6
R. SCOTT RUSSELL
To sum up, therefore, the basic radiological protection criteria which
should guide the planning of environmental surveys is that the highest
radiation doses which will be received by small groups of any age should
be estimated; this task can be accomplished by measuring the critical
nuclides in the critical foods.
3.
CRITICAL NUCLIDES AND CRITICAL FOODS
The nature of the operations carried out in any establishment determines
both the identity of the critical nuclides and the potential magnitudeofrisk.
The critical foods vary depending on agricultural practices and dietary
habits. In this discussion circumstances in the United Kingdom alone are
considered, and we are therefore concerned with a single broad dietary
pattern, the most important characteristics of which are that about 75 per
cent of the total dietary calctum which is of biological origin is usually
contained in dairy produce, and that milk is particularly important in the
diet of children.
With regard to the system of agricultural production it is of great signifi-
cance that whereas the entire milk supply of a localized community,
particularly in country areas, may come from nearby farms, many other
foods are normally obtained through commercial channels largely from
distant sources. It follows that in a localized emergency, such as might
result through the escape of radioactive materials from a nuclear establish-
ment, the significance of contamination of milk will be enhanced relative
to that of other foods.
The general behaviour of radioactive substances in food chains is now so
well known (see, for example, Refs. 4, 5, 6) that only a brief summary is
necessary :
A.
Release of Mixed Fission Products in Fission Yield
It can rarely be expected that fission products will be released in fission
yield, but this case is an appropriate starting point.
lodine 131 will be the critical nuclide in the early period and, if the
release occurs when cattle are grazing on open pastures, milk will be the
critical food. The thyroid glands of infants fed on fresh milk may receive
doses 20 times greater than those of adults. If deposition occurs during the
winter months, when cattle are receiving stored food, the risk will be very
greatly reduced.
Strontium 89 and strontium 90 are the other critical nuclides in mixed
fission products. The former is the dominant source of concern if the mean
age of the fission products is less than about 6 weeks and the release of
such material in appreciable quantity from a nuclear establishment is highly
unlikely. Milk will again usually be the critical food, though it is possible