Martin, R L

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Particle Accelerators, 1992, Vols. 37-38, pp. 453-457Reprints available directly from the publisherPhotocopying permitted by license only© 1992 Gordon and Breach Science Publishers, S.A.Printed in the United KingdomTHE FUSION-FISSION BURNERRONALD L. MARTINMartin Energy Systems, 901 S. Kensington, LaGrange, IL 60525.(Received 3 December 1990)The 14-MeV neutron spectrum from Inertial Confinement Fusion is unique in its ability to transmute thefission products of light-water reactors with the n, 2n reaction. This transmutation would occur in theinnermost of three independent concentric blankets of the reactor. Studies of fusion materials can also bedone in this blanket in much larger volumes than in accelerator facilities proposed for this purpose. Sincethe 14-MeV neutrons induce fission in all actinides, including 238U and 232Th, they can produce fissionenergy from spent fuel rods of LWR reactors and from heavy metals (U, Th) in the isotopic concentrationsmined. Generation of fission energy would take place in an intermediate blanket of the combinedfusion-fission hybrid reactor. Both inner blankets of the reactor act as neutron multipliers. Breeding oftritium and fissile material can therefore be done by moderating the neutrons to thermal energies in anouter blanket of the reactor. Heavy Ion Inertial Fusion machines appear to be much more valuable forthese applications than for the generation of fusion energy alone.INTRODUCTIONSeveral authors have suggested fusion-fission hybrid reactors l - 8 . The activity on thissubject seems to be confined to the decade of the 1970s with little or no followup inthe 1980s. Wolkenhauer (1972) pointed out that transmutation of the most dangerousof the fission products in the short term, 90Sr and 137Cs, would only be attractivewith 14-MeV neutrons by the n,2n reaction, and then only with neutron fluxes of1016 njcm 2 s. The latter represents a wall loading of 225 MWjm2 , much higher thanconsidered realistic by most reactor designers, who generally design for a 30-yearlifetime for the first wall.A neutronic analysis of an ICF fusion-fission hybrid was carried out by Berwaldand Duderstadt (1979). That design was seen as an actinide burner with a significantenergy production, operating at a wall loading of 5.7 MWjm2 . With a fusion neutronflux of about 2000 MWth, a fission energy of 22 000 MWth was indicated. Therecirculating power fraction was thus reduced by an order of magnitude over thatof a pure fusion device. The criticality eigenvalue, Kerr, was 0.874 in the fission blanketof about 5-m inner radius and a thickness of 25 cm. The actinides were contained ina liquid tin solution with the actinides having a volume fraction of 6.4%. Considerablemoderation of the fusion neutrons occurs so the neutron energy spectrum resembledthat of an LMFBR with a 14-MeV fusion peak.In view of the present concern about environmental and safety aspects of high-levelradioactive wastes (HLW) it seems compelling to reexamine the potential of inertialfusion by heavy ions (HIF) for solution of both the actinide and fission products453454 R. L. MARTINproblem. Considerable progress has been made in the past decade on technicalfeasibility issues of HIF. The capability of HIF for these applications should becompared with alternative concepts presently under consideration.The present paper will briefly discuss some issues on three goals for using HIF asan intense source of 14-MeV neutrons. The three purposes are represented by threeindependent blankets surrounding the point source, each of which could prove usefulin its own right. They also could be complementary when utilized together. Theseare the innermost blanket for transmuting fission products, the intermediate blanketfor fissioning actinides, and the thermal energy blanket on the outside. The sourceis assumed to be that produced by 4 MJ of heavy ions (A 2 200) on a DT targetwith a gain of 75. At a 10 Hz rate this represents a 14-MeV neutron flux of 3000 MW,or about 1.3 x 1021 neutrons per second. The intermediate fission energy blanketwill be discussed first.2 THE FISSION ENERGY BLANKETThe fission cross sections of all actinides are larger with 14-MeV neutrons than withI-MeV neutrons and are uniformly above 2 barns (except for 238U at 1 barn and232Th at 0.35 barn). Some of these cross sections are shown in Table 1. In addition,the neutron capture cross sections, even for 238U, are small at 14 MeV. Thus allactinides can be made to fission with the release of large amounts of energy. Thishas very important consequences in that spent fuel rods of LWRs would be suitablefuel without enrichment with fissile material. The possibility of utilizing 238U directlyin isotopic concentrations as mined would avoid the necessity of