The magnitude of the nuclear waste problem can be deduced from table 2.17 which shows the amount of spent fuels discharged in the OECD countries in 1992 [36].

We recall that nuclear power only accounts for 5.8% of the total world energy production. This small percentage will, however, lead to a spent fuel inventory of about 200 000 tons by the year 2020. The annual production of spent fuels amounts to about 8000 tons. This figure is to be compared with

the present spent fuel recycling capabilities of around 2000 tons per year, mostly by the COGEMA La Hague facility.

At present, two different strategic approaches are proposed for high — activity nuclear waste disposal:

1. Direct spent fuel element disposal, without any reprocessing. Such an approach is favoured by, among others, the US, Sweden and Swizerland.

2. Spent fuel reprocessing with the aim of optimized extraction of transuranics and fission products and, possibly, their transmutation by nuclear reactions into less radiotoxic or short-lived species. This approach is followed, notably, by the UK, Japan, France and Belgium.

In both cases, some sort of storage of radioactive wastes is needed. Two options are considered:

1. Deep underground storage with or without possible retrieval.

2. Surface or sub-surface storage.

It is clear that these two last solutions can only be temporary, since the half­life of many of the wastes exceeds, by far, the life span of civilizations. The proponents of such solutions argue that technical progress may allow a better evaluation of the safety or feasibility of alternative solutions. One should note, however, that such progress requires experimenting with deep underground storages in dedicated laboratories on the one hand, and separation and transmutation studies on the other hand. It may seem, there­fore, paradoxical that the most vocal advocates of temporary storage oppose both underground laboratories and reprocessing. The paradox can be understood as an aspect of a strategy aimed at pulling out of nuclear power altogether. Only after a withdrawal is obtained will the question of existing wastes be seriously examined. In that case the only possible solution will be deep underground disposal, but it is untimely to acknowledge that fact while fighting the anti-nuclear struggle!

In the future, the relevance of the two basic choices, direct storage or reprocessing, will depend on the development of nuclear power.

1. In the case of withdrawal from nuclear power in the near future, direct storage is the most natural choice. Reprocessing policies are consequences of investments made in the frame of the deployment of fast breeders. The phasing out or standing still of the breeder programmes raised the question of the future of the large reprocessing facilities like those of BNFL and COGEMA. It was found that using the separated plutonium as fuel in thermal neutron reactors had some advantages (decreased need for enriched uranium and reduced volume of the most active wastes) at a very modest cost [39]. An a posteriori policy of waste separation and transmutation followed.

2. At the present world level, nuclear power has only a marginal role in alleviating the waning of reserves and the environmental degradation problems associated with energy production. Long-term continuation of nuclear power would only be justified at a much higher level than at present. In that case, we have previously noted that breeding will be mandatory. Reprocessing will be necessary and the nuclear waste issue will be completely different. For example, using the values given above for scenario A2N in 2050, nuclear power would reach as much as 9000 GWe, more than 20 times more than at present. With the PWR technology, the annual discharge of spent fuel would rise to 260000 tons. Should this be disposed of underground, four sites equivalent to the US Yucca Mountain would be needed each year. Using fast reactor technology, both plutonium and uranium should be recovered from the spent fuel and, aside from tech­nological losses, the highly active wastes would be limited to fission products and minor actinides, i. e. about 9000 tons per year. Furthermore, given the existence of reprocessing facilities, it might be feasible to transmute minor actinides as well as some of the long-lived fission products.

Underground disposal

From the preceding, it seems probable that deep underground disposal will be necessary in all cases. It is, therefore, important to understand the

nature and amplitude of the hazards which might be associated with such a site. An order of magnitude of the dangers associated with deep underground storage can be obtained by comparing the activity of the stored wastes with the radioactivity of the earth’s crust. Assuming that the storage site is 500 m deep, the comparison can be made with that of the first kilometre of crust. The mean activity of the crust is around 1500 Bq per kg. Considered sites have areas of order 1 km2 corresponding to a crust activity of the order of 3.5 x 1015 Bq, a little less than half being due to 40K and more than half to thorium and uranium decay. The crust activity for the area of a country like France amounts to 1.7 x 1021 Bq. This activity has to be compared with that of the materials stored. We take the example of a storage of 100000 tons of irradiated fuel, corresponding to 50 years of operation of the nuclear reactors of France, a very highly nuclearized country. The activity of the storage is shown as a function of time in table 2.18 and figure 2.6.

10[8]‘

1011

Although this is a very rudimentary approach, the comparison of the activities of table 2.18 with the total activity of the crust of France shows that the average increase of radioactivity over France due to nuclear waste storage will remain very small at all times. Assuming that at least 100000 years are needed for a complete diffusion of the wastes one sees that, even locally, the dose which might be delivered to the most exposed population will not exceed a few times that due to natural radioactivity.

More precise diffusion calculations, such as those displayed in Appendix I, give the following results.

• At no time in the future, in a normal situation, will the dose delivered to the most exposed population exceed 0.25 mSv/year, i. e. a factor of ten below natural irradiation

• The main contributor to the dose is I. Almost all stored I will be released within a time span of approximately 1 million years. Due to their small solubility and mobility, actinides have a very small contri­bution.

• In case of an accidental situation, such as drilling a well through the repository and drinking the extracted contaminated water, the maximum dose to the most exposed population should not exceed a few mSv/year. In this case 9I remains an important contributor but Ra takes the lead for longer times. It is a descendent of 238U. In the case of reprocessing, its influence will decrease considerably.

• The amount of heating by the radioactive wastes at the storage site will have an essential role in determining the surface, and thus the cost of the storage facility. After 100 years plutonium and minor actinides will play the dominant role in heat production and the cost of their incineration will have to be evaluated in comparison with the ensuing cost saving of storage.

In order to have an estimate of the cost of deep underground disposal, we take the example of the US Yucca Mountain site which has been accepted as a site for deep underground storage of nuclear wastes in the US. The site would cover about 6 km2 honeycombed with about 100 km of tunnels [37], while the maximum storage capacity should be 70 000 tons. The cost of the site would be more than 15 billion dollars, corresponding to an additional cost of nuclear electricity of about 1 mil/kWh.