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Stuck Control Rod and Other Problems

Four months have passed since the July 16 Chuetsu-Oki Earthquake. As at October 31st, 2,997 problems had been found at the Kashiwazaki-Kariwa nuclear power plant. Gradually the full scope of the damage is emerging.

Details of the Earthquake
The fault in the seabed which caused the magnitude 6.8 earthquake is much longer than the 8 kilometers originally estimated by Tokyo Electric Power Company when it applied for a license. In fact, it is believed to be about 30 kilometers long. In the southern part of the epicenter, which was nearest to the nuclear power plant, the fault slopes downwards from the sea towards the land. In the vicinity of the plant the fault surface is at a depth of about 10 kilometers, although it is not certain whether the fault continues directly under the plant.

The seismic acceleration experienced by the plant as a result of the earthquake exceeded the design limit for almost all periods of the spectrograph (see diagram below). It was the first time that such a situation had arisen anywhere in the world. The seismic acceleration was greater for units 1-4 than for units 5-7. This was due to the strength of the latter half of the seismic pulse. It was not caused by an aftershock. There were some aftershocks, but much fewer than one would expect for such a large earthquake.

Diagram copied from 30 July 2007 TEPCO report

The opponents of the plant always called it "a nuclear power plant built on tofu". As they predicted, the ground of the site was horribly deformed by the quake. The ground was forced up a maximum of 60 cm, while it sunk as much as 160 cm elsewhere. As a consequence, the buildings themselves were displaced by the movement. The relative vertical displacement of the turbine building and the reactor building of unit 1 was 24cm. However, since the Geographical Survey Institute's reference point was destroyed by the quake, the absolute displacement is unknown. Nevertheless, it is clear that the plant was built on top of an active fold and that the judgments of both TEPCO and the government were incorrect.

Damage to the Plant
By and large the maximum shear strain on the reactor buildings was within their elastic limits. However, east-west movement of the fifth floor basement in the reactor 2 building was near the level where cracks could arise.

Unit 2 was being started up, and units 3, 4 and 7 were operational, but somehow the minimum functions of shutdown, cooling and containment were maintained (NIT 120).

The head of reactor 7 was removed and inspections of the core began on October 2nd. However, a 4-meter long control rod, which is required to meet the highest earthquake resistance standards, was stuck and could not be removed. This was the first irregularity found in an item of equipment within the core.

It was discovered that 200ml of water, which was slightly contaminated with radioactivity, had somehow leaked through the 2-meter concrete wall of the unit 7 spent fuel pool. This suggests that the stainless steel liner (6mm thick) might have been damaged. Leaks have been found in two places in the wall of the spent fuel pool of unit 7 and in 3 places in unit 1.

Unit 6 was closed for inspections when the earthquake struck. Signs of scraping were found in rotor blades in 11 stages of the 14-stage low-pressure turbine and 14 stages of the 18-stage high-pressure turbine. In other words, over half the moving blades in the turbines were damaged as a result of the earthquake. The gap between the moving blades and the stationary blades is very small, just 3mm in some places.

Unit 5 was also closed for inspections. However, when the fuel assemblies within the reactor core (13cm x 13cm x 4.5m, 300kg) were being moved to the spent fuel pool, 1 of the 764 assemblies could not be removed. It was one of the outermost assemblies. It is possible that the zirconium alloy cladding had been deformed and that it was stuck on the plate of the top lattice.

Also in unit 5, hangers supporting main steam pipes and recirculation pipes were found to be out of alignment.

As described above, more and more effects of the earthquake are emerging. However, the most frightening thing is that the extent of deformation ("plastic" (irreversible) deformation, as opposed to "elastic" (reversible) deformation) cannot be determined by visual inspection. It is likely that important equipment in the nuclear power plant suffered considerable deformation.Furthermore, it is impossible to check every location thoroughly. It must be concluded, therefore, that it would be very risky to restart the plant.

Yukio Yamaguchi (CNIC Co-Director)

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