Accelerator-driven subcritical facility:Conceptual design development

Accelerator-driven subcritical facility:Conceptual design development

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 562 (2006) 870–874 www.elsevier.com/locate/nima Accelerator-driven subcritica...

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ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 562 (2006) 870–874 www.elsevier.com/locate/nima

Accelerator-driven subcritical facility:Conceptual design development Yousry Gohara,, Igor Bolshinskyb, Dmitry Naberezhneva, Jose Duoa,c, Henry Belcha, James Baileya a

Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA b Idaho National Laboratory, P.O. Box 2528, Idaho Falls, ID 83403, USA c Pennsylvania State University, University Park, PA 16802, USA Available online 13 March 2006

Abstract A conceptual design development of an accelerator-driven subcritical facility has been carried out in the preparation of a joint activity with Kharkov Institute of Physics and Technology of Ukraine. The main functions of the facility are the medical isotope production and the support of the Ukraine nuclear industry. An electron accelerator is considered to drive the subcritical assembly. The neutron source intensity and spectrum have been studied. The energy deposition, spatial neutron generation, neutron utilization fraction, and target dimensions have been quantified to define the main target performance parameters, and to select the target material and beam parameters. Different target conceptual designs have been developed based the engineering requirements including heat transfer, thermal hydraulics, structure, and material issues. The subcritical assembly is designed to obtain the highest possible neutron flux level with a Keff of 0.98. Different fuel materials, uranium enrichments, and reflector materials are considered in the design process. The possibility of using low enrichment uranium without penalizing the facility performance is carefully evaluated. The mechanical design of the facility has been developed to maximize its utility and minimize the time for replacing the target and the fuel assemblies. Safety, reliability, and environmental considerations are included in the facility conceptual design. The facility is configured to accommodate future design improvements, upgrades, and new missions. In addition, it has large design margins to accommodate different operating conditions and parameters. In this paper, the conceptual design and the design analyses of the facility will be presented. r 2006 Elsevier B.V. All rights reserved. PACS: 28.41.I; 28.50.Dr; 29.25.Dz; 28.41.Kw Keywords: Accelerator driven system; Electron accelerator; Low enriched uranium; Subcritical assembly; Neutron source

1. Introduction Kharkov Institute of Physics and Technology (KIPT) in Ukraine has a plan to construct an accelerator-driven subcritical assembly using highly enriched uranium (HEU) fuel. The main functions of the subcritical assembly are the medical isotope production and the support of the Ukraine nuclear industry including reactor physics experiments and material research. Argonne National Laboratory (ANL) is studying the possibility of utilizing low enrichment uranium (LEU) fuel instead of HEU fuel without penalizing the subcritical assembly performance [1]. This paper describes the ANL work performed for this facility in Corresponding author. Tel.: +1 630 252 4816; fax: +1 630 252 4007.

E-mail address: [email protected] (Y. Gohar). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.02.158

preparation for a joint conceptual design activity between ANL and KIPT. Several studies have been carried out to investigate the main design choices and the system parameters for a satisfactory operating performance. Neutron source intensity, neutron spectrum, spatial neutron generation, and spatial energy deposition in the target material are examined as a function of beam parameters, target materials, and target designs. The main focus is to maximize the neutron production for the available beam power. Conceptual target designs have been developed based on these studies and the engineering aspects including heat transfer, thermal hydraulics, structure, and material requirements. The target geometrical configuration has been designed to maximize the neutron utilization in the subcritical assembly and to provide irradiation

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channels at the highest flux level. The neutron flux of the subcritical assembly design has been examined as a function of the uranium fuel enrichment, the uranium density, the reflector material and thickness, and the target material for Keff of 0.98. The facility design is configured to maximize its utilization, to use simple and efficient procedures for normal operation and maintenance procedures, to emphasize safety, reliability, and environmental considerations, and to accommodate future upgrades and new missions. The facility employs proven design techniques and operating procedures. The paper presents the key results from these analyses and the facility conceptual design. 2. Neutron source characterization and design In a subcritical system, the neutron source strength and the assembly subcriticality level define the neutron multiplication and the generated fission power. Therefore, the analyses are directed to maximize the neutron source strength. Beam parameters, target material selection, and target geometrical configuration are considered. For neutron production, high atomic number target materials are used with the electron beam. In addition, high melting point, high thermal conductivity, chemical inertness, high radiation damage resistance, and low neutron absorption cross section are the desirable properties for target materials. Previous physics studies [2] showed that uranium, tungsten, lead, and tantalum materials produce the highest neutron yield per incident electron. Based on the properties of these materials, neutron yield per electron, and the operating experience from different accelerator facilities, tungsten and uranium have been selected. The neutron yield from these materials has been studied as a function of the electron energy using MCNPX [3] computer code. The spatial energy deposition density per incident electron was examined as a function of the target length for different electron energies. The peak value occurs few millimeters away from the electron beam window. This peak decreases and shifts further from the electron beam window as the electron energy increases. The thermalhydraulics analyses require the use of thin layers of the target material to avoid high temperature and thermal stresses. This results in a large number of coolant channels, which reduces the target performance. Results from physics and thermal-hydraulics parametric analyses show that a beam power density of 2 KW/cm2 and electron energy X100 MeV satisfy the different design requirements. Reducing the beam power density increases the target cross-section area, which reduces neutron flux in the subcritical assembly. Decreasing the electron energy reduces the required target length and increases the peak energy deposition, which require very thin layers of the target material and reduces the neutron yield. The spatial distribution of the generated neutrons is analyzed to maximize their utilization. A fraction of the

