Journal of Nuclear Energy Science & Power Generation TechnologyISSN: 2325-9809

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Research Article, J Nucl Ene Sci Power Generat Technol Vol: 5 Issue: 1

Design of a Heavy Liquid Metal Neutron Spallation Target for Experimental Accelerator driven Sub-critical Reactor

Rawat RS1*, Swain PK1, Rai PK1, Tiwari V1, Satyamurthy P1 and Despande AV2
1ADS Target Development Section, Bhabha Atomic Research Centre, Mumbai, India
2Veermata Jijabai Technical Institute, Mumbai, India
Corresponding author : Ravindra Singh Rawat
Akashganga, Anushaktinagar, Mumbai, India
E-mail: [email protected]
Received: July 29, 2014 Accepted: January 02, 2016 Published: January 12, 2016
Citation: Rawat RS, Swain PK, Rai PK, Tiwari V, Satyamurthy P, et al. (2016) Design of a Heavy Liquid Metal Neutron Spallation Target for Experimental Accelerator driven Sub-critical Reactor. J Nucl Ene Sci Power Generat Technol 5:1. doi:10.4172/2325-9809.1000143

Abstract

In this paper detailed design of a liquid metal Lead-Bismuth-Eutectic (LBE) neutron spallation target for an experimental ADS reactor of ~30 MW with sub-criticality (k) of 0.975 is presented. The high energy beam consists of 650 MeV and 0.9 mA proton beam. The circulation of the liquid metal is based on gas lift method. Extensive numerical simulations have been carried out to optimise the target geometry, LBE/gas flow rate, beam parameters, neutron yield etc. This paper also includes time dependent two phase CFD analysis to study the effect of nitrogen gas flow rate on LBE flow rate, 3D thermal-hydraulic studies of liquid metal flow near the window and spallation region. The optimum asymmetric flow geometry at the bottom of the target to shift the stagnation zone to minimize the window temperature and estimation of thermo mechanical stress in the window has been carried out. In addition, spallation neutron generation and their energy spectrum, heat deposition distribution, spallation products and their activities have been estimated using high energy particle transport code FLUKA.

Keywords: Accelerator driven sub-critical System (ADS); Lead-bismutheutectic (LBE); Neutron spallation target; FLUKA; CFD; Window; Gas injection system; Two-phase flow; Water cooled safety jacket; Spallation radioactive products; Neutron yield; High energy proton beam; MPRR; LINAC

Keywords

Accelerator driven sub-critical System (ADS); Lead-bismutheutectic (LBE); Neutron spallation target; FLUKA; CFD; Window; Gas injection system; Two-phase flow; Water cooled safety jacket; Spallation radioactive products; Neutron yield; High energy proton beam; MPRR; LINAC

Introduction

Currently studies are being carried out at various nuclear research institutes of the world over to develop Accelerator Driven Sub-critical nuclear reactor systems (ADS) [1-3]. These systems have many attractive features like inherent safety, capability to transmute large quantities of nuclear waste, better utilization of thorium etc. The ADS system utilizes the neutrons produced in a spallation target irradiated by a high-energy proton beam to drive a blanket assembly containing both fissionable fuel and radioactive waste [4]. The spallation target is ideally conceived to be high atomic mass material and heavy density liquid metal like LBE (Lead-Bismuth-Eutectic alloy) which fit the requirements extremely well [5]. The novel feature of ADS is the presence of a neutron spallation target in the core of the reactor, which always operates under sub-critical conditions. A proton beam (Energy: ~ 1 GeV and Current: few to tens of mA) interacts with the target, which is located in the core of a subcritical reactor and produces spallation neutrons (~ 1019 n/s) that diffuse into and drive the reactor [6].
In this paper, detailed design of a heavy liquid metal neutron spallation target suitable for the proposed experimental Multi- Purpose Research Reactor (MPRR) operating under ADS mode is presented. The experimental reactor is of ~30 MW (thermal) and will operate at k = 0.975. Various target configurations have been studied with different beam energies, top injection and bottom injection for different geometries. Based on this analysis, it is proposed to have LINAC with proton energy of 650 MeV and beam current of 0.9-1.0 mA for generating the required spallation neutrons [7]. The target will be located at the centre of the reactor in a vertical cylindrical diameter space of 240 mm. Heavy density liquid metal LBE (leadbismuth- eutectic of 45% lead and 55% bismuth) will be used as spallation target. The schematic of the proposed Reactor is shown in Figure 1.
Figure 1: Schematic of proposed 30 MW ADS Experimental Reactor.

