1 FNS/P5-13. Temperature Sensitivity Analysis of Nuclear Cross Section using FENDL for Fusion-Fission System

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1 1 FNS/P5-13 Temperature Sensitivity Analysis of Nuclear Cross Section using FENDL for Fusion-Fission System Carlos E. Velasquez 1,2,3, Graiciany de P. Barros 4, Claubia Pereira 1,2,3, Maria Auxiliadora F. Veloso 1,2,3 and Antonella L. Costa 1,2,3 1 Departamento de Engenharia Nuclear - Universidade Federal de Minas Gerais Av. Antonio Carlos, 6627 campus UFMG , Belo Horizonte, MG Tel/fax: Instituto Nacional de Ciência e Tecnologia de Reatores Nucleares Inovadores/CNPq 3 Rede Nacional de Fusão (FINEP/CNPq) 4 Comissão Nacional de Energia Nuclear-CNEN Rua Gal Severiano, nº 90 - Botafogo; , Rio de Janeiro - RJ Brasil claubia@nuclear.ufmg.br, carlosvelcab@ufmg.br Abstract. Fusion energy has been presented as a clean alternative way of energy source. Furthermore, neutronics features from fusion associated with fissile systems, make favorable nuclear fuel regeneration and actinide transmutation due to the high-energy neutron flux. This work proposes a fusion-fission system (FFS) based on a Tokamak at operating temperature for transmutation of MA using reprocessed fuel by UREX+ technique spiked with thorium. The purpose is to follow the burnup of the MA inventory at operating temperature. The thorium cycle was due to the conversion from fertile to fissile nuclide and its abundance in crust earth. In this work, NJOY code was used to process cross sections at operating temperatures and they were used by the MONTEBURNS code, which links the ORIGEN2.1 and the MCNP to perform the depletion and modeling of the system. The results show a temperature sensitivity analysis of an FFS closest to its real material conditions at operating temperatures and one simulated at room temperature. The work establishes the neutronic modelling differences in the system, as well as, the temperature effect on the MA depletion and production nuclides for FFS. 1. Introduction In recent years, many concepts of hybrid systems have been a focus on transmuting the long-term minor actinides and fission products, which pose a hazard that, could remain during centauries [1]. The simulation of the fusion-fission system concept must be the closed to a real one, to ensure the transmutation effectiveness. Therefore, the better representation of the system during simulation enhance its performance, transmutation over MA and increase the possibility of adequate design [2]. Most important part is to represent each material at its corresponding work temperature especially in a Tokamak system where there is system exposed to high and low temperatures, such as the first wall material and superconductors [3]. Consequently, the neutron produced in the D-T plasmas pass through different materials at different temperatures. The insertion of a transmutation layer into a Tokamak can probably modify the neutron spectrum over the others Tokamak components. Through the years, research and development on materials make them less impure or the addition of an element improve the material performance. Hence, a continuously update of the materials should be made to respond to the improvement of them. This work aims to present the update of the fusion materials and their sensitivity under different temperatures, as well as, the neutron interaction in a Tokamak with transmutation layer and without it.

2 2 FNS/P Methodology The criticality calculations have been performed using the MONTEBURNS code [4], which links the MCNP [5] and the ORIGEN2.1 [6] codes. The flux calculations were made using MCNP, and the cross sections were generated at different temperatures with NJOY [7]. The library used was the FENDL3.1 [8] and the missing elements were completed with ENDF/B-VII.1 [9, 10]. The source multiplication factor (ksrc) of a subcritical assembly, driven by an external neutron source, can be expressed as the ratio of the fission neutrons and the fission neutrons plus the source neutrons. The MONTEBURNS code uses the source definition calculated from the value of the net multiplication obtained from the MCNP output file k s = (f mult 1) (f mult 1 (1) υ ) where fmult is the total neutron multiplication factor of the system and υ is the ratio of the source neutrons to the neutron lost to fission [4,5]. 2.1.Geometry Model The geometry is represented by the intersection of cylinders and planes to delimit boundaries of the transmutation layers, as well as the fusion device. This geometry was chosen due to its simplicity, its low relative error [11]. Furthermore, it allows simulating part of the device individually. Fig. 1 shows a 3D fusion fission reactor with the transmutation layer. The transmutation zone thickness is 20 cm beginning at 856 cm, defined in previously works [12]. The design proposed is different from the ones before presented; the purpose is to achieve the neutron flux track inside the transmutation layer. Fig 2. shows the components modeled on the Tokamak. Figure 1. Tokamak with transmutation layer in red mark Figure 2. Tokamak without Transmutation Layer

