Journal of Nuclear Materials 465 (2015) 820e834 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat The behaviour under irradiation of molybdenum matrix for inert matrix fuel containing americium oxide (CerMet concept)* * E. D'Agata a, , S. Knol b, A.V. Fedorov b, A. Fernandez c, J. Somers c, F. Klaassen b a European Commission, Joint Research Centre, Institute for Energy and Transport, P.O. Box 2, 1755 ZG Petten, The Netherlands b Nuclear Research and Consultancy Group, P.O. Box 25, 1755 ZG Petten, The Netherlands c European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany article info abstract Article history: Americium is a strong contributor to the long term radiotoxicity of high activity nuclear waste. Trans- Received 12 December 2014 mutation by irradiation in nuclear reactors or Accelerator Driven System (ADS, subcritical reactors Received in revised form dedicated to transmutation) of long-lived nuclides like 241Am is therefore an option for the reduction of 9 July 2015 radiotoxicity of waste packages to be stored in a repository. In order to safely burn americium in a fast Accepted 12 July 2015 reactor or ADS, it must be incorporated in a matrix that could be metallic (CerMet target) or ceramic Available online 14 July 2015 (CerCer target). One of the most promising matrix to incorporate Am is molybdenum. In order to address the issues (swelling, stability under irradiation, gas retention and release) of using Mo as matrix to Keywords: Uranium free fuels transmute Am, two irradiation experiments have been conducted recently at the High Flux Reactor (HFR) Minor actinides in Petten (The Netherland) namely HELIOS and BODEX. The BODEX experiment is a separate effect test, 10 Molybdenum where the molybdenum behaviour is studied without the presence of fission products using Bto Transmutation “produce” helium, the HELIOS experiment included a more representative fuel target with the presence Americium of Am and fission product. This paper covers the results of Post Irradiation Examination (PIE) of the two Boron irradiation experiments mentioned above where molybdenum behaviour has been deeply investigated HELIOS as possible matrix to transmute americium (CerMet fuel target). The behaviour of molybdenum looks BODEX satisfying at operating temperature but at high temperature (above 1000 C) more investigation should be performed. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction reaction on the opposite active matrix can contribute to the power produced by the nuclear reactor, generally, the inert matrix are Americium is a minor actinide that is contained in all the nu- used in ADS where power production is not an issue. Moreover, the clear waste produced by a nuclear power plant. Moreover, ameri- objective is to fission americium trapping the fission products in cium is one of the elements that contributes most to the the matrix which must withstand irradiation damages. In order to radiotoxicity of spent nuclear fuels. The transmutation of 241Am burn americium, the matrix is subject to an additional problem of will be in the next future the main option for the reduction of the internal gas pressures, caused by a relatively large production of mass and radiotoxicity of nuclear waste. Transmutation of minor helium. actinides may be realized in nuclear reactors as well as in Accel- Helium is produced by neutron capture of 241Am to 242Am which erator Driven System (ADS, subcritical reactors dedicated to decays via beta decay to 242Cm which in turn is a strong a-emitter. transmutation). Looking at the entire decay chain, a total of 75% of the initial 241Am A possible way to transmute Am is to incorporate it in a matrix. form He atoms. Due to the low solubility of He this results in a The matrix could be inert (like Mo, MgO, etc …) or active (like MOX significant internal gas pressure which the matrix material has to or U). The matrix is called inert for its transparency to neutron withstand. Based on previous experience [1] [2] [3], several inert matrices have been studied in the past: magnesium oxide (MgO), Yttria * Presented at the NuMat 2014 Conference, 27e30 October 2014, Clearwater, Stabilized Zirconium (YSZ), molybdenum (Mo). Metal fuel was the Florida, USA. original choice in early fast reactors (EBR-I, EBR-II, Fermi-1, and * Corresponding author. E-mail address: [email protected] (E. D'Agata). Dounreay Fast Reactor) because it is compatible with the liquid http://dx.doi.org/10.1016/j.jnucmat.2015.07.021 0022-3115/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). E. D'Agata et al. / Journal of Nuclear Materials 465 (2015) 820e834 821 À metal sodium coolant. Moreover, metal fuel give higher safety 1.5 mmol cm 310B in the matrix (6 pellets); À