APR: your source for nuclear news and analysis since April 16, 2010

Saturday, March 12, 2011

Fukushima Daiichi 1 observations

Officials have reported that some of the fuel elements may have been exposed while the core became partially uncovered. Loss of this cooling water may also be the reason that control room radiation levels spiked; the loss of the water as a neutron attenuator with decay still occuring in the fuel could lead to an increased radiation reading in an adjacent area.

Exposed and thus uncooled fuel elements can rupture from the pressure internally generated by fission product gases. The cladding on the outside of the fuel elements, having thus been compromised, can release fuel and fission products to the coolant. This has occurred a number of times in damaged reactor cores, and in reactor accidents. Cs and I have been detected at the site outside Plant 1, supporting the theory of fuel element damage very strongly.

Some sources are reporting the possibility of a metal-water reaction having generated hydrogen, resulting in a high enough concentration to be explosive. While radiolytic hydrogen and oxygen, and oxygen suppression hydrogen are likely culprits the metal-water reaction has in fact been difficult to produce in test and investigations. A very special set of circumstances is required to derive large amounts of energy from a zirconium hydride - oxygen reaction since these are not by any means the only materials involved and many things, like initial heat and formation of droplets all vary the likelihood and intensity of any reactions. However, it was surmised that metal-water reactions of this type did occur in the SL-1 and in the TMI-2 accidents.

Fuel damage, with release of fuel and or fission products to the environment, constitutes an accident. In the US NRC parlance, an accident with very severe circumstances that challenges plant design and safety equipment in an unusual way is a "Severe Accident."

The Draft Report of WASH-1250 in our library here indicates in so far as core melts are concerned, that only a totally uncooled core is likely to melt and begin to drop enough to contact and then begin to melt through the bottom of the reactor vessel ... However, this material, even for a totally uncooled 1000 Megawatt reactor core is unlikely to penetrate any further than 60 to 90 feet into the ground maximum and in fact in many plants may not exit the plant structure at all; there is no guarantee that the molten material would penetrate the outer limits of the physical containment.

The secondary containment of Fukushima Daiichi 1 must now be considered totally compromised, removing a very important protector for the environs and for personnel working in the vicinity to recover the plants affected. This also clearly means that if the situation were somehow to worsen in the primary that the defense in depth approach to safety is seriously degraded.

The flood cooling of the reactor with seawater and boric acid is undoubtedly the last resort action. While this renders the reactor unusable it should provide enough cooling to halt any core melt. The effluent will undoubtedly be highly difficult to deal with.

DEVELOPING... 4:10 PM Saturday

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