Barlow RJ

Bungau C

Cywinski R

Tygier S

English

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ACCELERATOR DRIVEN SYSTEMS FOR ENERGY PRODUCTION ANDWASTE TRANSMUTATIONCristian Bungau1,3, Roger Barlow1,3, Robert Cywinski2, Sam Tygier1,31Cockcroft Insitute, Daresbury Science and Innovation Campus, Warrington, UK2University of Leeds, Leeds, UK3University of Manchester, Manchester, UKAbstractDue to their inherent safety features and waste trans-mutation potential, accelerator driven subcritical reactors(ADSRs) are the subject of research and development in al-most all countries around the world. The neutrons neededto sustain fission are generated by the spallation processresulting from high energy protons impacting a target el-ement installed at the centre of the core. In the presentpaper the possible benefits of FFAGs as accelerator driversfor ADSR systems are analysed. FFAGs afford fast accel-eration as there is no need of synchronization between RFand magnets, high average current with large repetition rateand large acceptance. The present study also focuses onthe Monte Carlo studies of the reactor core design. The im-pact of the subcriticallity, target material and proton beamenergy on the ADSR performance was also examined. En-tirely novel ADSR configurations involving multiple accel-erator drivers and associated spallation targets within thereactor core have also been considered. Calculations werecarried out using the GEANT4 simulation code.INTRODUCTIONAccelerator Driven Systems (ADS) for transmutation ofnuclear waste typically require accelerators with protonbeam energy between 350 MeV and 1 GeV and deliver-ing proton fluxes of 5-10 mA for demonstrators, and 20-50 mA for large industrial systems [2]. Up to now, onlysector-focused cyclotrons and linear accelerators are ableto provide proton beam currents in the mA region. Recentstudies have shown that a Fixed Field Alternating Gradi-ent (FFAG) accelerator could meet these requirements andwould also be more cost-efficient to build and operate thana linac.The first part of this paper shows some preliminary stud-ies on the design of a non-scaling FFAG prototype thatcould be used to accelerate the proton beam to energiessuitable for an ADSR. The second part is focused on theneutronic aspect of the problem. It starts from a fast neu-tron ADSR design, using pure lead as spallation target,coolant and reflector in the framework of the ADSR pro-posed by C. Rubbia [1]. The simulation data is used to seeif one can improve the reactor design by changing the tar-get position and considering whether different designs withmultiple targets would improve the reactor performance.ACCELERATOR STUDIES FOR AN FFAGDRIVEN ADSRAn FFAG would be used to boost protons from a 70 MeVcyclotron, as used at PSI, to sufficient energies of up to 1GeV. An FFAG’s magnetic fields stay constant during theacceleration cycle. It is therefore possible to run it in acontinuous mode.It has been suggested [3] that it may be possible to designan FFAG lattice based on a synchrotron, with the dispersionminimised and the apertures wide enough to accommodatea wide range of energies.A lattice with 30 × 1 m cells, each with a focusingquadrupole, a dipole, a defocusing quadrupole and a drifthas been studied. The magnetic element lengths were con-strained to 200 mm, with 50 mm gaps and a 300 mm drift.The lattice was optimised to reduce the maximum distancebetween low and high energy closed orbits, and to reducethe time of flight variation.Figure 1: Tracks for energies from 70 (blue) to 500 MeV(red).Figure 2: Time of flight variation.Particle tracking was done using Zgoubi [4]. It followseach particle for many steps though the magnets, and re-TUPP147 Proceedings of EPAC08, Genoa, Italy08 Applications of Accelerators, Technology Transfer and Relations with Industry1854U03 Transmutation and Power GenerationFigure 3: Horizontal phase space for 10000 cells, after 1mm displacement, for energies from 70 MeV (Blue) to 500MeV (Red).Figure 4: Vertical phase space for 10000 cells, after 1 mmdisplacement, for energies from 70 MeV (Blue) to 500MeV (Red). Plots are artificially separated in displacement.evaluates the field and forces at each step. This is neededas trajectories are non-paraxial.The final values achieved were, dipole strength of 2.7142T, focusing gradient of 28.604 T/m and defocusing gradientof 19.398 T/m. The time of flight variation was 0.0674 ns,and the maximum separation of beams was 157.76 mm.Fig. 1 shows how the closed orbits say close to the centresof the magnets.Fig. 2 shows the time of flight variation, this must besmall so that the RF frequency can remain constant.To confirm that the closed orbits are stable, the startingcoordinates were offset by 1 mm horizontally and verti-cally. These particles were tracked for 10000 cells. Figures3 and 4 show the coordinates traced out in phase space. Theparticles remained confined to stable orbits in both cases.REACTOR CORE STUDIESThe reactor geometry for the proposed AcceleratorDriven System has been implemented into a GEANT4 sim-ulation code [5].In GEANT4 there are two intra-nuclear cascade modelsthat could describe the neutron spallation processes: theBertini and Binary cascade models and they both have beenpreviously validated against data from Los Alamos exper-iments. The validation plots of the Binary cascade modelare shown in Fig. 5 for different target materials includingPb.The validation plots for the Bertini model are shown inFig. 