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APPENDIX IV 1 15
Dounreay. Contamination of critical m aterials is also easily
measurable at Bradwell and Trawsfynydd nuclear power stations,
even though radiological exposure is lower than for Windscale.
These examples are among the best for monitoring schem es, and
those for the power station discharges illustrate two quite different
environments, the discharges being made to saline water from
Bradwell, and to fresh water from Trawsfynydd.
UKAEA W indscale,'Cumberland
This site contains the principal facilities of the United Kingdom
Atomic Energy Authority for reprocessing spent fuel, and waste
from .this plant is effectively responsible for the whole of the radioactivity
in the effluent; the reactor operations of the Calder Works
and the AGR development make nonsignificant contribution. Effluent
com position has been discussed by Howells and, though it is com plex
and subject to variations, four nuclides dominate the control
situation — ruthenium-106, zirconium -95/niobium -95 and, occasionally,
cerium -144.
Critical group of laverbread consumers
The critical group are eaters of laverbread, a foodstuff manufactured
from the edible seaweed of the genus Porphyra and eaten
in South Wales. The critical nuclide is ruthenium-106, although
ceriu m -144 som etim es occurs to an extent which is just significant;
it is then also considered. The critical organ is the GI tract, and
exposure to other organs from the combination of radionuclides is
relatively small.
The seaweed is harvested regularly over m ore than 30 m iles of
the Cumberland coast and concentrations of radioactivity in it are
significant over this whole area. The relative importance of
different collection centres varies: in an unpredictable manner and,
because of this, a chain of eleven sampling points is maintained
which spans all the important collection areas. Measurements
from these samples are sufficient to predict concentration elsewhere,
and the relationship between discharge rate and concentration of
ruthenium-106 in Porphyra has been well established by this type
of monitoring during recent years^
Samples are collected at monthly intervals, and analysis,
prim arily for ruthenium-106, zirconium -95 and niobium-95, is
carried out by gam m a-spectrom etry. Cerium-144 can be detected