Letter to the Editors Calculations of the tritium re-emission rate in the DEMO ®rst wall O.V. Ogorodnikova a , M.A. F utterer a , E. Serra b, * , G. Benamati b a Commissariat  a I'Energie Atomique, CEA/Saclay, DRN/DMT/SERMA, F-91191 Gif-sur-Yvette, France b ENEA Fusion Division, CR Brasimone, I-40032 Camugnano, Bologna, Italy Received 10 November 1998; accepted 15 December 1998 Abstract The deuterium and tritium re-emission ¯uxes from the ®rst wall (FW) of the DEMO reactor have been calculated. The in¯uence of temperature of the ®rst wall (FW), surface conditions, and trapping in neutron-produced defects on the tritium re-emission rate has been considered. Ó 1999 Elsevier Science B.V. All rights reserved. 1. Introduction In the case of controlled fusion by magnetic con- ®nement, as it will occur in the demonstration reactor DEMO, hydrogen isotope recycling from the ®rst wall (FW) in¯uences the plasma exhaust during discharge. When the fast hydrogen isotopes interact with the metal, the fraction of the implanted particles is directed back from the surface with the re¯ection coecient r and re- enters the plasma, while the remaining fraction pene- trates in the metal. The penetrated fraction diuses in the metal. When a diusing atom reaches the plasma- facing surface, it recombines with another atom at the surface to desorb as a hydrogen molecule. The hydrogen recycling is characterised by two components: (i) the fast one from backscattering and (ii) the other from surface recombination. Analytical models [1±3] and numerical codes [4±8] have been developed for the calculations of recycling, inventory and permeation of hydrogen iso- topes in fusion reactor design concepts. The purpose of the present investigation is to calculate the deuterium and tritium re-emission rates from the FW of the DEMO reactor due to hydrogen isotope surface re- combination. 2. Input data: diusivity, solubility and sticking factor for hydrogen isotope/®rst wall interaction It is assumed that the FW is represented by a thick- ness L 1  3  10 3 m of 7±10% Cr martensitic steel be- tween plasma and FW coolant. The martensitic steel F82H belongs to the 7±10% Cr martensitic steel class that have undergone some modi®cation in order to achieve better low-activation characteristics compared with those of MANET. The F82H steel is close to the material assumed to be used for the ®rst wall and structure for the demonstration fusion reactor DEMO. The data of deuterium solubility K D s  K s0 exp Q s =kT  and diusivity D D  D 0 exp E m =kT  in F82H steel are taken from [9] and are presented in Table 1. The tritium extrapolated value D T for the diusivity in the F82H steel is de®ned using the classical diusion theory: D T   m D =m T p D D   2=3 p D D : 1 The association desorption J 0i of the ith hydrogen iso- tope on the surface is usually described in terms of the `recombination coecient' K i r [10±16]: J 0i  2K i r u 2 0i ; 2 where u 0i is the concentration of the ith hydrogen iso- tope near the plasma-facing side. According to [14±16] K i r equals the relation of the adsorption coecient to the square of Sieverts' constant: K i r k ad  i =K 2 s : 3 Journal of Nuclear Materials 270 (1999) 368±371 * Corresponding author. Tel.: +39-0534 801 463; fax: +39- 0534 801 225; e-mail: emanuele@netbra.brasimone.enea.it 0022-3115/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 1 1 5 ( 9 9 ) 0 0 0 1 3 - 6