Basic Spallation Target Loop

The major components of the module are the window (the solid barrier through which the beam enters the target), spallation region above the window, riser pipe along with mixer for gas injection, annular down comer along with heat exchanger, cover gas region which also acts as a gravity based passive gas separator. The schematic of the target module and various subsystems are shown in Figure 2. Instead of mechanical pump or electromagnetic pump, the circulation of the liquid metal is achieved by the gas lift method [8]. This enhances the reliability of the system. In addition, an outer water cooled jacket will be provided around the target module to act as a safety jacket in case of LBE window failure [9]. The window is a very important component of the loop. Typically it is a hemispherical in shape, having a thickness of ~1.5 mm at the outlet and ~3 mm at the edges. One of the candidates for the proposed window material that will be tested in this facility is T91 steel [10]. The incident proton beam deposits 409 kW of heat (as estimated by FLUKA code presented later) in the target and 4.1 kW in the window for the above beam. An additional heat of 370 kW is deposited due to gamma radiation from the reactor.
Figure 2: Schematic of the target module and various subsystems.
The liquid metal is circulated to extract the heat deposited by the beam in the window and in the liquid metal itself. The circulation of the liquid metal is achieved in the loop as follows. In the mixer, located below the riser pipe, nitrogen (or argon or helium) gas is injected. This gives rise to two-phase mixture in the riser pipe and consequently leads to a density difference between the riser and downcomer pipes. This causes circulation of liquid metal in the loop. The riser height is designed in such a way that required flow rate of liquid metal is achieved. Both the phases enter the separator located at the top of the loop. Gas is separated here and taken to a gas outlet pipeline. The liquid metal flows down through the downcomer (annular pipe). At the top of the downcomer and below the separator, a heat-exchanger is located which extracts the heat from the liquid metal. At the bottom of the downcomer, where the liquid metal enters spallation region, there is a cut at slanted angle to break the flow symmetry [11]. This is to shift the stagnation zone at the bottom and provide convective heat transfer, where proton beam deposits bulk of the heat. Through the spallation region the liquid metal enters mixer zone. All the inlets and outlets to the target module have to be routed through top only. A water cooled safety jacket is provided around the target module, to contain the radioactive liquid metal under accidental scenario.

