Herbert Faust

Incineration of 241Am induced by thermal neutrons

Abstract An experimental study of the 241Am incineration in a high-intensity thermal neutron flux was carried out at the high-flux reactor of the Institut Laue-Langevin in Grenoble. The combination of nuclear γ -ray spectroscopy and off-line mass spectrometry methods made possible the measurement of several parameters of the transmutation chain and the first experimental determination of the unknown 242gsAm thermal neutron capture cross section, which plays an essential role in the 241Am incineration process. During a 19 days irradiation in a thermal neutron flux of 5.6×10 14 n /( s cm 2 ) , (46±5) % of the initial 241Am was transmuted by neutron capture of which (22±8) % was incinerated by nuclear fission. A value of the thermal neutron cross section of 242gsAm( n ,γ ) of (330±50) barns was obtained. We show that this keeps the option open to incinerate 241Am by high-intensity moderated neutron fluxes.

Mass and charge distributions in the very asymmetric mass region of the neutron induced fission of 238Np

Abstract The mass-separator Lohengrin was used to measure the yields of the light fission products with A=74−85 and their nuclear charge and kinetic energy distributions from the odd- Z compound nucleus 239 Np ∗ formed by double capture of thermal neutrons. The mass yield distribution reveals an influence of the fragment shell with N=50 affecting also the nuclear charge and kinetic energy distributions. An odd–even effect for protons is found in the very asymmetric mass division, increasing from 4% to 35% with increasing fission asymmetry. This is in contrast to findings in normal asymmetric fission (region of high fission yields) where no odd–even effect for protons was observed. An odd–even effect for neutrons is also found comparable in size with that for protons. The latter effect exists also in normal asymmetric fission and is at least partly attributed to prompt neutron emission from the fragments. Some information on the number of prompt neutrons emitted is also obtained. From this information and from the energy dependence of the odd–even effect for protons it is concluded that the very light fragments originate from a cold and nearly undeformed light sphere of a dumbbell scission configuration characterised by a double shell closure with 28 protons and about 50 neutrons. In contrast to the double shell closure at mass 132 ( Z=50 , N≈82 ) the two shell closures in the light sphere do not coincide at one mass but are realised at A=70 and 80, respectively. This leads to a layered structure of the light sphere of the dumbbell scission configuration.

The Mini-Inca Project: Experimental Study of the Transmutation of Actinides in High Intensity Neutron Fluxes

A new experimental installation has been recently commissioned at the High Flux Reactor of Institut Laue-Langevin (ILL) in Grenoble (France). It gives access to high intensity neutron spectra from pure thermal (5.6 1014 n/s/cm2) to epithermal (2 1015 n/s/cm2). Several of low mass (10 µg) mono-isotopic targets of actinide elements are in the process of being irradiated and analyzed by a number of techniques, from nuclear spectroscopy to off-line mass spectrometry and innovative double-deposit fission micro-chambers. In the present paper we will present the first experiments carried out at the thermal neutron spectrum installation with 242Pu, 241-243Am samples.

Actinide Fission and Capture Cross Section measurements at ILL: the Mini‐INCA project

Fission cross section of short‐lived minor actinides is of prime importance for the incineration of minor actinides in high and thermal neutron fluxes. But due to the shortness of their half‐lives, measurements are difficult to handle on these isotopes and the existing data present some large discrepancies. An original method has been developed, in the framework of the Mini‐INCA project at ILL, to measure the fission and capture cross sections of minor actinides with low error bars associated even for short‐lived isotopes. This method lies on a quasi on‐line alpha‐ and gamma‐spectroscopy of irradiated samples and on the use of fission micro‐chambers. Coupled to a very powerful Monte‐Carlo simulation, both microscopic information on nuclear reactions (total and partial cross sections for neutron capture and/or fission reactions) and macroscopic information on transmutation and incineration potentials could be gathered. In this paper, the method is explained in its originality and some recent results are gi...

Transmutation of 241Am in a high thermal neutron flux

Amongst the minor actinides issued from the spent nuclear fuel, {sup 241}Am is present in high concentration and contributes significantly to the long-term radiotoxicity of nuclear waste. A major uncertainty was present in the transmutation chain of {sup 241}Am when irradiated by a high intensity thermal neutron flux. This uncertainty was brought about by the poor knowledge of the {sup 242gs}Am neutron capture cross section. A dedicated experiment has been performed at the Institut Laue-Langevin in Grenoble, which gives a definitive experimental answer to this problem.

γ spectroscopy of neutron irradiated 241Am samples

241Am goes by neutron capture to the two 242Am states: the ground and the meta-stable. We have measured both thermal cross sections, with a relative uncertainty of 7%, by γ-spectroscopy of neutron irradiated 241Am samples. The results are: σcg.s.=σc(241Am(nth,γ)242Amg.s.)=636±46 barns and σcm=σc(241Am(nth,γ)242Amm)=60±4 barns. These measurements lead to a ratio σcg.s./σctot=0.914±0.007 with a total capture cross section σctot=696±48 barns. While the ratio is compatible with previous measurements, the total capture cross section σctot is increased by 16% with respect to the JENDL-3.2 value.

The Minor Actinide Transmutation‐Incineration Potential Studies in High Intensity Neutron Fluxes

In the framework of nuclear waste transmutation studies, the Mini‐INCA project has been initiated at CEA/DSM with objectives to determine optimal conditions for transmutation and incineration of Minor Actinides (MA) in high intensity neutron fluxes. Our experimental tools based on alpha‐ and gamma‐spectroscopy of irradiated samples and the development of fission micro‐chambers could gather both microscopic information on nuclear reactions (total and partial cross sections for neutron capture and/or fission reactions) and macroscopic information on transmutation and incineration potentials. Cross sections of selected actinides (241Am, 242Am, 242Pu, 237Np, 238Np) have already been measured at ILL, showing some discrepancies when compared to evaluated data libraries but in overall good agreement with recent experimental data.

γ spectroscopy experiments relevant for nuclear waste transmutation

γ-spectroscopy is an important tool also for nuclear waste transmutation studies. An experiment has been performed at ILL Grenoble to measure 241Am and 242gsAm thermal capture cross sections that play a major role for the transmutation of 241Am in a high thermal neutron flux, coupling γ-spectroscopy with mass spectrometry methods. σc(242gsAm), for which the three main evaluation data libraries were giving very different values, has been measured to be 280±40 barns. That confirms the low cross section value making possible the 241Am transmutation by thermal neutrons. In the MINI-INCA project, we plan to widen the measurement to other minor actinides of interest using γ and α-spectroscopy of neutron irradiated samples for thermal and epi-thermal fluxes up to 2.1015 n/s/cm2.