Controlled and uncontrolled wastes

Radioactive waste is material that contains, or is contaminated with radio­nuclides at concentrations or activities greater than the clearance levels set by the regulators, and for which no use is foreseen. The hazard associated with radioactive wastes depends on the concentration and nature of the radionuclides with those emitting higher energy radiation or being more toxic to life, being the most hazardous.

Radiotoxicity is the harmful effect of chemical substances as a result of their containing radioactive elements. The effect of ionising radiation emitted by the elements leads to changes in the metabolism and structure of living organisms. It is a measure of how harmful a radionuclide is to health. The type and energy of rays, absorption in the organism, residence time in the body, etc., all influence the degree of radiotoxicity of a radionuclide.

Alpha particles (He atoms) are very strongly ionising, so if they come into contact with atoms in a living tissue they can cause mutations, unusual chemical reactions in the cell and possibly cancer. Although the most ionis­ing, it is the least dangerous form of radiation as long as it is not ingested or inhaled, because it is stopped by, for example, a sheet of paper or skin so that it cannot penetrate into your body. Alpha radiation is most com­monly used in smoke detectors generated by americium.

Beta radiation is made up of an electron with high energy and speed. Beta radiation is more hazardous because it can also cause ionisation of living cells. Although it is less ionising than alpha radiation, it has the capa­bility to pass through living cells and can be stopped by an aluminium sheet. If beta radiation hits a molecule of DNA it may cause spontaneous muta­tion and cancer. It is used industrially in thickness measurement such as in paper mills and aluminium foil production.

Gamma rays are high frequency, very short wavelength electromagnetic waves with no mass and no charge. They are emitted by a decaying nucleus so that it can release energy allowing it to become more stabilised as an atom. Gamma rays have the highest penetrating power, only being stopped by a few centimetres of lead or a few metres of concrete. They are the least ionising of the radiations but this does not mean that they are not danger­ous. Gamma rays are likely to be emitted alongside alpha and beta radiation but some isotopes only emit gamma radiation. Gamma rays are useful because they can kill living cells and so be used to sterilise by, for example, destroying harmful bacteria. Gamma rays are also used in radiotherapy to kill off cancerous cells. They are also used to sterilise medical equipment, which is particularly useful in tools that would be melted by heat sterilisa­tion or compromised by bleaches and other disinfectants.

Radioactive waste is accompanied by significant levels of radiation, hence it requires not only immobilisation to prevent radionuclides spreading around the biosphere, but also shielding and, in some cases, remote handling. A waste with activity concentrations equal to, or less than, clear­ance levels is considered non-radioactive. Radioactive wastes are either controlled or uncontrolled.

Controlled wastes are largely a product of the nuclear fuel cycle (NFC) used to generate electricity for civil use (Fig. 1.1). Wastes are generated during ore mining and processing to access the uranium metal or oxide, its enrichment and synthesis into fuel (the front end of the NFC), the operation and running of the reactor (operations wastes) and from fuel removal, treat­ment and disposal (the back end of the fuel cycle). Front end waste is contaminated basically with naturally occurring radionuclides, whereas operational waste also contains fission and activated products (typically low level waste (LLW) and to a lesser extent intermediate level waste (ILW); these are defined in Section 1.3). Front end wastes include contaminated mining wastes and uranium hexafluoride tails from enrichment. Opera­tional wastes include spent filters and ion exchange resins, evaporator



1.1 Sources of radioactive waste (adapted from Ojovan and Lee, 2005). HLW = high level waste, ILW = intermediate level waste, LLW = low level waste, SRS = sealed radioactive sources.

concentrates and absorber rods. Back end wastes include sludges from storage ponds, typically cemented ILW and vitrified HLW from reprocess­ing or spent fuel if direct disposal is planned.

During the early part of the nuclear era, consideration was not given to disposal of radioactive waste. As a result some NFC wastes (now termed legacy or historic wastes) are ill-characterised and stored under conditions which are far from ideal. They comprise a vast range of materials, e. g. Pu — contaminated materials (PCM) such as paper, wood and plastics, fuel clad­ding, damaged and corroded fuel elements, old tools and equipment and assorted test samples often mixed together. Sometimes these have been stored under water and have degraded over time to form complex sludges and supernatant liquids.

Controlled non-NFC wastes include those from various applications of radionuclides in research, medicine and industry including spent sealed radioactive sources (SRS) of isotopes used in medical applications.

Uncontrolled wastes arise when unexpected events occur or where the level of care was not taken that would be expected today. At Hanford, one of two sites where the US stores its military (defense) wastes, poorly char­acterised highly active sludges were stored in massive single shell steel tanks


that eventually leaked (Fig. 1.2a). At Sellafield in the UK, some materials were stored in inappropriate open ponds (Fig. 1.2b) where ingress of atmos­pheric (salty) rain and organic matter (bird droppings, etc.) has added to the complexity of the problem. Uncontrolled wastes also arise from acci­dents such as at Chernobyl, Ukraine (Fig. 1.2c) and Fukushima, Japan. The financial cost of cleaning up such sites and others where accidental releases of radioactivity have occurred, such as in Fukushima, is enormous. Nonethe­less, the nuclear industry is now developing smart clean-up programmes and concepts (Fig. 1.3) and the knowledge gained from these mistakes has helped us be more proactive in dealing with uncontrolled waste.


(a) (b)

1.3 The Chernobyl NPP site (a) in 1986 soon after the accident and (b) a current view with protective sarcophagus in place.

Nonetheless, armed with sufficient resources, the results of decades of inten­sive research and international support progress can be made (Fig. 1.3).

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *