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Transmutation of 99 Tc or 129 I to stable 100 Ru and 130 Xe, respectively, may be accomplished by
neutron capture. Because no neutrons are produced in the transmutation process, introducing these
nuclides into a fission reactor will lower the reactivity or shorten the cycle, unless one increases the fuel
enrichment to compensate for the reactivity loss.
The neutron absorption cross-section of 99 Tc exhibits a strong resonance in the epithermal
range, while 129 I is a 1/v neutron absorber (see glossary). When a nuclide with a spectrum-averaged
one-group absorption cross-section σ is irradiated in a neutron flux ϕ, one may define the transmutation
half-life:
ln( 2)
T 1 / 2
=
σ ⋅ϕ
This expression for the transmutation half-life will be used below (see Table II.15), as it
characterizes the transmutation rate of the long-lived fission product in the targets. Because the fission
product will also be produced in the reactor, one has to consider the net transmutation rate, subtracting
the mass of the fission product produced in the fuel from the mass destroyed in the target.
The problems involved in the transmutation of 129 I are severe. Besides doubts about the
stability of the chemical form (see Section 2.2.2.3), the formation of gaseous xenon requires the target
to be vented, raising considerable safety issues.
2.4.1.2 Transmutation in fast reactors [153-155]
Transmutation of 99 Tc in fast reactors may be accomplished in several ways: in a special
moderated sub-assembly loaded at the periphery of the core or in the inner core, or in a non-moderated
subassembly loaded in the core. Moderation could be realised with a material like CaH 2 . Attention has
to be paid to the required fuel enrichment and to power peaking in the neighbouring fuel assemblies
caused by moderation. Although the capture cross section of 99 Tc in a fast neutron spectrum is relatively
low, transmutation in a fast reactor without moderation could be advantageous because of the very high
neutron flux and the limited power peaking. The consequences of introducing FPs should be evaluated.
Typical values of transmutation rates and half-lives are given in Table II.15. A fast reactor
with a power of 1 200 MWe could transmute the 99 Tc production of five 1-GWe-PWRs with
moderation, or the production of four PWRs without it. But this would need a huge 99 Tc loading,
leading to design problems and economic penalties.
To improve transmutation performance, a new concept of duplex pellet – a moderator annulus
surrounding a central 99 Tc zone – is being studied [156]. The moderated target subassemblies would be
loaded in the radial blanket region of the fast reactor. This concept seems promising, since a maximum
99 Tc transmutation rate was calculated to be more than double to reach about 10%/year.
The transmutation performance of 129 I has also been calculated [156]. 129 I was assumed to be
loaded as NaI with an isotopic concentration of 76% l29 I. In the most effective case, the transmuted
amount was 18 kg/year, which is about the output from three PWRs.
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