3 3 FNS/P Materials Table 1 shows the components, materials, and temperatures used in the simulation. Most of the materials and temperatures were assumed following [13,14]. The fuel loaded into the transmutation layer is a reprocessed spent fuel by UREX+ technique [15] and spiked with thorium. TABLE 1. MATERIALS AND TEMPERATURES FOR EACH COMPONENT [13-15] Components Material Composition Temperature (K) First wall Be-S65E/W1.1TiC inboard/outboard Heat Sink CuCrZr-IG Blanket module block SS316L(N)-IG (70%) + Water (30%) shield Vacuum Vessel SS316L(N)-IG 533 outer/inner shell Vacuum Vessel in wall SS304B7 (55%) + Water (45%) shield Thermal Shield SS304L 100 TFC outer/inner shell SS316LN 80 TFC SS316LN (47.6%)+SS316L(1.5%)+He/liq- 80 (12.9%)+Nb3Sn(6.3%)+r-epoxy(18%)+Cu (13.7%) Cryostat SS304L 95 Shield Concrete 300 Central solenoid SS316L(N)-IG 150 structure CS winding pack Jk2SS (54.7%) + SS316L(1.2%) Inconel(0.6%) + He/liq.(11.2%) + Cu(11%) + Nb3Sn(5.5%) + r-epoxy (15.8%) CS fill Nb3Sn 4.7 Coolant LiPb Clad HT Nuclear Fuel UREX+/Th RESULTS The results show the neutronic evaluation of the Tokamak with and without transmutation blanket. Figure 3 shows the neutron flux for a hybrids system based on a Tokamak with transmutation layer at work temperature and room temperature, as well as, the neutron flux for a Tokamak along their different systems at work temperature and at room temperature. The systems studied are: First wall, heat sink, shield block (SB), Transmutation Layer (TL) and Vacuum Vessel (VV). It can be seen that there is a big difference in the neutron flux along the different systems when is considered the transmutation layer on the Tokamak, but the differences were small for temperature variations.

4 4 FNS/P5-13 1E16 1E15 1E14 1E13 1E12 1E11 1E10 First Wall Heat Sink Before SB or TL After SB or TL End of SB Components Start VV Before VV filling Figure 3. Neutron flux along the FFS and the Tokamak Besides the small differences between the work and room temperature for the neutron flux in each system, in contrast, the criticality calculations as presented in Figure 4a, shows higher differences due to the temperature variations. The absolute difference of the multiplication factor between the working temperature and the room temperature is presented in Figure 4b. The highest difference is about pcm. a) 0.96 Multiplication factor k s Work Temperature Room Temperature 1500 Absolute difference of the k s (k s(work) -k s(room) )x Time (days) Time (days) Figure 4. a) Neutron multiplication factor at work and room temperature Absolute difference of the neutron multiplication factor between temperatures 3.1. Sensitivity Analysis This analyzes focus on cross section for each material and the neutron flux variations due to the temperature variations on the different components and materials from the hybrid reactor or the one for the Tokamak First Wall In this case, two different materials were used in the FW one based on Be and the other one on W alloy. Figure 5a presents the cross section of the first wall with the tungsten alloy. It can be found small differences around 10-4 MeV and the Doppler effect around 0.01 MeV. On the other hand, the neutron flux for this material is presented in the Figure 5b for the hybrid reactor and and for the Tokamak and. Where first can be appreciated two different types of differences one for caused by the insertion of the transmutation layer (hybrid system) and the other by the temperature influence. In the first case, the main difference can be seen between 10-4 to 10 MeV. In the second case, the difference is presented between 10-8 to 10-5 MeV.

