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As can be seen from Fig. 2, the total ingestion radiotoxicity arises from
the plutonium isotopes. At around 200 years, the plutonium radiotoxicity is
4.3 × 10 7 Sv (i.e. a factor of 180 (4.3 × 10 7 /2.4 × 10 5 ) above the reference value
indicated by the dashed line). Conversely, to reduce the waste toxicity to
reference levels on this timescale, the effective plutonium removal efficiency
needs to be 0.994 (i.e. 1 – 1/180). Similarly, at 200 years the americium radiotoxicity
is 9.4 × 10 6 Sv (i.e. a factor of 40 higher than the reference level). Hence an
effective removal efficiency needs to be approximately 0.975 (1 – 1/40). The
neptunium and uranium curves fall far below the reference levels.
For curium, it can be seen that the radiotoxicity at around 200 years is
actually below the reference level, so it may not be necessary to remove it from
the waste. However, this depends on the P&T strategy followed for the other
waste streams. The curium radiotoxicity follows from 200 to 5000 years a
plateau around the value of 10 5 Sv. If the separation and transmutation of the
plutonium and americium is efficient enough to significantly reduce the total
radiotoxicity of the waste, then the curium could be left in the waste (its own
radiotoxicity is then below the reference level).
Another approach to the evaluation of radiotoxicity evolution is
presented in Ref. [8], in which a procedure is proposed for the calculation of
radiation equivalence of waste and feed material in a fuel cycle with
transmutation.
2.1.2. Spent light water reactor MOX fuel
The recycling of plutonium into LWR MOX fuel generates a much more
radiotoxic spent fuel type, which up to now has remained in interim storage
awaiting either disposal or recycling if a second era of FR MOX use becomes
an economic reality within the next few decades. Since the decay heat of this
fuel is much higher than that of LWR UO 2 , the cooling period before recycling
must be prolonged by a few hundred years. The radiotoxicity of this fuel
follows the cumulated decay curves of 244 Cm, 238 Pu, 241 Am, 239 Pu, 240 Pu and
242 Pu. The other alpha nuclides (e.g. 243 Am, 245 Cm and 237 Np) are in this context
negligible.
2.1.3. Spent fast reactor MOX fuel
The alpha radiotoxicity of FR MOX fuel is approximately three times
higher than that of LWR MOX, taking into account the increased plutonium
concentration (~8–22%) in the fuel of a number of FRs that have been
operating over several decades (DFR, PFR, PHENIX, SUPERPHENIX), but
the radiotoxicity expressed per unit of energy produced is nearly the same.
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