Krypton and helium irradiation damage in neodymium–zirconolite M. Gilbert a,⇑ , C. Davoisne b , M. Stennett c , N. Hyatt c , N. Peng d , C. Jeynes d , W.E. Lee a a Centre for Advanced Structural Ceramics, Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK b Laboratoire de Réactivité et Chimie des Solides, CNRS-UMR 6007, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens, France c Immobilisation Science Laboratory, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK d Surrey Ion Beam Centre, Nodus Laboratory, University of Surrey, Guildford, GU2 7XH Surrey, UK article info Article history: Available online 13 December 2010 abstract A leading candidate for the immobilisation of actinides, zirconolite’s suitability as a potential ceramic host for plutonium disposition, both in storage and geological disposal, has been the subject of much research. One key aim of this study is to understand the effects of radiation damage and noble gas accom- modation within the zirconolite material. To this end, a series of ex situ irradiations have been performed on polycrystalline (Ca 0.8 Nd 0.2 )Zr(Ti 1.8 Al 0.2 )O 7 zirconolite samples. Zirconolite samples, doped with Nd 3+ (as a Pu surrogate) on the Ca-site and charge-balanced by substituting Al 3+ onto the Ti-site, were irradi- ated with 36 Kr + (2 MeV) ions at fluences of 1  10 14 and 5  10 15 cm 2 and 4 He + (200 keV) ions at fluences of 1  10 14 ,5  10 15 and 1  10 17 cm 2 to simulate the impact of alpha decay on the microstructure. Microstructural analysis revealed no damage present at the lower Kr + fluence, but that the higher 36 Kr + fluence rendered the zirconolite completely amorphous. Similarly, evidence of helium accumulation was only seen at the highest 4 He + fluence (1  10 17 cm 2 ). Monte Carlo simulations using the TRIM code predict the highest concentration of helium accumulating at a depth of 720 nm, in good agreement with the experimental observations. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Immobilisation of High Level Waste (HLW) radionuclides from spent fuel reprocessing has drawn much attention, both scientific and political, for several decades. With the small particle size of their oxide form, plutonium and the minor actinides are an excel- lent starting material for ceramic fabrication and considerable research has been done into the development of a variety of candi- date ceramic matrices for their sequestration [1]. A leading candidate host for immobilisation of actinides is zir- conolite, both as the major actinide host phase in SYNROC-C and also as a single phase ceramic waste-form in its own right. Nomi- nally CaZrTi 2 O 7 , zirconolite exhibits a wide range of stoichiome- tries and polytypes of general formula CaZr x Ti (3x) O 7 where 0.8 < x < 1.35 [2,3]. Polytype structures reported, both in natural and synthetic systems, include 2M (Fig. 1), 4M, 3O, 3T and 6T [4], all derived from an anion-deficient fluorite archetype. Zirconolite is well suited as a waste-form for plutonium dispo- sition. Its flexible composition allows incorporation of tri- and tet- ravalent actinides via substitution for a host ion. This substitution process can occur in two ways, either through isovalent substitu- tion of a tetravalent ion onto the Zr-site or via altervalent substitu- tion of a tri- or tetravalent ion onto the Ca-site (the latter in conjunction with incorporation of a charge-balancing species on the Ti-site), and in its most common natural form (zirconolite- 2M) a wide variety of natural lanthanide and actinide containing samples have demonstrated its high durability and stability [5]. Natural zirconolites, some in excess of 2.5 billion years old have been found to contain up to 20 wt.% ThO 2 and up to 14 wt.% UO 2 , showing not only good potential waste loadings but also excellent long-term stability against weathering. A major concern for any form of actinide host is the effect that radiation damage and fission product accommodation will have on the waste-form, particularly in the long term. The major source of damage in an actinide-bearing waste-form is from the a-decay of the actinides themselves. A single a-decay event involves ejection of a high energy a-particle (4.5–5.8 MeV) accompanied by a low energy a-recoil of the actinide nucleus (86 keV 235 U recoil from 239 Pu decay) [6]. On emission, the a-particle will travel over a range of 15–22 lm in the waste-form, with elastic collisions caus- ing displacement damage in the atomic structure of the host crys- tal. As the a-particle comes to rest it captures two electrons, resulting in the generation of helium within the waste-form. This build up of helium may lead to swelling, which can, in turn, result in increased internal stresses that can lead to cracking [7]. This ef- fect can be further exacerbated by the accommodation of gaseous fission products such as krypton, caesium and xenon. However, as most of the a-particle’s energy is dissipated through ionisation, its elastic collisions cause only 200–300 0022-3115/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.11.089 ⇑ Corresponding author. Tel.: +44 (0)7816830400; fax: +44 (0)2075946736. E-mail address: m.gilbert@imperial.ac.uk (M. Gilbert). Journal of Nuclear Materials 416 (2011) 221–224 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat