As suggested in Fig. 13.1, the first wall in a typical MCF reactor must directly face the fusion plasma. Hence, it will intercept the fusion neutrons, bremsstrahlung radiation and cyclotron radiation, as well as any plasma constituents which leak across the outer magnetic field lines. Therefore, the wall must maintain structural integrity against particle and electromagnetic radiation damage as well as against stresses induced by temperature gradients and pressure forces associated with vacuum requirements.

Radiation damage in a d-t fusion first wall is largely associated with the 14.1 MeV fusion neutrons. These neutrons have two important effects: i) knock-on collisions that displace nuclides from their normal lattice positions causing internal voids in the microstructure and ii) neutron capture reactions which result in 4He production and hence a build-up of helium pressure in the material lattice; the latter causes a volumetric swelling of the material since helium is relatively immobile and does not rapidly diffuse out of the structure except at very high temperatures. These (n, a) reactions typically have thresholds requiring neutrons of energy in excess of several MeV; thus, the swelling phenomenon is most pronounced in d-t fusion devices due to the high flux of 14 MeV neutrons.

Radiation damage effects may be contained by setting a limit on the neutron wall loading. The average loading, here represented by A„, is defined as

fnpA*Yr

An= ———————- (13.2)

A-w

where Рш(г) is the fusion reaction power per unit volume, Vc is the fusion core volume, Aw is the total wall area and fn is the fraction of the fusion energy carried by the neutrons; the commonly used units for this wall loading are MW m 2. For ICF, however, note that-due to the high density of the compressed pellet- neutrons can be absorbed to some amount in the dense fusion plasma. For reasons of simplicity, we neglect this affect here.

The above definition can be expressed in a more explicit form by using the appropriate equation for the power profile Pfu(r). Adopting the simplified geometry of an axisymmetric torus with circular cross section, and assuming only a radial dependence for the fusion power, gives for Eq.(13.2)

in the case of d-t fusion. Here 7 is the conversion factor of MeV/s to MW, a is the minor radius of the torus and R0 is its major radius. Note that it is the fuel ion densities as well as their temperature as a function of radius which enter as the determining space-dependent functions.

Limits on the neutron wall loading Л„ depend on the specific design and are most commonly set by radiation damage as it affects the component’s lifetime. Recent designs generally specify a range of 1 — 5 MW-m"2 for this parameter.

Additional considerations now need to be added. For example, the power associated with Л„ is not absorbed in the first wall itself since most of the neutrons are transmitted more deeply into the blanket with little attenuation. Thus, actual thermal wall loading must be evaluated separately based on (i) the incident bremsstrahlung and cyclotron radiation, (ii) direct neutron interactions, and (iii) the interactions associated with backscattered neutrons. Since the radiation is largely absorbed near the front surface of the wall, the surface temperature is strongly dependent on this power flow. Indeed, surface heat fluxes over 1 MW-m"2 may be difficult to transmit without exceeding surface temperature limits set by vaporization pressure considerations.