Spent fuel removed from a reactor after it has achieved a significant burn-up, is both highly radioactive and a rapidly diminishing source of heat.
Radioisotopic Power *
in watts per gram
* for use in thermo-electric generators.
Mostly from 'Chart of the Nuclides' (Lockheed Martin and GE Nuclear Energy)
There is choice of two main processes following discharge of spent fuel. One of these does not consider the immediate possibility of re-processing and the other does. The waste volume and waste management implications following this choice are quite different (section 8).
A. If reprocessing is not an option, then after a period of from 7 to 10 years of cooling in a water-filled storage bay, the spent fuel has cooled to just a few hundred watts to about 1 kW per tonne. It has also radioactively decayed sufficiently that it may be safely transferred and stored in monitored surface concrete silos or canisters for up to 50 to 100 years at each reactor site where this interim storage option is approved. During this interval, a political/regulatory decision may be made concerning final disposal; extending the duration of operation of this surface storage option; or revisiting the decision about re-processing.
B. If reprocessing is an option, then the spent fuel is resident in the spent fuel bay for long enough (about 5 months) that it may be safely transported to a re-processing facility. The less time that the fuel spends in storage the more valuable it is from the reprocessing and fuel value point of view, as some of the shorter half life transuranium nuclides are still present. The recovered fuel constituents - making up about 95% of the spent fuel - are recycled into the reactor, and the fission wastes are conditioned, solidified, and stored, pending removal to permanent non-retrievable disposal. The amount of low level waste produced apart from the fission waste is estimated to amount to a cumulative world total by the year 2000 of about 15,000 m3.
In either case, the interim storage period is in water-filled storage bays to provide continued cooling, radiation shielding and a medium in which the fuel assemblies or bundles can be inspected, monitored and identified, and can be observed and easily moved into storage racks.
With time, the cooled spent fuel, may be re-racked to conserve pool storage space if required; moved to an alternative location; or into approved dry storage once the heat output is sufficiently low not to present problems with structural deterioration or fuel cladding damage.
Dry Storage consists of specially designed concrete structures with walls typically of 1 meter-thick re-enforced concrete. These may be rectangular or cylindrical structures. They are designed be weather resistant, resistant to upsets due to extreme weather, earthquakes, intrusion or sabotage, and - once filled - are secured with tamper-proof seals placed by the IAEA. Safeguards usually include welded steel closures once the vaults or silos are filled, along with massive concrete closure plugs. Security also involves round-the-clock security monitoring of the site and frequent security inspections. Other monitoring takes place on individual vaults and silos by way of drainage lines to ensure no deterioration or leakage of the contents and no significant water ingress. Other site monitoring may include the placement of thermo-luminescent dosimeters around the facility, and air and groundwater monitoring in both surface run-off and in deep wells in and around the facility. None of the hundreds of facilities in existence as Dry-Fuel-Storage locations has ever shown any significant sign of either deterioration or upset, nor has there been any attempt made to clandestinely access or damage any of the facilities.
In the case of CANDU spent fuel, the spent fuel bundles are kept in the water-filled spent fuel bay for about 7 years before being transferred to secure Dry Fuel Storage facilities at each of the reactor sites. All such transfers and placements take place under the supervision of an IAEA international inspector who places interim seals on the storage containers until they are filled, and then affixes a 'tamper-proof' seal once the individual bunkers are filled, welded closed, and 'permanently' sealed.
The radioactive decay and decay heat of a typical CANDU natural uranium fuel bundle immediately following discharge at full power, and after burn-up of about 7800 MWdays/tonne of U, is shown below. After this burn-up the starting content of uranium-235 (0.72% is reduced to about 0.22%) and plutonium isotopes make up a total of about 0.4% of the fuel mass.