Annals of Nuclear Energy, Vol. 2, pp. 29 to 32. Pergamon Press 1975. Printed in Northern Ireland

THE FUSION BREEDER CONCEPT

PETER FORTESCUE General Atomic Company, P.O. Box 81608, San Diego, Cal. 92138, U.S.A.

(Received 12 August 1974)

Abstract--Some practical aspects and broader consequences of strategies that have been proposed for the association of fusion and fission processes are reviewed.

It is concluded more particularly that the build-up of the technology necessary for the ultimately desired substitution of fusion for the present fission source of nuclear power could be much advanced by an interim period of the application of fusion entirely to the breeding of U-233 for the large number of fission reactors that will be coexisting during this period.

Principal reasons for this view are the considerable technical easements of this approach, and the very large gain in effective energy release per fusion event, which offers economic advantage long before achievement of the classic definition of the fusion "break even" point.

I N T R O D U C T I O N

Freedom from the highly undesirable fission by- products associated with the only presently available access to nuclear power is surely a prime attraction of fusion as an alternate basic energy source. It is therefore perhaps surprising to hear of proposals to bring the fission process back into the picture as, for instance, by introduction of uranium or thorium into the fusion reactor's blanket material. Such a step would not be taken lightly, or without prospect of very material reward. Nevertheless, several authors (Leonard, 1973; Horoshko, 1974) have referred favorably to the potential reward of such hybrid schemes.

The objective here is to review briefly the thinking associated with these ideas and, more importantly, to distinguish among the number of modes of assoc- iation of the fusion and the fission processes, ranging from use of fertile material to boost fusion blanket heat production to the extreme case of the operation of a fusion plant purely as a fissile fuel factory, avoiding the technical problems of direct fusion plant power production altogether.

This note is, in fact, more particularly concerned with the case for this latter route, and some of the broader issues involved in its choice.

G E N E R A L O B J E C T I V E S

The basic incentive for considering any kind of combination of fission and fusion technology is that the net energy released or eventually made available, per initial fusion event, can be enormously increased if interaction of the accompanying freed neutron with a fertile or fissile atom is allowed. This increase in energy yield comes from a basic twelvefold energy release advantage for fission as compared

with D.T. fusion, supplemented by neutron multi- plication effects which can occur both in the fusion reactor's blanket, and to an even greater extent in fission reactors fed by the fissile material so produced. Total energy release gains of nearly 100 can, in principle, accrue from appropriately associated operation of fusion and fission reactors, so we are thus talking of no small effect. Indeed, at first glance, this might look like an offer too good to refuse, but the matter is not so simple as this.

It must be remembered that the primary fuel for fusion (deuterium) is both unlimitedly available and even cheap by the appropriate standards, so the prospect of even a hundredfold reduction in supply can amount to no big deal in the area of raw fuel conservation and cost. The secondary material (lithium) is relatively cheap and in ample supply. The value of the fuel in fact arises virtually wholly in the effort required to prepare it for the moment of fusion, and this includes its part conversion to tritium (for D.T. fusion) and, above all, the provision of the special environment necessary to get fusion at all.

So long as the cost of the equipment necessary to provide these functions remains high, it will clearly pay handsomely to increase the energy yield, for the number of fusions needed rather than the number of fissions ultimately produced sets this element of the c o s t s .

Another practical handicap of the low energy yield accompanying pure fusion is that the required neutron flux per unit power output is correspondingly very high and this poses very severe materials problems. The basic improvement in energy yield provided by introduction of the fission process, reflected in terms of reduced total neutron dose per

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30 P. FORTESCUE

unit of energy produced, represents a practical advantage which is certainly not small and could even spell success or failure.

So, in short, reduction of plant cost and minimiza- tion of engineering difficulty, rather than fuel supply considerations, are main incentives for mixing fusion and fission. There is, moreover, a special interest in the extreme case of taking no useful power at all from the fusion plant, in that just dumping the fusion heat (which might be only a few per cent of the total made available by the fuel produced) enormously eases technical problems by avoiding need for high temperatures and continuous duty.

