6. TRANSMUTATION
6.5. Transmutation issues of long lived fission products
Transmutation of LLFPs is a very difficult task, because the neutron capture cross-sections to transmute the radionuclides into short lived or stable nuclides are very small. Moreover, each neutron absorption is a net neutron loss without a compensating fission. This way, very long irradiation periods are necessary to obtain a significant depletion. Dedicated reactors with high thermal neutron fluxes and/or dedicated accelerator driven transmutation facilities (e.g. using resonance neutron absorption) are the only possible options for carrying out this very expensive endeavour.
In the short term, after discharge of the fuel, the main fission products determining the thermal load and the overall radioactivity of HLW are 137Cs and 90Sr. These two isotopes, with half-lives of 28–30 years, are not considered in P&T operations since their radioactive life is limited to about 300 years.
The fission products that play an important role in the long term dose to humans originating from the back end of the fuel cycle are, in order of radio-logical importance, 129I, 99Tc, 135Cs, 93Zr, 95Se and 126Sn. The associated risk varies according to the type of repository host formation.
Time (a)
Transmutation efficiency eT (%)
Actinide mass (normalized to the initial 241Am + 243Am)
Total actinides
241Am
238Pu
243Am eT (burnup)
242Pu 244Cm
FIG. 22. Composition of an irradiated americium target and transmutation efficiency as a function of fluence in an FR [9].
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90 100
237Np (%)
Cumulative nuclides (%)
+ Cm-244 + Am + Other Pu + Pu-238 + U-234 + Fissium Np-237
FIG. 23. Transmutation rate of a neptunium target in a MYRRHA type ADS [9].
Some activation products are also of importance in the determination of dose to humans (i.e. 14C and 36Cl).
6.5.1. Iodine-129
Transmutation of 129I is a difficult task because it has a moderate to small thermal cross-section (27 barns). Up to now, no thermally stable iodine matrix has been found, and most of the calculations have been performed for NaI, CaI2 and CeI3. Each of these candidates has its limitations, but CeI3 seems the most promising. Two opposite safety requirements have to be fulfilled: on the one hand the confinement of the iodine compound in the target capsule during irradiation and on the other hand the discharge of the produced xenon. A vented capsule with an iodine filter is to be investigated. The upscaling of such complex irradiation procedures to industrial quantities is not obvious.
6.5.2. Technetium-99
Technetium-99 is one of the most important LLFPs that occur in spent fuel and in several waste streams from fuel reprocessing. Due to its long half-life (213 000 years) and the diverse chemical forms in which it can occur, its radiological significance is important if the repository surroundings are slightly oxidative. In reducing conditions of deep aquifers, it is remarkably stable and insoluble as technetium metal or TcO2.
If 99Tc is a real radiological hazard in some repository conditions, new separation technologies need to be developed. Transmutation of 99Tc is
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90 100
Cumulative nuclides (%)
+ Other Cm + Cm-244 + Other Pu + Pu-238 + U-234 + Fissium Am
Am (%)
FIG. 24. Transmutation rate of an americium target in a MYRRHA type ADS [9].
possible if it is present as a metallic target, since the transmutation product is inactive 100Ru. Taking into account the very small thermal cross-section of 20 barns it is important to have a high thermal neutron flux, a high loading in the reactor and an optimized moderator to target radius.
6.5.3. Caesium-135
Caesium occurs in several isotopic forms: 137Cs, 134Cs, 133Cs and 135Cs. In terms of radiological significance 137Cs is the major constituent of HLW.
Caesium-135 has a very long half-life (2 million years), but its concentration is a million times lower. Caesium is very mobile in the geosphere if not conditioned into a suitable matrix, for example glass. At present no 135Cs separation is envisaged since isotopic separation from the highly active 137Cs would be necessary in order to isolate this radionuclide. Transmutation to stable 136Cs in order to deplete 135Cs is very difficult, since stable 133Cs and 134Cs are present in the fission product mix and would generate new 135Cs during long term irradiations.
6.5.4. Zirconium-93
Zirconium-93 is to a certain extent similar to 135Cs, since it has a very long half-life (1.5 million years) but is present as a relatively small fraction (14%) of the total zirconium load present in the fission product mix. Separation of 93Zr involves the development and operation of isotopic separation procedures. In the longer term, it could be used in IMFs (remote fabrication).
6.5.5. Tin-126
Tin-126 has a half-life of 100 000 years, is partly soluble in HLLW from aqueous reprocessing but occurs also as an insoluble residue (similar to technetium). Its isolation involves a special treatment of the HLLW and the use of isotopic separation techniques. Recently, it has been proposed to add this isotope to lead spallation targets.
6.5.6. Carbon-14
The transmutation of 14C has not yet been considered in the P&T context.
Theoretically, the 14C released from the spent fuel could partly (about 50%) be recovered from reprocessing off-gases. There is, however, not enough knowledge about the chemistry of 14C in dissolver conditions to improve this figure. Once transformed into a solid target, for example barium carbonate
(BaCO3), it could be stored for an infinite period. The cross-section of 14C for thermal neutrons is nearly zero. Transmutation by charged particles in high energy accelerators is a theoretical alternative in some cases, but the practical feasibility and economy of such approaches are questionable.
Phototransmutation at 5–6 MeV can theoretically be carried out on caesium and strontium radionuclides in HLW. This technology is still at the fundamental level and will need to be further investigated [85, 86].