The hybrid reactor discussed above is characterized by a blanket which sustains both fissile fuel breeding as well as fission reactions. This latter function assigns some fission reactor characteristics to the blanket and consequently may impose similar safety considerations such as possible criticality or loss-of-coolant accidents and fission product release. In order to minimize these problems, it is possible to design a blanket which tailors the neutron spectrum in order to maximize fissile fuel breeding processes. The bred fuel could then be used in various companion fission reactors so that the fusion breeder reactor could be viewed as a "nuclear fuel factory" much as uranium mining and enrichment plants presently serve this function. Indeed, some additional fuel service features could be introduced if desired. For example, the fuel could be enriched in the fusion neutron-driven blanket to a desired level with a minimum of fission product accumulation-that is, rejuvenated in-situ while it is still retained in its cladding-for direct insertion into a fission reactor.

Another appealing extension can be conceived of by drawing upon the d-d rather than the d-t fusion reaction. Though the plasma conditions become more
demanding, the absence of tritium fuel breeding in the blanket and handling does render this fusion cycle very appealing and makes all of the fusion neutrons potentially available for breeding. The scheme is the following

We begin by recalling the d-d and d-h fusion reactions in addition to the base d-t reaction. If only deuterium fuel is supplied and if the reaction product tritium is consumed at its rate of production, then the overall fusion reaction cycle may be represented by

d + d —4 t + p

d + t —4 n + cc d + d —4 n + h 5d —4 2n + cc + p 4- h.

In the above processes, the charged products a and p will be, to a large extent, retained in the plasma and the two neutrons will enter the blanket and breed fissile fuel. While the energetic a and p serve only for plasma heating, the bred helium-3 (h) can either be recirculated in the fuel to provide added energy release via d-h reactions or it could be extracted and used as fuel for small fusion reactors optimized for the reaction

d 4- h —4 p 4- cc. (15.24)

The appeal in this kind of d-h satellite fusion reaction is that the fuels and reaction products are not radioactive and, additionally, neutrons are produced only diminutively by d-d side reactions so that the chamber walls would suffer less activation. Further, the energy associated with the charged particle reaction products would be suitable for transformation into electricity by direct conversion techniques.

These features suggest that small d-h fusion reactors might be placed near populated sites to provide radiologically cleaner and smaller size nuclear energy sources. One difficulty with the satellite approach, however, is that the amount of 3He bred by the hybrid is limited so that the satellite power would only be a fraction of that supplied by the hybrid-client reactor complex. Nevertheless, this could be attractive for specialized applications requiring small electrical plants.

Additionally, and as illustrated in Fig. 15.6, one may conceive of one large central parent d-d reactor simultaneously breeding fissile and fusile fuel for a distributed system of various fission and fusion satellite reactors.

Problems

15.1 If fissile fuel burning were to be incorporated as a fourth layer in Fig. 15.2, where would it be most effectively inserted based on neutron energy considerations?

15.2 Confirm the correctness of the Qfi / (1 — Cfi) term leading to Eq.(15.5).

15.3 Confirm that Eq.(15.21) follows from Eqs.(15.19).

15.4 Undertake a power balance analysis for the Symbiont/Satellite system of Fig. 15.6.

15.5 Estimate the d-h satellite to hybrid power ratio for the system described in Fig. 15.6.