We can of course at present only dimly appreciate the real problems of fusion power plants and, to this extent, must realize that technological progress may well overcome presently apparent difficulties.

The desirability of a mixed energy source phase of power production may therefore, in the end, prove to be a transitory one. But even so, the period involved may well be a long one, in which case the opportunity thus afforded for fusion to make an early and useful, if not total, contribution to the world's energy needs should considerably accelerate the development of the necessary technology. In particular, the prospect of getting 50-100 times the energy release per fusion event should greatly advance the date by which fusion consumption and generation power can be considered as breaking even.

All present indications are that, by the time fusion technology is ready, there will already be enormous deployment of fission reactors, either operating or under construction; thus such a phased introduction of fusion is desirable anyway. The further pro- duction of fission products associated with the continued use of fission power in this interim period might add little to the then current accumula- ted total world fission product disposal problem. Indeed, a period of working with fission-fusion plant could actually reduce the eventual world fission product pool if its earlier possible intro- duction sufficiently speeded the development of pure fusion.

With these general observations in mind, we may now look more specifically at kinds of fission-fusion association that have been proposed, the mech- anisms involved, and the importance of their differences.

MODES OF FISSION-FUSION ASSOCIATION

All approaches to supplementation of the basic 17.58 MeV energy release associated with the D.T.

fusion reaction depend on multiplication of the one neutron freed by this reaction, by access to (n-2n), (n-3n) and fission reactions, and on taking advant- age of the much larger (192 MeV) energy yield of fission. All these effects can contribute substantially to the total energy yield, and can be provided by the simple incorporation of fertile atoms (~32Th or 2asU) into the material of a suitably cooled blanket external to the fusion chamber vacuum system, with provision for preferably continuous extraction of bred fissile material.

A very important point here is that these energy- augmenting effects need not be confined to the fusion reactor's blanket. Indeed, the neutron multiplica- tion occurring in the recycled fuel of a high conver- sion ratio fission reactor, topped up by the fissile material produced in a fusion reactor's blanket, can far exceed that occurring directly in this source.

A further benefit conferred by the admission of enhanced neutron multiplication and fission pro- cesses into the picture is that it is no longer necessary or desirable to breed new tritium in situ, which eliminates the main case for use of a lithium coolant. This feature could represent a considerable tech- nological easement. Use of lithium in the vastly extended quantity needed to allow it to double up as both coolant and tritium source might, in fact, eventually greatly strain resources of this material.

Transferring the function of tritium production to the fission reactors of an associated fission and fusion reactor system is in any case desirable, for in this way full benefit is taken of effective neutron multiplication occurring both in the fusion reactor's blanket, and from the fission reactor's internal conversion ratio.

The schemes that we are looking at, then, essen- tially comprise a D.T. fusion chamber, a surrounding cooled blanket incorporating fertile material in which some neutron multiplication, fission and energy release occur, and an associated but not necessarily adjoining fission reactor. This reactor yields the rest of the total energy made available, and also houses the tritium breeding arrangements.

Within this framework, there however remains an important degree of freedom in the choice of residence time for the fusion blanket fertile material which can considerably modify the behavior of the system. If we wish to encourage the largest energy release at the fusion plant itself and minimize the size of associated fission reactors needed, we can slow the passage of fertile material through the fusion blanket and thereby acquire a high build-up of fissile material there. This avenue, in the limit, would lead in effect to a fusion source sitting inside a

The fusion breeder concept 31

critical fission reactor, an extreme which of course would not be contemplated. Indeed, merely boosting the fusion plant's energy output, somewhat by progress in this direction, although attractive from the standpoint of total energy derived, poses sub- stantially increased technological problems, rather than the easements which are so much desired.

At the other end of the scale, we can so hurry the blanket fertile material passage that very little fissile build-up occurs, and local energy generation is minimized. This route, which in the limit treats the fusion plant simply as a kind of fuel factory, would be favored if the fission reactors supplied had a high conversion ratio and could provide a cheaper, more efficient, or otherwise more practically desirable form of power plant. Use of thorium rather than uranium addition to the fusion blanket would be favored in this case, both to minimize fast fissions and therefore heat production there, and to provide ~38U rather than plutonium to secure the highest associated fission reactor breeding ratio. Interest in this mode of operation is strengthened by the prospects in this direction that seem to be offered by HTGR-type reactor, which at the period of practical fusion contribution may well be in thoroughly established association with gas turbines, highly efficient bottoming cycle additions, and the high conversion ratio benefits of 233U fissile feed--the latter, in the absence of fusion plant, being derived from fast breeder blankets.

