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information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise

Monju: How does it work?

September 25, 2016

News Navigator: How does the Monju fast-breeder nuclear reactor work?


The Japanese government is moving toward decommissioning the Monju prototype fast-breeder nuclear reactor in Tsuruga, Fukui Prefecture. The Mainichi answers common questions readers may have about what kind of reactor Monju is, and the state of international research on other fast-breeder reactors.

Question: The Monju reactor is supposedly a power generating device, but how does it work?

Answer: The reactor uses one of three high-speed neutrons that are released when plutonium-239 undergoes nuclear fission, causing more plutonium-239 to undergo nuclear fission and creating heat. The other two neutrons are collided with uranium-238 -- which is not usable by normal nuclear reactors -- to create more plutonium-239. The reactor is called a "fast-breeder" because it uses "fast" neutrons to "breed" more nuclear fuel.

Q: What were the original research objectives at Monju?

A: Generally, the development process of fast-breeder reactors is to create an experimental reactor followed by a prototype reactor, a testing reactor and then a practical-use reactor. Monju is at the second of these stages. Its research objectives included improving nuclear safety and reducing nuclear waste.

Q: What are other countries' fast-breeder reactor programs like?

A: There are few countries that are actively involved in this kind of research. One example is Russia, which has been running its prototype reactor "BN-600" since 1980 and in 2015 it began power production at a testing reactor called "BN-800." Russia aims to have a practical-use reactor by around 2030. Meanwhile, since 2011, China has been generating power at its testing reactor "CEFR," and it is also aiming for a practical-use reactor by around 2030. India also planned to start a prototype reactor this year, but its plan has fallen behind schedule.

Q: What about in developed countries?

A: France is planning to begin running a reactor called ASTRID around the year 2030. However, rather than producing nuclear fuel, this reactor is primarily aimed at shortening the radioactive life of nuclear waste products, recovering resources and otherwise dealing with the issue of nuclear waste. France is aiming for commercial operation of the reactor in the 2040s.

On the other hand, the United States, after putting its prototype reactor development plans on indefinite hold in 1977 due to concerns about costs and nuclear proliferation, canceled its fast-breeder reactor plans. In 1991, Germany canceled its construction of a prototype reactor, partially due to financial difficulties. In 1994, the United Kingdom shut down its prototype reactor as well.

Fast-breeder reactors use sodium for cooling, which reacts violently when exposed to water or air, making it difficult to handle, and accidents have occurred. Another point against fast-breeder reactors is that for the time being there is little concern that uranium used for fuel at nuclear plants will run out, reducing the need for creating more nuclear fuel. (Answers by Shuichi Abe, Science & Environment News Department)

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