Mihama (Japan)

Map of Mihama

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unit-1: 320 MW PWR unit; constructed by Westinghouse; grid connection in 1970.
unit-2: 470 MW PWR constructed by MHI; grid connection in 1972.
unit-3: 780 MW PWR constructed by MHI; grid connection in 1976.

Facilities in Mihama

plantreactor typconstruction startoperation startshut down

During a periodic inspection, cracks were found on the inside of the primary coolant inlet piping nozzle stub of steam generator A.
(source: http://cnic.jp/english/)


"While operating at 50% power for inspection and maintenance, an alarm indicated an increase of the safety valve outlet temperature of pressurizer-B in Mihama-1. The reactor was shut down manually to check for a possible leak from the valve sheet."
(source: http://cnic.jp/english/)


"In Mihama-1 while conducting a visual inspection during periodic inspection of the auxiliary building exhaust stack, it was discovered that two drain pipes on the base of the exhaust stack had become detached and that there were cracks in the base plate. This exhaust stack is located beside the reactor containment vessel. The cracks were caused by fatigue from repeated vibration."
(source: http://cnic.jp/english/)


A section of steam pipe that blew out Monday, killing four workers at a Japanese nuclear power plant, had not been inspected in 28 years and had corroded from nearly half an inch to a thickness little greater than metal foil, authorities said.

Although the carbon steel pipe carried 300-degree steam at high pressure, it had not been inspected since the power plant opened in 1976. In April 2003, Nihon Arm, a maintenance subcontractor, informed the Kansai Electric Power Company, the plant owner, that there could be a problem. Last November, the power company scheduled an ultrasound inspection for Saturday.

"We thought we could postpone the checks until this month," Akira Kokado, the deputy plant manager, told reporters at Mihama. "We had never expected such rapid corrosion."

But on Monday, four days before the scheduled shutdown for the inspection, superheated steam blew a two-foot-wide hole in the pipe, scalding four workmen to death and injuring five others seriously. The steam that escaped was not in contact with the nuclear reactor, and no nuclear contamination has been reported.

Initial measurements showed that the steam had corroded the affected section of pipe from its original thickness of 0.4 inches to 0.06 inches, less than one-third the minimum safety standard. Kansai Electric said in a statement that the pipe "showed large-scale corrosion."

"We conducted visual inspections but never made ultrasonic tests, which can measure the thickness of a steel pipe," said Haruo Nakano, a Kansai Electric spokesman.

In response, Japan's nuclear and industrial safety agency ordered ultrasound inspections at four other power companies that own nuclear plants with the same type of pressurized water reactors. The inspections will involve nearly half of Japan's 52 nuclear power plants.

(source: New York Times Aug 11 2004)
Steam leaked due to rupture of the condensate water piping of the secondary
system and the reactor automatically shut down. It had possibility to challenge safety
requirement of the reactor immediately. Therefore, the evaluation is judged to be as the
level 0+. However, because lack of safety culture was recognized, this resulted in the
evaluation of the level 1.)

(source: http://www.nisa.meti.go.jp/text/kokusai/050603.pdf)


Reactor Mihama-1 manually shut down due to primary coolant valve leak rate increased; cracks found at sea water drain pipes.
(source: http://cnic.jp/english/)


Coolant leak from reactor water level meter housing.


Condenser clogged.


Marine organism found in EDG cooler. EDG removed from line.


Power dropped due to failure of water amount adjustment in FW heater during test operation.


488 SG tubes found damaged during inspection.


Neutron monitor failed during rated power opertaion


manual shutdown because of an increase in off-gas activity due to SG tube leak


SG water level dropped due to introduction of foreign object into main FW bypass control valve. scram


Double-ended guillotine break of SG tube. Unit was operating at full power, when secondary circuit activity increased. 70 m³. of primary coolant have been lost to the secondary side. Pressure drop in the RCS caused ECCS to kick in. Staff was confronted with more unexpected situations: PRZ relief valves failed to open, when operators called on it to reduce RCS pressure following the injection of ECCS. The ruptured tube had been found without any defect by last years inspection. Mihama's SG were considerd relatively healthy (only 6% of the tubes plugged) according to the company 5 GBq of noble gases were released. Later it was said by the company that the release was about 8% of the annual limit: 148 MBq into the air and 7 MBq discharge into seawater. An official report of the Ministry for Trade and Industry followed in June 1991, saying that 55 tons of primary coolant escaped from the damaged SG to the secondary system, 1,3 tons of steam escaped from the SG's relief valve. the releases to the environment are reported as : 22 GBq noble gases; 379 MBq iodine; 7,5 MBq liquid substances. This accident was a proof agaianst the "leak before break theory", because there was only very short time between detecting the leak and the pipe rupture.


corrosion pin holes found in 6 SG tubes during inspection


damage to 18 bolts of primary coolant outlet vane found during inspection