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neutron current leaving the target material in the beam direction interacts with the water coolant and the reflector material under the target assembly without reaching the subcritical assembly. Also, a fraction of the neutron current from the electron beam window exits the system through the vacuum beam tube or interacts with the beam tube structure. The rest of this current reaches the subcritical assembly by leaking from the beam tube. The results show that the use of high electron energy reduces the neutron fraction that leaves in the beam direction. It should be noted that the neutron current from the beam window is larger than neutron current from the other end of the target, which calls for locating the target midpoint under the subcritical assembly mid-plane to increase the chance for the neutrons leaving the beam window to reach the subcritical assembly. The use of high-energy electrons in the range 100–200 MeV is beneficial for improving the system performance. Increasing the electron energy in this range enhances the achieved performance. The neutron spectrum from the target was analyzed to quantify the high-energy component as a function of electron energy. The calculated neutron spectra show that the peak value of the neutron spectrum is about 1 MeV. The high-energy component of the spectrum is not sensitive to the electron energy and its magnitude is very small. Also, the neutron yield from the target was analyzed as a function of the target length for the 200 MeV electrons. For tungsten target material, the number of neutrons per electron increases as the target length increases to reach a maximum value at about 0.069 m. Extra tungsten target length acts as a neutron absorber and reduces the neutron yield. The neutron loss rate due to the extra target length is very small since the neutron fraction leaving the bottom surface is only about 0.07. The balance of the generated neutrons is not sensitive to the extra tungsten target length. Uranium target has a different performance with respect to the target length because of the fission reactions caused by the generated neutrons. In the first section of the target length, most of the neutrons are generated from the electron and neutron interactions. As the electron beam vanishes inside the target material, the fission reactions caused by the bottom neutron fraction slightly increase the neutron yield. A 0.08-m target length is adequate for the uranium target. Heat transfer and thermal-hydraulics parametric studies were performed to define the target mechanical configuration, the size of the water coolant channels, and the temperature distribution in the target materials. The target material has a cylindrical geometry and its axis coincide with beam tube axis. The water coolant flow direction is perpendicular to the target axis. This arrangement results in a stack of disks forming the target design. The water coolant channels between the target disks have a constant thickness of 1.75 mm based on a maximum water temperature increase of less than 5 1C. Each target disk is cooled from both sides to minimize its thermal deformation. The water coolant channels are connected in parallel to the input and output manifolds. The parametric study

ARTICLE IN PRESS Y. Gohar et al. / Nuclear Instruments and Methods in Physics Research A 562 (2006) 870–874

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Table 1 Disk thicknesses for tungsten and uranium targets Target material

Disk thickness (mm)

Tungsten Uranium

4.5 4.2

3.0 3.0

3.5 3.0

5.0 4.2

8.0 6.0

20.0 12.0

25.0 24.0

— 24.0

Fig. 1. Exploded view of the tungsten target.

