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Neutron Spallation Studies for an Accelerator Driven Subcritical Reactor

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Neutron Spallation Studies for an Accelerator Driven Subcritical Reactor Proceedings of PAC09, Vancouver, BC, Canada TU6PFP029 NEUTRON SPALLATION STUDIES FOR AN ACCELERATOR DRIVEN SUBCRITICAL REACTOR Cristian Bungau, Roger Barlow, University of Manchester, Manchester, UK Adriana Bungau, Robert Cywinski, University of Huddersfield, Huddersfield, UK Abstract not take into account the nuclear structure effects in the inelastic interactions during the intranuclear cascade and Nuclear power production can benefit from the develop- therefore the code modelling of interactions at energies ment of more comprehensive alternatives for dealing with much below 100 MeV is questionable [2]. This becomes long-term radioactive waste. One such alternative is an an important issue when dealing with thick targets, as al- accelerator-driven subcritical reactor (ADSR) which has though the primary neutrons are produced by the high en- been proposed for both energy production and for burn- ergy proton beam, these are relatively low energy neutrons ing radioactive waste. Here we investigate the effects of which can produce further spallation processes inside the the size of the ADSR spallation target on the total neutron target leading to secondary neutrons. yield integrated over the neutron energy and emission an- For the Geant4 simulations, we selected the Liege intra- gle. The contribution to the total neutron yield from the (n, nuclear cascade model together with the independent evap- xn) neutron interactions is evaluated at proton beam ener- oration/fission code ABLA. This model has been added re- gies between 0.4 and 2 GeV.Calculations have been carried cently to the Geant4 code and has been validated against out with the GEANT4 simulation code using the Liege in- experimental data for spallation processes in many differ- tranuclear cascade model and the results are compared to ent heavy elements [4]. This model is valid for proton, the the LAHET/MCNP code package predictions. neutron, pion, deuteron and triton projectiles of energies up to 3 GeV and heavy target materials (Carbon - Uranium). INTRODUCTION It models the Woods-Saxon nuclear potential, Coulomb Due to their inherent safety features and waste transmu- barrier, non-uniform time-step, pion and delta decay cross tation potential, accelerator driven subcritical reactors are sections, delta decay, Pauli blocking and utility functions, the subject of research and development in many countries making it an independent code. The Liege model is largely around the world. The ADSR consists of three parts: the free of parameters and is preferred by validation and, com- accelerator, spallation neutron target and sub-critical reac- pared to the other theoretical models available in Geant4 tor core. The spallation target is at the heart of any acceler- (Binary and Bertini being currently the most widely used), ator driven system. it is more data driven [5]. Because the ADSR is operated in a subcritical state, the The MCNPX data with which our GEANT4 simulations target system has to provide the neutrons needed to sus- are compared have been taken from reference [2]. In all tain fission. These are generated by the spallation process the simulations described below, a 60 cm long Pb target resulting from high energy protons impacting the spalla- was considered. tion target installed at the centre of the core. Therefore the target materials must have high neutron production ef- RESULTS ficiency. One of the best candidate target material is lead or a lead/bismuth eutectic. Figure 1 shows the dependence of the number of neu- In the present work, the GEANT4 simulation code [1] trons on the target radius, both for neutrons produced and has been used to calculate the total neutron yield for leaving the target volume. This is shown separately for spallation reactions in Pb for proton energies between the primary neutrons generated by the initial protons, sec- 0.4 and 2 GeV. The results of this work provide also a ondary neutrons generated by the primary neutrons inside benchmark of the GEANT4 simulation results against the the target and the total number of neutron induced. It can MCNP/LAHET (RAL model) code output published in ref- be seen that the main “source” of neutrons inside the target erence [2]. is not the initial proton, but the primary neutron. The big difference in the number of produced neutrons COMPUTATION DETAILS and the number of neutrons leaving the target volume is due to the absorption of low energy generated neutrons. While GEANT4 provides an extensive set of hadronic physics the Bertini model does describe accurately the proton in- models for energies up to 10 - 15 GeV, both for the intra- duced spallation processes, there is concern over its