Design of Target Module

For proper design of the target module, it is required to carry out Computational Fluid Dynamic (CFD) analysis in the entire module. The complexity of the flow includes;
i. Turbulence
ii. Regions of single and two phase
iii. Two phase flow separation
iv. Modeling of free surface in the separator
v. Non-symmetric flow geometry
vi. Time dependent analysis.
In view of extreme complexity, flow analysis was carried out in two stages;
i. 2D (axis-symmetric): two phase time dependent flow analysis in the entire target module to determine liquid metal flow rate as a function of gas flow rate [12]. This study included carry under of gas in to downcomer, carryover of liquid in gas.
ii. 3D- single phase (liquid metal): flow analysis near the window region of the target module. ANSYS FLUENT software was used for the analysis [13].
Gas injection system for liquid metal circulation
In the present analysis nitrogen gas is used for designing the gas injection system although argon or helium gas in principle can be used for the circulation of LBE. For this part of the analysis, the heating of liquid metal and gas (nitrogen) are neglected as their contribution to circulation is small as compared to gas injection. Both fluids are assumed to be at the same temperature and energy equation was not solved. Both the fluids are assumed to be at 220 0C temperature (average temperature of the fluids in actual system). In the actual system, due to buoyancy effects, there will be small enhancement of the liquid metal flow.
The computational domain considered is shown in Figure 3. The domains are mixer, riser, separator, downcomer, spallation region. In actual system the gas is injected into the module through large number of holes through a distributer. However in the modelling gas enters through concentric annular rings (width ~5.5 mm, spacing ~5.5 mm).
Figure 3: Schematic of spallation target (computational domain).
The present two-phase flow analysis is mostly bubbly flow (i.e. void fraction is less than 0.25) [14]. For this type of flow regime the best suitable model is Eulerian multiphase model. In this model phases are taken explicitly and separate equations are written for each phase and the interaction between the phases is also considered. The model take into account that each phase can have different properties and different velocities.
The actual flow is 3D and requires very large computational time. For analysis purpose an axis-symmetric geometry of the target is considered, which gives the advantage in computational time. Time dependent analysis has been carried out until steady state solution is obtained.
Separate equations of continuity and momentum for each phase are solved simultaneously, together with the drag and other forces, which describes how the phases interact with each other and with the walls of the duct. In the simplest analysis only one parameter (usually velocity) is allowed to differ for the two phases while conservation equations are only written for the combined flow.
Governing equations
The flow is assumed to be turbulent. The buoyancy effect is neglected. The governing equations are written in Cartesian coordinates.
Volume fraction equation: Volume fractions represent the space occupied by each phase, and the laws of conservation of mass and momentum are satisfied by each phase individually. The derivation of the conservation equations can be done by ensemble averaging the local instantaneous balance for each of the phases. The volume of phase, is defined by
equation(1)
equation(2)
αq is the void fraction of qth phase [15]. In a two-phase system, the phases are represented by the subscripts 1, which is liquid phase (continuous fluid) and 2 is gas phase (dispersed fluid).
Mass conservation: The continuity equations for the phases can be written as,
equation(3)
ρq is the physical density of the qth phase. It is assumed that there is no generation of phases or mass transfer between the fluids.
Momentum conservation: The momentum balance for phase q yields
equation(4)
It is assumed that there are no external forces other than gravity.
In the above equation, is the phase stress-strain tensor, is acceleration due to gravity
equation(5)
equation(6)
Where, Kpq (Kqp) is the inter phase momentum exchange coefficient.
Here,μq is the shear viscosity of phase q, equation is an interaction force between phases, and P is the pressure shared by all phases. equation and equation are the phase velocity of phase p and q respectively.
For this liquid – gas flow analysis, secondary phase (gas) is assumed to form bubbles. The exchange coefficient the gas-liquid mixture is given as
equation(7)
Where, bubble relaxation time equation , is defined as
equation(8)
Where is the diameter of the bubbles or droplets of phase ‘p’
Drag function, f, is a function of drag coefficient (CD) and relative Reynolds number (Re). Based on Schiller and Neumann, [16] the following equation is used for calculating ‘f’
equation(9)
Where
equation = 0.44 Re >1000                                            (9)
The relative Reynolds number (Re) for the primary phase q (liquid) and secondary phase p (gas) is obtained from
equation (11)
Transport equations for κ−ε model: Two phase κ−ε model has been used for simulating turbulence [17]. The turbulent kinetic energy k, and its rate of dissipation ε, is obtained from the following equations.
equation (12)
equation (13)
Where the mixture density and velocity are computed from
equation (14)
equation (15)
The turbulent viscosity is computed as follows,
equation (16)
For the production of turbulence kinetic energy, the following equations are used:
equation (17)
The Various coefficients are given as below [17],
equation

