A nuclear fusion core may be surrounded by a blanket in which the neutrons from the fusion reactions sustain tritium breeding as well as fissile fuel breeding and — to a varying extent-fission energy production. The fissile fuel bred in such a hybrid is to supply the fuel requirements for "client" fission reactors and, by the consequent energy credit generated, alleviate some energy balance constraints on the fusion component.

15.1 Conceptual Description

The potentially recoverable energy from a fusing domain consists of electromagnetic radiation, the kinetic energy of charged nuclear particles, and the kinetic energy of neutrons.

The neutron is of particular interest for the following reasons: while its kinetic energy can be utilized-that is converted into thermal energy as it slows down by nuclear collisions in the blanket-the neutron itself remains a valuable particle to induce selected nuclear reactions. As previously discussed, the tritium for fuel self-sufficient d-t fusion reactors must be bred by neutron capture in lithium. Hence, for d-t fusion, one essential use of the fusion neutron is to breed tritium in order to close the fuel cycle. The reactions and reaction linkages involved are

(15.1)

Further thought suggests other productive uses of the 14.1 MeV fusion neutron. With its high kinetic energy, the fusion neutron has access to numerous (n, xn) neutron multiplying reactions, Fig. 15.1, and these reactions can result in a number of neutrons in excess of what is required for tritium breeding. Consequently, the spare neutrons can be used for other purposes; alternatively, if a fuel cycle such as catalyzed-D described in Ch. 7 is used, tritium breeding is not required so that the entire neutron population is available for other purposes. An interesting system concept is to surround the fusion chamber with a blanket of fertile nuclear fuel, that is 238U or 232Th, so that the neutrons can breed fissile fuel and/or aid in sustaining fission reactions in the intrinsically subcritical blanket.

Since each fission event yields -200 MeV, the blanket serves the function of energy multiplication and can also serve as a "fuel factory" for fission reactors. This concept can be further clarified by considering three dominant classes of neutron reactions which, for illustrative purposes, are assumed to occur in separate regions of the blanket.

Immediately adjacent to the fusion core, a region is envisioned with a high concentration of isotopes possessing a significant (n, xn) neutron multiplication cross section (i. e., x > 1) as shown in Fig. 15.1; the dominant reaction is therefore of the type

n+ AZ^>xn + A~X+1Z, X>1 (15.2)

where AZ represents a typical neutron multiplier. A location close to the fusion core capitalizes on the high energy of fusion neutrons to increase the neutron population by (n, xn) reactions which require neutron energies in excess of specific thresholds.

The next blanket layer is taken to contain fertile materials (232Th, 238U) which transmute into fissile materials ( U, Pu) by the processes
(15.3a)

or, more simply

n + g-^f (15.3b)

with g representing the fertile fuel and f denoting the fissile nuclei bred, some of which may be fissioned by neutron absorption and thereby generate significant amounts of energy.

The outer most blanket region is used for tritium breeding by neutron capture in lithium via

(15.4a)

or

n + £->t, (15.4b)

where £ denotes the lithium fuel.

It is thus evident that the hybrid blanket produces both energy and fissile fuel, and depending on the design objectives, one of these functions can be emphasized. The ratio of fissile fuel nuclei bred per unit fusion energy released is therefore an important parameter for the characterization of such a system. For example, in designs emphasizing the fuel factory approach where energy production is de-emphasized so the plant need not be a key contributor to an electrical network-and thereby also freeing it for a more flexible operating schedule-this ratio would be maximized. This may be accomplished by selecting materials in order to minimize fission reactions in the blanket while maximizing fissile breeding reactions.

These three dominant processes and the corresponding blanket domains are depicted in Fig. 15.2. Other arrangements are possible, but the order chosen here for the various regions follows a functional pattern intended to make enhanced use of the fusion-source neutron energy: (i) neutron multiplication is most productively accomplished with high-energy neutrons and hence the blanket section designed for this process occurs close to the fusion core for immediate access to the 14.1 MeV fusion neutrons before they slow down; (ii) next, fissile fuel breeding is best accomplished with intermediate energy neutrons; (iii) finally, after the neutrons have slowed down, the 1/v-dependence of the neutron capture cross section of 6Li ensures efficient tritium breeding.