What is high level waste ?
High level waste is fuel that has been used in the fission process. It is radioactively extremely dangerous. When it is removed from the reactor vessel it is stored in special “fuel pools”.
After ten years in a fuel pool it is “cool” enough to be moved into thick-walled casks which can be stored above ground for up to 40 years. Ten years has been chosen because in that period the bulk of the radioactivity from the short lived nuclides has decayed. This is known as dry storage and also interim storage.
A two-reactor pressure water reactor power station like Koeberg generates approximately 32 tons of spent fuel each year. Over a 40-year lifetime that would add up to 1 280 tons.
Each spent fuel assembly contains radioactive materials which fall into three categories.
The first category contains the fission products (such as caesium, iodine, stroulium, and xenon) which are created when uranium or plutonium nuclei are split. They are the most radioactive components of spent fuel when it leaves the reactor vessel for the fuel pool but they decay to low levels relatively quickly and after 1 000 years only about 400 GBq (10 curies) of 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 materials which take 10 000 years to decay to about 800 GBq (200 curies)
The last category contains the structural materials of the fuel assemblies which become radioactive through irradiation by neutrons. They only add a small amount of radiation to the whole spent fuel assembly total and decay in about 500 years to less than 200 GBq (5 curie)
How quickly does the radioactivity decay ?
A remarkable feature of spent fuel is that after one year of storage only 0.92% of the radioactivity remains in the assembly because the radioactive nuclides in the material decay so quickly.
After 10 years, which is the earliest time at which the assemblies would be taken out of the fuel pool, only 1% 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 others actinides. After this period the radioactivity has decayed to below what would have been there had the uranium been left undisturbed in the ground.
Radioactivity decays until it is harmless. In other words, spent fuel eventually gets rid of its own toxicity – unlike chemical toxic waste, which must be pro-actively treated to make it harmless.
Decommissioning
Power station operators have a choice regarding the decommissioning costs they incur. They can decommission the entire station soon after shut down and accept the cost of dealing with highly radioactive material. Or they can dismantle the non-radioactive parts of the site buildings immediately and leave the central block around the reactor itself for perhaps 50 years u and accept the cost of maintaining it for that period.
There are nuclear stations that have been shut down and decommissioned throughout the world . All decommissioning activities are supervised by independent nuclear safety authorities. In South Africa’s case this authority would be the National Nuclear Regulator . They establish safety requirements for the decommissioning workers and the public and ensure that they are adhered to. Great care is taken to prevent the spread of possible radioactive dust and liquid effluents
All organisation operating nuclear stations including Eskom are obliged to set aside funds to cover even tual decommissioning. These funds are set aside from the profit generated by the power station itself.
The options for Koeberg as far as decommissioning is concerned are:
It can be shut down, sealed and left under close security for as long as its reactor systems inside the containment buildings are radioactive
It can be shut down and the containment buildings filled with concrete to prevent any radioactivity present in the buildings from escaping
Under the Greenfield Option the Koeberg site can be rehabilitated to be left exactly as it was before the power station was built. This could only be done if the two radioactive cores and containment buildings were cut up and put into containers and buried, along with the waste and spent fuel.
After removal of the radioactive components the site could be used for another nuclear power station or a conventional power station.
It could be used as an industrial complex as it has all the infrastructure.
All the equipment within the containment buildings could be stripped of radioactive materials, and the buildings then used again.
Re-racking
Eskom has, since it began operating, looked at various options for storing it’s spent fuel.
The station is an exact copy of a French Pressurised Water Reactor (PWR). As such the station was originally designed to store spent fuel on site for only 5 years whereafter the fuel would be sent off for reprocessing. This process was and remains a very expensive option. During the late 1980’s the fuel pools were re-racked in order to store fuel for a 10 – 15 year period. This was state of the art technology at the time.
The fuel pools at Koeberg have now reached capacity and they will no longer be able to take any more spent fuel.
In 1996 after looking at options such as reprocessing, buying special storage casks and building a facility in which to house these casks or putting high density racks into the fuel pools a decision was taken to go for the high density racks. New technology now enables us to pack more spent fuel into racks making it possible to store all the spent fuel that will be generated over the lifetime of the station (approximately 40 years). These would number 11 000 for both units.
New racks
Two different storage regions will be created in the spent fuel pools. The first region will have 210 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. Criticality (the start of the fission process) is further controlled by using neutron absorbing materials in the construction so that the number of thermal neutrons in the region is always below that required to get 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 is a stainless steel which contains as part of its chemical composition 1.7% boron. Boron is a very good neutron absorbing material.
The second region will contain the bulk of the spent fuel. The assemblies will be closer together since this will be fuel that has spent a longer period in the reactor and hence will have a lower residual amount of fissile uranium.
The racks in this region are constructed of the same materials as those in region one.
Interim measures
Due to delays in the re-racking project a decision was taken to use certain interim measures during the refueling outage in April 2000 and January 2001. Four spent fuel casks will each temporarily store 28 spent fuel assemblies. These casks were bought in 1996 and are specially designed to shield radioactivity. The casks were bought to provide Koeberg with flexibility in decision making should the occasion arise.
The casks
The casks weigh 97 740 kgs and are made of Ductile Cast Iron. The cast iron walls are 358mm thick. This thickness is optimal for three safety requirements
Providing strength needed for the cask wall
Providing shielding from the radiation emitted by the spent fuel and
Being manageable for transportation.
A layer of polyethylene rods has been built into the inside wall of the cask to provide a shield against the neutrons emitted by the fuel.
The cask is also designed so that the remaining thermal heat in the fuel assemblies is dissipated naturally. The advantage of this is that no heat removal systems which will require monitoring or maintenance are necessary. The heat losses occurs in the same way as it does when a cup of tea is allowed to cool down.
The casks are tested to cover three areas:
The structural ability of the cask to withstand strains caised by the use for which it is intended
The durability of the materials
The performance of the cask under unexpected or accident conditions
Prototypes of the Eskom casks have been dropped at various angles from a height of nine metres to see whether they broke or developed cracks through which fuel or radioactivity could escape. They remained sound.
They have been smashed into by a train locomotive traveling at a speed of 100kms/hr.
The re-racking project will continue and once all the racks have being installed the spent fuel in the casks will be moved back into the fuel pools and will remain there for the lifetime of the station.
