Monju Restart Delayed Again Nuke Info Tokyo No. 126

On 20 August 2008 the Japan Atomic Energy Agency (JAEA) announced a change to its schedule for completing plant confirmation tests for the Monju Prototype Fast Breeder Reactor (FBR, 280 MWe). JAEA said that it now aims to restart Monju around February 2009. This represents a delay of about 4 months. The previous target of October 2008 was announced in August 2007.

Trucks transporting plutonium fuel to Monju May 2008

The latest delay arose as a result of the very long time taken to check equipment for detecting sodium leaks. Because these checks have taken such a long time, the fuel in the reactor core has degraded to the point where it cannot reach criticality. Consequently new fuel has to be fabricated. This article summarizes the issues involved.

Sodium Leak Alarm
Monju has been out of action since a sodium leak caused a fire in the plant in 1995. In the last couple of years, the sodium leak detector alarm has gone off repeatedly in various locations. The first time was on 7 August 2007 in the room housing the tubing of the main circulation pump for the secondary system. JAEA explained that the wiring was severed due to a manufacturing error. The second time was on 28 August 2007. The location of the problem was again the secondary system, but on this occasion it was the room housing the steam generator in the A loop of the secondary coolant. Once again it was a false alarm, this time caused by a loose screw in the internal base plate. JAEA replaced the screw and said that it had altered the structure so that the screw could not come loose again. Then on 13 January 2008, there was another false alarm in the same room as on 28 August 2007. This time JAEA said that alarm went off because of a change in pressure and temperature.

JAEA had submitted a report to the government on 13 October 2007 saying that its safety assessment was complete. The report stated that all items identified by the government as requiring safety checks had been fully checked. The Nuclear Industrial and Safety Agency (NISA) accepted the report in February 2008 and JAEA was preparing to restart the plant when, on 26 March 2008, a “contact-type sodium leak detector” alarm went off an in the primary system. The alarm continued for two hours and six minutes. As it turns out, there was no sodium leak, but JAEA did not inform Fukui Prefecture, Tsuruga Town, Mihama Town and other municipalities for three hours and the delay in notification became a major issue.

This was not the end of the false alarms. Two days later, on March 28, the alarm was set off again by the same detector and it continued to go off intermittently thereafter. When the detector was inspected it was discovered that the head of the detector (an electrode) was bent as a result of a manufacturing error.

The detectors in question are referred to as “contact-type sodium leak detectors (CLD) with sealant”. In response to directives from Fukui Prefecture and NISA, JAEA inspected all CLDs with sealant in the primary and secondary systems. Besides bent heads, the inspections identified numerous errors, including loose sealants, and incorrectly inserted detectors. There are a total of 252 CLDs with sealant in the primary and secondary systems. Of these, 31 had bent heads and 98 were incorrectly inserted. JAEA will replace all 252 of this type of detector.

By July 24, JAEA had carried out checks on 1,350 other types of detectors (besides CLDs with sealant) and checks on 3,000 other items produced by the same company that manufactured the faulty CLDs with sealant.

On April 19, the head of NISA said, “In view of the fact that there are issues in common with the 1995 sodium leak accident, we have no choice but to treat this as a very serious problem.”

Alarms Continue
One would have thought that a thorough inspection had been done, but more alarms went off on June 19 (not reported) and July 4 (reported one hour late) in the C loop of the secondary system, on August 22 in the primary system, and on September 6 in the sodium flow adjustment tank in the secondary system. It is as if the sodium leak detectors are sounding the alarm that JAEA is not qualified to operate Monju. On August 21, an article in the Fukui Shimbun reporting the delayed schedule carried the following headline: “Even February 2009 restart is dubious”. The headline may turn out to be prophetic.

While Monju Remains Idle
Shipping of fuel for Monju began in July 1992 and nine shipments were carried out up to March 1994. Reprocessing to extract plutonium and fuel fabrication predate these shipments. The plutonium used in Monju comes from spent fuel from light water reactors (LWR). Generally “fissile plutonium” represents 70% of the total plutonium in spent LWR fuel. Pu-241 represents about 11~15% of total plutonium. Pu-241 is “fissile”, but with a half-life of just 14 years it decays rapidly to become Americium-241. Even though this Pu-241 was considered “fissile” when the fuel was fabricated, Monju has not operated for 13 years since the December 1995 sodium accident. Since the fuel was fabricated before then, by now over half of the original “fissile” Pu-241 has decayed into Am-241. Furthermore, even though Monju only operated for a brief time up to a maximum of 40% of full capacity, that further reduced the amount of fissile plutonium. The net result is that JAEA says that it is impossible to restart Monju using the fuel currently loaded.

JAEA intended to load 78 “new” fuel assemblies, which were fabricated by 1996, in order to achieve criticality and an output of around 10% of full capacity. Of these 78 “new” assemblies, 48 (including 2 spares) were stored at Monju and 32 were stored at JAEA’s reprocessing facility in Tokai Village, Ibaraki Prefecture.

JAEA transported the 32 assemblies from Tokai to Monju in two road shipments, 18 assemblies in May this year and 14 in July. In 2005 the Nuclear Reactor Regulation Law was amended to strengthen nuclear security. On 28 November 2005 an advisory notice was issued by the Ministry of Land, Infrastructure, Transport and Tourism, the Ministry of Economy, Trade and Industry (Nuclear Industrial and Safety Agency) and the Ministry of Education, Culture, Sports, Science and Technology. This notice made it possible to conceal information about the route, security measures and so on related to the shipment of nuclear fuel. By rights, therefore, the shipments should have been carried out in secret, but details were leaked giving people opposed to the restarting of Monju and people concerned about the danger of the shipments an opportunity to protest.

Even then, Monju will not be able to achieve output of 40% of full capacity. The replacement fuel is only to start up the plant. In that case it would be better to use new fuel, but apparently researchers are excited about the prospects of a world first experiment in starting an FBR with a large amount of Americium in the fuel. However, using Monju to carry out such superfluous experiments increases the risk of further accidents.

Due to the extended time taken to deal with the false alarms from the sodium leak detectors, the restart of Monju has been further delayed. The result is that, even using the 78 “new” fuel assemblies, Monju won’t be able to achieve criticality. The reason why JAEA delayed the restart to February 2009 was to allow time to fabricate and load new fuel. According to JAEA, at least three new assemblies have to be fabricated and loaded in order to achieve criticality. With this they say they will be able to achieve 10% of full capacity. Thereafter, they will have to load more new fuel. In order to achieve 40% of full capacity they need to load 24 new assemblies and for 100% capacity they need to load an additional 57 new assemblies. At the time of writing, fabrication of these assemblies had not begun and according to media reports it is doubtful whether they will even be able to fix the problem of the sodium leak detectors in time for a February restart.

Hideyuki Ban (CNIC Co-Director)

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