The ingestion radiotoxicity of an element is a measure of the biological con­sequences of its ingestion. The radiotoxicity is defined as

R(Sv) = Fd(Sv/Bq)A(Bq) (3.138)

where R(Sv) is the radiotoxicity in Sievert per mass unit, Fd(Sv/Bq) is the dose factor in Sievert per Becquerel activity and A(Bq) is the activity. For 1 kg mass

1 32 1019

A(Bq/kg=w^) -г (3Л39)

where A is the atomic mass of the element. The International Commission on Radiation Protection (ICRP) has evaluated the dose factors [65], some of which are given in table 3.7 [66].

Fission products decay by fl radiation, while transuranic elements decay essentially through a radiation. For the same disintegration rate, a emitters are much more radiotoxic than fl emitters, as can be seen from table 3.7, with the exception of 129I which has very peculiar biological properties, with a very high affinity for the thyroid gland.

The use of ingestion radiotoxicity as a measure of noxiousness is subject to question. For example, in the case of underground storage, the probability for the radioactive species to enter the biosphere is of paramount importance. Plutonium and, generally, other actinides, have very low mobility, especially in clay, so that they contribute little to the radiotoxicity released to the bio­sphere. In contrast, elements like niobium, technetium and iodine are very mobile and are, potentially, the chief contributors to radiotoxological release from deep underground storage. Calculations such as those given

in the Appendix show that, indeed, the dominant contribution to the future possible exposition of populations comes from 129I.