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is higher, but the neutron economy is less efficient due to absorption. Even with
P&T integrated in the nuclear fuel cycle, a substantial amount of radioactive
material (activation products, MAs and traces of uranium and plutonium) will
always accompany the bulk of the fission products.
Neutron irradiation of higher actinides also leads to neutron capture and
the formation of other higher actinides. If the resulting half-life of these
actinides is shorter, the radiotoxicity of the target increases proportionally. An
example is the transmutation of 237 Np, which leads to the generation of 238 Pu,
resulting in an increase of the radiotoxicity of the transmuted fractions that is
proportional to the ratio of the half-lives (2 × 10 6 and 87 years, respectively). In
a waste management option the risk of this irradiation product, which is less
soluble and shorter lived, is lower than that of the initial target, but the radiotoxicity
is four orders of magnitude higher.
The reverse can also occur, for example with 244 Cm with a 18.1 year halflife,
which is generated in high burnup fuel. After ten half-lives (~180 years),
99.9% of a separated curium target will have decayed into 240 Pu, with a half-life
of 6560 years. By transmutation of a 244 Cm target it would partially be
transformed into 245 Cm, with a half-life of 8500 years, having a slightly reduced
radiotoxicity compared with the natural decay product 240 Pu.
The licensing period for the proposed Yucca Mountain repository is
10 000 years. Partitioning of MAs followed by advanced conditioning is a
complementary technology, which should be investigated. It has to be
thoroughly examined to determine those radionuclides that benefit from transmutation
and which transmuter would provide the cleanest end product. In the
partitioning–conditioning scenario the partitioned fractions remain available
for possible transmutation in the future, if this technology is fully implemented.
Transmutation with mainly fissioning is currently the most effective
technology to reduce the actinide inventory and consequently its radiotoxicity
and long term hazard.
2.1.7. Natural and archaeological analogues
Nuclear waste disposal is an extreme example of radionuclide
‘packaging’. One of the responsibilities of the nuclear industry is to
demonstrate that an underground repository can contain nuclear waste for very
long periods of time and that any releases that might take place in the future
will pose no significant health or environmental risk. It must be taken into
account that the engineered barriers that initially contain the waste will
degrade and that some residual radionuclides may return to the surface in low
concentrations at some time in the future due to groundwater movement
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