Radioactive waste is divided into three broad categories:

This section deals principally with the last category which is released into the environment.

There are five main sources of radioactivity to the environment and the table linked below summarises some of the most common radionuclides in relation to these sources.

Primordial and natural sources. The Earth=s crust contains primordial and natural radioisotopes, such as uranium and thorium, which may produce radioactive decay products. Cosmic radiation entering the Earth=s atmosphere replenishes the Earth's supply of radionuclides (Kennedy et al 1988).

Nuclear weapons testing. Testing and use of nuclear weapons in the atmosphere has occurred since the Second World War. Nuclear explosions result in the presence of fission products, including man-made radioisotopes, in the atmosphere and on the Earth=s surface. Releases into the atmosphere have resulted in widespread contamination of the soils and oceans because of atmospheric circulation and fall-out.

Nuclear power generation. The generation of electricity from nuclear power stations results in low level discharges of radioactivity to the atmosphere, low level radioactive liquid effluent discharges to surface waters and the generation of solid radioactive waste from the normal functioning of the power stations. There is also the potential for uncontrolled releases of radioactivity from accidents, such as Chernobyl (1986).

Nuclear reprocessing industry. The use of nuclear fuel in power stations and in other uses generates spent nuclear fuel and solid radioactive waste that is stored or reprocessed at nuclear reprocessing installations. Two such installations exist in the UK: Sellafield (England) and Dounreay (Scotland). These installations also give rise to low level discharges of radioactivity to the atmosphere, low level liquid radioactive effluent discharges to surface (principally marine) waters and solid radioactive waste. There is also a risk of uncontrolled releases of radioactivity from accidents, such as at Windscale (Sellafield) during 1957.

Various military, industrial, medical and research establishments. Radioactivity is also released into the environment from various smaller sources in the form of low level atmospheric and liquid discharges and potentially from accidental releases. These include:

• military establishments where nuclear weapons are located or nuclear powered vessels are based;
• industrial providers of radioisotopes for medical, industrial or research use (for example Amersham International plc in the UK), and users of radioisotopes including hospitals and research establishments.

With the exception of primordial and natural sources, all the above are point sources of radioactivity into the environment. However, in terms of entry to the aquatic ecosystem, there is a combination of point and diffuse sources. Discharges of low level liquid effluent discharges to surface waters can be considered as point sources. Discharges of low level discharges and accidental releases to the atmosphere can result in a widespread distribution due to atmospheric circulation, such that fall-out to the aquatic environment is effectively a diffuse source of contamination.

The principal diffuse source of radioactivity to the aquatic environment is from atmospheric fall-out and the main point source is from the nuclear reprocessing industry. Estuarine systems, in particular, are sinks for organic matter from both freshwater and marine origins and, as such, accumulate radionuclides that are associated with organic matter. They are also very productive and act as a nursery and feeding area for fish, birds and macro-crustaceans. As such, there is a pathway for accumulated radionuclides to enter the food web where potential impacts may occur and possibly result in the exposure of Mankind to this source of radioactivity.

The responsibility for monitoring levels of radioactivity in the marine environment lies with the Environment Agency in England and Wales, SEPA in Scotland and the Environment and Heritage Service in Northern Ireland. In addition, the competent authorities for food safety (MAFF, Scottish Executive and DANI) are responsible for monitoring radioactivity in food organisms (including algae, shellfish and fish). MAFF publish an annual report jointly with SEPA on >Radioactivity in food and the environment= (i.e. MAFF 1998) that summarises the results of government surveillance. The main dischargers (i.e. British Nuclear Fuels at Sellafield) also monitor their own discharges and report the results annually (i.e. BNFL 1997).

Radionuclides are found in measurable quantities in the water column, suspended sediments, sea-bed sediments and the biota (Kershaw et al 1992).

Kennedy et al (1988) reported studies where detectable levels of ¹³⁷Cs were found in sand flats, Arenicola sand flats and coastal embayments and saltmarshes of the Solway estuary and levels of ¹³⁷Cs and ²⁴¹Am in the Ravenglass estuary (see table below) .

Kershaw et al (1992) reported studies in the Esk estuary where detectable levels of ¹³⁷ Cs, ¹⁴⁴Ce, ¹⁰⁶Ru, ⁹⁵Zr, ⁹⁵Nb and Pu were found. Radionuclides from Sellafield have also been found in the Wyre estuary and in the water column and sediment of the Ribble estuary (Kershaw et al 1992).

