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Prospects for Kashiwazaki-Kariwa Units 1 and 5

Tokyo Electric Power Company's (TEPCO) Kashiwazaki-Kariwa (KK) Nuclear Power Station Unit 6 (ABWR 1,356MW) resumed commercial operations on January 19 this year. KK-7 (also ABWR 1,356MW) resumed commercial operations on December 28 last year, so 33% of the 8,212MW seven-unit KK Nuclear Power Station's total capacity has been brought back on line since the July 2007 Chuetsu-oki Earthquake.

Residents of Niigata Prefecture are concerned about the safety of the station. They did not agree to the resumption of commercial operations of these two reactors. The deliberations of the prefecture's subcommittee looking into equipment-related issues have not concluded. Nor have issues related to earthquakes and ground condition been resolved. Nevertheless, the focus of the central government's and the prefecture's deliberations has moved on to the next stage and now equipment integrity and seismic safety for units 1 and 5 are being considered. Whereas units 6 and 7 are ABWRs, units 1 and 5 (as well as units 2, 3 and 4) are BWRs (1,100MW each). Moreover, units 1 and 5 were shaken more vigorously than units 6 and 7. Hence, the prefecture's deliberations are proceeding cautiously.

Of all the reactors, unit 1 incurred the largest horizontal seismic movement: 680 Gal at the base plate of the reactor building in the east-west direction. Unit 1 also incurred the second largest vertical seismic movement: 408 Gal at the base plate of the reactor building, compared to 488 Gal for unit 6.It also suffered the largest number of defects: 693 non-conformities (18.5%) identified, compared to 246 (6.6%) for unit 7 and 275 (7.4%) for unit 6.

During the deliberations regarding the seismic safety of unit 1, the issue was raised of whether or not during an earthquake the control rods would insert as designed. Based on data from vibration testing of a single fuel assembly, TEPCO asserted that scram specifications would be met for relative displacement up to 40mm. However, several members of Niigata Prefecture's equipment integrity and seismic safety subcommittee were not satisfied. They were concerned that there was a lot of random variation in the measured data, the analysis was not conservative and there was no guarantee that the control rods would insert safely.

The debate about plastic deformation is likely to be reignited (see NIT 125). During deliberations in relation to unit 7, TEPCO and the central government concluded that plastic deformation below about 2% was undetectable. They claimed that although it is not possible to identify slight deformation incurred by metal materials, on the whole the deformation incurred was in the elastic range. Although there was a grey zone, they proclaimed the plant safe. However, unit 1 suffered much greater seismic stress than unit 7, so the debate about plastic deformation should be raised again.

There was also discussion about the strength of the concrete. The issue related to judgments about whether there was any seismic resistance margin. The design basis strength of the concrete in unit 1 is 240 kg/cm2, but integrity and seismic safety are being considered on the basis of an excessively large figure for the so-called "actual strength" of 450 kg/cm2. Concrete gets stronger for a period of time after it is initially poured. However, it is not certain when it reaches its peak strength, or how strong it becomes. Furthermore, the value for "actual strength" would vary depending on where the sample was taken from.

For the time being deliberations concerning Unit 1 are being prioritized. Unit 5 will not resume operations until deliberations concerning unit 1 are completed. At this stage the prospects for both units are still very unclear.

Yukio Yamaguchi (Co-Director CNIC)

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