WASTE MATTERS FACT SHEET 19: RADIOACTIVE WASTES

Transcription

1 WASTE MATTERS FACT SHEET 19: RADIOACTIVE WASTES You should have already covered the basic theory of radioactivity in your first year chemistry subject. However, some brief revision is probably appropriate. EXERCISE 1 (a) What is radioactive decay? (b) Give examples of radioactive emissions. (c) What is meant by the term half-life? (d) What are the three ways by which radioactivity can be measured? Health effects of radioactive waste Radiation is invisible and odourless, making it seem more deadly than asbestos fibres or pools of oily liquids. The radioactive emissions are highly energetic, and have the potential to damage cells and genetic material. It is known as ionising radiation, because of its ability to remove electrons from atoms. When this happens in a molecule, bonds break, and the molecules becomes changed. If this molecule is an essential molecule to a cell, such as DNA, then the problems can be severe. The body s ability to repair damage is dependent on how much damage to cells has occurred, and whether it is continuing (acute or chronic exposure). Nor is everyone is affected equally. One of the insidious aspects of ionising radiation is that it does not require ingestion or inhalation like most other toxic substances. Simply being near radioactive material can cause the effects described above, though certainly the problems are reduced if the decaying isotopes are not inside your body. This above effects are purely to do with the radiation generated by radioactive wastes. They do not include the chemical toxicity of some of the components, such as plutonium, which is highly carcinogenic. It is important to realise that there is a constant, natural level of radiation, which normal healthy individuals can cope with. It is when levels rise that sufficient cell damage can occur which the body cannot repair. Table 1 compares exposure doses from a variety of sources. The unit of measure the rem is a measure of the potential damage caused by the radiation, and takes into account not just how much is absorbed, but also how much damage the different particles can cause.

2 TABLE 1 Radiation exposure Source Background radiation Dose (millirem) 80 / year Long-distance air flight 5 Chest X-rays (four) 20 Radon gas from soil and granite Radiation-caused sickness or death Recommended maximum chronic exposure Recommended maximum acute exposure 200 / year 100, , / year 500 / year It has been shown that single exposures of up to 250 rads cause minor health problems, such as vomiting and hair loss, but will not lead to long-term problems for most people. Above this, the problems become more severe in the medium term. Similar initial symptoms result, and while they fade away, they are likely to return after a few weeks, and it is likely that bone marrow where the white cells that fight infection are produced has been irreversibly damaged. The death rate is around 50% for this level of exposure. Higher exposures (around 2000 rads) cause a more rapid death (within 2 weeks) almost without exception, while exposures above 5000 rads cause instant death. For those that survive single acute exposures or experience chronic exposure, there are delayed effects which are much harder to pin down to radiation. The incidence of cancer is increased by increased exposure to ionising radiation. This may not be obvious for many years. Two examples one of acute and the other of chronic illustrate the point. Radioactive radium was used in watches to provide dials that glowed in the dark. The workers who were responsible for painting the material onto the watches showed a very high incidence of bone tumours. Two years after the dropping of atomic bombs on Hiroshima and Nagasaki to end WW2, survivors began showing very high rates of leukaemia and other cancers. This effect continued for at least ten years. Classes of radioactive waste The most common method of classification for radioactive wastes is by the level of radioactivity in that waste. This is measured by the number of decays per unit time (as Bequerels). Each of the classes below has maximum activities for each type of radioactive emission, but they will be listed here because they are not internationally recognised: low-level (LLW) this has radioactivity enough above natural levels to require some special attention before being disposed to landfill; it requires very limited shielding for protection of workers or the public intermediate-level (ILW) these have significant levels of radioactivity and can contain long-half isotopes, but don t generate significant heat or have high enough levels of the most dangerous isotopes (eg plutonium); protective shielding is required high-level (HLW) highly radioactive, with toxic elements and capable of generating large amounts of heat; special high-efficiency shielding is required Just to confuse matters, the widely-used terminology for the first two classes have been changed by the International Atomic Energy Agency to exempt level (for lowlevel) and low and intermediate level (for intermediate level). Sources of radioactive waste Radioactive substances are widely used through commerce and industry, not just by nuclear power generators and weapons manufacturers. Waste Matters 19. Radioactive Wastes Page 2 of 6