The extreme approach to such a fusion-fission association would be to so minimize the fusion- produced energy (which might, at 17 MeV per fusion, amount to only few per cent of the total), that the heat from the fusion plant could be simply dumped, or relegated to low temperature application.

The objectives of this mode of operation would be threefold:

(1) The effective neutron multiplication in a high gain HTGR with 233U feed is substantially higher than that obtainable by in situ fission in the fusion blanket.

(2) Engineering ploblems of the fusion reactor could be immeasurably eased by dropping the requirements for high temperature and/or highly pressurized coolant, and need for in situ tritium production. This would, in particular, open the way to accommodate continuous fuel transport by simple solution in the fusion blanket coolant.

(3) It would avoid need for redeveloping the whole power chain, and much more readily permit pulsed fusion operation.

FAST BREEDERS VS FUSION AS A FUEL SOURCE

The rather extreme scenario just mentioned amounts in fact to passing the role of fuel production from the fast breeder to the fusion plant. It is therefore necessary to give some preliminary thought to the relative potential of fusion plant and fast breeders as fuel factories. Put more simply, granted that a fusion plant can operate as a fission fuel factory, why encourage this if the fast breeder can already accomplish the same thing. There are in fact some fundamental reasons for expecting fusion eventually to provide the more economic approach to fuel manufacture. Principal among these are:

(a) The fusion plant has to carry a negligible fissile inventory.

(b) The primary neutron supply comes from relatively cheap and ample resources.

(c) The fusion process frees many times more neutrons for breeding per unit energy generated than does the core of even the highest gain FBR. It is thus less encumbered by economic necessity to usefully employ this energy production, which is not the prime purpose of a fuel factory.

(d) The inherent safety of the fusion process could considerably reduce expenditures in this area.

(e) The fusion plant lends itself very well to continuous refueling, whereas this is virtually impossible in a fast breeder using solid fuel.

CONCLUSIONS The eventual development of fusion power, with

its particularly attractive prospect of nuclear energy without fission products, may well be speeded substantially by an interim period of direct associa- tion with the fission process.

A specially interesting case of such association is the relegation of the fusion plant to the sole duty of fertile-to-fissile material conversion, the eventual energy from which could be over 50 times the fusion energy released in its production. Such an approach could speed the eventual development of pure fusion power production by substantially reducing the degree of technical advancement necessary to achieve the net energy gains required to justify wide industrial support.

We cannot of course be sure at this time that practically attractive pure fusion power plant will ever materialize, but it seems at least clear that production of fissile fuel in a much easier task to which the fusion process is indeed particularly well suited.

It is therefore suggested that understandable preoccupation with the "clean power" aspects of

32 P. FORTESCUE

fusion should not inhibit attention to fusion- fission combinations which might provide the fastest route to the desired goal, and indeed even thus reduce the eventual cumulative world pool of fission products.

REFERENCES

Horoshko, R. N., Hurwitz, H. and Zmora H. (1974) Ann. nucl. Sci. Engng 1, 223-232.

Leonard, Jr., B. R. (1973), Nucl. TechnoL 20, 161- 178.

See also American Nuclear Society and Atomic Energy Commis-

sion, First Topical Meetinff on The Technolosy of Controlled Nuclear Fusion, San Diego, Calif., April 16-18, 1974.

Draper, E. L. and Gage, S. J. (1972), The fusion-fission breeder: Its potential in a fuel-starved thermal reactor economy. Technology of Controlled Thermonuclear Experiments and the Engineerin 8 Aspects of Fusion Reactors, Austin, Texas, Nov. 20-22, 1972, Conf. 72111.

Hiifele, W. and Start, C. (1974), d. Br. nucl. Energy Soc. 13, 131-139.