5 5 FNS/P5-13 First Wall Be S-65E Figure 5. a) W1.1TiC Cross section at different temperatures first wall neutron flux for the hybrid system and the Tokamak Heat Sink The material for the used in the heat sink is a copper alloy CuCrZr-IG, the cross section of this material at a different temperature is present in Figure 6a. The cross section of this material decreases when the temperature is increased. It can be seen that at work temperature the cross section is below the cross section at room temperature. Therefore, the neutron flux between 10-5 to 14.1 MeV for the system with transmutation layer is higher than the one without it. On the other hand, the only thing that affects the flux is the presence of the transmutation layer on the system, which increases the flux over this component. Heat Sink - CuCrZr Figure 6. a) CuCrZr Cross section at different temperatures heat sink neutron flux for the hybrid system and the Tokamak Block Shield The block shield was assumed with a mixed composition of SS316L(N)-IG (70%) and water (30%). Figure 7a shows the cross section at different temperatures. The one with higher temperature has lower cross section than the room temperature. Nevertheless, as shown in Figure 7b, the effects on the neutron flux can be seen just for energies below 10-6 MeV Vacuum Vessel Figure 8a presents the cross section at work temperature and room temperature for the material used in the vacuum vessel. As shown in Figure 8b, in this component, the neutron flux differences between the hybrid reactor and Tokamak decreases and becomes one close to the other.

6 6 FNS/P5-13 Shield Block Figure 7. a) SS316L(N)-IG (70%) + water (30%) Cross section at different temperatures block shield neutron flux for the hybrid system and the Tokamak Vacuum Vessel Figure 8. a) SS316L(N)-IG Cross section at different temperatures block shield neutron flux for the hybrid system and the Tokamak With and Without Transmutation Layer The influence of the transmutation layer inside of a Tokamak changes the neutron flux profile over the reactor. As shown in Figures 9 and 10, there is an increment in the fast range, but they decrease for lower energies and in the both materials, the coolant and the nuclear fuel. The neutron flux for the systems without transmutation layer presents a flatten behave, for the coolant inside the transmutation layer. On the other hand, when the transmutation layer is added, the neutron flux behave in a decreasing form modifying the neutron spectrum. The differences between temperatures for each the system with and without transmutation layer appear for low energies below 10-5 MeV. Coolant LiPb - LiPb - SS316NIG+H 2 O SS316NIG+H 2 O Figure 9. a) LiPb Cross section at different temperatures Coolant neutron flux for the hybrid system and the Tokamak

7 7 FNS/P5-13 Nuclear Fuel - (UREX+) Fuel Fuel SS316NIG+H 2 O SS316NIG+H 2 O Figure 10. a) Reprocessed Fuel Cross section at different temperatures Nuclear fuel neutron flux for the hybrid system and the Tokamak Sensitivity The fuel depletion sensitivity due to the differences in temperature for transmutation layer is shown in Fig.11. It represents the difference between fuel depletion mass by nuclide between the work temperature MWT and the mass for the room temperature MRT. The highest difference is for the 239 Pu and 242m Am. In other words, these nuclides have a higher transmutation at work temperatures, but also it might function in the other way and produce more nuclides such 238 Pu. mass difference M= M WT -M RT (kg) Th-232 Pa-231 Pa-233 U-232 U-233 U-234 U-235 U-236 U-237 U-238 Np-237 Np-238 Np-239 Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Am-241 Am-242m Am-243 Cm-242 Cm-244 Cm-245 A Figure11. a) Mass difference between the work temperature and room temperature 4. CONCLUSION The transmutation layer insertion inside of a Tokamak contribute to an increment in the neutron flux over the different components, which increases the neutron damage probability over delicate component such as the FW. The absolute difference of the ks shows the fuel sensitivity at different temperatures. There is a strong neutron influence over the different components in the hybrid systems due to insertion of the transmutation layer in the Tokamak system. Most of the differences in the neutron flux between the work temperature and the room temperature appear for low energies, in spite of the differences in the cross sections. Some material has stronger effects with temperature changes such as the LiPb, the SS316L(N)-IG + H2O and the copper alloy. There is a strong temperature influence on the depletion of the nuclear fuel loaded. The most sensitive nuclides are 238 Pu, 239 Pu and 242m Am.