breeding andseparation of fissile material.While the reactor concept proposed by Berwald (1979) appears feasible for thefission energy blanket, an alternative scenario might be to preserve the hardness ofthe 14-MeV neutron spectrum as much as possible because of the attractive con-sequences. The fuel could be contained as solid fuel rods in a thin cylindrical blanketwith a thickness such that 50% or less of the 14-MeV neutrons would interact withactinide isotopes to cause fission. Since each fission would release about 200 MeV,TABLE 1Fission Cross Sections (barns)1 MeV 14 MeVTh-232U-238Np-237Pu-239Pu-240Am-243Cm-2440.0020.0151.51.71.51.02.00.35*1.0**2.42.42.22.52.5* + 1.5 barn n,2n** + 1.5 barn n,2n and n,3nTHE FUSION-FISSION BURNER 455a great deal of energy would still be produced, and perhaps 7-8 metric tonnes (T)of actinides would be fissioned in one year. This would be equivalent to thetransuranic (TRU) content (with fission products and uranium removed) of700-800 T of spent fuel rods. In view of an estimate of 20,000 T of spent fuelrods in cooling ponds presently in the U.S., this result would have a significant impacton the spent fuel rod problem. Proposals which only deal with the minor actinides(TRU without Pu isotopes) appear to do little to solve this problem because 900/0of the TRU wastes are Pu and removal of it from spent fuel rods represents asignificant proliferation and disposal problem.The spent-fuel-rod problem is brought into sharp focus, however, by consideringthat 100 light water reactors (LWR) in the U.S. are each producing about 30 T ofspent fuel rods per year. What clearly seems required is gradual replacement ofexisting reactors with a second generation of reactors that eliminate this problem(such as the Integrated Fast Reactor being developed at Argonne National Labor-atory-which also burns all actinides) and resolving the existing spent-fuel-rodproblem over several years with the best means possible. To permanently dispose ofspent fuel rods by burial would be to ignore the enormous energy content theyrepresent, particularly in the form of 238U.The design of the thin fission energy blanket proposed cannot be optimized withoutdetailed knowledge of all cross sections (including elastic and inelastic), considerationsof cooling and energy extraction, detailed neutron transport calculations, etc. Thedegree of criticality is also a key issue because of the desire to keep the K factor low(perhaps 0.5) in order to avoid the safety questions of dealing with a near-criticalblanket. All these questions are beyond the scope of this paper. It appears, however,based on these ideas, that a realistic blanket design could be achieved because of thevery large neutron flux of the HIF source.3 THE FP TRANSMUTING BLANKETThe purpose of the fission-product-transmuting blanket is the elimination of the mostdangerous of the fission-product isotopes. The most serious of these are 90Sr and137CS, with half lives of 29 and 30 years, respectively. They are produced in sufficientquantities to dominate the fission-product hazard in spent fuel rods for the first fewhundred years. Also important are the long-lived isotopes 1291 and 99Tc. The firstthree of these four can be transmuted with the n,2n reaction into isotopes whichhave much shorter half lives and therefore decay quickly to stable isotopes (98Tc isalso a long-half-life isotope, so the n,2n reaction with 99Tc is not useful. A thermalneutron spectrum with the n, y reaction is useful for transmuting 99Tc and will bediscussed later).The threshold for the n, 2n reaction in Sr is about 12 MeV and is somewhat lowerfor heavier isotopes. Assuming a I-barn cross section and a neutron flux of1016 njcm2 s, the reaction rate per atom is then 10- 8 js, or about 12 times faster thanthe natural decay rate of 90Sr or 137CS. The required mean residence time ofradioactive isotope in the transmuting blanket is then 3.2 years. If the stable reactionproducts are removed and replaced with the radioactive isotope when 10% of the456 R. L. MARTINblanket loading is transmuted (~every 4 months), then the transmutation rate isnearly constant and one full blanket loading is transmuted in 3.2 years of full timeoperationAchieving a flux of 1016n/cm2s will require the first wall of this transmuting blanketto be one meter fron1 the implosion. The survival of the first wall under thesecondition is a major question in the feasibility of such a blanket. While the intenseneutron flux does not appear to be a short-term problem because the neutronspenetrate the first wall, the x-rays and target debris produced in the implosion areabsorbed on the surface and will vaporize any material. A liquid surface that can bevaporized and replaced between each target implosion thus appears essential. With300 MJ of neutrons per implosion, the x-rays and target debris are estimated at 80 MJand 20 MJ, respectively. This problem has been studied by Hassanein9 and aconceptual solution incorporated into the HIBALL reactor design with a liquid filmof LiPb formed by the seepage through porous tubes of SiC fibers. Alternative designsinclude the Li waterfall concept of Maniscalco. 