18.1.4 General principles

Two basic options exist for the safe, long-term management of unwanted material:

1 dilute and disperse for liquid and gaseous wastes of low hazard

2 concentrate and contain for solid wastes

In everyday life we see the former deployed through chimney stacks and sewer pipes and the latter through landfills. Guidance that is specific to radioactive waste is provided through the Safety Standards of the International Atomic Energy Agency (IAEA). Its Requirements for disposal24 cover a wide range of issues including duties laid upon the government, regulator and WMO. They emphasise the importance of safety and safety assessment in planning and implementing disposal solutions. In general terms they promote an approach that may be described as concentrate and contain until the radioactivity in the waste has decayed to safe levels.

Included in the IAEA Requirements are four that are probably best seen as design objectives. These are the provision of containment, isolation, passive safety and multiple safety functions. Whilst all four are ordinary English words and phrases, in the context of radioactive waste disposal they have specific meanings. Containment relates to the radionuclides in the waste: it may be provided by physical and/or chemical means. There is an expectation that containment will persist until the hazard has been significantly reduced by radioactive decay. In addition, for heat-producing wastes, containment should be maintained whilst heat production is at a level that could adversely affect safety performance. Isolation concerns the waste as a whole and the need to keep it separate from the human environment. This means that the prime considerations are (a) erosion, i. e. exposure of the waste by removal of the covering of rock or soil and (b) inadvertent human intrusion — the possibility, for instance, that humans might drill or dig into the waste whilst, say, exploring for water or minerals. Passive safety is the idea that safety should not be dependent upon human intervention. If at some future date, knowledge of the facility were to be lost, for example, humans and the environment must still be fully protected; this is a fundamental assumption in post-closure safety assessment. Multiple safety functions refers to the need to provide diverse physical and chemical means of achieving containment and isolation. This is a recent development of the older concept of a multiple barrier system. In both cases the longevity of the safety functions or barriers must be commensurate with that of the hazard.

The notion of multiple barriers and safety functions may be illustrated through the design of a hypothetical package for metallic reactor components where atoms lying within the body of the metal have become radioactive through neutron capture. Let us imagine that this waste is cut into pieces, placed into a metal container and cemented into position (‘encapsulated’) using a free-flowing cement grout. The container is then closed with a lid, lifted into position alongside other containers in the repository and backfilled with more grout. This system provides four barriers to radionuclide migration: the waste itself, the encapsulation grout, the container and the backfill grout. The waste and the container constitute physical barriers that work by diverse means. The former must dissolve and the latter must be penetrated by corrosion before radionuclides can be released. The two grout layers provide both physical and chemical containment. Physical containment results from the low permeability of the grout, which reduces the flow of water to and from the waste. Chemical containment acts to retard radionuclide migration through two effects:

1 The high pH conditions (caused by dissolution of cement solids) reduce the solubility of the actinides.

2 The porous structure of the cement provides surfaces on which many radionuclides can sorb thereby retarding their migration.