As seen in Table 1, the first wall must handle high plasma surface heat fluxes under normal operation and volumetric heat loadings due to the penetrating neutron and electromagnetic radiation. Surface heat loading is dependent on line-of-sight distance from the plasma and can be as high as several MW m— . These surface and volumetric heat loadings will induce temperature gradients on the PFMs and corresponding thermal stress, and stresses at the interface between the PFM and the heat sink. For example, if one assumes the ideal case of a 2.5 cm thick, infinitely wide graphite plate that is perfectly bonded to a 50 °C copper heat sink, the thermal stress at the graphite-copper interface for a heat flux of 5 MW m—2 has been shown to be 200 MPa.3 The ability of the PFC to withstand this heat flux and thermal stress will depend both on the material prop­erties and the component design. The two most obvi­ous design parameters are the thickness of the PFM and how it is attached to the heat sink. The critical material property of thermal conductivity, which to a great extent can be engineered to optimize con­duction to the heat sink, is a strong function of temperature. As discussed later in Section 4.18.3, this property and other performance properties such as elastic modulus and strength are also highly depen­dent on radiation-induced displacement damage. A typical design for a fusion reactor divertor is shown in Figure 2. In this design, the heat flux strikes the surface of CFC composite blocks and the heat flows into a water-cooled copper tube that has been brazed inside the block. The PFC is bolted to a stain­less steel support structure. This configuration ofPFC is called the monoblock structure, as compared to the flat plate and saddle types inset into Figure 2.

To provide a quantitative comparison of candidate PFMs, a number of figures of merit (FoMs) have been derived, one of which may be written as follows:

K Sy

a E(1 — v)

where K is the thermal conductivity, sy the yield strength, a the thermal expansion coefficient, E the Young’s modulus, and n the Poisson’s ratio. High values of FoMth provide guidance to superior
performing candidate materials. Figure 3 shows a comparison of the three primary candidate PFMs: graphite, beryllium, and tungsten. Graphite has been further broken down into fine and coarse-grained (Poco and H451 respectively) graphites, and a high-quality one-dimensional (1D) fiber architecture (MKC-1PH) and a balanced weave 3D fiber architecture (FMI-222) CFC. In Figure 3, it has been assumed that the high thermal conductivity direction for the 1D CFC is oriented at a normal angle to the surface of the PFC. From Figure 3, it is apparent that the graphites and graphite fiber composites, which possess higher strength and thermal conductivity, exhibit thermal FoMs considerably higher than either beryllium or tungsten. Thus, strictly from a thermal stress point of view, high-conductivity and high-strength graphite materials would be considered superior under normal operating conditions for fusion PFCs.