These studies indicate that synthetic fuel production by direct radiolysis cannot compete economically with other production methods. Low G-values and radiation contamination of products are given as reasons. (MOW

Booth, L. A.

Finch, F. T.

Frank, T. G.

Pendergrass, J. H.

UNT Digital Library

LA-uR -77 -z SS& IvMTEIITITLE: REALISTIC ASSESSMENT OF DIRECT RADIOLYSIS FOR SYNTHETIC FUELSPRODUCTION USING FUSION RADIATION SOURCESAUTHOR(S): J. H. Pendergrass, L. A. Booth, F. T. Finch,and T. G. FrankSUBMllTED TO: ZlldMiami International Conference on AlternativeEnergy Sources, 10-13 December 1979, Miami BeachFloridaBy acmptmsm of this article, the publisher recognizes that tlwU,S Gowmrrem remim ~nonexclusiw. royalty.frealiamseto publish or reprodum the published fem. of this contribu.fion, or to ●ow others to do so, for U.S. Gowrmnent pur-fmses,Ttm Los Alamos Scientific LalmrWo~ requests thai the pub.Ilsher idanti~ this ●sicle m work perforrrmd under th w$.pmsof the U.S. Deparfnwmof Energy.L4kl!llKLO:;ALAMOS SCIENTIFIC LABORATORYPost Off Ice Box 1663 LOOAlamos, New %lexlco 87545An Affkmatfve ActIon/Equal OPpoflunitY-Form No, 636 R3St. No. 2629UNITED STATESn-mm-+”.”- -- --------REALISTIC ASSESSMENTGF DIRECT RADIOLYSISFOR SYNTHETIC FUELS PRODUCTION USINGFUSION RADIATION SOURCESJ. H. Pendergrass, L. A. Booth. F. T. Finch,and T. G. FrankUniversity of CaliforniaLos Alamos Scientific LaboratoryLos Alamos, New Mexico 87545 USAThe use of fission reactor radiatior~, including neutrons and gamma pho-tons, but with emphasis on fission fragment: which carry off -8C% offission energy release, has been extensively investigated for directradiolytic production of large volume chemicals. Potential products in-clude synthetic fuels by wdter or carbon dioxide decomposition, fixednitrogen from air, and ozone from oxygen. Interest in fission reactorradiolysis has largely vanished because of apparently insurmountableobstacles to simultaneous economical achievement of high efficiency inconversion of radiant energy to stored chemical energy of radiolysisproducts and acceptable levels of product radioactivity contamination.More recently, the use of fusion reactor radiations for direct radiolysishas been of interest. Fusion reactor radiations are apparently moresuitable for radiolytic chemical production, but many obstacles to com-mercialization using fission radiation sources must also be faced withfusion sources, albeit to a lesser degree.Because the conditions required for useful thermonuclear energy releaseare less extreme for deuterium-tritium (DT) fusion than for all otherfusion reaciions, at least the first few generations of fusion reactorsare expected to burn deuterium and tritium. up to -80% of primary DTfusion energy release is represented by 14-MeV neutron kinetic energy,with the remainder carried off by x-ray photons and energetic ions.Further fusion reactor energy release results from exoergic reactions offusion neutrons with reactor blanket, coolant, and structural materials.The highly penetrating nature of fusion neutrons permits utilization of amajority of fusion reactor energy release in blankets surrounding reactorcavities independent of cavity phenomenology. However, these highlypenetrating neutrons also present prcblems. Many radiolysis reactionsare conducted most efficiently in the gas phase, but gases have suchsmall stopping powers for high-energy neutrons that either very thickblankets or high pressures, which generally reduce radiolysis efficien-cies and ~mpose undesirable reactor Structural requirements, will be re-quired to provide adequate neut~on energy absorption. Tritium breedingin lithium-containing blankets surrounding fusion reactor cavities suf-ficient, or nearly sufficient, to close the tritium fuel cycle may benecessary for economic viability. Simultaneous tritium breeding, preven-tion of proauct contamination with tritium and other radioactive specie:1 JHPproduced by neutron interactions with the radiolysis mixture and reactorstructure, and use of a high percentage of total fusion reactor energyrelease for radiolysis appears to represent an extraordinarily difficultengineering problem with no apparent solutions in sight.Radiolysis efficiency for a particular radiation and a particular radi-olysis product can be expressed in terms of a G value, defined as thenumber cf molecules of that product produced per 100 eV of radiant energydeposited in the reaction mixture. The ratio of experimental G values tomaximum G values for endothermic reactions, corresponding to completeutilization of radiant energy, gives the efficiency of radiant energyutilization. Jbnong the highest G values reported for H20 decompositionto give H2 and 02, C02 decomposition to given CO and 02, and HC1decomposition to give H2 and C12 are -15, -10, and -8 respec-tively. The corresponding maximum G values are 40.3, 34.5, and 101.2,which result in respective radiolysis efficiencies of -37%, - 29%, and-8%. This suggests that only topping cycle applications or combinedradiolytic-thermochemical cycle applications, for which HC1 radiolysishas been proposed, can be economically viable. If this is the case, thenthermal energy rejected from such cycles must be available at tempera-tures high enough to permit efficient further utilization.Significant