Thursday, March 17, 2011

The Japanese Nuclear Crisis

While tragic in many ways, the Japanese nuclear crisis is interesting in even more ways. Design of nuclear reactors have to meet criteria often unfathomable. Events with so low a probability that they would be insignificant, have to be considered. Since few things in life have zero probability, the design elements of nuclear reactors and containment of waste are causing considerable panic.

Designers have to include one element that is extremely difficult to predict. The impact of heroic measures. Emergency workers are valiantly risking their lives to control the situation. Well intentioned, but not very prudent actions are the result in this case.

The type of reactors involved have design features to contain and minimize dangers. Once a significant amount of damage is done to the fuel, partial meltdown, the design of the pressure vessel and concrete containment building should handle the maximum amount of molten radiative fuel possible. The possibility of all the fuel melting, then all the molten fuel melting through the metal pressure vessel, then all of that fuel remaining molten as it falls to the concrete floor of the containment building dry well are extremely remote. That is the basis of the design, and extremely remote possibility.

Should the emergency actions cause water to be introduced into the "DRY" well, the possibility of a steam explosion that can spread the heavier, more dangerous, radioactive elements increases. The Japanese military are dumping sea water on the concrete containment buildings which increase the potential of the disaster, not decreases the potential.

The shape, weight and design of the concrete plug at the top of the containment building takes even this remote possibility into consideration. The plug is heavy enough to not move out of position until pressure in the containment building is high enough to produce potential damage to the containment building. If the plug is removed by a major steam explosion, the shape and size of the containment structure to the plug location, is designed to allow pressure from the steam explosion to be relieved without destroying the containment structure. More water than normal, can create a jet of steam from the containment building until either the water evaporates or the fuel is sufficiently cooled. While the jet is evidence of some cooling of the fuel, it is also evidence of more radioactivity being spread. Health impact wise it is best not to dump sea water on the reactor.

The spent fuel cooling pools is another overly complicated situation. There is a remote possibility that the spent fuel can return to criticality. There is a remote possibility that the Zircaloy cladding of the spent fuel rods can "burn" or oxidize. Both of these are "Extremely" remote possibilities. Most people should have the common sense to report the possibilities in terms that the general public can relate to. The criticality probability is equivalent to winning the Irish sweepstakes, being struck by lightning a couple of times and then still being able to spend the money without your ex-wife getting half of it. The probability of the cladding fire is much greater. While I don't have all the information needed to give a more exact estimate, it is more like winning the Irish sweepstakes and not having to give your ex-wife half.

There is also a huge misunderstanding about how much and what kind of radiation is a problem. Too much too fast is without a doubt a problem, but how much is too much too fast? A good guideline is 5000 millirad per year and 10,000 millirads in rapid doses. Below those numbers there is no measurable increase in cancer risk, per the NRC fact sheet on average dosage. There is a large gray area between 10,000 millirads and 50,000 millirads, the point where significant risk is verified. Even the significance of that risk is dependent on the individual.

So while the situation is tragic, it is also a good learning experience.

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