HELIOS: Irradidation of U-Free Fuels and Targets for Americium Transmutation

Abstract

Americium is one of the radioactive elements that contributes to a large part of the radiotoxicity of nuclear spent fuel. Transmutation by irradiation in nuclear reactors of long-lived nuclides like 241Am is, therefore, an option for the reduction of the mass and radiotoxicity of nuclear waste. The analysis of previous irradiation experiments which were carried out with targets of MgAl2O4+11wt 241Am showed that the release/trapping of helium is the key issue for target design. In fact, in those experiments a significant volume swelling was observed which was partly attributed to the production of helium, which is characteristic for 241Am transmutation. These findings led to the conclusion, that a new experiment should be designed in such a way that helium shall be released from the target already during irradiation. Such release of helium might be achieved either with a fuel temperature kept sufficiently high during the whole irradiation or with release paths for helium created by inclusion of tailored open porosity in the targets. A new irradiation experiment called HELIOS is currently being designed and will be carried out in the High Flux Reactor (HFR) in Petten (The Netherlands) in the frame of the 4-year project EUROTRANS of the EURATOM 6th Framework Programme (FP6). The main objective of the HELIOS irradiation is to study the in-pile behaviour of U-free fuels and targets such as CerCer (Pu, Am, Zr)O2 and Am2Zr2O7 MgO or CerMet (Pu, Am)O2 Mo in order to gain knowledge on the role of the microstructure and of the temperature on the gas release and on fuel swelling. The irradiation temperature will be high enough to be able to tune the release of a significant fraction of helium produced so that the material swelling can be minimised as much as reasonably possible. Besides, the irradiation duration has been chosen as a compromise such to ensure that the central temperature in the (Pu,Am,Zr,Y)O2 pellets be always higher than that of the (Am,Zr,Y)O2 pellets in order to be able to investigate, during the Post-Irradiation Examinations (PIE), the influence of the higher irradiation temperature on the helium release. The HELIOS irradiation experiment is planned to be carried out in the HFR core and shall last 300 full power days starting in the first quarter of 2007. The proposed irradiation position is a high flux position of the HFR core, which has a thermal flux of about 1 1018 m-2s-1 and a total neutron flux of about 6 1018 m-2s-1. The use of a high flux position is required in order to transmute a substantial fraction of the 241Am, within the planned duration of the HFR irradiation. In the present paper the fabrication procedure and the development of the HELIOS irradiation, its rationales and objectives are described and discussed.JRC.F.3-High Flux and Future Reactor