Information from Eskom
There are three levels of waste at Koeberg, South Africa's only nuclear power station. These are caegorised as: low level waste, intermediate waste and high level waste (spent fuel).
Low level waste comprises of refuse that may or may not be contaminated with minute quantities of radioactive material. This waste is usually in the form of clothing, plastics, insulation material, paper and coveralls and is generated in the controlled radiological areas of the power station. These items are sealed in clearly marked metal drums and stored on site until they are shipped to the Vaalputs national nuclear waste repository that is run by the South African Nuclear Energy Corporation (NECSA). Vaalputs is the national repository for low and intermediate level waste some 500 km north of Koeberg.
Intermediate level waste consists of evaporator concentrate, spent resins, filter cartridges and contaminated scrap metal. This waste is more radioactive than the refuse but much less radioactive than spent fuel. It is mixed in a specific way with concrete and sealed into appropriately marked concrete drums. These concrete drums are shipped to Vaalputs. If a shipment of these concrete drums were involved in a road accident and fell from the truck and fractured at point of impact, the radioactive materials encapsulated within the concrete would retain the contents without leakage, with no threat to the public or environment.
High Level Waste comprises the metal and mineral waste left over once spent fuel has been reprocessed to extract any re-usable Uranium or Plutonium. HLW has been around since mankind started its large-scale nuclear activities.
Spent nuclear fuel is radioactively extremely dangerous and therefore needs to be safely housed. When it is removed from the reactor vessel it is stored in special “pools” known as fuel pools.
At Koeberg the two pressurised water reactors generate approximately 32 tons of spent fuel each year. Over a 40-year design lifetime of the plant this would add up to 1 280 tons. Each spent fuel assembly contains radioactive materials that fall into three categories.
The first category contains the fission products such as Caesium, Iodine, Strontium, and Xenon which are created when Uranium or Plutonium nuclei are split. They are the most predominant radioactive nuclides of spent fuel when it is removed from the reactor vessel and transferred to the spent fuel pool where they decay to low levels of radioactivity relatively quickly. After 1 000 years only the longest-lived fission products such as Iodine 129, remain.
In the second category are the actinides, which are isotopes of Uranium and heavier metals including Plutonium. These are long-lived nuclides which take 10 000 years to decay.
The last category contains the structural materials of the fuel assemblies which become radioactive through irradiation by neutrons. They add a small amount of radiation to the spectrum of the spent fuel assembly total and decay in about 500 years.
A remarkable feature of spent fuel is that after one year of storage only 1% of the radioactivity remains in the assembly because the radioactive nuclides in the material decay so quickly. After 10 years, only 0,5% of the original radioactivity remains.
What is left after 10 000 years of storage is about 0,0002% of the radioactive content and most of that would be Plutonium and other actinides. After this period the radioactivity has decayed to below what would have been there had the Uranium been left undisturbed in the ground.
During the 1990s Koeberg took a decision to go for the high-density storage racks for its spent fuel assemblies. New technology enables us to pack more spent fuel into racks making it possible to store all the spent fuel that will be generated over the design lifetime of the station (40 years). These would number approximately 3 000, depending on how many refueling outages Koeberg has. This re-racking project was completed in 2002.
Two different storage regions have been created in the spent fuel pools. The first region has 360 positions in three racks and will store the most reactive fuel. This is the fuel that has spent the least amount of time in the reactor and therefore contains relatively large amounts of U235, which could still undergo fission.
In this region the fuel assemblies are further apart so that there is no chance that they may start a spontaneous fission reaction. Using neutron-absorbing materials in the construction further controls criticality (the start of the fission process) so that the number of thermal neutrons in the region is always below that required to start a chain reaction. The racks are made up of stainless steel with plates of borated steel attached to the outside surface of each stainless steel storage channel. Borated stainless steel contains 1,7% boron as part of its chemical composition. Boron is an excellent neutron absorbing material.
The second region contains the bulk of the spent fuel. The assemblies are closer together since this fuel has spent a longer period in the reactor and hence has a lower residual amount of fissile uranium. The racks in this region are constructed of the same materials as those in region one. In 1996 four spent fuel casks that can house 28 spent fuel assemblies each were bought. These casks are dual purpose transport and/or storage casks and are specially designed to contain the radioactivity associated with 10-year old spent fuel assemblies.
Due to delays in the re-racking project a decision was taken to use these casks as an interim contingency measure prior to the refueling outage in April 2000 and January 2001, in order to ensure that there would be enough space in the spent fuel pools to allow fuel unloading to occur .
The empty casks weigh 97 740 kg and are made of ductile cast iron. The cast iron walls are 358 mm thick providing the structural strength needed, heat dissipation as well as shielding from the radiation emitted by the spent fuel assemblies. A layer of polyethylene rods are contained inside the wall of the cask to provide a shield against the neutrons emitted by the fuel.
The cask design ensures that the remaining thermal heat in the fuel assemblies is dissipated naturally. The advantage of this is that no heat removal systems that will require monitoring or maintenance are necessary. The heat losses occur in the same way as it does when a cup of tea is allowed to cool down.
Eskom has already commented on the draft nuclear radioactive waste policy that has been drawn up by the Department of Minerals and Energy Affairs. Eskom is in the mean time looking at all the options available world wide for the handling of spent fuel but will have to abide by the measures laid down in this radioactive waste policy for final storage of spent fuel.
Contract Fani Zulu, Eskom, Tel: (011) 800-8111, firstname.lastname@example.org