|NEA Issue Brief: An analysis of principal nuclear issues|
|No. 3, January 1989|
The term high-level radioactive waste (HLW) generally refers to the highly radioactive wastes requiring permanent isolation from man's environment that arise as a byproduct of nuclear power generation. In countries where the spent nuclear fuel arising from reactor operations is chemically reprocessed, the radioactive wastes include highly concentrated liquid solutions of nuclear fission products. These are later solidified, generally in a glass matrix in a process known as vitrification, although other solidification processes are possible. Both the liquid solutions and the vitrified solids are considered HLW. If the spent nuclear fuel is not reprocessed, it, too, is considered as HLW to be disposed of by appropriate means (see reference 5). Because HLW contains relatively high concentrations of both highly radioactive and extremely long-lived radionuclides, special disposal practices are needed. Although the relative amount of HLW is small with respect to the total volume of radioactive waste produced in nuclear power programmes, it contains 99% of the radioactivity in this volume. Furthermore, it takes about 10,000 years for the radioactivity of such wastes to decay to the level which would have been generated by the original ore from which the nuclear fuel was produced, should this ore never have been mined.
Although certain reprocessing wastes and spent fuel are almost invariably considered the only sources of HLW, there are other waste types that, because of their level of radioactivity, may require a similar degree of isolation from man's environment, and therefore should be borne in mind when discussing radioactive waste disposal options. By far the most important of these other waste types is generally referred to as alpha-bearing wastes (also called transuranic (TRU) waste) because of its relatively high concentration of long-lived radionuclides that emit alpha particles as they decay. Indeed, these wastes are produced in volumes greater by a factor of 5-10 than HLW. A main difference between such wastes and HLW, however, is that TRU waste does not generate intense levels of radioactivity and heat.
HLW, whether spent nuclear fuel or vitrified reprocessing waste, generates such intense levels of both radioactivity and heat that heavy shielding and cooling is required during its handling and temporary storage. The wastes are therefore best stored in specially engineered cooling pools or vaults for several decades prior to disposal. While stored, both the temperature and radioactivity of the wastes gradually decrease, simplifying their handling and disposal considerably.
Storage cannot be relied upon in the long-term to provide the necessary permanent isolation of the wastes from man's environment, and future generations should not have to bear the burden of managing wastes produced today. Seen from this perspective, while disposal of HLWis not an urgent technical priority, it is nevertheless an urgent public policy issue. These political aspects have led to the need for the nuclear industry in recent years to demonstrate the feasibility and safety of HLW disposal and, in some countries, laws have been implemented that require operational HLW disposal capability in the next 15-50 years. In particular, the Federal Republic of Germany and the United States plan to begin disposing of HLW in the early 2000s, France by about 2010, Belgium, Canada, Finland, Japan, Spain, Sweden, and Switzerland by about 2020, and the United Kingdom somewhat later. All HLW produced so far is currently being stored; no permanent disposal has yet occurred.
Among the options discussed for disposing of HLW, an international consensus has emerged that deep geological disposal on land is the most appropriate means for isolating such wastes permanently from man's environment (see references 1-3). However, the full range of options also includes disposal in geological formations under the deep ocean floor, disposal on the ocean floor, disposal in glaciated areas, extraterrestrial disposal, and destruction by nuclear transmutation. In addition, extended storage, whether at production sites or in a centralised store, may, in principle, be considered an acceptable waste management strategy, provided it is not supposed to be perpetuated for longer than feasible and safe and is to be replaced by a more permanent solution at a later date.
The basic requirement for any geological formation is its ability to contain and isolate the radioactive wastes from man's environment until the radiotoxicity of the wastes has decayed to non-hazardous levels. In order to increase the safety of geological disposal, most such disposal concepts rely on a system of independent and often redundant barriers to the movement of radionuclides in an effort to provide a high degree of assurance that exposures to man will remain at acceptably low levels. These barriers generally include (1) the leach-resistant waste form itself, (2) corrosion-resistant containers into which the wastes are encapsulated, (3) special radionuclides- and groundwater- retarding material placed around the waste containers, commonly referred to as backfill, and (4) the geological formation itself -- the principal barrier -- which should both retard the transport of radionuclides in circulating groundwater, and isolate the waste from man's environment.
There are five important reasons why deep geological disposal on land has evolved into the disposal method of choice for virtually every country with a nuclear power programme.
Disposal in geological formations under the stable, deep ocean floor, also called subseabed disposal, is conceptually similar to deep geological disposal on land, but there are a few notable differences. Whether the waste is emplaced in the relatively soft near-seabed unconsolidated sediments, or in the underlying consolidated sediments or even deeper basalt, the emplacement technology is not entirely defined. A major difference, however, would be the enormous dilution capacity provided by the ocean, should the containment system prematurely fail and allow substantial releases of radionuclides to the ocean floor. Another significant difference is that this disposal would be ideally suited for the establishment of international cooperative activities, although using the high sea, which is common property, represents a major political complication. Nonetheless, subseabed disposal is currently the only other disposal option under serious consideration as an alternative to deep geologic disposal on land.
