March 1995
In February 1995 the first of a number of shipments of vitrified high-level waste departed from France for Japan. This waste belongs to four Japanese power utilities who are responsible for its safe storage and eventual disposal.
The waste arises from spent fuel which has progressively been shipped by these utilities to France for reprocessing over the last 16 years.
Reprocessing separates the waste, particularly the high-level waste containing nearly all of the radioactivity in spent fuel, from the uranium and plutonium which are recycled as fresh fuel. A shipment of separated reactor-grade plutonium was returned from France to Japan in 1993.
The origin of High-Level Waste in the nuclear fuel cycle
For most of the world's nuclear reactors, uranium oxide concentrate from the mine is first converted into uranium hexafluoride so that it can be enriched. Natural uranium contains only 0.7% U-235 (with 99.3% U-238), but this needs to be increased to about 3% U-235 for use in a nuclear reactor. After enrichment, the uranium, as an oxide, is made into fuel pellets which are assembled into rods for use in the reactor core.
The fuel stays in the reactor for three or more years during which time it is altered by the fission process. Some of the U-235 is 'burned' and produces energy as heat. This results in the formation of fission products, - atoms of around half the original atomic weight and which are generally highly radioactive. Some of the U-238 captures neutrons and through a series of radioactive decay stages, isotopes including Pu-239 and Pu-241 are formed. These two isotopes, like U-235, are fissile and much of them is 'burned' in the reactor to produce about one-third of the total energy. Some Pu-240 is also formed, along with other transuranic elements (elements of higher atomic number than uranium).
After three years or more these various changes in the fuel assemblies cause the efficiency of the nuclear reaction to be reduced. Consequently every year or so about one third of the fuel assemblies are removed and replaced by new ones. The spent fuel is then stored under water in ponds at the reactor site while it cools and the initially intense radioactivity starts to diminish.
A number of countries simply regard this spent fuel as waste. These countries, notably USA and Sweden, therefore aim to store spent fuel for several decades until a lot of the radioactivity has decayed. They then intend to dispose of the fuel elements in an underground repository.
However, several countries, notably Japan, France, Germany and UK, currently reprocess their spent fuel so as to return the useable uranium and plutonium to the front-end of the fuel cycle. They are then left with about 3% of the quantity as high-level waste, which includes almost all of the radioactivity from the spent fuel.
Vitrification
To enable safe storage and transport, the high-level waste is mixed with molten borosilicate glass and poured into 1.3 metre high stainless steel canisters. The waste becomes locked into the matrix of the glass as it cools, making it stable and resistant to leaching. Lids are then welded on to the canisters to seal them.
Each canister contains 150 litres of glass weighing 400 kilograms. Some 14% of the content is high-level waste derived from the reprocessing of about 1.3 tonnes of spent fuel. The thermal output of each canister in the 1995 shipment is less than 1.5 kilowatts.
Transport
The stainless steel canisters containing high-level waste are transported in specially-engineered, heavily shielded steel and resin containers called flasks. Each weighs approximately 100 tonnes. Those used for the high-level waste are very similar to the flasks used for transporting the spent fuel from Japan to Europe in the first place. A flask holds 28 canisters of vitrified waste.
The shipment to Japan in February 1995 includes one flask containing canisters of high-level waste and five empty spent fuel flasks being returned to Japan.
The ships involved are 104-metre, specially designed double-hulled vessels used only for the transport of nuclear material. Three ships belonging to a British company associated with BNFL have been approved for the transport of vitrified residues, and conform to all relevant international safety standards.
Reprocessing arrangements
A total of ten Japanese electric utilities have contracts with the French company Cogema to reprocess their spent fuel. These Reprocessing Service Agreements date from 1977-78. (Other contracts are with British Nuclear Fuels Limited, - BNFL.)
After the spent fuel has been in storage for some time at the reactor site, it is shipped to France, via the Panama Canal, for reprocessing. There have already been over 140 such shipments. All the high-level waste from reprocessing the spent fuel will eventually be returned to Rokkasho in Japan for long-term (30-50 year) storage prior to ultimate disposal.
So far one shipment of plutonium recovered from spent fuel reprocessing has been returned to Japan. This was reactor-grade material, with about 30% Pu-240 in it and therefore useable only as a reactor fuel. It is not useable for nuclear weapons. In future the plutonium will probably be returned as a mixed oxide (MOX) fuel, in which the plutonium is mixed with enriched uranium and fabricated into fresh fuel elements ready for use in a power station reactor.
Japan has a small (210 tonnes/year) reprocessing plant already in operation at Tokai, associated with the Monju fast neutron reactor. They are building a much larger reprocessing plant at Rokkasho but this will not be operational until about the year 2000.
Meanwhile Japanese spent fuel is reprocessed by Cogema in France and by BNFL in the UK. Japanese utilities have contracts with these for the reprocessing of some 7000 tonnes of spent fuel. A total of more than 3000 canisters of high level waste will be returned to Japan, in about 110 flasks. Two thirds of this will be from Cogema and the rest from BNFL, starting in 1998.
Japan's Energy Policy
The February 1995 shipment of high-level waste from France to Japan underlines the fact that the Japanese nuclear industry is mature in its technical arrangements for closing the fuel cycle and responsible in its dealing with waste.
Nuclear power provides about one-third of Japan's electricity, and with the enhanced efficiency brought about by reprocessing spent fuel to recycle the uranium and plutonium, it represents a major part of Japan's endeavours to achieve maximum self sufficiency in energy. The Japanese see this in both commercial and ethical terms, avoiding the depletion of fossil fuels and maximising the utilisation of uranium. More recently the policy has enabled them to commit to much greater reductions in greenhouse gas emissions than countries such as Australia.