Safety_Series_041_1975 - gnssn - International Atomic Energy ...

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This publication is no longer valid Please see http://www.ns-iaea.org/standards/ 118 APPENDIX IV of the discharge point are caesium -134 and -137 and ruthenium-106, with sm aller amounts of strontium-90. The maximum degree of exposure to fishermen from eating this fish is less than 0.2% of the derived working limit. Bradwell nuclear power station This was one of the first com m ercial nuclear power stations in the United Kingdom and is now well established, although the effluent com position has not yet quite reached equilibrium. The Blackwater Estuary, which receives low -level liquid radioactive waste from the station, is well known for its oyster fisheries, whose operation sets a practical lim it on discharges. The critical group for this discharge is to be found among the oyster fishermen them selves, who eat the local product in larger quantities than anyone else. This is a common feature of critical groups where locally caught fish or shellfish is the critical material. Other potential exposure routes are of trivial significance: no other shellfish or fish is collected in important quantities, and use of the foreshore is insufficient to make external exposure important. The critical radionuclide for oyster consumption is zin c-65, although others are detectable to a sm aller but just significant extent — particulary cobalt-6 0, iron-55 and phosphorus-32. This is a cla ssic case of a minor constituent of the discharge being the critical radionuclide due to a very high reconcentration factor — in this case 105. Basic monitoring to establish that control of discharges is adequate requires a minimum of effort and consists principally of monitoring the critical material, oysters, for zinc-6 5. Two species of oyster are farmed in the estuary, one of Portuguese origin (C rassostrea angulata) being cultivated in addition to the native species (Ostrea edulis). Research has shown that the highest con centrations of zinc-6 5 and also other radionuclides in oysters are found at the nearest com m ercial bed to the power station, where only the native species is cultivated. Higher concentrations are assimilated at a second site of special layings in the immediate vicinity of the outfall on the B arrier Wall, but these oysters are not sold for human consumption; they were laid for the deliberate purpose of indicating the maximum conceivable concentration in any oysters in the estuary. The site is very poor for maintaining oysters in a healthy, growing, condition and layings cannot be left long enough to attain biological equilibrium with zinc, but experience has shown that the basic aim of the layings is regularly achieved.
This publication is no longer valid Please see http://www.ns-iaea.org/standards/ APPENDIX IV 119 Both sites are sampled quarterly for the native species, the Portuguese species also being collected at the B arrier Wall position. Routine analysis consists of a gross beta assay followed by gammaspectrom etry for zinc-65, which also shows the presence of some other nuclides. At the present low degree of contamination the gross beta assay is predominantly due to natural radioactivity — potassium-40 — although phosphorus-32 has made a significant contribution on occasions, particularly in B arrier Wall samples. Specific analysis has to be made by chem ical separation to verify the importance of most of the various radionuclides present, although much of the information has been produced prim arily for the laboratory's radioecological research program me in the estuary. Trawsfynydd This nuclear power station occupies a unique position in the United Kingdom since it is the only such unit discharging its low - level radioactive waste to fresh water — Lake Trawsfynydd — which is also the source of the station's cooling water. This has posed new environmental problem s in waste control, although the principles of application are the s!ame as those for sites discharging their liquid wastes to saline waters. The critical route to public exposure is by consumption of fish from Lake Trawsfynydd, which thus involves only internal exposure. At the present stage in the station's operational history the critical radionuclides are caesium -137 and caesium -134, to the extent that all others have so far been of trivial significance, although the situation could still change since the station has not yet reached operational equilibrium. However, work by Preston, Jefferies and Dutton has shown that reconcentration factors for the caesium isotopes are very large (about 4 X 103), so that they are likely to remain the critical radionuclides unless the relative discharges of, for instance, phosphorus-32 (which displays sim ilarly high r e concentration factors) should increase considerably. The lake is noted for trout, which is the principal species fished, but a small minority of the critical group, who are essentially local anglers and their fam ilies, eat perch. Both species are normally sampled at quarterly intervals and specific analysis for caesium isotopes is the m ost important measurement, although a gross beta assay is carried out as a routine. This would easily show any significant quantities of beta emitters such as phosphorus-32.
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