margins against ceramic fuel and in particular Mo based fuel has 1.5 mmol cm 311B in the matrix (2 pellets); been largely used as fuel in Material Test Reactor (MTR). This paper No B-dopant in the matrix (2 pellets). will report the results of two irradiation experiments: HELIOS [4] and BODEX [5] looking in detail the Mo matrix which could be The amount of boron adopted in the pellets has been calculated considered the most promising inert matrix [3]. in order to have a sufficient generation of helium in a relatively Both HELIOS and BODEX irradiation experiments have been short irradiation time. developed within the EURATOM 6th Framework Programme (FP6), The particle distribution of the Mo2B in the Mo matrix is also as part of the 4 year project EUROTRANS which ended in 2009. The more or less homogenous, the size of the Mo2B particles varies from aim of EUROTRANS was to carry out a first advanced design of an 0.4 to 5 mm. Porosity of the matrix is less than 2% and the pore size experimental facility demonstrating the technical feasibility of is also in the range of 0.4e5 mm. transmutation in an Accelerator Driven System (XT ADS), as well as All pellets irradiated had an equal height and diameter to accomplish a generic conceptual design of a modular European (5 Â 5 mm). After sintering, the dimensions and properties of the Facility for Industrial Transmutation (EFIT) [6]. pellets were measured. The density of the pellets was measured in Knowing already that the release/trapping of helium is the main two ways; geometrically and by helium-pycnometry, see Table 2. issue for the design of the target containing americium [7,8],both The experiment was irradiated in the Pool Side Facilities (PSF) of experiments had the purpose to study this effect using different the HFR. The PSF is a facility next to the core, which allows moving techniques. the experiment from and to the core, controlling the neutron flux Although both experiments contained several capsules with and temperature during irradiation. different inert matrixes, this paper will focul on Mo behaviour. The temperature in the experiment was also controlled chang- BODEX used inert matrixes doped with 10B to emulate the ing gas mixture (helium and neon) in the gas gap between the presence of Am and generate helium. Indeed, due to the large cross- capsule, shroud and containment tube and it was maintained section of 10B for thermal neutrons, a large amount of helium can be constant during the whole irradiation. produced in a relative short irradiation time, providing fast insight For monitoring the required parameters, one of the two capsules in the behaviour of such matrix materials. Another advantage is the containing Mo was connected to a pressure transducers and each small amount of 10B needed to produce representative amounts of capsule had two dedicated thermocouples, positioned as close as helium. By using 10B it is also possible to study the matrix structural possible to the capsules, to measure the temperature during irra- behaviour avoiding the complex chemical interaction of a large diation. Additionally, fluence-monitoring sets were embedded in number of fission products, hence, separate effects due to the the shroud surrounding the capsules for neutron dosimetry. presence of fission products. Additionally, the absence of fissile Each capsule contained 5 pellets, kept apart by 6 spacers for material greatly simplifies the handling and increases the possi- positioning the pellets and increased heating purposes. Three pel- bilities in the Post Irradiation Examinations (PIE). The table below lets were doped with a 10B-compound, one pellet was doped with shows the main differences between the helium produced by boron an 11B-compound and one pellet consists of the molybdenum only. (in BODEX) and americium (in HELIOS) reaction with neutrons. The volume surrounding the pellets and spacers was filled with The main objective of the HELIOS irradiation project was to neon gas to keep a clear distinction between filling gas and the study the in pile behaviour of U free fuels and targets in order to helium produced by irradiation. gain knowledge on the role of the microstructure and of the tem- The fluence of each capsule was determined by using activation perature on the gas release and on fuel swelling. monitor sets. These monitor sets consist of different metal wire This paper will describe briefly the experiments and will present pieces that have an activation reaction at a specific energy range. the results obtained during the PIE (visual inspection, dimensional The different activation energies are chosen in such a way that the measurement, gas release and microanalyses) with particular spectrum can be reconstructed.