defined the thickness of the different target disks assuming a maximum surface temperature of 80 1C for the electron energy in the energy range of 150–200 MeV. This temperature was selected to provide a margin of 65 1C away from the boiling temperature. To satisfy these design requirements, the 100-KW electron beam with 2-KW/cm2 uniform beam power density needs 7-m/s water coolant velocity and 4-atm coolant pressure. Table 1 shows the disk thicknesses for tungsten and uranium target materials. A clad material is utilized for the uranium disks to avoid water coolant contamination with fission products. Fig. 1 shows an exploded view of the tungsten target disks and the configuration for the coolant manifolds. The power density distribution in the uranium target assembly is shown in Fig. 2 which is calculated using LEU subcritical assembly with uranium density of 3 g/cm3 and beryllium reflector. The neutron source strength is 3.3  1014 n/s for 100 KW electron beam with 200 MeV electron energy. 3. Subcritical assembly characterization and design Since the neutron source strength and the subcriticality level are fixed, the fuel design, the uranium enrichment, and the reflector material selection are the main variables under consideration for maximizing the neutron flux field. The fuel design of the Kiev research reactor [3] is utilized in the analyses. The targets disks are placed in an aluminum tube surrounded by hexagonal canisters that match the fuel geometry and serve as coolant manifolds and irradiation

Fig. 2. Uranium target power density distribution.

channels. LEU with 20% uranium-235 and HEU with 90% uranium-235 are considered. Two uranium densities of 1 and 3 g/cm3 are used for both the LEU and HEU fuels. The different combinations are referred to as LEU-1, LEU-3, HEU-1, and HEU-3. The analyses focused on the possibility of utilizing LEU fuel instead of HEU fuel without penalizing the subcritical assembly performance. The beam tube is located at the center of the subcritical assembly and its axis coincides with the centerline of the subcritical assembly. Four of the six channels around the target disks are used as coolant manifolds for the target assembly. The other two channels are used for material irradiation. In the current analyses, these two channels are filled with beryllium to increase the neutron production from photonuclear reactions and neutron multiplication. Several three-dimensional models of the subcritical assembly including the detailed target design were developed to define the subcriticality level and the neutron flux distribution. The target and fuel assemblies are modeled explicitly without any geometrical approximation or material homogenization to get an accurate prediction of the subcritical performance. MCNPX computer code with continuous energy data libraries and Sða; bÞ thermal data was used for the analyses. Two reflector materials, beryllium, and water were considered. Parametric analyses

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were performed to assess the number of fuel assemblies required to get Keff of 0.98 for different combinations of the reflector and target materials, fuel density, and fuel enrichment. The corresponding neutron flux fields were also calculated. The results show that the use of uranium target requires the smallest number of fuel assemblies to achieve the desirable Keff value. For example, the number of fuel assemblies is 21 with uranium target versus 24 assemblies with tungsten target for the case of beryllium reflector and LEU-3 fuel. This smaller number of assemblies is due to the extra neutron multiplication produced by the natural uranium target material. The subcritical configurations with water reflector require a larger number of fuel assemblies as compared to the configurations with beryllium reflector. This is due to the excellent reflective properties of beryllium. For example, the required number of fuel assemblies with water reflector is 38 versus 9 with beryllium reflector for the tungsten target and HEU-3 fuel. The small number of fuel assemblies limits the subcritical assembly flexibility to study different geometrical configurations. In addition, the fuel assembly arrangements with beryllium reflector and uranium target are significantly asymmetric. In order to compare the performance of the subcritical assembly for irradiation experiments, with the LEU and the HEU fuels, the average neutron flux in the two radiation channels around the target was calculated. The results show that no significant difference between the subcritical assemblies with HEU and LEU fuels was observed. For example, the maximum difference between neutron fluxes from the LEU-3 and the HEU-1 subcritical assemblies is 3%. However, the subcritical with HEU fuel has an extremely small number of assemblies, which limits its utilization for reactor physics experiments and generates 20 15

Neutron Flux, n/cm**2.s 2.3E+13 2.2E+13 2.1E+13 2.0E+13 1.7E+13 1.8E+13 1.6E+13 1.5E+13 1.4E+13 1.2E+13 1.1E+13 1.0E+13 9.0E+12 7.8E+12 6.6E+12

10 5 0 -5 -10 -15

-15

-10

-5

0

5

10

15

20

Fig. 3. Total neutron flux map perpendicular to the beam tube averaged over the active fuel length of the subcritical assembly. The configuration has the uranium target, LEU Fuel (3-g/cm3 uranium density), 21 fuel assemblies, and beryllium reflector.

