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4. IMPACT OF P&T ON RISK ASSESSMENT AND WASTE MANAGEMENT
4.1 Introduction and definitions of radiotoxicity and risk
4.1.1 Risks in the back-end of the fuel cycle
In speaking of radioactive waste management strategies, one frequently mentions (1) the risks
associated with each strategy, and especially those inherent in it, (2) the extent of these risks, and (3) the
radiotoxicity of the waste.
While the definition of the risks merely demands precision of language, risk assessment is very
difficult. The term “radiotoxicity” is often qualified, without clarification, as “potential” or “residual”,
and it is often confused with risk. These terms must, therefore, be explained.
According to the nomenclature of the World Health Organisation, risk is a quantified
evaluation of the danger, and is expressed in terms of probability. Hazard is defined as the cause of a
detriment, and is not quantifiable.
The danger that concerns us here arises from the radiation emitted by radionuclides and the
effect of exposure to it on living matter.
The hazard posed by the chemical toxicity of the radioactive element is generally much less
significant and seldom taken into consideration.
In a given set of conditions, exposure of matter (tissue or organ) to radiation delivers an
absorbed dose, D (Gy) which is in principle measurable. The unit is joule per kg, and it is called as gray
(1 Gy = 1 J /kg).
In the case of high exposure (several grays) delivered in a short time interval, the effects are
known, and are somatic effects of a deterministic nature. These effects appear above a threshold and
their gravity depends on the dose. In theory, the probability of their occurrence is one above the
threshold and zero below it. However in practice the threshold for various deterministic effects is a
broad range of values rather than a single number.
In the case of low exposure (i.e. smaller than doses at which deterministic effects appear)
delivered over an appreciable time interval, cancers and genetic effects can be induced, although they
may appear only after many years or even decades. These are probabilistic effects with a likelihood
considered by the ICRP to be proportional to the dose, without a threshold.
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