3 Low-level waste Low-level wastes accounts for about 90% of radioactive waste around the world by mass but only 1% by activity. It is usually created by limited contact between normal materials and radioactive substances. There are a number of different sources, such as: nuclear power plant operation - contaminated machinery parts, protective clothing or gloves worn by workers, filters, or other materials used to treat radioactive liquids, waste paper, and other materials laboratories at universities and research institutions contaminated test tubes and glass containers, clothing, shoes or gloves worn by technicians and researchers, waste from animals used in experiments and their carcasses, and paper towels and other rubbish medical facilities contaminated syringes, linens, paper products, and waste liquids, as well as protective clothing worn by both hospital personnel and patients from the use of radioisotopes for treatment and diagnosis industrial applications radioactive materials are used extensively to measure the thickness of materials, as catalysts in chemical plants and as tracers in flowing streams The nuclear research facility at Lucas Heights in south-western Sydney, operated by ANSTO (Australian Nuclear Science and Technology Organisation) produces radioactive products for use in Australia, as well as for its own research, in its nuclear reactor. It lists the following as low-level wastes generated on-site: tissue paper disposable gloves plastic tubing solids removed by centrifuging from waste water Intermediate-level wastes These are mainly derived from the nuclear power industry, with materials in direct and prolonged contact with highly radioactive substances. This includes packaging, machinery, pollution-control equipment etc. A distinction between short- and long-lived intermediate wastes is important for the long-term management of the waste. ANSTO lists the following as intermediatelevel wastes generated on-site: cans irradiated in the reactor alumina used in the processing of radioisotopes ion-exchange resins used to purify the reactor cooling water liquid waste from the production of the medical radioisotope technetium-99 High-level wastes These are the wastes left over from the processing and use of radioactive materials in power stations and weaponry. By mass, they are relatively minor, but they account for 97% of the activity in the world s radioactive waste. The radioactivity of high-level wastes will remain dangerously high for thousands of years. ANSTO lists the following as high-level wastes generated on-site: 3-4 spent fuel rods, each containing 280 g of uranium are generated each month 7.5 Assessment of radioactive wastes in NSW The principal legislation for the control of radioactive materials in NSW is the Radiation Control Act 1990 (the Radiation Act) and Regulation (2003). The Radiation Act requires those who use or sell radioactive substances to hold a licence for all high activity sources that, when requiring disposal, correspond to the types of sources classified as Hazardous Waste in these guidelines. The Radiation Act also controls the disposal of such sources. Any person handling radioactive sources in this category requires a Radiation Act licence to do so. The Radiation Control Regulation (RCR) provides a classification system for radioisotopes into four different groups Waste Matters 19. Radioactive Wastes Page 3 of 6

4 (Group 1-4), based on the potential hazard. This division takes into account the half-life, activity and chemical toxicity of the isotope. The NSW Waste Guidelines require assessment of radioactive waste using the RCR scheme. If the specific activity is above a level of 100 Bq per gram or contains radioisotopes specified in the RCR, then the waste is hazardous. If the level is below the limit, then a calculation of the total activity ratio and the specific activity ratio (suing the subdivision of isotopes from RCR) is done. The results of these calculations, together with the nature of the waste (liquid or solid) leads to classification. Where the ratios fall below a certain value, the waste is classified by its other characteristics (see Factsheet 16). Disposal of radioactive wastes Any waste which has radiation levels sufficiently above background need special attention. The level of attention is very much dependent on the level of waste involved. Low-level wastes require little more than careful landfill, while high-level wastes are a problem that still requires a satisfactory solution. The basic approaches are: concentrate-and-contain dilute-and-disperse delay-and-decay EXERCISE 2 What do you think each of these terms means? Which is applicable to the three levels of radioactive waste? Concentrate and contain Low Intermediate High Dilute and disperse Low Intermediate High Delay and decay Low Intermediate High Radioactive waste disposal/storage in Australia Each year Australia produces about 45 cubic metres of radioactive wastes arising from these uses and from the manufacture of the isotopes - about 40 m 3 low-level wastes and 5 m 3 intermediate-level wastes. These wastes are now stored at over a hundred sites around Australia within cities and towns, because no one wants a central repository built near them. The Commonwealth government has tried a number of times to acquire land to build a national facility for all wastes, but has failed because of the NIMBY attitude (Not In My Back Yard). The most recent was a plan, after a major investigation, to use the former missile testing site at Woomera in outback SA as a location for a low-level waste repository. The SA government successfully blocked the purchase in the High Court. The reason: we (as in the people of SA) don t want other people s waste being transported to our state (it s much better sitting in sheds and basements dotted throughout towns and cities across the country). Waste Matters 19. Radioactive Wastes Page 4 of 6

