An investigation by Chubu into the status of spent fuel stored in the spent fuel pool from the Unit 1 reactor of the Hamaoka nuclear power plant found cracks and other deformations among the fuel rods. The Unit 1 reactor is currently undergoing decommissioning work and the spent fuel was transferred to the Unit 5 spent fuel pool, and some of the damage is thought to have occurred during the transfer from one spent fuel pool to another.
Using a fiber-optic scope, workers inspected the fuel assemblies which had been transferred to the Unit 5 spent fuel pool in late January, and found new scratches, beyond damage that had been observed in 1994.
Prior to the Fukushima nuclear disaster, the Hamaoka nuclear power plant was considered by many experts to be the most dangerous nuclear power plant in Japan. Both Hamaoka Unit 1 and Unit 2 are BWR Mark 1 reactors, and were permanently shut-down in January of 2009 due primarily to seismic concerns. An earthquake of magnitude 8.0 or higher is estimated 87% likely to hit the area within the next 30 years.
In 1994, workers detected radioactive materials in the Unit 1 spent fuel pool, and discovered a damaged fuel assembly was the source of the leak. In April 1995, workers attempted to further inspect the spent fuel assembly and discovered that cracks had formed in the circumferential direction on one of the fuel rods in the damaged fuel assembly. Each fuel assembly holds 60 fuel rods.
Chubu determined that water had infiltrated the interior of the fuel rods through the holes, which made the entire strength of the fuel rods weaker, leading to the cracks that appeared in the spent fuel rods. The utility assumed that the tensile load endured by the fuel rods when the fuel was moved for inspection may have contributed to the cracking damage. In 2005, workers at the Hamaoka nuclear power plant found the spent fuel pool had foreign matter in it.
Source: Chubu Electric