The waste disposal in an ice sheet need not rely on storage periods longer than some hundreds of years. Three hundred years after dumping, the radioactive power of the fission products has decreased to about 10-4 times the value of two-year-old waste. Six hundred years after dumping, it has decreased to about 10-6 times the two-year value. There are only four radioactive fission.isotopes with half-lives between six years and 60000 years: 85Kr (10 years) has practically disintegrated after 300 years. 90Sr (with its daughter 90Y) and 137Cs (both 30 years) are reduced to 10-3 after 300 years and to 10-6 after 600 years. 151Sm (85 years) has an extremely low disintegration energy; the waste contains only a very small percentage of this isotope.

Radiation and thermal power of all fission products with long half-lives (more than 60 000 years) are many orders of magnitude smaller than those of all other fission products in waste that has been stored for several years. Furthermore, long-lived fission products have almost no radiation other than β-radiation. Future research is necessary as to whether and to what extent such long-lived isotopes, and possibly other isotopes (c.g. 239Pu or 14C), have to be separated and as to how it could be done in the safest and most economical way. The technology of separating and recycling 239Pu, an extremely valuable fissionable fuel, is being developed in view of the increasing importance of breeder reactors. The separate disposal of long-lived isotopes would not raise serious thermal or handling problems; for example, they could be deposited in a highly concentrated form into a deep geological formation

Should the waste be retrievable or not? That is ultimately a philosophical question. Which is more reliable, man or Nature? Should we trust that our descendants will have sufficient knowledge and goodwill to keep the waste safe and not misuse it—or should we rely more on Nature not to bring the waste into the biosphere by unexpected catastrophic events ?

The proposed ice-sheet disposal—be it in deep ice layers or near the surface—avoids the main dangers of both aspects. Under normal glaciological conditions the waste containers are practically irretrievable from the beginning (deep-layer deposit) or after some centuries (near-surface deposit). If, however, a catastrophic climatic change should melt away the ice sheets very quickly, the ablation melts off one after the other of the upper layers while the deep layers still remain cold. Under these circumstances the containers are “self-retrieving”: they come to the surface of (the ice or of the ice-freed bedrock and can easily be picked up. Further research on such a melt-out process and on the durability of the waste containers and their solidified contents should be carried out.