Geometry

The details of the geometry used in the simulation are shown in Figure 3. This was arrived after analyzing many configurations (i) varying the distance between free surface and exit port in the separator; (ii) varying the gas exit area in the separator (small area chokes the gas flow leading to pressure built up in the separator and enhance carry under at higher flow rates); (iii) varying the gap between window and riser bottom (decides the velocity of jet entering spallation region). Even though, heating effects are not simulated for this part of the analysis, it becomes essential to account for hydraulic resistance due to the presence of heat exchanger in the downcomer. A solid annular concentric pipe is included in the downcomer which simulates the additional pressure drop arising due to heat exchanger.
Boundary conditions
Inlet is defined as mass flow inlet with uniform velocity. The mass flow rate of nitrogen is varied from 0.25 to 2.0 g/s and volume fraction of nitrogen is one. Outlet is defined as pressure outlet type with backflow normal to the boundary. The backflow volume fraction of nitrogen is specified as one. Turbulence intensity is specified 10% at the inlet. For velocity field no slip boundary condition is used at the wall.
Grid generation
It is not possible to generate structured grid for the considered entire target geometry, so block-structured grid with 36000 quadrilateral cells has been used. The mesh interval size used is 1 mm. The grid at the inlet, outlet is meshed with interval size of 0.5 mm.
Estimation of gas flow rate for various liquid metal flow rates
Initially the geometry of the module was finalized after varying different parameters (radii of riser, downcomer, height of liquid level in the separator, exit area for the gas in the separator, beam diameter, gap between the bottom of the riser and window, etc [12]. In the optimum geometry we can achieve flow rate of LBE up to 60 kg/s with the injection of 1.2 g/s gas flow rate. The effect of nitrogen gas flow rate on LBE flow rate is shown in Figure 4.
Figure 4: Effect of nitrogen flow rate on LBE flow rate.

Thermo-fluid Single Phase (Liquid Metal) 3D Flow Analysis near the Window Spallation Region

