15.14 Nuclear Generation and Waste Disposal

One of the products supplied by Worcester Porcelain Co. was calcium fluoride tubes used for the separation of uranium isotopes. High purity ceramic crucibles are also used extensively in the processing of uranium and its alloys. Even the fuel elements for higher temperature nuclear reactors are ceramic, made from moulded uranium and thorium oxides. The UK’s Advanced Gas-cooled Reactor fuel pellets have enriched uranium oxide formed into pellets that are fired at 17-1800 ºC supported in molybdenum “boats”. One pellet is equivalent to 1.5 tons of coal.

AGR nuclear fuel pellets - courtesy BNFL

Ceramics are also used for control elements and rods (boron carbide), moderators and reflectors of the neutrons (beryllia) and in shielding (boron carbide and nitride). Ceramics are even used as the restraints for the superconductors used as magnets in nuclear fusion power reactors.

Ceramic devices are used in nuclear instrumentation. As an example, because of the high Curie point of lithium niobate (1210 ºC), it is used for high temperature vibration detection in heat exchangers in nuclear reactors.

The Gas Cooled Fast Reactor is an advanced reactor used for power generation. It operates at much higher temperatures than current nuclear reactors, typically with an 850 ºC outlet temperature. Withstanding this level of temperature for extended periods in the core of the reactor is beyond the capability of most metals so ceramics are used that are resistant to high temperature distortion. One version of the nuclear fuel for this type of reactor is a mixed oxide ceramic honeycomb frit contained in a ceramic cladding. The cladding is made of silicon carbide as a composite of bulk ceramic and reinforcement fibres. The fuel is made in the form of pins 10 mm diameter that are stacked to form a fuel rod.

Safe storage of nuclear waste is a very important issue to protect our descendants while the nuclear isotopes decay to safe levels, sometimes taking decades or centuries. The principle is to encapsulate the waste in a glassy ceramic that is sufficiently robust to maintain its integrity indefinitely. The main source of high-level waste is spent fuel. This is reprocessed to remove any remaining unused uranium, leaving the waste fission products as a nitrate solution that is concentrated by evaporation. This is initially stored in stainless steel tanks within massive concrete structures to provide the necessary shielding. There are several methods of converting this waste into a safe passive state. It can be processed by the Waste Vitrification Plant, where it is first calcined in a rotating kiln to produce dry granules. This is then mixed with molten borosilicate glass in a large crucible heated to 1050 ºC. A version called Synrock uses a combination of titanates tailored for particular waste products.

Encapsulated nuclear waste, models of high
level waste puck and Synrock puck
- courtesy BNFL

High level waste canister
- courtesy BNFL

The glass/metal oxide, fission product matrix can then be poured into special stainless steel canisters, and once the contents are solidified they are sealed. The canisters, containing about 35% by weight waste product, are placed in a long-term concrete storage facility, convection cooled to remove the residual radiation heat emission, and is designed to be safe for a minimum of 100 years.

An alternative method is to pyrolyse (burn) the nuclear waste in a fluidised bed furnace using alumina powder as the bed material and superheated steam at 700-750 ºC as the fluidising gas. Clay and other additives are used to produce a variety of highly durable end products such as boro-silicate and alumino-silicate glasses and magnesium silicate and titanate ceramics. The volume of the encapsulated radioactive waste is reduced and it is rendered easier to handle.