¢._~--9 ELSEVIER Journal of Nuclear Materials 233-237 (1996) 852-856 jnurnalof nuclear materials Stability of the Be-steam reaction and its impact on safety F. Andritsos *, D.A. Sarigiannis htstitute jbr ,~vstems, lulormutic.v and SaJbty. C.E.C. Joint Research Centre. 21020 lspra, ltal,v Abstract Beryllium is one of the major candidate materials for the plasma-facing components of ITER. At high temperatures it represents, however, a considerable safety issue because of its high toxicity and its highly exothemfic and fast oxidation ira case of steam or air ingress into the vacuum vessel of the reactor. Such conditions might occur after a plasma disruption. The work presented here attempts to analyze the transient behavior of the thermochemical system of a reacting plasma-facing surface, identify the reaction runaway conditions, and assess its impact on overall ITER safety. 1. Introduction Beryllium (Be) is one of the candidate materials for the coating of the plasma-facing components of ITER, mainly due to its beneficial properties with regard to Bremsstmhlung radiation (low-Z) and its high thermal conductivity. In the case of air or steam ingress into the vacuum vessel of the reactor it represents, however, a considerable safety issue both due to its high toxicity, and its highly exothermic and very fast oxidation at high temperatures. The chemical reaction of Be with steam or air being exothennic and its rate being strongly dependent on the temperature [1], it is feared that, under certain post acci- dental conditions, it might run away (i.e. the heat produced by the chemical reaction being more than what can be taken away from the reaction front, thus resulting to a temperature increase leading to even higher reaction rates). Such conditions could be realized after a plasma disrup- tion, when part (or even the whole) plasma-facing surface of the first wall (FW) is heated up to temperatures as high as the melting temperature of beryllium. A simultaneous (or subsequent) steam or air ingress in the vacuum cham- ber could cause the Be-steam/air reaction to run away. Corresponding author. Tel.: + 39 332 789599; fax: +- 39-332- 789392; e mail: fivus.andritsos@jrc.it. E-mail: denis.sarigiannis@jrc.iL To date, however, big uncertainties characterize the knowl- edge of both the physics of the plasma quench mechanisms in ITER, and the determination of the most affected areas of the FW and/or the divertor. In order to get a better understanding of the phenomenon and a relative measure of the quantities involved, an extensive parametric study was undertaken, making use of numerical modelling tech- niques and, where possible, analytical calculations. The ultimate goal of this activity is to assess the influence of the various FW design parameters and the effect of the shutdown/disruption scenarios (initial condi- tions of the transient) and the cooling/relaxation mecha- nisms (boundary conditions) to the transient evolution of the Be steam reaction, mainly with regard to its stability and the overall quantity of H 2 produced. Initially [2-5], our work concerned the TAC-4 design of ITER. The work reported here concerns the Interim Design Report (IDR) design. 2. Chemical source kinetics The rate of energy released from the beryllium oxida- tion reaction per unit of first-wall area can be readily calculated as the product of the heat of the reaction ( A Il k ) witla the reaction rate corresponding to the temperature (T) at which the phenomenon takes place, dQ"/dt = Alia(T ) * ,-('/). Using the standard heats of fom]ation of the chemical compounds and substances involved in the Be oxidation 0022-3115/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PII S0022-3115(96)00t46 8