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20 15 Power, W/cc 4.2E+01 4.0E+01 3.7E+01 3.4E+01 3.2E+01 2.9E+01 2.6E+01 2.4E+01 2.1E+01 1.8E+01 1.6E+01 1.3E+01 1.1E+01 7.9E+00 5.3E+00 2.7E+00 1.2E-01 4.9E-02 2.7E-02

10 5 0 -5 -10 -15

-15

-10

-5

0

5

10

15

20

Fig. 4. Power density distribution perpendicular to the beam tube averaged over the active fuel length of the subcritical assembly. The configuration has the uranium target, LEU Fuel (3-g/cm3 uranium density), 21 fuel assemblies, and beryllium reflector.

difficulties for reactivity measurements and calibration procedures. Thus, it is desirable to use the LEU fuel instead of the HEU fuel to avoid such difficulties. The calculated neutron spectra show that about half of the neutrons in the irradiation channels have energies below 100 keV and the other half are in the energy range of 100–20 MeV. The neutron fraction with energy above 20 MeV is very small. The use of beryllium reflector produces higher flux level in the irradiation channels relative to the water. The use of uranium instead of tungsten as a target material doubles the neutron flux because of the extra neutron yield as shown from the target analyses. Fig. 3 shows the neutron flux map averaged over the active fuel length of the subcritical assembly. This configuration has the uranium target, LEU-3 fuel, and beryllium reflector. The corresponding power deposition distribution is shown in Fig. 4 for the 100 KW beam. 4. Subcritical facility conceptual design The facility conceptual design is configured to maximize its utilization for medical isotope production, experimental studies, and material characterizations. Two irradiation channels are located between the target and the subcritical assembly fuel where the peak value of the neutron flux is obtained. The target assembly is designed for fast replacement procedure to allow the use of different target materials and designs without modifying the configuration of the subcritical assembly. The target assembly has separate coolant loop, which provides flexibility to accommodate heat load changes resulting from different target materials or designs. The target replacement procedure does not perturb the subcritical assembly or the experimental hardware.

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Fig. 5. Section through the subcritical assembly showing the main components of facility.

The subcritical assembly has a pool design configuration, which provides full access to the subcritical configuration to maximize the facility experimental capabilities. The fuel replacement procedure is fast and it can be performed without opening the shield cover of the subcritical assembly. In addition, loading and unloading material capsules do not affect the target or the subcritical fuel assemblies. Safety, reliability, and environmental considerations are included in the facility conceptual design. For example, redundancy and large design margins are utilized to accommodate design modifications and future upgrades. Standard proven design and operating procedures are utilized to avoid development time and cost. In addition, this approach shortens the license and construction times and it permits the use of standard commercial components. Fig. 5 shows a vertical cross-section of the facility featuring the main components. 5. Conclusions An accelerator-driven subcritical assembly with 100 KW electron beam power has been developed. The target design

analyses defined the electron beam parameters and the target performance. Tungsten and uranium are the selected target materials for the electron beam. Electron energy in the range of 100–200 MeV provides an optimum performance for this application. A low-pressure water coolant is used for the target and the subcritical assembly, which facilitates the system design and satisfies its functions. A beam power density of 2 KW/cm2 is selected based on the target analyses to satisfy the engineering requirements and to minimize the target cross-section area. Several three-dimensional geometrical models were developed for the subcritical assembly including the target design to analyze and define its performance. Kiev research reactor fuel design is employed for the subcritical assembly. The analyses were performed with LEU and HEU fuels. The analyses defined the required number of fuel assemblies to achieve Keff of 0.98 and the corresponding neutron flux and power density maps for different design choices. The subcritical assemblies with LEU fuel utilize more fuel assemblies, which provide more experimental flexibility relative to HEU fuel. The obtained neutron flux levels from LEU and HEU are comparable. The obtained results show that the use of the LEU fuel is preferred relative to the HEU fuel. The conceptual design of the facility has been developed based on a pool configuration to maximize its experimental capabilities. Safety, reliability, and environmental considerations are included in the facility conceptual design. Standard proven design and operating procedures are utilized to avoid development time and cost. Further activities are underway to analyze the use of different fuel materials, fuel geometries, reflector materials, and enhance the facility design to increase the neutron flux in the irradiation channels and the facility experimental capabilities Acknowledgments Argonne National Laboratory’s work was supported by the US Department of Energy, Office of Global Nuclear Material Threat Reduction (NA212), National Nuclear Security Administration, under contract W-31-109-Eng-38. References [1] Y. Gohar, J. Bailey, H. Belch, D. Naberezhnev, P. Strons, I. Bolshinsky Accelerator-driven subcritical assembly: concept development and analyses, in: RERTR-2004 International Meeting on Reduced Enrichment for Research and Test Reactors, November 7–12, 2004 Vienna, Austria. [2] W.P. Swanson, Improved Calculation of Photoneutron Yields Released By Incident Electrons, Health Phys. 37 (1979) 347. [3] J. S. Hendricks, G. W. McKinney, L. S. Waters, et al., MCNPX, Version 2.5.e, Los Alamos Report, LA-UR-04-0569, February 2004.