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APPENDIX IV 121
Indicator materials chosen by the laboratory are usually species
of seaweed and thus not all environments are amenable to monitoring
in this way. F or instance, barren shingle beaches, such as those
around Sizewell, Suffolk, do not provide the conditions necessary for
seaweeds to grow. Fortunately, such areas are relatively rare and
there is sometimes a range of seaweeds from which to choose. The
essential requirements of an indicator m aterial are that it should
be a sedentary species and exhibit high reconcentration factors.
Most commonly found seaweeds exhibit useful concentration factors
for a number of radionuclides found in discharges. The Fucus weeds,
particulalry F. vesiculosus, F. serratus and F. spira lis, grow in
profusion on many parts of the coasts of the United Kingdom and
readily concentrate zinc-54, iron-55, cobalt-60, zirconium -95/
niobium-95, ruthenium-106 and iodine-131. It is from this group
of seaweeds that sampling m aterials for indicator monitoring
program m es are normally chosen.
The best example of use of an indicator material in the laboratory's
work in this present context is provided by the seaweed
F. vesiculosus in the vicinity of Hinkley Point, Somerset. This
station has been at power since 1965 and discharge rates so far
have been low, no contamination having been detected in any of the
potentially critical m aterials. Locally caught fish and shrimps are
the most important in this respect, and also the foreshore which
might becom e contaminated by gamma-emitting radionuclides and
could thus, as a working area of fisherm en, give rise to external
exposure. During 1968 traces of zinc-65, iron-55 and cobalt-60
were detected in the weed.
These data can be used to make a quantitative estimate of
exposure of the critical group if values of appropriate concentration
factors in both critical and indicator m aterials are known. The
seaweed concentrations indicate concentrations in the local water
m ass, from which the actual concentrations in critical m aterials
can be calculated. Consequently, this procedure is not lim ited to
radionuclides found in the indicator m aterial and concentrations of
any constituent in the critical material can be estimated within
reasonable lim its if full data are available on the com position of the
effluent being discharged.
From recent data it appears that at Hinkley Point, Somerset,
the critical nuclide will probably be phosphorus-32 if the present
discharge regim e is maintained, with minor contributions from
chrom ium -51, caesium -137 and antimony-124. This is the first
indication of the identity of the critical radionuclide and though the