6 for the same target materials.Figure 5: Neutron yield produced by 256 MeV protons.Histograms - Binary Cascade predictions, points - data [6].Figure 6: Neutron yield produced by 256 MeV protons.Histograms - Bertini Cascade predictions, points - data [6].Binary Cascade reproduces the data rather well for alltargets for energies above 50 MeV, while below 50 MeVthe neutron evaporation becomes important for light tar-gets (Fig. 5). For Pb targets there is very good agreementbetween the data and the model predictions over the wholeenergy range. In contrast to Binary Cascade, the Bertinimodel reproduces only the general trend of the data above50 MeV (Fig. 6), but does well below 50 MeV for all tar-gets.The Binary Cascade model has been used in our code todescribe the nucleons and pions interactions at intermediateProceedings of EPAC08, Genoa, Italy TUPP14708 Applications of Accelerators, Technology Transfer and Relations with Industry U03 Transmutation and Power Generation1855Figure 7: Longitudinal section through the lead-cooledsubcritical reactor. The target is made of lead, and the reac-tor is surrounded by a Pb reflector. The 1 GeV proton beamof 10 cm radius hits the target from the top.z (cm)-80 -60 -40 -20 0 20 40 60 80dE/dz (J/cm)00.20.40.60.811.21.4-610×dE/dz - energy released along z (central target)Figure 8: z-dependence of the energy released inside thecore for a central target position.energies (65 MeV - 6.1 GeV) and the pre-equilibium modelfor lower energies (20 MeV -70 MeV).As already indicated the model of the subcritical reactoris based on the Energy Amplifier proposal by C. Rubbia[1]. It consists of a reactor with Th232/U233 fuel, with leadas the coolant, moderator and reflector material. Also thetarget is made of lead and has a diameter of 20 cm anda length of 48 cm. The longitudinal section is shown inFig. 7. The core is surrounded by ZrO2 and Pb reflectors.The z-axis is defined as the axis along the direction of the-150 -100 -50 0 50 100 150-150-100-50050100150-910-810-710Energy Distribution in X-Y planex(cm)y(cm)-150 -100-50 050 100150-150-100-50050100150-1010-910-810-710-910-810-710Energy Distribution in X-Y planex(cm)y(cm)E(J)Figure 9: Energy distribution inside the core - transversalplane.incoming proton beam (Fig. 7).The z-axis dependence of the energy released inside thecore is shown in Fig. 8, for a central position of the target.The maximum of the energy distribution is at z≈10 cm.Therefore, lowering the Pb target by 10 cm will result in anoptimum energy distribution inside the core.The energy distribution within the transverse plane (X-Y) is shown in Fig. 9 and it shows that the largest fractionof energy released inside the reactor core region is releasedin the central part of the core and that very few fission neu-trons reach the outer regions.CONCLUSIONThe first part of the paper showed preliminary studies forthe design of a non-scaling FFAG which is believed to bethe best candidate for an ADSR unit. It has been shownthat the FFAG design can stably hold beams of a suffi-cient range of energies, however variables such as magnetlengths, positions and face angles, the existing lattice couldbe further optimised. It may be possible to use an opticscode to give quick approximate parameters before usingthe more complete tracking code. It will also be necessaryto add acceleration in to the model.The second part of this paper describes a first attempt touse the GEANT4 simulation code to simulate an Accelera-tor Driven Subcritical Reactor, and presents few attempts toimprove the initial design proposed by Rubbia [1], whichwas based on the idea of using a standard nuclear reac-tor design. This study showed that by lowering the Pb tar-get one can get a better energy distribution inside the core.It also showed that the energy output inside the reactor islocalised in the central part of the core and therefore thedesign could be improved by considering multiple targetsinside a homogeneous core, leading to a more uniform en-ergy distribution inside the reactor.After implementing the multiple-targets geometry intothe code, the next steps would be to simulate the reactorpower output distributions for the Th232/U233 fuel reactorand for different fuel enrichment values, and though differ-ent initial values of the criticality keff .REFERENCES[1] C. Rubbia et al., Conceptual design of a fast neutron operatedhigh-power Energy Amplifier, CERN/AT 95-44-ET (1995).[2] Jean-Luc Biarrotte et al., A reference accelerator schemefor ADS applications, CNRS/IN2P3, Institut de PhysiqueNucle´aire, 91406 Orsay, France.[3] H. Owen, Daresbury Laboratory, Daresbury, UK - privatecommunication.[4] F. Me´ot, Zgoubi, http://sourceforge.net/projects/zgoubi .[5] GEANT4: A toolkit for the simulation of the passage of par-ticles thorugh matter, http://www.geant4.org/geant4/ .[6] V. Ivanchenko et al., Nuclear Instruments and Methods inPhysics Research A525 (2004) 402.TUPP147 Proceedings of EPAC08, Genoa, Italy08 Applications of Accelerators, Technology Transfer and Relations with Industry1856U03 Transmutation and Power Generation

A reference accelerator scheme for ADS applications, CNRS/IN2P3, Institut de Physique Nucle´aire,

Conceptual design of a fast neutron operated high-power Energy Amplifier,

GEANT4: A toolkit for the simulation of the passage of particles thorugh matter,

Nuclear Instruments and Methods

Accelerator driven systems for energy production and waste transmutation

Accelerator driven systems for energy production and waste transmutation

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