Having established the overall target geometry and liquid metal flow rate as a function of gas flow rate, 3D liquid metal flow analysis was carried out in the bottom region of the target module. In addition to the single phase momentum and continuity equations, the steady state energy equations have been solved to obtain the temperature distribution. The governing equation for steady state single phase energy equation neglecting the work done by viscous forces is given below:
equation (18)
Here, ρq, Cpq, κeff is the density, specific heat and effective thermal conductivity of the respective materials. is the volumetric source term and T is the temperature. For Single phase analysis is set equal 1. In solid domain equation (18) is solved with equation. Here,
equation
κ is the molecular conductivity.
κt is the conductivity due to turbulent transport and is given by:
equation
equation is the turbulent viscosity.
Prt is the turbulent Prandtl number which is taken 0.85 in this present simulation.
Flow analysis was carried out in the bottom of the target module covering spallation region and window (Figure 3). The required nonaxis symmetry of the riser pipe at the bottom (to reduce/eliminate the stagnation zone) is explicitly introduced. After carrying out analysis for different tilt angles and the minimum gap at the bottom between riser and window, the geometry is finalized. The flow is turbulent and k-ε turbulent model is used. Analysis was carried out both for uniform circular and Gaussian profile for the proton beam of energy 650 MeV and current 0.9 mA. After carrying out the various analyses beam radius was optimized to 35 mm of uniform cross-section. The energy deposited by the beam in the window and liquid region was estimated using FLUKA code [18]. The data was converted into a functional form and given as a input to the CFD code.
FLUKA code was run to obtain the detailed heat deposition in the window, LBE, and structural material for 650 MeV and 0.9 mA of proton beam. The given beam parameters leads to beam power of 585 kW which is distributed among various components of the target system. Major part of the energy is deposited as heat in LBE and Window. However, the beam power in proposed LINAC may vary from 585 kW to 650 kW depending on the beam current varying from 0.9 mA to 1.0 mA. The detail heat deposition data for two different geometries as obtained from FLUKA analysis is given in Table 1. T91 is chosen as window material and SS316 for the other parts of the target module. Additionally, 370 kW of heat is deposited in the target due to gamma radiation from the reactor. For this preliminary analysis, this heat has been assumed to be distributed uniformly in the entire target module located within the reactor. The distribution of heat for various zones is shown in Figure 5. This data forms the input for CFD analysis.
Figure 5: Contours of heat distribution in various zones.
Table 1: Heat deposition in different regions of the target for two different geometries.
The steady state solution has been obtained for two cases of geometry, one with flow gap of 7 mm and other with 10 mm between window and bottom surface of inner pipe (riser). In each case the slanting angle of the riser pipe bottom surface is varied with 5 degree, 10 degree to 15 degree cut angle. The analysis is carried out with different flow rates for each case study and slanting angle.
For case of 7 mm flow gap, it is observed that with increasing the slanting angle the maximum temperature of the window is increasing. The tangential component of the velocity jet along the window surface which is originating from the narrow gap of the slanted surface decreases with increasing slanted angle. Hence residence time of the liquid metal is more at the maximum heat deposition region of the window results in increase of temperature. However, with increasing flow rates, the maximum window temperature decreases.
For case of 10 mm gap, keeping the flow rate and slanting angle same as that of 7 mm case, the maximum temperature of the window is more as compare to the corresponding lower flow gap case (7 mm). This is due to shifting of stagnation zone towards the region of maximum heat load in window. Similar flow behaviour has been observed with increasing slanted angle as in the case of 7 mm flow gap case. However the pressure drop is less as compare to the lower flow gap (7 mm flow gap case) due to increase in flow cross section.
The contour of the velocity distribution and velocity vectors in a mid plane near the window region is shown in Figures 6 and 7 for case of 10 mm flow gap with 10 degree slanting angle and 45 kg/s mass flow rate. The corresponding temperature distribution is shown in the Figure 8. It has been observed that the pressure drop decreases with increasing cut angle of the slanted surface but the maximum temperature of the window is increasing. The steady state maximum window temperature, pressure drop for various case studies are shown in Tables 2 and 3. The maximum temperature of the window for 10 mm flow gap is 705 K for 15 degree slanting angle and flow rate of 45 kg/s and the corresponding pressure drop 4430 Pa. Since the design temperature for the upper limit of window temperature is 723 K, this case may not be a optimum target module due to very less margin to the design value. Hence for a conservative estimate the target module having 10 mm flow gap and slanting angle around 10 degree will be a suitable design with sufficient margin. The maximum window temperature for 10 mm flow gap and 10 degree slanting angle is 661 K which is well within the design parameters and hence recommended as an optimum target module. In view of pressure drop, the geometry that leads to low pressure drop for a given flow rate is encouraged for a typical gas driven system. The range of pressure drop for different flow rates (45 kg/s to 55 kg/s) in the case of 10 mm flow gap and 10 degree slanting angle of the riser pipe at the bottom surface is 5488 Pa to 6620 Pa. The required flow rates with the estimated preesure drop studied in this present analysis can easily be achievable by gas injection method and hence recommended for optimum target geomery.
Figure 6: Velocity contour for case of 10 mm flow gap and 10 degree slanting angle of the Riser bottom surface at the mid plane.
Figure 7: Velocity vectors for case of 10 mm flow gap and 10 degree slanting angle of the riser bottom surface at the mid plane.
Figure 8: Temperature contour for the case of 10 mm flow gap and 10 degree slanting angle of the Riser bottom surface at the mid plane.
Table 2: Maximum temperature and pressure drop for 7 mm flow gap.
Table 3: Maximum temperature and pressure drop for 10 mm flow gap.

Thermo-mechanical Stress Analysis

The steady state temperature distribution obtained from thermal hydraulic simulation is used in ANSYS model to calculate the thermomechanical stress. The Von Misses stress [19] obtained for various geometries and the results are summarised in the Tables 4 and 5. The simulation result indicates that maximum thermal stress of window decreases with increasing flow rate and increasing the slanting angle for a given flow gap. However, the maximum stress is 95 MPa for 5 degree slanting angle and 7 mm flow gap with flow rate of 45 kg/s. This value is well below the allowable stress which is 600 MPa and hence thermal stress is no issue for the current configuration of the target geometry. The stress distribution due to unequal flow distribution is not symmetrical in window which is as shown in the Figure 9 for case of 7 mm flow gap and 5 degree slanting angle for the flow rate of 45 kg/s. The stress distribution in window in not symmetric due non symmetric temperature distribution for the case of 7 mm flow gap and 5 degree slanting angle as shown in the Figure 9 for the flow rate of 45 kg/s.
Figure 9: The stress distribution in window for case of 7 mm flow gap and 5 degree slanting angle for flow rate of 45 kg/s.
Table 4: Maximum Von-Mises stress of the window for 7 mm flow gap.
Table 5: Maximum Von-Mises stress of the window for 10 mm flow gap.

