Lithium and tritium diusion in lithium oxide (Li 2 O), a molecular dynamics simulation Heriberto Pfeier a, * , Jorge S anchez-S anchez b , Luis Javier  Alvarez b a Departamento de Qu  õmica, Universidad Aut onoma Metropolitana-Iztapalapa, Av. Michoac an y la Pur  õsima s/n, Iztapalapa, C.P. 09340, M exico D.F., Mexico b Laboratorio de Simulaci on de Materiales, Instituto de Matem aticas, Unidad Cuernavaca, Universidad Nacional Aut onoma de M exico, Apdo. Postal 273-3, Admon 3., C.P. 62251 Cuernavaca, Morelos, Mexico Received 29 December 1999; accepted 14 April 2000 Abstract Molecular dynamics simulations of Li 2 O were performed in the microcanonical ensemble at several dierent tem- peratures in order to study the lithium diusion process, and a preliminary exploration of the diusion of tritium was performed. Dierent Li/O molar ratios were used to investigate the role of non-stoichiometry in the Li diusion processes. The mechanism of lithium diusion as a function of temperature is proposed based on the analysis of our simulations and a model is proposed to explain the overall behaviour of the lithium diusion coecient as a function of temperature. Our simulations suggest what is the role of hydrogen in the tritium release from breeder ceramic materials. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction There are a variety of breeder ceramic materials that could be employed in fusion reactors. All these materials have lithium as a common element. Lithium may be found as an isotope, 6 Li, in a concentration of 7.4 wt%. When this isotope is irradiated with thermal neutrons, the following ®ssion reaction occurs [1]: 6 Lin; a 3 H; E  4:8 MeV: 1 In this reaction one tritium atom, 3 H, is formed and could be used as fuel in nuclear fusion reactors through a reaction between tritium and deuterium atoms pro- ducing 17.59 MeV per tritium atom, through [1,2] 3 H  2 H ! 4 He  n: 2 Synthesis, characterisation, evaluation of radiation damages, and lithium and tritium diusion have been studied in breeder ceramic materials considered for tri- tium production such as lithium oxide, lithium alumi- nates, silicates and zirconates [3±11]. Experimentally the diusion coecients vary between 5 and 6 orders of magnitude, and this is caused by a variety of factors such as host crystal size, morphology, purity of the sample, and temperature [12±16]. Tritium release is produced when the samples are purged with argon or helium, and further favoured when the gas is enriched with about 0.1% of hydrogen [10]. Even though all these studies represent a great eort to understand the mech- anisms of diusion of these important species, the exact mechanism by which tritium diusion is activated and blocked as temperature rises in the interval 300±1200 K remains largely unknown. In sintered solid breeding materials, tritium release is composed of a series of mi- gration steps and they still remain to be elucidated. Simulation techniques may be very useful to elucidate the mechanism of tritium release [9]. However, some important characteristics of the system are to be deter- mined before one is able to properly simulate the be- haviour of tritium in the Li 2 O lattice. As a ®rst step towards this end we present in this paper molecular dynamics simulations of the Li 2 O structure in order to study the role of non-stoichiometry in the temperature Journal of Nuclear Materials 280 (2000) 295±303 www.elsevier.nl/locate/jnucmat * Corresponding author. E-mail address: pfei@xanum.uam.mx (H. Pfeier). 0022-3115/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 1 1 5 ( 0 0 ) 0 0 0 6 3 - 5