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Monju reactor set for decommissioning lacks sodium removal method

In this Oct. 20, 2017 file photo, the Monju prototype fast-breeder nuclear reactor is seen in Tsuruga, Fukui Prefecture. (Mainichi)

The Japan Atomic Energy Agency (JAEA) has revealed that the Monju prototype fast-breeder nuclear reactor in Tsuruga, Fukui Prefecture, which is set to be decommissioned, was designed without any consideration of ever having to remove liquid sodium from inside the reactor vessel.

While removal of the radioactive liquid sodium is a key task in terms of early phase decommissioning of the nuclear reactor, the JAEA is likely to be unable to specify a method for extracting the sodium in its decommissioning plan that is due to be submitted to the Nuclear Regulation Authority (NRA).

Water is normally used as a nuclear fuel coolant in power plants but in the case of the Monju reactor, liquid sodium is used instead to increase production of plutonium. However, liquid sodium ignites when it comes into contact with air, and it causes an explosive chemical reaction when it is mixed with water. In 1995, some liquid sodium leaked from the Monju reactor, causing the reactor to be shut down for a long period of time.

According to the JAEA, the primary coolant system facility for nuclear fuel at Monju is covered with an alloy wall, and workers cannot get close to the reactor vessel. Furthermore, the reactor vessel is designed in such a way that the amount of liquid sodium does not go below a certain level so that nuclear fuel is not exposed. As a result, of about 760 metric tons of liquid sodium used for the primary coolant system, several hundred inside the reactor vessel cannot be extracted.

Since operations at the Monju reactor were commenced in 1994, liquid sodium inside the reactor vessel has never been removed.

In an interview with the Mainichi Shimbun, a senior official at the JAEA acknowledged that the reactor had been designed without any regard for removing liquid sodium from the reactor vessel, saying, "When the reactor was being designed, the main priority was to finish the project quickly. Decommissioning was not taken into account." In addition, the liquid sodium has been exposed to radiation, making it difficult for humans to approach it and perform tasks.

The JAEA intends to consider ways of extracting the sodium by a specialist decommissioning division to be set up as early as fiscal 2018. However, the NRA expresses concern, stating that, "There are no holes through which to extract sodium from the reactor, and there are no completely safe methods for removing the sodium."

Keiji Kobayashi, a former instructor at Kyoto University Research Reactor Institute who is an expert on the Monju reactor, states, "It has to be said that there are defects in the design. It is also thought that the number of staff members who understand the structure of the reactor is declining. If the sodium is removed erratically, it could lead to a major accident."

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