzor, cast doubt on the safety of the old VVER reactors operating in that country. He wrote about the report here, quoting from page 315 of the original document, which can be found, in Russian, here. The report notes that the areas of the VVER reactors where the greatest stresses occur are not fully verified by ultrasound testing, as in other reactors.  The report doesn’t specify whether that’s because they can’t be tested, or that they simply aren’t being tested. But the report says that verifying the safety of these areas is an “open question,” and an urgent one when assessing how long the reactors should remain in service.

Ozharovskiy declined to say whether he thought Armenia’s or any of these reactors without containment could be called the world’s most dangerous.  “I don’t think it’s a good game to rank nuclear reactors,” he said. “In my opinion, all of them are dangerous.”

(Related: “How is Japan’s Nuclear Crisis Different?”)