Monju (Japan)

Map of Monju

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280 MW FBR in operation since 1994; shutdown after an accident in 1996

Facilities in Monju

plantreactor typconstruction startoperation startshut down

Monju sodium leak. Increasing doubts over STA´s fact-finding efforts:
Since the Monju accident PNC has released three accident reports and the STA two. On June 7, PNC conducted a second sodium leak and combustion accident test.
STA concluded that the thermocouple sheath broke because of a design error by the manufacturer.
By correlating observations of the broken surfaces, the results of fatigue experiments on a thermocouple in water and Monju´s operational record, STA has speculated that the crack first appeared in the thermocouple between April and June 1992. By March 1995 the crack had passed right through the metal and over half way around the circumference.
The leaked sodium melted parts such as a ventilation duct and a catwalk, and was piled up on the floor. A major issue here is the temperature attained at that time. This is because under the safety screening that anticipated sodium leaks, analysis results say the floor temperature must not exceed 400 C.
The reports say that Monju´s floor reached 700 to 750 C, but did not melt.
With the accident and its cover-up having been revealed, the public is more vociferous than ever in calling for freedom of information. Although STA and PNC have both promised to do so, concrete information has yet to be released despite the public demands for it.


On the engineering front, PNC Power Reactor Development Headquarters Deputy Director Osamu Yanagisawa said investigations have shown faulty design of the thermocouple well is the likeliest cause of the accident, particularly after another well was found with signs of cracks
"I assume the same theory applies to all the thermocouples," he told reporters. There are 48 thermocouple wells in the secondary heat transport system (SHTS), 16 each in three loops, officials said. If that is the case, Yanagisawa said, there will be many phases of work needed before Monju can restart, including redesigning the sodium boundary design, and improving operations manuals. Other systems including ventilation must also be reassessed.
The PNC Vice President said PNC plans to downsize for the time being production of plutonium at its Tokai plant so that no excessive plutonium will accumulate. Japan pledged earlier to inventory only the plutonium it needs.
PNC admitted that the reactor will not restart for the next 15 month.


A second thermocouple sheath has been found with crack indications at Monju in what investigators now call "faulty designing" for the prototype fast breeder reactor (FBR).
Each of the three SHTS loops has 16 thermocouples installed in the piping.
The discovery of the second damaged thermocouple sheath indicates that "faulty design" is more and more likely the cause of the accident, in which the 15-centimeter tip of the exit thermocouple sheath snapped.


The government agency that supervises the owner of the Monju prototype fast breeder reactor (FBR) last week harshly reprimanded what it called questionable operating procedures, inadequate manuals, and sloppy crisis management in the wake of the December 8 sodium leak there.
The Science & Technology Agency's (STA) special investigation team also criticized the agency's own on-site inspection regime.
However, the investigation has shown, that much less sodium leaked from the Monju piping than originally thought: less than one metric ton, versus early estimates of up to 3 MT.
Meanwhile, the remaining portion of a broken thermocouple in the Monju secondary heat transport system (SHTS), through which the sodium leaked into the surrounding room, was cut loose from the piping and trucked to the Tokai Laboratory.
The report, released by the STA Nuclear Safety Bureau Director, focused on 14 points related to three major areas concerning the leak: its cause, operator response, and post-accident management.
The team found faults, insufficiency, sloppiness, and inefficient maneuvers in all three areas.
The team estimated that the most likely cause of the leak is rupture of the 15-centimeter thermocouple thimble, with metal fatigue the probable culprit. The two most likely factors leading to fatigue, the team said, are stress of synchronized vibration of sodium flow with the inherent vibration frequency of the Thimble, and repeated thermal stresses.


The 600-billion yen (US $6-billion) unit remained shut down with all control rods inserted, cooled by sodium at 200 degrees C circulating in the A and B loops, officials said. The C loops of the primary and secondary systems were sealed in argon after being drained.
Symbolizing the anti-Monju sentiment mounting in Tsuruga, a local report complained that the reactor is consuming "an enormous amount of electricity," which it said is enough for some 27.000 households - 4.000 more than the number of households in Tsuruga - just to stay idle.


Representatives of antinuclear activists seeking a court order to stop Monju operation will be allowed into the room, that houses the SHTS piping, officials said.
At least seven persons who are plaintiffs in the lawsuit will be able to enter the room accompanied by three officials of Kanazawa District Court, including a judge, STA officials said January 23.


Sodium leak in the secondary heat transfer system.The escaped sodium covered some 4400 square meters and is estimated to be between 1 and 2 tons. By end of December the company had recovered 280 kg sodium oxide and other compounds.
Cover up of the accident escalated the technological failure to a national scandal:
Two inspection teams entered the room containing the SHTS, took videos and photos, but they did not inform outside inspectors, the Majors of surrounding townships and the press.
Science & Technology Agency's inspectors also dicovered violations of operating procedures.
The cover up has developed into to a threat to Japan's plutonium recycling policy.
A large volume of liquid sodium coolant, estimated at between two and three metric tons, leaked from the secondary system of Japan's prototype fast breeder reactor (FBR) Monju December 8, creating what experts deem the gravest crisis ever for the country's plutonium use program.
As of December 12, PNC had not determined the exact location nor the exact volume of the sodium leak, which occurred in the C loop of the secondary cooling system.
The reactor was running at 43% power.
A second inspection, however, found the volume and density of the white smoke increasing. The chief operator decided to consider the incident as "a medium-scale" leak and moved to manually stop the reactor.
Monju contains roughly 1,530 metric tons (MT) of sodium: slightly over 760 MT in the primary system, some 220 MT in the secondary system B loop, and another 270 MT in the C loop where the leak occurred. Another 150 MT are kept in a storage tank.
A spokesman for the nuclear reactors division of the Commissariat a l'Energie Atomique (CEA) in Paris said that the Monju sodium leak was the biggest ever from an FBR. All earlier leaks from western fast reactors were limited to a few kilograms of sodium. "Three tons is a huge leak" for such a reactor, said one French expert, indicating the break must have been large.


Monju restarts. The 280-MW prototype fast breeder reactor (FBR) Monju achieved criticality again on November 7 to conduct a series of pre-commissioning inspections and tests.


Pump failure caused shutdown at 20 % of nominal capacity;


Shutdown to replace part of the units water/steam system whose design shortfalls made it unable to go beyond several % of power. PNC plans two major modifications: 1. to alter the structure to supress water vortexes in the flash tank and 2. to enlarge the drain pipe to improve flow.
Skeptical citizens have already collected more than 800 000 signatures against operation of Monju.
Restart: 8.5.95.


Grid connection pushed back because of troubles with steam supply system, which may require replacement of components.


Initial load of Pu-fuel is showing less reactivity than designed because of depletion of shorter half-life Pu-141 (which is due to quality deficiencies at PNC's Tokai Pu-fuel production facility).


Monju achieved first criticality on April 5, 1994. Test operation is scheduled to end in December 1995.
Construction costs: US$ 6000 -7000 million