Background:

After the end of WWII, Japan was persuaded by the Allies to embrace nuclear power – partly as a way of expiating the horror of the Atomic Bomb explosions at Hiroshima and Nagasaki, but mainly as a way of achieving energy independence. To avoid the necessity of purchasing fuels off-island, the Japanese were urged to embrace the nuclear reprocessing option. By routinely extracting plutonium and unfissioned uranium from irradiated nuclear fuel, they were informed, new fuel could be fabricated by blending the recovered plutonium with the unfissioned uranium.  This new plutonium-based fuel is called MOX – an acronym for Mixed Oxide fuel. As a result, Japan developed no plans for the long-term storage of irradiated nuclear fuel.  Spent fuel was to be regarded as an energy resource rather than as nuclear waste. And as it happens, spent MOX fuel contains even larger amounts of highly toxic radioactive substances than spent uranium fuel from conventional reactors.

The Fugen advanced converter reactor started up in 1978. It was the first reactor in the world to use a full MOX fuel core. It had 772 MOX fuel assemblies, the most for any reactor anywhere. It has received the title of a historic landmark from the American Nuclear Society. The Fugen reactor boiled ordinary water as in standard boiling water reactor (BWR) but used heavy water as a moderator as in a CANDU reactor. The Fugen reactor  was shut down permanently in 2005.

The Monju prototype fast breeder reactor (1994-2016) was designed to produce more plutonium as a byproduct than the plutonium it uses as a fuel. That’s why it’s called a “breeder”.  But for this breeding process to work, the fuel has to be much more “enriched” in fissile material.  It has to be nuclear-weapons-usable material, capable of sustaining a nuclear chain reaction using “fast neutrons”, without the use of any moderator. That’s why it’s called a “fast” breeder.  So, instead of using water as a.coolant (which inevitably slows down the fast neutrons) the Monju reactor uses liquid sodium (a liquid metal) as a coolant.  Needless to say, the liquid sodium becomes highly radioactive, just as the primary coolant of any recator is highly radioactive.

The Monju reactor has been inoperative for most of the time since it was first built. It last operated in 2010, and last year it was decided top decommission it.  The Japanese have no idea at present how to dispose of the spent fuel or the radioactiuve sodium from Monju. Radioactive sodium is yet another nuclear conundrum.

At Fukushima-Daiichi nuclear power plant, Unit 3 (one of the reactors that suffered a complete core meltdown) was fuelled with a heterogenous core of uranium oxide fuel and plutonium-based mixed oxide fuel (about 6 percent of the fuel was MOX).  Unit 3 was the only one of the six reactiors at Fukushima-Daiichi that had MOX fuel in its core.

Gordon Edwards.