Radionuclide accumulation in saltmarshes is controlled principally by the physical processes associated with tidal flow and sediment deposition (Horrill 1983), but the type of vegetation present also has an effect on accumulation rates - vegetated areas accumulate radionuclides, such as americium, caesium and plutonium at faster rates than unvegetated areas. A large number of other factors can also affect accumulation rates, to the extent that variability within and between different saltmarshes can be wide. However, the relative stability and high biological productivity of saltmarsh sediments (away from tidal channels) favours the accumulation of plutonium and caesium isotopes, with highest activities often being associated with fine-grained mud flats, such as those in the Solway Firth (Kennedy et al 1988)

Some radionuclides have been found to accumulate in the biota. In particular, benthic algae, molluscs (mussels, winkles, limpets, whelks, scallops, queens), crustacea (crab, lobster, Nephrops, shrimps) and fish (including plaice, cod, flounder, herring) have been found to accumulate some radionuclides based on monitoring information collected by MAFF in the Irish Sea (Kershaw et al 1992). The principal concern has been to determine the risk to the human population and so the fish and shellfish species selected for monitoring have been commercially important ones. These species have been found to accumulate a number of radionuclides but the most important appear to be ¹⁰⁶Ru and ¹³⁷Cs. Both have been found to accumulate in fish muscle (plaice) and in crab Cancer pagurus hepatopancreas and muscle tissue. Crabs were found to accumulate ¹⁴⁴Ce and ⁹⁵Zr/⁹⁵Nb in addition to ¹⁰⁶Ru and ¹³⁷ Cs. The most significant uptake route for these species is believed to be via the diet.

The table below summarises pre-Chernobyl levels of ¹³⁷Cs in birds collected from coastal sites in and around Cumbria.

The quantities of some of the shorter-lived fission product nuclides discharged to UK coastal waters, such as ⁹⁵Zr and ¹⁰⁶Ru, have declined since the early 1970s, whilst discharges of ¹³⁴Cs and the longer-lived ¹³⁷Cs reached peak values during 1974-1978, since when they too have declined. Discharges of ²⁴¹Am peaked between 1971 and 1975, and of ²⁴¹Pu between 1970 and 1980. Further reduction in Am and Pu discharges have occurred since 1994, in contrast to ⁹⁹Tc, ¹²⁹I, ⁶⁰Co and ¹⁴C, the quantities of which have increased.

The fate and behaviour of radionuclides in the marine environment is determined by the fate and behaviour of the element concerned. For example, if an element is adsorbed to sediment particles, then the radionuclide of that element will behave in the same way.

The radioactive elements will not be destroyed in the environment and radioactivity will be emitted from whatever compounds are formed with the element. The duration that the energy will be emitted is governed by the half-life of the radionuclide which can range from hours to hundreds of years.

There are a number of important factors that determine the environmental effects of radionuclides. Radioactivity is a form of energy released from radioactive elements and the potential for damage depends on the amount of energy absorbed by an organism. In radiation risk assessments, the amount of energy absorbed is termed the absorbed dose (measured in Grays (Gy)). Factors affecting the absorbed dose are the identity of the radionuclide, the type of radioactivity, the chemical form of the radionuclide, the exposure pathway to the organism and the biochemistry of the organism.

There are a number of different forms of radiation, including alpha and beta particles, gamma and x-rays each with different levels of energy. Radionuclides emit some of these forms of radiation in different proportions over different lengths of time (related to the half-life of the radionuclide. In order to compare the absorbed dose from different radionuclides, the estimate in Grays is commonly (though not always) converted by a quality factor to a dose equivalent (measured in sieverts (Sv)). Effectively, this takes into consideration the different biological effects of different types of radiation.

Polikarpov (1998) proposed a conceptual model of radiation effects in the environment, relating dose rates to effects at the individual, population and community level. The model comprises four zones:

1. Radiation well-being zone: natural background levels of radiation up to a dose rate of 0.005 Gy yr^-1;
2. Physiological masking zone: where minor radiation effects at the individual level occur between 0.005 Gy yr^-1 to 0.05 Gy yr^-1.
3. Ecological masking zone: where effects of radiation at the population level have been detected between 0.05 Gy yr^-1 and 4 Gy yr^-1.
4. Damage to ecosystems zone: where community level effects (reduction in the number of organisms, elimination of radiosensitive species and impoverishment of communities) have been detected at concentrations above 4 Gy yr^-1.

