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Table II.17 Effective dose coefficients of fission products FD RN [166]
Element Nuclide Sv/Bq (ingestion)
Strontium
Zirconium
Technetium
Iodine
Caesium
90 Sr 3.4×10 -10
93 Zr 8.6×10 -10
99 Tc 6.4×10 -10
129 I 1.1×10 -7
135 Cs 2.0×10 -9
137 Cs 1.3×10 -8
The general radioactive characteristics of the spent fuel as source term are calculated by
computer programs (ORIGEN 2.1 or ORIGEN 2.S, KORIGEN and APOLLO). For a given standard
burn-up (e.g. 40 or 50 GWd/tHM) the programs calculate for each element involved the mass,
radioactivity, residual heat power, radiotoxicity etc. which are compiled in tables or graphs (see
Annex E). Figure II.19 shows the typical evolution for each of the major components, actinides, fission
products and activation products, as a function of time [168].
The total radioactivity of the spent fuel expressed per tHM 500 years after unloading will
amount to about 200 TBq, which is 700 times lower than after one year cooling. During the first
200 years the radioactivity will be mainly due to the fission products and will drop from 1.4×10 5 TBq
after one year cooling to 200 TBq. After 200 years the actinides contribution (~300 TBq/tHM) becomes
dominant and the radioactivity will decay very slowly. It will naturally decay to 100 TBq/tHM after
1 000 years and to 13 TBq/tHM after 25 000 years.
The residual heat will vary from about 2 kW/tHM one year after discharge to 65 W/tHM after
1 000 years. The radiotoxic inventory expressed as ingestion hazard (Sv/tHM) follows very closely the
total alpha radioactivity inventory as a function of time. For UO 2 the curves for the individual elements
are shown in Figure II.20. The plutonium isotopes and their decay products determine the radiotoxic
inventory up to 1 000 000 years. In an extremely long time interval of several million years, Np and the
daughter products of the uranium isotopes, determine the remaining radiotoxic inventory.
In the case of the RFC, the reprocessing operation and recycling of Pu as LWR-MOX
significantly reduces the radiotoxic inventory of the HLW. In this case the long-term radiotoxicity is
essentially determined by the minor actinides (Np, Am and Cm, see Figure II.21) and the long-lived
fission products 99 Tc, 126 Sn, 79 Se and 135 Cs (see Figure II.22). An important long-lived fission product
129 I does not appear in this graph since it is discharged into the ocean as a consequence of the
reprocessing operations. Between 100 and 10 000 years the most important radiotoxicity contribution in
the HLW comes from Am and Cm isotopes. Beyond that period both isotopes have significantly
decayed to 239 Pu, 240 Pu (daughters of 243 Cm, 244 Cm) and 237 Np (daughter of 241 Am) which become
predominant.
The radiotoxic inventory of spent MOX fuel as a function of time is shown in Figure II.23.
Since the Pu content of 7 spent UO 2 assemblies is put into one MOX fuel element, the actinide
radiotoxic inventory of a spent MOX fuel element is much higher (about 8 times) than in spent UO 2 fuel
elements. Beyond 1 000 years the radiotoxic inventory of actinides will decrease smoothly from
5×10 8 Sv/tHM after 1 000 years to 1.2×10 8 after 10 000 years and 5.0×10 6 after 100 000 years.
Figure II.19 Radioactivity of PWR type spent fuel (4.1% 235 U, 40 GWd/tHM)
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