i~fluences otlthe efficiencies of radiolytic processes in-clude (1) type of radiation, (2) radiation intensity or dose rate, (3)temperature and pressure, (4) radiolysis product concentrations, and (5)whether or not back reactions are inhibited by phenomena that separateexcited reaction products until they are deactivated, by continuous re-moval of reaction products from the reaction mixture, or by prompt de-activation of reaction prodticts with so-called scavengers. Some of thesepromote radiolytic production of desired substances, but others hindertheir production. The listed G values were obtained under conditionsthat favor the desired radiolytic reactions, but which may also iniolve,(because radiolytic processes are stil 1 only imperfectly understood) hid-den factors that hinder radiolysis and which if eliminated or counter-acted might lead to higher G values.The first influence is generally characterized in terms of the linearenergy transfer or LET, the mean energy deposition density along a radia-tion track. Heavy charged particles have large LET values, e.g., - 3 to25 eV/A for alpha particles of various energies and fiss;cm fragments(mean), whereas gatrrnaana x-ray photons, beta particles, accelerateddeuterons and neutrons exhibit small LET values, e.g. ,-O.02 to 1.0ev/A. For many radiolysis reactions, radiolytic efficiency increaseswith increase in LET value. Fusion neutrons interact with radiolysismixtures and transfer energy in a variety of ways to readily convertneutron energy to a mixture primarily of high LET charged particlekinetic energy and low LET gamma photons. Considerable experimentalevidence suggests that very-high-dose-rate, pulsed irradiation, charac-teristic of inertial confinement fusion can give substantially higher G2 JHPvalues than does steady irradiation, characteristic of magnetic confine-ment fusion, for the same time-averaged radiation fluxes. Although thereare some apparent exceptions, G values generally decrease with increasein gaseous radiolysis mixture pressure, probably because of increasedback reaction rates, but higher densities result in thinner blankets forgaseous radiolysis mixtures. Higher temperatures can result in higher Gvalues for at least some radic’lysis reactions. For example, G values forwater vapor radiolysis jump sh,~rply at about 2500C. However, theorysuggests that it is a one-time ieffect on!y and that further increases intemperature will not lead to further sharp jumps. The highest G valuesare obtained when radiolysis product concentrations are low, correspond-ing to low back reaction rates. Back reactions can also be stimulated byradiation, so that steady state concentrations are often largely deter-mined by the type and intensity of the radiation, rather than by ordinarythermodynamic considerations. On the other hand, low product concentra-tions can mean increased recycle of reactants and expensive productseparations. Scavengers, such as HC1 and HBr, that are simple, readily-reconstituted (required because of lddiolysis of scavengers themselves),inorganic compounds substantially increase G values for hydrogen produc-tion by water radiolysis. Scavengers were used to obtain the listedmaximum G vaiues, but more effective scavengers may exist. Dispropor-tionation of 92 and 02 between phases in liquid water-steam mixturesreportedly enhances G values for Hz production by separation of radi-olvsis products, but the enhancement may be solely a result of the temp-erature effect previously discussed. On the other hand, a similar effectmay be useful with other radiolysis systems. There is experimental evi-dence that inhibition of diffusion of dissociation products that occursin critical regions due to vanishing of chemical potential driving forcesfor diffusion at critical points can significantly reduce rates of re-combination of radiolysis products. Unfortunately, critical temperaturesfor many radiolysis systems of interest are rather low and efficientutilization of reject heat is thus precluded. Nonetheless, attractiveradiolysis systems with higher c;-itical points may be discovered. Aprincipal reason for low G values for many radiolysis processes is thatmuch of the radiant energy is transformed into molecular excitationenergy short of that necessary to cause dissociation. Lasers could beused to selectively and efficiently introduce into the excited speciesthe small additional amounts of energy required for dissociation.Our, ~nclother studies, of the economics of synthetic-fuel production bydirect radiolysis, all of which ignore some of the technology problems,suggest that direct radiolysis cannot compete with other methods ofsynthetic-fuel production, including coal gasification and liquefaction,production of synthetic crude from tar sands and oil shales, and fusion-driven thermochemical and electrolytic hydrogen production. Principalbarriers to cormnercialization of radiolytic processes includ? low Gvalues and radioactivity contamination of products.JHP

Realistic assessment of direct radiolysis for synthetic fuels production using fusion radiation sources

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Realistic assessment of direct radiolysis for synthetic fuels production using fusion radiation sources

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