HKNIC - High level waste: alternative solutions
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High level waste: alternative solutions

Since the quantity of high level waste or HLW produced by nuclear power generation is very small, it is feasible to isolate it in a stable geological formation for several thousand years until its radioactivity has fallen to natural levels. Nevertheless, the very long term storage of HLW has generated regulatory and monitoring issues, as well as public and political controversies, in some countries. As a result, other methods of long term HLW management are being explored in order to reduce the required storage time.

HLW is a mixture of numerous substances. The bulk of HLW is made up of fission products, which are the lighter elements produced after uranium has undergone nuclear fission. A much smaller amount belongs to a group called actinides – a sub-section of the periodic table that consists of 14 elements including uranium, neptunium, plutonium, americium and curium. These are produced when uranium and its products absorb neutrons to become other actinides in a process known as “transmutation”. In turn, these actinides undergo further transformation or decay to become different elements. The radioactivity of some of these elements declines faster than the others. Actinides are the primary source of the long term radioactivity in HLW.

High level waste: alternative solutions

Source : IAEA, 1992 Radioactive Waste Management

 

Since the radioactivity of fission products other than a very few nuclides such as technetium 99 and cesium 135 declines to natural levels typically over only several hundred years, research is underway to separate out the long-lived fission products and transmutation products in HLW, in a process called “partitioning”. The remaining HLW constituents could then be stored for a much shorter period of a few hundred years, and only the much smaller quantities of long-lived constituents would need longer term storage.

 

Research is also being carried out on transmuting some of the long life transmutation and fission products by nuclear bombardment into less radiotoxic, shorter life products which would require a far shorter period of storage.

Complete transmutation could be carried out in a repeated process of partitioning and transmutation in a fast neutron reactor or a particle accelerator-driven device with high-energy neutron bombardment.

 

Recently, scientists have developed the idea of making use of particle accelerator and fission reactor technologies to produce electricity and transmute long-lived HLW into radioactive materials with shorter lives, so reducing the period required to isolate the waste from the environment.

An accelerator-driven device relies on a high-energy proton beam produced in a particle accelerator hitting a metal target to produce high-energy or “fast” neutrons. The neutrons will cause fission in the nuclear fuel as well as transmute long-lived transmutation and fission products into other types of nuclides that ideally will have shorter lives, and through a repeated process of partition and transmutation, the amount of long-lived radioactive material can be minimised.

The concept of partition and transmutation is attractive since it will shorten the length of time required to store HLW. Commercial application of the technology is not likely to materialise in the near future, however, since it requires new reprocessing techniques for the effective separation of the various elements so that stable nuclides are not returned to the reactor where they would be be transmuted into radioactive ones. The repeated partition and transmutation process would also require a long operating cycle to achieve its objective of a substantial reduction in radiotoxicity, which would require a long term political, social and economic commitment.