Group Findings Unit 3
Ex-vessel steam explosion:
Information gathered from the unit 3 explosion and events before and after the explosion event indicate a probable ex-vessel steam explosion and secondary hydrogen explosions occurred at unit 3. There are indications that sea water being injected into unit 3 may have flowed into the containment drywell rather than into the reactor vessel. This would have flooded the drywell, setting up the conditions that could cause an ex-vessel steam explosion as melted fuel fell down into the water.
The explosion at unit 3 appears to have two distinct components, “the fiery plume shaped like a fist that punches out through the top of the south sunlit wall.they blasted out along roughly 45˚ angles” and the mushroom cloud of fuel-dirtied steam. TEPCO’s assumption has been that the entire blast was a hydrogen explosion in the upper refueling deck of the reactor. Moriyama et al cites the steam explosion issue: “The steam explosion caused by the contact of molten core and coolant[water] is recognized as one of the potential threats to the integrity of the containment vessel during a severe accident of light water reactors and one of the important sources of uncertainty in the evaluation of frequencies of large early fission product releases.” The tons of water vaporized as melted fuel fell through the reactor bottom into the water, creating a steam mushroom cloud that triggered the secondary hydrogen explosions.
As part of the steam explosion the containment well cap momentarily lifted allowing a significant portion of the seawater to escape before falling shut again. The cap gasket was damaged and allowed steam to leak out for weeks as seen in both heat images and photos. Images show the distinct under pressure steam leaks at two points near the reactor well. There was also a significant pressure drop in the drywell seen at the time of the explosion indicating the ejection of steam and pressure from containment. This pressure change challenges TEPCO’s position that unit 3’s explosion was only a hydrogen explosion involved on the refueling deck.
The theory is described in great detail by SimplyInfo member Ian Goddard here. He has also provided the videos below to help illustrate the theory.
MOX factors that may have contributed at unit 3:
Background: A scandal erupted around 1999 when it was discovered that a shipment of MOX sent to Japan by BNSF was flawed and that BNSF had covered up the defects. Further investigation found that the process used at BNSF was the same as what was used at 3 other MOX facilities in the EU including Belgonucleaire’s P0 plant in Belgium, Cogema’s Melox and Cadarache (CfCa) facilities in France. Belgonucleaire made the MOX assemblies loaded into unit 3 at Fukushima Daiichi. As of 2001 production falsifications were found at Belgonnucleaire and an investigation was launched against that company by Japanese authorities. Dr Frank Barnaby claimed in a 2001 report for Greenpeace that the quality of the MOX fabrication at Belgonnucleaire was even worse than at BNSF. Some of the data falsifications include faking pellet diameter testing among others. The data falsification was denied by the power company and Japanese government and was only pursued further after constant pressure from environmental groups.
Belgonnucleaire is owned by Cogema and Framatome. MOX fuel is made by a process referred to as MIMAS. MIMAS takes plutonium and uranium in a mixture of 30% plutonium, remainder uranium and “scrap” this is milled for many hours. This end product is called MasterMix. The MasterMix is then mixed with natural or depleted uranium in a blending (not milling) process that has agglomerates of 30 per cent plutonium Mastermix in a UO2 matrix. A number of issues can impact the ability for powders to be completely and properly mixed including humidity, binder concentrations and particle size. The quality control processes themselves may be unable to accurately detect fuel pellets that are not completely and properly mixed. Then there is the deliberate falsification of quality control data known now to have gone on at these MOX plants.
Mox quality, flaws and safety risk:
The issues with pellet production can cause pellets of variable plutonium content that can cause issues with core neutronics. Pellets with more plutonium than others or pellets with inhomogeneous distribution of plutonium can cause concentrations of plutonium in spots along the fuel rod. The current quality control standards can miss pellets with a faulty plutonium mix. Plutonium “hot spots” along a fuel rod can damage the cladding. This can also lead to cladding rupture in accident conditions in a reactor. The MOX fuel at Fukushima was produced at Belgonenucleaire in Cherbourg. The mixing process used by Belgonenucleaire had worse homogeneity problems than BNFL’s process that was already highly criticized.
TEPCO’s checks to confirm fuel quality did not provide a wide enough scan to actually catch plutonium concentration spots.
“In a TEPCO report, dated 24th February 2000, it is stated that 32 pellets were checked for homogeneity out of a total of 430,000 (for Fukushima-I-3 reactor fuel). And even for each pellet only a thin slice, representing a very small fraction of the volume of the pellet, is examined.”
A number of concerning conclusions were made in 2001 by Dr. Frank Barnaby related to MOX fuel in a possible accident. With the limited information from the disaster at Fukushima Daiichi it is not possible yet to conclude if any of these happened, they do raise some serious concerns about MOX and reactor accidents.
- “Because the thermal conductivity of MOX, compared with UO2, is reduced, the energy stored in the fuel rods in a loss-of-coolant-accident is increased.”
- “Higher temperatures also increase the release of fission gases from MOX fuel and increase the pressure in the rods; plutonium hot spots may affect the behaviour of MOX fuel65 and the cladding of MOX rods during reactivity accidents,”
- “The different concentrations of fission products and actinides in MOX fuel may increase the severity of a reactor accident; the larger amounts of actinides in MOX fuel the decay heat of the fuel rods will be greater.”
- “In the context of accidents in reactors fueled with MOX, it should be noted that, although MOX ceramic melts at a temperature of about 1,800 degrees Centigrade, surface oxidation occurs at the much lower temperature of about 250 degrees Centigrade if the fuel is exposed to air. At relatively low temperatures, exposed MOX pellets produced respirable-sized particles following relatively short exposure periods. For example, 1.87 per cent of the initial mass was rendered respirable when MOX fuel was exposed at 430 degrees Centigrade for 15 minutes, compared to 0.01 per cent at 800 degrees Centigrade.68 A particle with a diameter less than 3 microns can be inhaled into the human lung, with a resultant substantially increased public health risk of lung cancer due to the alpha radiation.”
Unit 3 continues to be of considerable interest due to the extent of the damage, high radiation and a variety of still unknown factors that will help better understand the details of the accident.
2010 MOX assemblies inspection
Greenpeace report on MOX fuels
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