Water Cooled Jacket to contain LBE under Accident Failure of Window

A water cooled safety jacket around the target module will be provided to contain the radioactive LBE in case of target module failure. The issues involved in the design are: (i) fitting the jacket within the space available, (ii) extracting heat deposited by proton beam in the circulating water and container material, (iii) the shape and geometry at the bottom is in such a way that leaked LBE is collected away from the impinging proton beam, (iv) minimum thermo-mechanical stresses in the container material, (v) optimum water flow rate. Figure 3 shows the schematic of target loop with Water cooled safety Jacket parameter for the design of water jacket is as given in Table 6. It is proposed to fill the gap between Jacket and Target module with helium.
Table 6: Parameter for the design of water jacket.
Design criteria for water jacket
Water jacket wrapped around the Target [9]. Proton beam enters the target module through water jacket. As a result heat is also deposited in the water jacket which has to be removed. AlMg3 has been chosen as water jacket container material. Schematic of water Jacket is as shown in Figure 10 and their corresponding dimensions in Figure 11. The annular ring of Jacket is divided into two parts. One part is for inlet (Down-flow) and other part is for outlet (Up-flow). Along the axial direction flow dividers are provided, they extend radially at the bottom. Inlet water enters into proton beam heating region (cross-flow) of hemispherical dish shape and after extracting heat water enters to Up-flow annular region. This shape prevents leaked LBE from the window of target module to interact with the proton beam. The leaked LBE will be contained at the bottom part of the jacket where Leak detectors are located. Shape optimization was the major issue in the Water Jacket Design. Shape of the Water Jacket should be more spherical and steeper so that leaked LBE comes down quickly with minimum exposure to the proton beam. Two configurations of flow divider have been studied with 10 and 20 percent blockage at the annulus region.
Figure 10: Schematic of Water Jacket.
Figure 11: Dimension of Water Jacket.
Thermal-hydraulic study of water jacket
Heat deposition by the proton beam in the water and inner and outer container material was estimated by the FLUKA code. Proton beam deposits 2.52 kW, 2.68 kW and 1.25 kW heat in inner wall, outer wall and water region of safety jacket respectively which is as shown in Figure 12. Since water has been chosen as the coolant, computational fluid dynamic study has been carried out for various flow rates and various water inlet temperatures [9]. As the flow is turbulent, model has been chosen for analysis. CFD analysis shows that 1 kg/s flow rate and 305 K inlet temperature of water is optimum to cool the water jacket for 10 percent blockage, since for this case stresses (Maximum stress=24.6 MPa) in jacket are much less than the allowable limit of AlMg3 (201 MPa). Temperature distribution for this case is as shown in Figure 13.
Figure 12: Heat distribution in the various zones of Water Jacket.
Figure 13: Temperature Distribution in the Water Jacket.