With regard to other disposal options that have been discussed in OECD countries, disposal of HLW on the ocean floor in some kind of highly engineered containment would not be internationally acceptable at this stage. Disposal in glaciated areas, in Antarctica for example, would require substantial changes to international legal and political agreements. Disposal into space would provide the greatest degree of isolation from man's environment, but its practicality, cost, technological complexity, and potential risks all argue against it at the moment. Finally, nuclear transmutation, the conversion of long-lived radionuclides into shorter-lived or even stable nuclides, is not considered feasible in the near future.
The remainder of this paper will address only deep geological disposal on land, as this is currently the preferred disposal option throughout the OECD countries and, indeed, worldwide.
Disposal of HLW is preceded by some period of interim storage, either on site or at a centralised location, during which time the temperature and radioactivity of the HLW decrease systematically. Movement of the wastes to the disposal site will be necessary, and this may be accomplished using specially constructed collision- and fire- resistant shipping casks, transported via designated ship, train, or truck, according to national circumstances. Finally, special waste packaging is envisaged in most disposal concepts, either at the disposal site or at some interim site. It should be noted that liquid HLW must be solidified prior to its transport, packaging, or disposal.
During disposal, the individual waste packages will be lowered down shafts or transported into the repository through sloping tunnels. Once at the repository, the waste packages will be emplaced into holes predrilled into the sides or floor of the repository using equipment developed for this purpose. In most concepts, these holes will then be backfilled with suitable material. Filling the repository may require anywhere from 10 to 50 years or more, depending on the individual nuclear programme. Finally, the repository itself will be backfilled and sealed, including all shafts, boreholes, and tunnels which may have been drilled during repository construction. With a suitable choice of waste packaging, backfill, and geological environment, the radioactive materials should remain isolated from man's environment for many tens of thousands of years at least.
The long-term safety of HLW disposal can be systematically assessed through predictive modelling of the gradual failure of the engineered barriers (i.e., the waste form, waste package, and backfill) and the subsequent transport to man's environment of radionuclides by circulating groundwater. Such safety assessments must be based on a good physical understanding of the processes involved in the release and transport of radionuclides, as well as those acting on, or likely to act on, the repository and the geological formation. In addition, the potential interplay between these processes must be understood (see reference 4). Finally, substantial site investigation efforts will be needed, involving the collection of data at the surface as well as in situ, at the proposed repository location.
As a preliminary step to in situ studies, several OECD countries have developed Underground Research Laboratories in representative geological environments to demonstrate the safety of the geological disposal option. These laboratories are being used to provide data in support of generic safety assessments, to evaluate engineering feasibility, and to develop and refine techniques for site investigation. Laboratories in Belgium (Mol), Canada (Lac du Bonnet), the Federal Republic of Germany (Asse), Sweden (Stripa), and Switzerland (Grimsel) have been the focus of major international or bilateral cooperative research programmes (see Reference 7).
There are many parameters which affect the cost of HLW disposal, most importantly the size of the nuclear programme. Other parameters of concern that may vary from one estimate to the next are the depth of burial, the length of time the HLW cools prior to emplacement, the type of waste package used, the need to design for waste retrievability, and whether the disposal involves spent fuel or vitrified reprocessing wastes. Despite variations in these parameters, most estimates conclude that the cost of deep geological disposal will represent only a few per cent of current electricity generating costs.
According to the "polluter-pays" principle, the cost of HLW disposal is properly financed by the nuclear utilities. In many countries, a special waste fund has been established to cover the cost of HLW disposal according to which the utility may pay either an annual fee or an amount that in some way corresponds to the relative amount of HLW produced. The establishment of such a fund is important because disposal of HLW will not occur until many years - several decades in most cases - after its production. Whatever the system of financing, a general principle is that future generations should not have to pay for disposal of the wastes generated today.
The NEA has been concerned with the problem of high-level waste disposal for more than a decade and this topic has developed in recent years into a priority area of its programme. Its principal role is to assist its Member countries in the further development of methodologies to assess the long-term safety of radioactive waste disposal systems and to increase confidence in their application and results. This is done through the exchange of information and experience among national experts, joint studies of issues important for safety assessment (identification of potentially disruptive events, treatment of uncertainties), the development of related computer models (in particular for probabilistic events) and data bases (used to assess the behaviour of radioactive materials in the geosphere), and their validation at an international level.
The NEA also sponsors international research and development projects (the Stripa Mine Project, in Sweden, and the Alligator Rivers Project, in Australia) and co- ordinates activities of its Member countries involving in situ research and site investigations (see reference 6). It ensures that working links are maintained at the international level between performance assessment projects, field projects and underground research laboratories, through specialised working groups operating in the Agency's framework. The ultimate purpose of this international effort is to reach the level of scientific understanding required to ensure that nuclear waste disposal systems will be able to contain and isolate the radioactive materials so that they will not cause any harm to man or his environment either now or in the future. Such co-operative programmes are also aimed at enhancing confidence in the quality of the safety analyses upon which the acceptability of nuclear waste disposal is to be judqed.