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Transmutation is technically feasible


Can the minor actinide, americium-241, 3
be transmuted by thermal neutrons?


Figure. Isotopes formed during 241Am irradiation in a thermal neutron flux. The dotted arrows in bold indicate at what point incineration through fission becomes possible. The transmutation process is more effective when fission occurs after the fewest successive neutron captures.

Figure. Isotopes formed during 241Am irradiation in a thermal neutron flux.
The dotted arrows in bold indicate at what point incineration through fission
becomes possible. The transmutation process is more effective when fission
occurs after the fewest successive neutron captures.
• • • • •


As part of their research on the transmutation of long-lived waste, teams from the Physical Sciences and Nuclear Energy divisions of the CEA and the Laue-Langevin Institute (ILL) in Grenoble have joined forces to find a solution to a major problem in the chain of nuclear reactions involved in the transmutation of americium-241 (241Am), induced by thermal neutrons. Amongst the minor actinides, it is the most significant in terms of quantity and the most radiotoxic for a period of 2,000 years (it has a half-life of 432 years), at which point, the neptunium generated by its decay takes over in the long term, after 200,000 years. So far, it has been impossible to determine whether transmutation was feasible with thermal neutrons. In its transmutation chain, the capture cross-section of the ground state of americium-242 (242gsAm, gs standing for "ground state") varies by a factor of 20 in the most commonly used nuclear databases. The US database, ENDF-B/VI, gives a value of 253 barns, while its European counterpart, JEF-2.2, quotes 5,500 barns. These values lead to totally contradictory results in thermal-neutron transmutation simulations. In the former case, transmutation is theoretically possible, as 242gsAm fission has the upper hand over capture, leading to 243Am, a non-fissile isotope that would therefore require an additional capture to be transmuted through fission. In the second case, it is not feasible because it demands an excessive input of outside neutrons. It should nevertheless be stressed that this difference, which has a crucial impact on the neutronic properties of an incineration system with a high thermal-neutron flux (approx. 2 x 1015 n/s/cm2), has virtually no effect in a conventional reactor, where the concentration of 242gsAm remains very low because of the low neutron flux and the absence of 241Am in the initial fuel.

The discrepancy between databases is explained by conflicting theoretical evaluations, due to the lack of experimental values. The very short half-life of 242gsAm, 16.02 hours, rules out the use of conventional experimental methods. A solution has been set up at the ILL, which owns the reactor with the highest thermal-neutron flux in the world. Using a 241Am target, the high flux forms enough 242gsAm to have a strong impact on the transmutation chain. Furthermore, the perfect neutron thermalization of the ILL reactor implies that experimenters have no need to make corrections that would decrease the measurement accuracy.


(1) Branching ratio: the branching ratio of an isotope A to an isotope B is the fraction of isotope B formed for a decay of isotope A.


At the start of the transmutation chain (figure), it can be seen that the 242gsAm capture cross-section determines the amount of 243Am formed during the irradiation sequence. During irradiation, 241Am transmutes through successive neutron captures and radioactive decay into other, possibly fissile, actinides.

The aim of the experiment was to observe changes in a set of 13 samples, each containing 30 µg (or 10-6 g) of 241Am, irradiated in two neutron fluxes of different intensities (5 x 1014 n/s/cm2 and 3 x 1013 n/s/cm2) for periods from 30 minutes to 24 days. In each sample, the quantity and isotopic composition of americium, curium, and plutonium nuclei, and fission products, greatly depends not only on the 242gsAm capture cross-section value, but also on that of other cross-sections. By measuring the isotopic composition of each irradiated sample, the transmutation of 241Am in a high-intensity thermal-neutron flux can thus be determined experimentally.

Unfortunately, it is not easy to determine the isotopic composition. After irradiation, the samples are highly radioactive, making it impossible to handle them or take direct measurements by gamma (g) spectrometry. The solution is to combine an X- and g-ray measurement of one sample after a very brief irradiation period (30 minutes), with that of another sample irradiated for 24 hours but cooled for 8 months. The spectra of the two samples can be compared to determine the precise 241Am capture cross-section value, another critical and uncertain parameter in the transmutation chain. Most analyses were carried out by thermal-ionization mass spectrometry (TIMS) or inductive-coupled plasma mass spectrometry (ICP-MS) after six to nine months of cooling at CEA/Saclay. This method gave the isotopic composition of the americium, curium, and plutonium nuclei and fission products of the different samples. This method offers sensitivity of several tens of nanograms (10-9 g) and very high precision (between 0.5 and a few per cent).

Results were compared with the predictions obtained for various cross-section values of 241Am and 242gsAm. This comparison confirmed the low theoretical prediction for the 242gsAm cross-section, with a value of (330 ± 50) barns and, for 241Am, a value of (696 ± 48) barns, with a branching ratio(1) (between the formation of the ground state and the metastable state of 242Am) of (0.914 ± 0.007). In addition, after 19 days' irradiation in a thermal neutron flux of 5.6 x 1014 n/s/cm2, (46 ± 5)% of the initial 241Am was transmuted through neutron capture, of which (22 ± 8)% was incinerated through fission. Consequently, it remains theoretically possible to transmute 241Am using a high-intensity thermal-neutron flux.

Gabriele Fioni, Michel Cribier
and Frédéric Marie
Department of Astrophysics,
Particle Physics, Nuclear Physics,
and Related Instrumentation
Physical Sciences Division


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