High Level Waste (HLW) Treatments and Short-term Storage
There are two methods of disposal of spent fuel rods these being reprocessing and vitrification with storage on site.
Reprocessing of High Level Waste
The fuel rods require replacing every four to five years, the spent fuel rods are very radioactive, requiring extreme caution in handling. The Thorpe Center at Sellafield in UK has been involved for some time in reprocessing used fuel both from here and abroad. The plutonium and uranium components are extracted from the irradiated fuel, the uranium being converted back to pellets for reuse. The plutonium was mixed with uranium producing MOX fuel, (this process has now been shelved) the remainder of the spent fuel being vitrified and stored on site.
Processing of High Level Wastes
The spent fuel rods as well as being highly radioactive are still very hot, and will continue to produce thermal energy for some time as they decay.
For this reason they are stored underwater in ponds specifically designed for this purpose which keep them cool and prevent the escape of radiation. They will remain in the ponds for four years, by which time the will have radioactively decayed and cooled sufficiently to proceed to the next operation.
All the following processes are carried out in a hot cell, a building constructed exclusively to contain radiation but enabling observation and operation of the process taking place within.
The liquid spent fuel is pumped from the cooling ponds to a liquor tank and from here into a rotating cylinder device which is suspended in a furnace. Here the liquor is calcined by subjection to high temperatures whilst continually rotating, thereby converting the liquid waste into a dry powdery substance.
This is then mixed with powdered borosilicate glass and fed into an induction furnace or a specialized melter such as a Cold Crucible Mixer (CCM) and heated to 1100C, causing both components to become molten. The molten mixture is held at this elevated temperature whilst being continuously stirred, both operations effectively binding the waste and glass in about eight hours.
The molten substance is then poured into a stainless steel container (resembling an old style milk container used by dairy farmers) and left to solidify. An air-tight lid is then welded on and the container subjected to high pressure water jetting to remove any contamination from outside of the container.
Sometimes the containers are covered in layers bentonite clay before being transported to an underground store where they can remain in safekeeping for up to fifty years.