8 8 FNS/P5-13 Acknowledgments The authors are grateful to the Brazilian research funding agencies, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES, Comissão Nacional de Energia Nuclear - CNEN, Conselho Nacional de Desenvolvimento Científico e Tecnológico -CNPq (Brazil), and Fundação de Amparo a Pesquisa do Estado de Minas Gerais -FAPEMIG (MG/Brazil), and International Atomic Energy Agency- IAEA, for the support, References 1. W.M.Stacey, Nuclear Reactor Physics, Wiley, Weinheim, (2007). 2. W.M. Stacey, Tokamak D-T fusion neutron source requirements for closing the nuclear fuel cycle, Nuclear Fusion, Vol. 47, pp , (2007) 3. Y. Wu and FDS Team, CAD-based interface programs for fusion neutron transport simulation, Fusion Engineering and Design, Vol. 84, , (2009). 4. MONTEBURNS 2.0 code, RSICC Peripheral Science Routine Collection, Los Alamos National Laboratory 5. X-5 Monte Carlo Team, MCNP A General Monte Carlo N-Particle Transport Code, Version 5, Volume II: User s Guide University of California, Los Alamos National Laboratory. (2003) 6. ORIGEN2, User s Manual, ORNL/TM-7175, Oak Ridge National Laboratory, NJOY99.0- Code System for Producing Pointwise and Multigroup Neutron and Photon Section from ENDF/B Data, RSICC, (2000). 8. International Atomic Energy Agency-Nuclear Data Section, FENDL-3.1b Fusion Evaluated Nuclear Data Library Ver.3.1b,Vienna, (2016) 9. M. B. Chadwick, M. Herman, P. Oblozinsky, et al., "ENDF/B-VII.1 nuclear data for science and technology: Cross sections, covariances, fission product yields and decay data", Nuclear Data Sheets, 112(12): , (2011). 10. Brookhaven National Laboratory, ENDF/B-VII.1 Evaluated Nuclear Data Library (2015). 11. Velasquez, C.E. ; Pereira, C. ; Veloso, M.A.F. ; Costa, A.L. Modelling effects on axial neutron flux in a Tokamak device, Progress in Nuclear Energy (New Series), Vol. 2014, pp. 1-8, (2014). 12. C.E. Velasquez, C. Pereira, M.A.F. Veloso, A.L. Costa, Layer thickness evaluation for transuranic transmutation in a fusion fission system. Nuclear Engineering and Design, Vol. 286, pp (2006) 13. ITER, Plant Description Document (PDD)- G A0 FDR R H.HU, Y. Wu, M.Chen, Q.Zeng, A.Ding, S.Zheng, Y.Li, L.Lu, P.Long, FDS Team, Benchmarking of SNAM with the ITER 3D model, Fusion Engineering and Design, Vol.82, pp , (2007) 15. Cardoso, F., Pereira, C., Veloso, M.A.F., Silva, C.A.M., Cunha, R., Costa, A.L.,. A neutronic evaluation of reprocess fuel and depletion study of VHTR using MCNPXand WIMSD5 code. Fusion Science and Technology, Vol. 61, pp , (2012).