structural materials in nuclear fusion technology ... Abstract. The fusion technological consequences .... The irradiation conditions deal with 5 full-power years.
Fusion Engineering and Design Fusion
Engineering
and Design
29 (1995) 219-224
Comparative radiological assessment of SiC,/SiC composites as structural materials in nuclear fusion technology H.W. Scholz a, M. Zucchetti
b
’
Institute for Advanced Materials, CEC Joint Research Centre, I-21020 Ispra, VA, Italy b Dip. di Energetica, Politecnico di Torino, C. Duca d. Abruxi 24, I-10129 Torino, Italy
Abstract The fusion technological consequences of the radioactive decay behaviour of Sic-based structural material were analysed based on a complete chemical analysis of an industrial SiC,/SiC composite. Radioactivation simulation was performed with the FISPACT-2 code and EAF-2 cross-section library, taking n-spectra from the SEAFP reactor study for first wall and blanket. The decay of radioactivity in SiC,/SiC after shut down and derived radiological quantities are presented and compared with those of SEAFP structural materials. The effective dose equivalent (EDE) to the population from accidental release of activated material was computed with the GENII code. SiC,/SiC performs favourably. Contact dose rates and y-dose rates inside the plasma chamber are given. Assuming an upper limit of IO4 Gy h-’ for robotized maintenance, SiC,/SiC renders the first wall a zone where immediate repair would be possible. For this short-term activation limit, an elemental “critical list” is presented. Long-term decay heat generation and contact dose rates show which material fulfils waste limits. Although Al-26 is generated in the first wall, SiC,SiC is a safely disposable material, owing to other characteristics such as inertness against corrosion.
1. Introduction Thermonuclear fusion reactors offer the possibility of optimization of the elemental content of structural materials to maintain limited radioactivation under operation, and to give favourable resulting radiological characteristics. In recent years, a number of mostly theoretical studies have yielded different alternatives for future radiologically advantageous structural materials, often designated low activation materials (LAMS). The ARIES reactor study gave an important boost to the consideration of Sic-based ceramic matrix composites as structural materials for future fusion reactors [ 11. Continuously reinforced Sic-fibres in a CVI-SIC matrix (SiCJSiC) were proposed. Since the experimental or at 0920-3796/95/$09.50 I(; 1995 Elsevier Science S.A. All rights SSDIO920-3796(94)00261-4
reserved
least extrapolated mechanical and physical properties under radiation conditions specific to fusion were often regarded with priority, some radiological advantages of rather uncommon “outsider” material choices are only now beginning to be investigated in more experimental research programmes. These materials include SiC,/SiC and chromium alloys. Instead of changing some alloying elements in essentially conventional steel compositions, some novel class of material could be developed able to meet the specific and manifold demands of fusion reactor application. SiC,/SiC, for instance, already serves as a structural material in many aerospace applications demanding reliability, and is produced on a scale of 100 t year-’ [2] by, for example, only one of today’s producers in Europe. Its thermomechanical
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H. W. Scholz,
M. Zucchetti
/ Fusion Engineering
properties combined with its oxidation resistance [3] surpass those of all other structural materials. Although a comparable broad database on its irradiation behaviour is presently not available, intensive research is currently taking place in order to assess its potential for fusion technology. This work is intended to clarify the motivation for this time and cost demanding development procedure, covering such key fusion technological issues as reactor safety and operability.
2. The SiC,/SiC
material and its purity
Experimental analyses of the purity of industrial 2D SiCr/SiC material considered in this work were published by Scholz et al. [4]. Table 1 reports the chemical composition and complete impurity control, considering conservatively the results of nuclear y-spectrometric and chemospectrometric methods used. Some methodological information was given in Ref. [5]. The samples used in these analyses were not laboratory material, but were derived from industrial production batches, and can thus be considered typical for the process technology used in industry for large parts of several metres. The emerging purity of this SiC,/SiC is understandable regarding the inherent synthetic character of all the “raw materials” or, better, precursor materials which are used in the production of the fibres [6] and the chemical vapour infiltrated matrix [3]. In all the radiological calculations of the work presented here, a conservative approach to spectrometric detection limits in the chemical analyses was applied. Calculated detection limits (marked with a “ < ” sign in Table 1) for impurities which were not found experimentally are conservatively included as if they were existent quantities.
3. Irradiation conditions: the SEAFP reactor The Safety and Environmental Assessment of Fusion Power (SEAFP) deals with the design of a fusion power reactor in which environmental issues are taken into particular account during the conceptual design activity. More details on this power reactor study may be found in Ref. [7]. The reference blanket for this reactor has L&O as breeder, Be as multiplier and first wall protection, V-5Ti as structural material and He as coolant. The irradiation conditions envisaged for this reactor are at present the most appropriate for evaluation of the low-activation characteristics of long-term fusion reactor materials. The SEAFP reactor first wall (FW) exposure conditions have been simulated (see
and Design 29 (1995) 219-224
Table 1 Chemical composition and detection limits of impurity control for SiC,/SiC composite, produced by the Societe Europeenne de Propulsion (s.e.p.), from Ref. [4] Chemical element
Si C 0 N Au Cl Xe Kr Fe Ar Ir OS Al Tb Ni Na Nd K Ti Cd, Ho, Nb Zn cu Mn Ba Y Sn W V Pb Pd Se Zr Rb, MO Bi, Ce co AS Sb U CS AS Cd La Hf, Ta, Yb Eu Cr LU SC, Sm
Detection
limit
(‘ < ‘) or revealed
Result
(wtparts)
Error
6.57 3.01 3.98 6.20 5.58 3.08
