Investigation of the radiation effects upon the stability of fusion magnet coils

The deleterious effects of the D-T fusion radiation environment upon the stability of the cable-in-conduit conductor (CICC) magnet coils have been both qualitatively and quantitatively investigated. Until now, no systematic and accurate analysis of the fluence dependence of the stability of these coils has been performed, and designs have been primarily concerned with the stability of the coils at start-up. The analysis presented here shows that stability as a function of fluence (reactor operating time) degrades much more quickly than previously anticipated. This rapid degradation of coil stability has potentially profound design ramifications. The basis for the present analysis has been a code called MagRad, specifically developed for the purpose of predicting the stability of a fusion magnet coil as a function of fluence, given the coil geometry, flow parameters, and initial materials characteristics. Radiation has significant effects upon some of the basic materials parameters of the coils, such as the stabilizer resistivity and the critical temperature and upper critical field of the superconductor. The code, CICC, developed by R.L. Wong, together with the Dresner formulation for the limiting current, have been incorporated as reliable predictors of the stability of the coil at start-up, which is used as input for MagRad. Most recent data is used with respect to radiation effects upon the materials properties of the coil. Significantly, inappropriate assumptions used in the semi-analytical form which predicts upper critical field as a function of fluence (which has hitherto been widely accepted and used in stability codes) have been corrected in this present study, and a new and much improved empirical form which represents a fit to the data is presented. That the new form is more suitable than the previous one can be clearly seen in that while the previous form gives a peak upper critical field, B_c20, for binary Nb₃Sn of about 63 T at a fast neutron fluence of about 25×10¹⁸ n/cm², the new form mirrors the data which gives a peak B_c20 of about 25 T at a fast neutron fluence of about 4×10¹⁸ n/cm² (at zero fluence B_c20 is about 24 T). Additionally, these inappropriate assumptions are discussed in a qualitative manner, and correction is given to the underlying theory. In its primary functional capacity MagRad has been used to analyze the stability of a possible International Thermonuclear Experimental Reactor (ITER) Engineering Design Activity (EDA) coil design, as a function of both fluence and superconducting material