valid- nuclear cascade region and for modelling of evaporation. ity in describing the low-energy neutrons induced spalla- In MCNP/MCNPX codes, the Bertini model is used by de- tion. This model is the default MCNPX intra-nuclear cas- fault for nucleons and pions, while the ISABEL model is cade model for protons and neutrons, and has been chosen used for other particle types [3]. The Bertini model does by the authors in reference [2]. The spallation neutrons en- Applications of Accelerators U03 - Transmutation and Power Generation 1351 TU6PFP029 Proceedings of PAC09, Vancouver, BC, Canada Neutron spallation - 1GeV p - Pb target Total Neutron Yields Ep(GeV) = 0.4 (geant4) Ep(GeV) = 0.4 (mcnpx) 120 primary neutrons produced 100 Ep(GeV) = 0.8 (geant4) primary neutrons out of target Ep(GeV) = 0.8 (mcnpx) secondary neutrons produced Ep(GeV) = 1.2 (geant4) 100 90 Ep(GeV) = 1.2 (mcnpx) secondary neutrons out of target YIELD (n/p) Ep(GeV) = 1.6 (geant4) total neutrons produced 80 Ep(GeV) = 1.6 (mcnpx) total neutrons out of target 80 YIELD (n/p) Ep(GeV) = 2.0 (geant4) Ep(GeV) = 2.0 (mcnpx) 70 60 60 40 50 40 20 30 0 0 50 100 150 200 250 300 20 TARGET DIAMETER (cm) 10 0 Figure 1: The variation of the calculated total neutron yield 0 20 40 60 80 100 120 140 160 180 200 with the target radius (GEANT4). TARGET DIAMETER (cm) Primary and secondary neutrons spectra Figure 3: The variation of the total neutron yield with the htemp target diameter for different proton energies (GEANT4 vs 105 Entries 148647 Mean 21.74 RMS 60.9 MCNPX). 4 htemp 10 Entries 208208 Mean 5.435 RMS 20.29 103 Total Neutron Leakage (En > 20 MeV) (Ep = 2.0 GeV) primary neutrons 5 2 4.5 Ep(GeV) = 0.4 (geant4) 10 Ep(GeV) = 0.8 (geant4) secondary neutrons 4 Ep(GeV) = 1.2 (geant4) Ep(GeV) = 1.6 (geant4) 10 3.5 Ep(GeV) = 2.0 (geant4) 3 1 0 200 400 600 800 1000 2.5 # OF NEUTRONS ESCAPE En (MeV) 2 1.5 Figure 2: Spallation neutrons energy spectra (GEANT4). 1 0.5 00 20 40 60 80 100 120 140 160 180 200 TARGET DIAMETER (cm) ergy spectrum is shown in Fig. 2, for a 1 GeV proton beam and a 20 cm diameter target. There are 14.8 primary neu- Figure 4: The leakage of high-energy neutrons versus tar- trons per incident proton with an average energy of 21.7 get diameter for different proton energies (GEANT4). MeV and 20.8 secondary neutrons per projectile with an average energy of 5.4 MeV. Figure 3 shows the variation of the total number of neu- protons escaping the 10 and 20 cm targets extends almost trons escaping the target per incident proton, for different to the initial proton energy, for protons escaping from the proton beam energies and different target diameters. The back end without undergoing an inelastic scattering, while number of neutrons increases steadily with the target diam- for the 100 cm target the proton energy loss is at least 0.7 eter, reaching a plateau at about 100 cm diameter. GeV. The leakage of high-energy (En > 20 MeV) neutrons per Figure 7 shows the comparison between the Geant4 and incident proton as a function of target diameter is shown in MCNPX predictions for the total number of protons and Fig. 4. The average energies of the spallation neutrons from high-energy neutrons escaping from the target per incident the (n, xn) interactions is 6 MeV (see Fig. 2) and since the increase in total neutron yield with target diameter in Fig. 3 Leakage of Neutrons is due primarily to the contribution of these neutrons, they 4 10 cm diameter (geant4) 3.5 10 cm diameter (mcnpx) will not contribute to the leakage of high-energy neutrons 20 cm diameter (geant4) 20 cm diameter (mcnpx) 3 and hence the decrease in the number of high-energy neu- 100 cm diameter (geant4) 100 cm diameter (mcnpx) trons with increasing target diameter shown in Fig. 4. 2.5 Also of interest is the cumulative spectral distribution of 2 1.5 all the neutrons escaping the target volume. Figure 5 shows # of Neutrons Escape Above En 1 the distribution of the number of neutrons above energy En 0.5 per incident proton, for a 1.6 GeV proton beam. It can 0 10-2 10-1 1 be seen that the majority of these neutrons have energies Neutron Energy (GeV) below 100 MeV. The proton leakage cumulative spectral distribution for Figure 5: The cumulative energy spectrum of neutrons es- 1.6 GeV protons is shown in Fig. 6. The energy of the caping from the target (GEANT4 vs MCNPX). Applications of Accelerators 1352 U03 - Transmutation and Power Generation Proceedings of PAC09, Vancouver, BC, Canada TU6PFP029 0.6 TOTAL # OF PROTONS ESCAPE (Ep = 2.0 GeV) 0.5 TOTAL # OF PROTONS ESCAPE (Ep = 1.6 GeV) 0.4 TOTAL # OF PROTONS ESCAPE (Ep = 1.2 GeV) TOTAL # OF PROTONS ESCAPE (Ep = 0.8 GeV) 0.2 0.4 geant4 0.3 geant4 geant4 0.4 geant4 mcnpx mcnpx 0.15 mcnpx mcnpx 0.3 0.2 0.1 0.2 0.2 # OF PROTONS ESCAPE # OF PROTONS ESCAPE # OF PROTONS ESCAPE # OF PROTONS ESCAPE 0.1 0.1 0.05 0 0 0 0 0 50 100 150 200 0 50 100 150 200 0 50 100 150 200
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