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Questions Over the Design of Fukushima Nuclear Plant

The reactor design at the Japanese plant has long been superseded because of safety failings.
March 15, 2011

The New York Times is reporting that the design of the containment structures used at the earthquake- and tsunami-damaged Fukushima nuclear power plant in Japan dates back the 1960s, and has long been criticized as less safe than other types of structure for trapping radiation and preventing molten fuel from escaping into the environment. According to the article, the original design uses less concrete and other materials than competing containment designs.

But the report also includes a logical error and omission that may exaggerate the potential problems.

What isn’t made clear in the report is whether the design has been retrofitted. In the U.S., power plants with the “Mark 1” containment structure have been retrofitted to help make up for design flaws. Some retrofitting may also have taken place at the plant in Japan. (The design has since been superseded by Mark 2 and Mark 3 designs.) The report acknowledges, “It is not clear precisely what modifications were made to the Japanese boiling water reactors now failing.”

Also, it does not follow that because the containment structure is “smaller and less expensive” that it will fail. It may just be a better engineered design. What’s more, the strategy the Japanese plant operators are using of flooding the reactor and containment structures with water could prevent the system from failing. I’ve pasted some comments to this effect from nuclear engineering experts below.

This is all to say that the containment system might still work, or at least not fail catastrophically. (There have already been radiation leaks, but these aren’t necessarily to do with the design flaws that had been pointed out, and are much lower than the levels of what would happen with a complete meltdown.)

The design might also fail. If it does, the credibility of the nuclear industry and its regulators would be undermined. According to the Times, the U.S. Atomic Energy Agency passed up an opportunity to ban the flawed design, in part because it would have harmed the nuclear power industry. The Times quotes David Lochbaum, director of the Nuclear Safety Program at the Union for Concerned Scientists, as saying, “Not banning them might be the end of nuclear power.”

Here’s a take on the possibility of the containment system failing from two nuclear engineering experts, Andrew Kadak, from MIT, and Mitchell Farmer, a researcher at Argonne National Labs:

“Once the core melts, there is likely to be water in the bottom of the reactor vessel into which the melted fuel will fall. That will harden the corium [melted fuel and cladding] and cool it. As long as the water is there, the corium will be cooled with a hard outer shell slowly cooling the rest of it. Hydrogen may be generated but the key issue is the ‘meltdown stops’. This was demonstrated at TMI [Three Mile Island] so this is not theory but fact.

“The key in any stage of this is the application of water. Even if the core is partially uncovered, as occured at the Japanese plants, the introduction of sea water was through core sprays which injected water above the core providing some cooling which quickly evaporated which is why the water level was not being quickly restored. The longer the period preventing a major melt, the less heat generated which requires less cooling. Should the containment stay intact - recall the core is still in the vessel - the liklihood of a major release is greatly reduced. This is very much like TMI even though the containment design is different.”

“In the unlikely scenario that the molten core material (corium) were to melt through the bottom of the reactor vessel and discharge into the containment, the material can interact with the underlying concrete basemat. This scenario has been under intense research for many years following the accident at TMI with the intent of supporting accident management planning for existing plants just like the ones in Japan. This research has indicated that if water is present as an initial condition on the basemat floor, then there is a relatively high probability that the material can be quenched for a fairly wide range of melt pool depths. The Japanese have participated in this research from the beginning and are very familiar with these findings, along with their ramifications for accident management planning.”

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