Computer Simulation Studies Of Radiation Damage Stability (Fission-Track Annealing) and He Diffusion In Apatite Materials

Lead Research Organisation: University College London
Department Name: Chemistry


The stability of radiation damage and the diffusion of gas in natural and synthetic calcium phosphate (apatite) is of major importance to environmental, geoscience and engineering problems, including nuclear waste storage. Deciphering the thermal histories of Earth's upper crustal rocks is an essential requirement for understanding geodynamic processes, long-term landscape evolution and in assessing hydrocarbon maturity in sedimentary basins. Key methods in thermal history reconstruction utilise apatite, measuring the accumulation and stability of nuclear radiation damage (fission-tracks FT) and of radiogenic helium - (U-Th)/He chronometry. Interpretation of sample data requires precise knowledge of the stability of the tracks and helium at elevated temperatures which at present is derived from empirical measurement of the fractional track shortening (annealing) or helium loss under different time - temperature conditions. Varying substitutions in the calcium phosphate (apatite) lattice produce differences in the FT stability and gas diffusion.This project will employ a parallel approach developing and employing a range of complementary computational methods to investigate radiation damage and gas diffusion in calcium phosphate materials, where the key issues are:The development of models for the interaction of impurity cations and anions, substituted in the calcium phosphate lattice;Modelling of He diffusion in the pure and defective lattice;Simulation of the processes of generation and annealing of nuclear fission tracks in different apatite structures.The outcome of the project wi4 be an improved and detailed understanding on the atomic scale of the influence of composition on the formation and stability of nuclear fission tracks and the diffusion of helium in apatite, an approach which can be applied subsequently to study damage and diffusion in increasingly complex materials.


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Description The project has concentrated on two important systems and two processes:

(i) Modelling radiation-induced fission tracks in apatite minerals

Apatites commonly contain fission tracks from natural radation damage, which anneal in a narrow temperature band, making these minerals exceptionally good materials for unraveling the thermal history of crustal rocks. They are also promising materials for the storage of nuclear waste. One major concern is the unknown effect of composition and impurities on the experimentally observed fission track annealing rates, which was therefore the main subject of the computational research. First, we have investigated in detail the effect of composition on the structures and properties of a comprehensive series of apatites. Second, we have developed new computational methodology to create a fission track from a simulated "spike", which is closer to real life than the previously applied primary ion "knock on" effect. Finally, we have developed a computational annealing technique, which uses a system of harmonic springs to accelerate the crystallization of the lattice after a fission event.

(ii) He diffusion in zircon

We have investigated the diffusion of He in zircon ZrSiO4, a ceramic material with great potential for the long-term storage of radioactive waste, using two complementary techniques. It is important to understand the behaviour of He in the lattice as its presence can lead to swelling and eventual failure of the material, thus jeopardizing its effective use as a safe storage material. Accurate Density Functional Theory calculations have been carried out on He incorporation and diffusion pathways in the perfect lattice and the zircon structure containing oxygen vacancies, the results of which have also been used to derive a set of interatomic potential parameters to study the behaviour of He in zircon containing impurity ions. Energy minimization calculations were then carried out on the effect of U and Pu impurities on He incorporation and transport in the zircon lattice, which were found to trap the He in their vicinity, thus increasing the activation energies for diffusion. Finally, we have employed molecular dynamics simulations to study the dynamic behaviour and to calculate He diffusion coefficients, where we have also taken into account the presence of Sr impurities in the zircon lattice. Finally, we have compared the He diffusivity in the zircon lattice with other silicate minerals, namely alpha-quartz SiO2 and forsterite MgSiO4.

Highlights of the project thus include:
1) Derivation of a full set of transferable and consistent interatomic potentials for the modelling of the complete range of apatite minerals and its application to substituted apatite solid solutions;
2) Development of new methodology to model the formation of fission spikes in complex materials, exemplified by a range of apatite materials;
3) Development of an accelerated crystallization methodology to investigate fission track annealing, which reproduces experimental trends in apatite and has been generalized and applied to silica glasses. This adaptation has been implemented in the DL_POLY molecular dynamics code;
4) Modelling of He diffusion in pure and substituted zircon mineral, and the effect of defects from radiation processes on the trapping of He in the lattice.

The project has led already to 8 publications in peer-reviewed scientific journals and two conference abstracts, with another two papers in preparation for submission later this summer. The PDRA and student on the project have both presented their work in poster and oral presentations at a number of national and international conferences, including RSC Solid State Group meetings in the UK, the 2007 Goldschmidt Geochemistry conference, Cologne, and the 2008 MRS meeting in Boston, USA.

A comprehensive final report was submitted to the EPSRC upon completion of the grant.
Exploitation Route Peer-reviewed publications in scientific journals

Conference presentations
Sectors Environment

Description Peer-reviewed scientific publications, conference presentations and international conferences.
First Year Of Impact 2007
Sector Environment
Impact Types Cultural