Estimation of neutron yield, spallation products and radioactivity

It is essential to estimate radioactivity, spallation products etc. for the shielding purpose, remote handing, process design, replacement of the target module etc. FLUKA code has been used to estimate the neutron yield and radioactive products generated in the target, window and other structural materials. T91 is chosen as window material and SS316 as other parts of the target module. Number of histories taken for the analysis is 106. Details of the geometry used for FLUKA analysis is as shown in Figure 14. In FLUKA simulation the mixture region is modeled with 80% of LBE and 20% of nitrogen by volume fraction.
Figure 14: Geometry for FLUKA analysis.
Neutron yield and energy distribution
Figure 15 shows spallation neutron energy distribution from LBE target using 650 MeV energy proton beams which has been estimated using FLUKA. The dominant neutron energy lies in the range of 10-12 GeV to nearly 10-4 GeV which is mainly due to evaporation process of the excited nucleus that emits nucleons or light nuclei such as D2, T3, He3, Li and Be etc. The higher energy part is due to the intra-nuclear cascade ranging from 10-3 to ~ 0.65 GeV. The total number of neutrons produced is 7.14×1016 for 0.9 mA proton beam.
Figure 15: Spallation neutron energy distribution.
Activity and various Spallation radioactive products in LBE
A large number of radioactive products like Pb203, Bi205, 206, 207, 210, Hg197, Rb83, Sr85, Y87, Zr88, etc. are produced in LBE and Window material including volatile spallation products like Po210, Hg203, Tl204, Sr90, etc. The various products produced in the spallation reaction are as shown in the Figure 16.
Figure 16: Total activity distribution of spallation products for different mass number in LBE.
Analysis has been carried out for one year of continues irradiation. The activity increases with irradiation time and reaches saturation level at different irradiation time for different structural material. The builds up of saturation activity for LBE, window and structural material and their decay after irradiation is shown in Figures 17-19 respectively. The saturation activity for LBE, window of T91 material (the composition of the material is presented in Table 7) and Structural material of steel (SS316) is 7.2×1015, 2.3×1013 and 3.1×1013 Bq respectively. The saturation activity reaches after 60 days, 330 days and 335 days of continuous irradiation for LBE, window and structural wall respectively.
Figure 17: Saturation activity in LBE during irradiation time (one year) and activity after irradiation in different cooling time.
Figure 18: Saturation activity in window (T91 steel) during irradiation time (one year) and activity after irradiation in different cooling time.
Figure 19: Saturation activity in structural material steel during irradiation time (one year) and activity after irradiation in different cooling time.
Table 7: Composition of the T91.
In the Table 8, major volatile products generated in LBE and their activity is presented. Polonium, Mercury, Thallium and Strontium isotopes are major contributors. In Table 9, the non-volatile activity contributors are presented. Lead, Bismuth and Mercury isotopes are the major contributors. The activity in the window is presented in the Table 10. The major contributors are the isotopes of Rubidium, Strontium, Yttrium and Zirconium isotopes.
Table 8: Activity of the radioactive products (Volatile) in LBE for different cooling times.
Table 9: Activity of the radioactive products (Non Volatile) in LBE for different cooling times.
Table 10: Activity of the radioactive products produced in Window for different cooling times.

Summary and Conclusion

Detailed study has been carried out to design a LBE spallation target for an experimental ADS reactor of 30 MW class. Total spallation neutron generation and their energy spectrum, heat deposition distribution, spallation products, have been estimated using high energy particle transport code FLUKA. The total activity in LBE, Window (T91), and structural material (SS316) and saturation activity of major spallation products including volatile gaseous products have been estimated for 1 year of beam irradiation. 3D CFD (Computational Fluid Dynamic) thermal hydraulic simulations have been carried out for different target geometries to obtain optimum Window temperature distribution and pressure drop for various flow rates. The required gas flow rate for LBE circulation has been obtained by carrying two phase CFD analysis. The geometry has been optimised by varying slanting angle of the Riser bottom surface and different flow gap between window and Downcomer/Riser. Based on the thermal hydraulic simulation, thermo mechanical stresses in window have been estimated. Finally a suitable target module which can be operated by gas injection as well as pump driven system with optimised geometry and flow rate has been recommended. Although the target is designed for a specific reactor but in general is generic one which can be used for any type of reactor. The analyses presented here for the LBE target design is fairly rigorous with respect to thermal-hydraulics, radioactivity analysis, prediction of spallation products and calculations of thermo-mechanical stresses in the window and the nearby components. However, additional issues like fabrication details, remote target handling system, shielding and post irradiated target handling systems are still to be addressed. Some of these issues are associated with the detail design of the ADS reactor system and have to be addressed in a coupled manner. Also the FLUKA code is a well established high energy particle transport code, some benchmarking of the code’s capability to predict radioactivity, spallation products, neutron yield etc has to be carried out at these proton energies (585-650 MeV) interacting with liquid LBE.

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