Gabriele Fioni

Ternary fission yields of 241Pu(nth,f)

Abstract Ternary events in the thermal neutron induced fission of 241Pu(n,f) were studied with the recoil separator LOHENGRIN at the high-flux reactor of the Institut Laue Langevin in Grenoble. Yields and energy distributions could be determined for most isotopes of the elements hydrogen to oxygen. Also several heavier nuclei up to 30Mg could be observed. Yields were measured for 42 isotopes, for further 17 isotopes upper limits could be deduced. For the first time the halo nuclei 11Li, 14Be and 19C were found in neutron induced fission with yields of some 10−10 per fission.

Development of Fission Micro-Chambers for Nuclear Waste Incineration Studies

The INCA (INCineration by Accelerator) project of the Directorate for Science of Matter of the French Atomic Energy Authority (CEA/DSM) aims to outline the ideal physical conditions to transmute minor actinides in a high intensity neutron flux obtained either by hybrid systems or innovative critical reactors. To measure on-line the incineration rates of minor actinides, we are developing an innovative double deposit fission chamber (DDFC) working in current mode. Our method is based on a comparison between the isotope under study and a reference material whose nuclear parameters are well known, as 235 U and 239 Pu. This new fission chamber will be used in the High Flux Reactor in Grenoble/France in a neutron flux of 1.2 10 15 n . cm -2. s -1 for 50 days, the operating cycle of the reactor. These specific experimental conditions require substantial modifications of the existing chambers. The first experiment will be carried out in fall 2000.

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.

Comparative Analysis of ENDF, JEF and JENDL Data Libraries by Modelling High Temperature Reactors and Plutonium Based Fuel Cycles

The gas turbine modular helium-cooled reactor (GT-MHR)1) is known probably as the best option for the maximum plutonium destruction in once-through cycle, even though the industrial fabrication of coated particle fuel still has to be proved. We perform detailed simulations along these lines by comparing different sets of data libraries in terms of keff eigenvalues, the length of the fuel cycle, neutronic characteristics and the evolution of fuel composition in particular. In all cases the same Monteburns code system2) is used making our results dependent only on the evaluated data tables. We show that in general the performance of GT-MHR is not considerably influenced by the choice of the data libraries employed. Nevertheless, a number of major differences among ENDF, 3) JENDL4) and JEF5) data files are identified and quantified in terms of the averaged one-group cross sections both for military (MPu) and civil (CPu) plutonium based fuel cycles.

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.

Transmutation and Neutron Flux Studies with Fission Chambers in the MEGAPIE Target

8 fission micro chambers will be inserted inside the central rod of the 1 MW liquid Pb‐Bi MEGAPIE target in order to study the transmutation of two major actinides and to measure the neutron flux at a level of 5%. These chambers were developed for high neutron fluxes and tested at Laue Langevin Institute.

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.