Problems at KK-3 and KK-7
1. KK-3 Problems
Niigata Prefecture's subcommittee on equipment integrity and seismic safety began its review of the integrity of Kashiwazaki-Kariwa (KK) Unit 3 (BWR, 1100MW) on December 13, 2010. However it is likely to be a long road to a restart.
When the Chuetsu-oki Earthquake struck on July 16, 2007, KK Units 3, 4 and 7 were operating at full power. It is thought that Units 3 and 4 were affected much more than Unit 7, which on December 28, 2009 became the first reactor to resume commercial operations after the earthquake (see NIT 135).
The biggest problem with KK-3 is that the spectrographic record for the reactor building does not match the simulation results. Since Tokyo Electric Power Company (TECPO) does not understand how the reactor building responded to the Chuetsu-oki Earthquake, it is unable to guarantee the reactor's safety in the event that it is struck by another earthquake.
The following table compares the seismic motion observed at the base mat of KK-3 with the design basis.
(Gal is a unit of acceleration. Gravitational acceleration at the earth's surface is 980 Gal.)
The Chuetsu-oki Earthquake was a magnitude Mj6.8 earthquake. If the design basis was so inadequate for this quake, imagine what would happen if the plant was struck by a magnitude 7 class earthquake.
The observed values for seismic motion do not match the simulated values in the 0.1-second and 0.5-second periods. TEPCO carried out calculations which considered whether the base of the reactor building was rigid, or whether it had some flexibility, and the degree of elasticity in the ground in which the building is situated. It also compared the results with those for KK-6 and KK-7. However it is yet to come up with a consistent and rational explanation.
The data shows that the behavior of KK-3 was different from KK-2 and KK-4 on either side. Clearly KK-3 was pushed up to a considerable degree, but the reason has not been discovered. The local people have always believed there were problems with the ground itself.
Cracked foundation piles
KK-3's exhaust stack has 52 foundation piles. Only four of these have been checked and cracks were found in all four. The biggest crack was 2mm wide and 2.08mm long. Kotaro Kuroda, a member of the subcommittee on equipment integrity and seismic safety, suggested that this should be assessed as level IV damage, but TEPCO assessed it as level II on the grounds that there was no exfoliation. One wonders about the condition of the other foundation piles. Clearly they should be checked.
2. KK-7 Problems
A leak of radioactivity from a fuel assembly in KK-7 was identified on September 10, 2010. TEPCO continues to operate the reactor with the leaking assembly, even though it has not been able to identify the cause of the problem. Since then, the radioactivity count at the off gas monitor continues to hover around 10 CPS (counts per second) compared to a normal reading of 1 CPS. In response to a question by committee member Masahiro Koiwa during the December 13 meeting of the subcommittee into equipment integrity and seismic safety, TEPCO admitted that a small amount of radioactivity continues to leak from the reactor. Even if the amount is not large, it is significant that TEPCO was forced to admit that it is operating a reactor that is leaking radioactivity.
Damaged control rods
On November 1, cracks were found in a spent control rod. Since then, more cracked control rods have been found. On January 7 TEPCO announced that it had discovered 28 damaged rods. This represents 61% of the 46 spent control rods stored in KK-7's spent fuel pool. We are very concerned about those currently in use in KK-7. TEPCO does not know how many of those might be cracked.
The control rods in question are the hafnium flat tube type. In 2006, problems with this type of control rod were found in Fukushima I-6 (see NIT 111). The insertion function of the 23 hafnium flat tube type control rods currently in use in KK-7 is being checked once a month, but that is all. We believe this is insufficient. TEPCO claims the control rods are functioning normally.
At this stage TEPCO considers the cause of the cracks to be cumulative neutron irradiation.
Yukio Yamaguchi (CNIC Co-Director)
Return to NIT 140 contents