Monju Fast Breeder Reactor
Of the 23 research reactors in Japan, Monju prototype Fast Breeder Reactor is the most noteworthy. The plant was plagued by accidents, missteps and scandals involving falsification of documents. Over its 22-year lifetime, it operated just 250 days.
The 300 MWe “fast” reactor (one in which the chain reaction is maintained by high-energy neutrons) is so called because the energy spectrum of the neutrons is said to be fast. A breeder is a reactor that is configured so as to produce more fissile material than it consumes. A fast reactor can be designed and operated to be either a net breeder or a net burner. A thermal reactor is a net burner of nuclear fuel, but — and this is very important — all uranium-fueled thermal reactors are prolific breeders of plutonium.
Fast neutron reactors generate power from plutonium by using the uranium-238 in the reactor fuel assembly instead of needing just the fissile U-235 isotope used in most reactors. If they are designed to produce more plutonium than they consume, they are called Fast Breeder Reactors (FBR). If they are net consumers of plutonium they are sometimes called "burners". For many years the focus was on the potential of this kind of reactor to produce more fuel than they consume, but today, with low uranium prices and the need to dispose of plutonium from military weapons stockpiles, the main interest is in their role as incinerators.
The Monju reactor started up in April 1994, but a sodium leak in its secondary heat transfer system occurred during performance tests in December 1995. A temperature sensor broke and sodium leaked from a secondary sodium loop and caught fire. Some minor damage was caused by the burning sodium, and combustion products were spread through a portion of the building; cleaning them up took almost a year. The accident was classified as Category 1 on the international scale of 0 to 7 (with 0 being the least serious) by a committee of independent specialists.
Monju used liquid sodium for cooling and heat transfer, which makes the system intrinsically safer than one that uses water. That is because the molten sodium runs at atmospheric pressure, which means that there is no internal pressure to cause the type of accident that has to be carefully designed against in an LWR: a massive pipe rupture followed by “blowdown” of the coolant when the loss of pressure lets it the water flash into steam. Although sodium is not corrosive like water is, it burns wildly in the presence of water, even water vapor. Sodium ignite spontaneously in air, and reacts violently with water to give sodium hydroxide and hydrogen, which ignites spontaneously. Although in many ways Sodium is difficult to handle chemically, in some respects it is more benign overall than very high pressure water.
The Monju reactor was shutdown and was not operated for some time thereafter. The reactor was repaired within a year or so, but its restart was held up ever since by a series of non-technical problems. In November 2005, an Investigative Commission on the Safety of Monju was established under Nuclear and Industrial Safety Agency (NISA), to examine the safety of activities that would take place if Monju was to re-start.
A thermal reactor starts out with no plutonium at all, and soon has a lot of it, created when neutrons are absorbed by U-238, which leads to Pu-239. In the process, though, it burns more fuel (mainly uranium) than it gives back as plutonium, and therefore is not called a breeder.
By 2004 the role of the fast breeder reactor prototype Monju has expanded to meet the challenges of the 21st century. Instead of being merely a demonstration of an economical, safe, environmentally responsible source of energy, as originally designed, it had also the capability to be transformed into a unique International Irradiations Test Facility. The potential for Monju's role in the Generation IV nuclear energy systems development, and fast reactor research and development area is clear. Its incomparable fast neutron spectrum density will be a major interest not only for Sodium Fast Reactor but for all the Generation IV concepts.
A proposal was submitted to restart Monju in 2008. The Monju reactor re-commenced operations in May 2010, after a string of administrative and other nontechnical snafus. For a while it was scheduled to go on the grid in 2013, but the Fukushima incident led to a change in the Japanese attitude towards nuclear power.
In December 2016 Japan decided to pull the plug on Monju. The Monju nuclear plant cost 20 billion yen ($170 million, 163 million euros) per year, and its operating costs were about to go up. The reactor would have required costly upgrades to meet new safety standards that were implemented after the meltdown at Japan's Fukushima reactor in 2011.
Japan still hoped to have another fast-reactor demonstrator plant by 2028 (with pledged U.S. DoE and French assistance) but has put off deciding on construction. The area around the Monju facility will be turned into a research center for nuclear technology, including plans to explore a different type of fast breeder reactor. Tokyo planned to continue to co-develop a fast-breeder demonstration reactor that had been proposed in France, while research would continue at another experimental fast-breeder reactor, Joyo, which was a Monju predecessor.
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