10 The first wall in the HIBALLconcept was 3 m from the implosion. At this distance the neutron flux would bemarginal for this transmutation application, so innovative ideas to achieve theshortest distance and highest flux possible are required. Even the possibility of havingto replace the blanket periodically because of accumulated neutron damage shouldnot be ruled out.A flux of 1016/cm2 s of 14-MeV neutrons over an area of 105cm2 would obviouslyserve the needs of a fusion materials study facility. The volume available for materialdamage studies is several orders of magnitude greater than that possible withaccelerator driven D-Li sources, even for 1000 rnA of deuterons at 35 MeV. Evenlower fluxes, hence larger radius of the first wall and larger useful volumes might beuseful for this application.If the thickness of the fission-product-transmuting blanket be such that no morethan 25 % of the 14-MeV neutrons interact via the n, 2n reaction, then nearly 750/0would be transmitted to be incident on the fission-energy blanket with the full energy.One or two elastic scatterings would not seriously degrade the 14-MeV energyspectrum (the energy loss is about 1.5%, average, per elastic scattering). Inelasticscattering, however, must be taken into account. The rate of transmutation of fissionproducts depends only on the total neutron flux rate and the blanket thickness.Assuming a 25% reaction rate, a mass of 2 T of fission products would be transmutedper year. This is to be compared to the production rate in a 1000-MWe LWR plantof a total of 64 Kg of 137CS, 90Sr, and 1291 per year. If practical chemical processingwere not able to separate the radioactive isotopes from the stable species, then theproduction figure would be 188 Kg/yr, and one transmuting blanket would stillhandle the output of 10 LWRs.4 THE THERMAL ENERGY BLANKETBoth of the two inner blankets are neutron-multiplying blankets so that emergingfrom the fission-energy blanket is a very large neutron flux; its energy spectrum is aTHE FUSION-FISSION BURNER 457combination of the n, 2n reaction spectrum, a fission spectrum, and a residual 14-MeVfusion peak. These neutrons can be moderated to thermal energies in an outer blanketand utilized to transmute 99Tc and to breed tritium for the fusion reaction, inaddition to supplying heat.5 CONCLUSIONHigh-level radioactive wastes from fission reactors are a serious worldwide problem.The potential intense source of 14-MeV neutrons represented by inertial confinementfusion machines driven by heavy ions appears to be attractive for addressing thisproblem. It can extract energy from all actinides including 238U and transmute themost dangerous fission products. It therefore has the potential of impacting thespent-fuel-rod problem of LWRs and of greatly extending the usable energy resourcesof uranium. It could include a fusion materials study blanket with a volume sufficientto acquire engineering characterization of materials under irradiation.Considerable development of the accelerator, fusion target, and reactor design isrequired to produce credible designs for such a facility. A major worldwide effort isrequired if definitive answers are to be achieved in 4-5 years with a prototype machinein 10-15 years. This time scale represents the author's view of what is feasible,provided that conservative design and reliability are given precedence over issues ofminimizing cost. The increased probability of success with a higher-energy driverthan is normally proposed, capable of igniting simpler targets to lower gain, seemsto outweigh the additional costs involved. The fusion-fission concept presented seemssufficiently promising that it should be compared with other proposals to addressthe high-level radioactive waste problem.REFERENCES1. L. Gore, Nucl. Sci. Eng. 53 319-323 (1974).2. K. Chi and R. Klein, Trans. Aln. Nucl. Soc. 23 262-263 (1976).3. L. Jenquin, Trans Am. Nucl. Soc. 23 549-551 (1976).4. W. Seifritz, Trans. Am. Nucl. Soc. 23 27-29, (1976).5. b. B"ender and J. Lee, Trans. Am. Nucl. Soc. 23 24-25 (1976).6. A. Cook and J. Maniscalco, Trans. Am. Nucl. Soc. 23 25-26 (1976).7. W. Wolkenhauer, Trans. Am. Nucl. Soc. 23 92-93 (1976).8. D. Berwald and J. Duderstadt, Nuclear Technology 42 34-50 (1979).9. A. Hassanein, J. Nucl. Mater. 141-143, 221-225 (1986).10. J. Maniscalco, in Proceedings of the Heavy Ion Workshop, Brookhaven National Laboratory, p. 73(1977).

The fusion-fission burner

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