Consider now the case of a d-t fusion power device equipped with a tritium breeding blanket, the purpose of which is to breed tritium for the core. We will retain the steady-state operational mode description and also assume that the bred tritium is extracted from the blanket and supplied to the core on a time scale very short relative to the tritium decay half-life; hence, the decay of tritium need not be incorporated into the analysis. Figure 14.4 provides a schematic of the relevant particle flows and reaction rates.

The two important reaction rates are tritium destruction by fusion in the core

Rd, = Nu, cN,,c <w>dt d3r (14.19)

k:

and tritium breeding in the blanket

Rnu = J J4 (v„) — v„ [an6(v„) • N6 + an7 (v„) ■ N7 ]dvnd V. (14.20)

у. v

rb v n

Here Vc and Vb are the core and blanket volumes, Nn is the neutron density, vn is the neutron speed, N6 and N 7 are the lithium-6 and lithium-7 densities and (J„6 and o„7 are the corresponding neutron absorption cross sections for tritium breeding.

Replacement of the burned tritium requires

Rnl. t — Rdt ■

That is, the tritium production rate must be equal to or exceed the tritium destruction rate. In practice, tritium losses in the overall cycle by radioactive decay and transport into containment walls will occur so that for operational reasons R*n(>, > R*dt; this is incorporated in the tritium breeding ratio C, by requiring

*

C, = > 1 • (14.22)

Rdt

We will next consider some dynamical aspect of the tritium inventory in the blanket which incorporates tritium decay and transport.

Extraction