This model is not confined to the marine environment but has been developed using responses in the marine, freshwater and terrestrial environments.

The most detailed study of potential environmental effects of radioactivity has been the investigations into the impacts of the Sellafield discharges on the marine environment (summarised up to 1992 by Kershaw et al 1992).

While it must be assumed that any exposure to radiation carries some risk of harm, for marine organisms, if the damage to individuals is not manifest at the population level, and does not damage the overall reproductive capacity of the population, then the effect may be regarded as being of little significance (Kershaw et al 1992). In a comprehensive review of radiation effects reported in Kershaw et al (1992), the lowest dose rate at which minor radiation induced disturbances of physiology or metabolism might be detectable was about 400 mSv hour^-1. The dose rates around Sellafield were at least an order of magnitude below those which would be expected to elicit effects under controlled laboratory conditions and about two orders of magnitude below those which might be expected to have an effect at the population level during the period of maximum discharges (Kershaw et al 1992). There have been no conclusively demonstrated effects at the population level of the radioactive discharges from Sellafield on the marine environment.

An exhaustive literature review on the effects of radioactive substances on marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Kershaw et al 1992 and Parrett 1998).

Parrett (1998) considered the following issues in a consideration of the effects of radioactivity on North Sea fish stocks:

• lethal effects;
• effects on reproductive success;
• genetic effects.

Studies reported in Parrett (1998) indicated that the range of lethal levels in adults of different species of fish was in the range 3.75 to 100 Gy, and for invertebrates ranging from 0.2 to above 500 Gy. Earlier developmental stages have been identified as more susceptible and mortality of fish embryos has been shown to occur at about 0.16 Gy.

The effects on reproductive success in fish that have been demonstrated include sterility, reduction in counts of primordial germ cells and reduced testicular weight. The lowest dose rate at which effects of chronic radiation exposure on fertility of aquatic invertebrates and fish were demonstrated was about 0.25 mGy hour^-1 (Parrett 1998). The implied mechanism for these effects was damage to germ cells and the induction of dominant lethal mutations in gametes.

Mutation rates increase in relation to radiation exposure and so therefore does the chance of deleterious mutations occurring. While natural selection will act to keep these mutations at low level in the gene pool, some expression might occur in the short-term (in the form of sterility or dominant lethal mutations) or in the long term (in the form of >genetic disease.=)

Despite these types of effects being demonstrated in laboratory conditions, there is no evidence of the consequences of this expression at the population level in fish or macro-crustaceans. Dose rates in the order of 10 mGy hour^-1 are considered acceptable for the protection of aquatic populations. This assumes some damage to individuals but not to the extent that this would affect the population as a whole (Parrett 1998).

Little is known of the processes involved in radionuclide uptake and retention to be able to predict those species which will be most efficient at accumulating environmental radioactivity. However, a number of generalisations can be made:

• Reproductive stages and growing tissues are the most sensitive to radioactivity, notably the eggs of marine fish (Kershaw et al 1992).
• Like more typical pollutants, such as persistent organics and heavy metals, radioactive isotopes can be bioaccumulated, both within primary producers and by uptake through the food chain.
• Bacteria, fungi and some lichens tend to be relatively tolerant to radioactivity.
• Amongst fauna, mammals appear to be the most sensitive, followed by birds, and then insects.
• Environmental radioactivity is not known to have produced deleterious effects in the growth patterns of plants and animals, so radioactive isotopes are probably less important than many of the other contaminants listed in this document.

The lowest dose rate at which minor radiation-induced disturbances in physiology or metabolism might be detectable is about 400 µSv.hour^-1 (IAEA 1976), approximately an order of magnitude greater than the dose rates measured around Sellafield (Kershaw et al 1992) which is the largest radionuclide source in the British coastal environment. Despite this, wading birds and their prey are potential accumulators of radionuclides, so a precautionary approach is desirable. The following nuclide concentration factors have been estimated for a range of marine biota (see table below from Preston and Jeffries 1969):

Potential effects include:

• accumulation of radionuclides in sediments (particularly in estuaries) and in biota;
• exposure of organisms to ionising radiation at dose rates greater than background levels (if a precautionary approach is adopted).

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