5 CASE STUDY IN STUPIDITY In 1989, the National party government, against much community opposition, built a facility for LLW on the outskirts of Brisbane to replace a shed (an old air raid shelter) in the middle of Brisbane CBD. When the government changed in the 1991 election, the incoming Labour government closed the facility without it ever being used. Presumably the waste continues to be stored in the old shed, which was (is?) poorly lit and ventilated, and had boulders falling on it from an adjacent cliff! Low-level wastes Many countries have built special facilities for the temporary storage of this waste. Once it reaches a low enough activity level (generally 6 months to 2 years), it can be sent to landfill. There are a number of approaches to reducing the volume of the problem, given the nature of the waste. These include: size reduction shredding, cutting or fracturing the materials reduces the overall storage volume required compaction simpler than shredding for materials that are too hard, bulky or even too soft, such as drums and tissues evaporation if the liquid is not radioactive, it can be removed to leave the solid as an easier material to store filtration as with evaporation, but dissolved radioactive substances will remain in the liquid separation more sophisticated separation techniques, such as reverse osmosis, ion exchange and precipitation ANSTO has about 5000 drums of low level waste and about 150 are generated each year. The drums are stored in a dedicated building on racks designed to withstand seismic forces. Over the past few years, ANSTO has been successful in reducing the amount of low-level wastes produced each year. Most of ANSTO's drummed waste is suitable for disposal in a repository for lowlevel wastes. In some cases, the wastes will need to be conditioned, or specially packaged, prior to disposal. ANSTO is carrying out research into the use of cement for conditioning and immobilising such wastes. Intermediate-level wastes These wastes are often treated (to achieve volume reduction) and/or conditioned (waste immobilisation) prior to disposal. A variety of safe and effective treatment options are available including chemical precipitation, incineration and compaction. These may be followed by immobilisation in materials such as concrete, bitumen, or polymers. Disposal methods for treated and conditioned wastes are typically in: shallow concrete-lined trenches engineered surface structures mine cavities deep geological repositories Safe near surface disposal of ILW has been practiced in many countries for over 30 years. Actual facilities include: Barnwell (USA) Vaalputs (Sth Africa) Drigg (UK) Forsmark (Sweden) Morsleben (Germany) Konrad (Germany) The isolation period is usually up to 300 years, thus requiring serious control of the disposal site. ANSTO stores its intermediate-level wastes on-site in the following way. Solid wastes are kept below ground in specially designed concrete pits. Liquid wastes are stored in shielded tanks. A project is underway to solidify this waste by chemical treatment and evaporation. The solid resulting from this process will be suitable for storage for at least 50 years. ANSTO is working on a process to convert this solid into a durable form for indefinite storage or disposal. A number of options are being evaluated to achieve this objective, including incorporation into Synroc or cement. Waste Matters 19. Radioactive Wastes Page 5 of 6

6 High-level wastes Spent fuel rods from nuclear reactors are not completely worn out by their decay, but rather the level of radioactivity has decreased sufficiently to make them less effective in the power generation process. When they are removed from the reactor, they are too hot radioactively and thermally to handle immediately. The normal approach is to store the rods under water for at least one year, before anything else is done with them In Europe and Japan, the rods are reprocessed, which means that the stillactive proportion of the uranium is separated from the decay products, which include plutonium, a substance considered to be the most toxic ever found or produced. Other valuable radioisotopes are also potentially recoverable. Reprocessing is at best economically marginal, and still generates high-level wastes, though in smaller quantities. The disposal of high-level radioactive wastes is a major problem without a perfect solution. Various methods have been proposed, but each is an isolation approach. Remember that it is not possible to destroy this waste. Only time can take away its radioactivity. The most widely used approach is to bind the waste in an impervious glass or rock-like matrix. ANSTO has developed a process known as SYNROC (synthetic rock), which incorporates the waste species into the crystalline structure of the rock. After this, the immobilised waste is sealed in a corrosion-resistant container, such as stainless steel and stored in a remote locality, preferably underground in a geologically-stable area. At least 1,000 years is required for the radioactivity level of these wastes to decrease to levels found in a uranium mine. These storage facilities do not exist at the moment, though many are being investigated around the world. reactor. The heat and radioactivity in the spent fuel decrease rapidly. After about one year, the fuel is transferred to another pond where the ends of each element, which do not contain any uranium, are cut off. The section that contains the uranium is stored under water for a further three to four years until the heat has decreased sufficiently to permit dry storage. As of mid 1997 ANSTO had about 1630 spent fuel elements, most of which are kept in dry storage either in an engineered facility or in storage flasks. The spent fuel storage facilities are checked every three months by a safeguards inspector from the International Atomic Energy Agency. ANSTO's in-ground fuel storage facility consists of 50 holes, 16 metres deep, each lined with a stainless steel tube sealed at each end. Each tube, which can hold up to 22 spent fuel elements, is filled with dry nitrogen gas to minimise corrosion of the fuel. ANSTO stores its high-level wastes on-site in the following way. Every four weeks, three or four spent elements are removed from the reactor, using a specially designed transfer flask, and put into a small pond adjacent to the Waste Matters 19. Radioactive Wastes Page 6 of 6