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Vaalputs: SA’s nuclear crypt
Happy is the land that has no history. And happy is the nuclear facility that makes no news. The Vaalputs nuclear waste depository, in sparsely populated and near-desert Namaqaland, has for fifteen uneventful years (against an annually negotiated fee) been receiving low and medium level nuclear waste from Eskom’s Koeberg nuclear power plant near Cape Town. (Necsa’s own low and intermediate level waste is currently being stored at Pelindaba, west of Pretoria, but negotiations could lead to permission to dispose of this material too at Vaalputs.)
The guiding principle with low or intermediate level waste is to contain the emitting material in an appropriate container, which should be placed in a geological formation to trap any material that escapes through leakage. The hazards to health associated with a well-managed depository are actually very low. High level waste (essentially spent nuclear fuel) emits a great deal of heat and requires special treatment; but this at present is not within Vaalputs’s brief. Koeberg’s high level waste is currently stored on site.
Vaalputs is operated by Necsa’s nuclear liabilities management division. Manager Elwin Raubenheimer explains that the sound management of a nuclear waste depository starts with site selection. The Atomic Energy Board (the predecessor to Necsa) followed accepted international geological and environmental criteria, generally endorsed by the International Atomic Energy Agency (IAEA), when establishing the nuclear waste disposal facility around 1986. These include rainfall, rate of flow of underground water from the site, earthquake risk, mining potential, agricultural potential, population density in the vicinity, permeability of the underlying strata and their susceptibility to corrosion.
The Board acquired Vaalputs, some 10 000ha, large enough to receive all possible low and intermediate level nuclear waste from as many as five nuclear power plants the size of Koeberg (nearly 2 000MW generating capacity) over their possible lifetimes. It is located in one of the driest and least densely populated parts of the country.
The site is underlain generally by clay (a layer of 15-25m), a suitably impermeable geological formation. This is important as back-up in nuclear waste disposal. Low level waste is packed in steel drums, but the site planning allows for the inevitable deterioration of the drums through rust. However, at a correctly chosen site, all radioactivity will be safely contained even in the event of leakage of containers for intermediate level waste. Relevantly, a strange discovery was made in Gabon – a deposit of uranium of such high concentration that it “went critical” two billion years ago, in effect forming a natural nuclear reactor. This burned until the fuel available in its vicinity had been consumed, whereafter activity ceased. The products of the nuclear reaction were immobilised in the immediate vicinity, where they remain in the rocks to this very day. This remarkable event suggests that fears about the safety of storage of low and intermediate level nuclear waste – or even of high level waste—bear no relationship to the actual risks.
Regrettably, some composite containers for intermediate level waste put in place in the earliest years have developed leaks. The enhancement of the containment structures and repairs to the leaking containers are under negotiation with Eskom. It must be emphasised that leaking material has been contained within the site and presents no hazard.
The safety system is further reinforced by a variety of monitoring procedures. A watch is kept on such aspects as ground water. Nuclear Liabilities Management maintains boreholes in and around the area. The presence of radioactive isotopes would be a warning sign of the movement of radioactive material. The air above the site is also monitored, as is the rock under the storage trenches and even the vegetation growing on the site. The same is done for the general neighbourhood. In fifteen years, there has never been a problem. Remote handling of material is unnecessary with low and intermediate level waste. Operators must, however, wear protective clothing and have their health monitored at suitable intervals. It is additionally inherent in the phenomenon of radioactivity that it declines over time. Considering the mix of materials being stored at Vaalputs, it is safe to say that after 300 years the level of radiation at the site will have fallen off to insignificant levels.
The procedure at Vaalputs is to excavate large trenches of seven metres in depth, each trench being gradually filled up with one type of storage container – steel drums for the low level waste. The preparation of intermediate level waste for storage is more complex. This is vitrified within concrete containers, in turn placed inside a metal casing, surrounded by a further concrete casing. Low level waste typically consists of gloves, clothes, paper and cleaning material and medium level waste of resins, filters and smaller components.
When a trench has been filled, it is capped with the original top layers of earth carefully preserved from the time of excavation. Thereafter the original vegetation reconstitutes itself. Ironically, the site managers have observed that the cessation of sheep farming, and even – after heavy rains – cattle grazing, has brought about a great improvement in the state of the grass cover. The Vaalputs management has been able to reintroduce several species of buck to the area, notably springbok and gemsbok. These graze contentedly and healthily, regardless of the buried load of radioactive material. It is worth mentioning the possible fate of the depleted fuel from Koeberg. This could be reprocessed either locally or overseas, which is both expensive and still leaves a residue of radioactive material requiring storage. Alternatively, it can be safely stored down mineshafts, say 500m in depth, providing a sufficient depth of rock to contain the radiation well away from the surface. The SA Department of Minerals & Energy is drafting legislation to govern the management and disposal of nuclear waste, including – most importantly -- spent nuclear fuel.
Vaalputs is located well within the boundaries of the African tectonic plate, where the risk of substantial earthquakes is at a minimum. Such records as are available about the seismic history of the area bear out this view. No seismic event worse than moderate (about 5.5 magnitude) has ever been recorded in the vicinity since record keeping began. The site can be regarded as geologically stable.
Although Necsa is responsible to SA’s National Nuclear Regulator, the management of Vaalputs enjoys a cordial relationship with the IAEA, which has frequently visited Vaalputs.