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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.