DESIGN AND THERMAL-HYDRAULIC PERFORMANCE OF A HELIUM COOLED TARGET FOR THE PRODUCTION OF MEDICAL ISOTOPE 99M TC

Transcription

1 DESIGN AND THERMAL-HYDRAULIC PERFORMANCE OF A HELIUM COOLED TARGET FOR THE PRODUCTION OF MEDICAL ISOTOPE 99M TC Keith Woloshun, Gregory E. Dale, Charles T. Kelsey IV, Eric R. Olivas, Michael A. Holloway, Ken P. Hurtle, Frank P. Romero, Dale A. Dalmas, Angela Naranjo, LANL, Los Alamos, NM, USA Sergey D. Chemerisov, ANL, Argonne, IL 60439, U.S.A., James Harvey, NorthStar Medical Technologies Abstract 99m Tc, the daughter isotope of 99 Mo, is the most commonly used radioisotope for nuclear medicine in the United States. Under the direction of the National Nuclear Security Administration (NNSA), Los Alamos National Laboratory (LANL) and Argonne National Laboratory (ANL) are partnering with NorthStar Medical Technologies to demonstrate the viability of large-scale 99 Mo production using electron accelerators. In this process, 99 Mo is produced in an enriched 100 Mo target through the 100 Mo(γ,n) 99 Mo reaction. This process utilizes a stack of thin 100 Mo wafers cooled by high pressure (22 MPa) and high velocity (nominally 300 m/s) helium. Peak heat flux as high as 1000 W/cm 2 has been observed. This paper describes the design and performance (test results) of the target. INTRODUCTION Electron beam impingement on a 100 Mo target will be used for the production of 99 Mo for medical imaging. The optimum beam conditions and target size continues to evolve. Nonetheless, target cooling tests have been conducted in a thermal performance test using an electron beam accelerator at Argonne National Laboratory. For these tests, the beam was nominally 6 mm FWHM circular pattern, with a target diameter of 12 mm. The tests and analysis reported here are for helium cooled targets. Previous attempts to cool with water, reported at the AccApp11 conference [1], were successful. However, unacceptable corrosion and dissolution of the target in the water coolant mandated a change to gas cooling. The very high power density and total power in the targets, now 120 kw, demands high heat transfer and high strength/high temperature containment materials, most importantly for the target window, the beam entry surface. Helium coolant was chosen for its favourable properties, and a high pressure system has been designed to increase density and thereby increase overall heat transfer coefficient. In order to establish experimental validation of the approach in a short period of time, the first helium cooling test, the 4-Disk Mo Target Test, was conducted open loop. This foreshortened target test allowed for proof of concept in an open system without excessive helium mass flow. Subsequently, a closed loop helium system, described below, was completed. This will be used to for a second test with a full length target of 25 disks. This test and performance predictions are also discussed in detail below. A repeat of this test at higher beam energy, consistent with evolving optimum isotope production scheme is planned.. THERMAL TEST OF HELIUM-COOLED, 4-DISK MO TARGET A foreshortened natural Mo target, comprised of 4 Mo disks 1 mm thick, 12 mm in diameter and cooled by helium was tested. The objective of this helium cooled target test was to determine the net effective heat transfer coefficient as a function of mass flow rate and to determine the maximum heat flux at the design point of 2 MPa and kg/s. Beam energy for this test was 17 MeV, while optimum energy for 99 Mo production is 35 to 42 MeV. While facility upgrades to this optimal energy are in progress, this interim test serves to establish cooling measurements for production plant design optimization. An illustration of the test target is shown in Figure 1. The Mo disks are held so that there is a 0.5 mm gap for the helium coolant on each face of the disks. Thermocouples penetrate to the center of each Mo disk. The housing that encloses and holds the Mo is made from Inconel 718. The front and back faces of the housing are, like the Mo, directly in the beam. These faces are 0.6 mm thick, and, as shown, cooled only on one side. Figure 2 is a photo of the target in the holder and positioned in the beamline. The 4 thermocouples penetrate the top flange. The housing is made from Inconel 718 for its high temperature strength. In addition to the 4 thermocouples inside the target disks, helium pressure, temperature, temperature rise (the red cylinders on either side of the target holder in Figure 2) and mass flow rate are measured. Helium pressure is set with a regulator, and flow rate is adjusted with a control valve downstream of the target. Heat transfer coefficient as a function of flow rate is shown in the graph in Figure 3, based on the Colburn correlation for closed channel coolant flow and equivalent hydraulic diameter. This calculation is based on an assumption of uniform velocity in all channels. This validity of this assumption was verified computationally by CFD analysis using the ANSYS code, as shown in Figure 4. Using this heat transfer coefficient and heating distribution predicted by MCNPX calculations, Solidworks finite element

2 software was used to predict the beam current at which the peak temperature of 1000 K in the face is reached as a function of helium flow rate. Beyond 1000 K the material strength is too low to withstand the thermal and mechanical stresses. The determination of the limiting temperature for this test is complicated somewhat by the fact that at higher beam currents the temperature gradient through the front face is quite large. At the 680 µa peak current imposed during the test the temperature gradient through the 0.6 mm thickness of the face is 181 K. Thermal profile of the exterior of teh front face under these conditions is shown in Figure 5. On the basis of the otherwise over conservative design, and the fact that thermal stresses induced by this gradient are secondary stresses, the inside temperature is taken as a guideline for these experiments. Figure 1: Test target. Gas flow enters and exits through rectangular tube extensions, with the target proper at the center. Mo target disks are shown in the light crosssection in Detail B. He flows in 0.5 mm wide gaps between the Mo disks. Figure 2: Photograph of the test target installed in the beamline. Heat transfer coefficient as a function of flow rate is shown in the graph in Figure 2, based on the Colburn correlation for closed channel coolant flow. This calculation is based on an assumption of uniform velocity in all channels. This validity of this assumption was verified computationally by CFD analysis using the ANSYS code, as shown in Figure 3. Using this heat transfer coefficient and heating distribution predicted by MCNPX calculations, Solidworks finite element software was used to predict the beam current at which the peak temperature of 1000 K in the face is reached as a function of helium flow rate. The determination of the limiting temperature for this test is complicated somewhat by the fact that at higher beam currents the temperature gradient through the front face is quite large. At the 680 µa peak current imposed during the test the temperature gradient through the 0.6 mm thickness of the face is 181 K. Thermal profile of the exterior of teh front face under these conditions is shown in Figure 4. On the basis of the otherwise over conservative design, and the fact that thermal stresses induced by this gradient are secondary stresses, the inside temperature is taken as a guideline for these experiments. Test Set-up and Results Helium flow is once through, starting with a 20 bottle farm through a regulator set at 300 psig. This corresponds to standard liters, or 20 minutes of run time at the maximum flow rate of 17 g/s. Pressure is monitored at the high pressure side of the regulator to track gas usage. Flow is then through 19 mm tubing, passing through a pneumatic shut-off valve and a water heat exchanger, then through the target. Downstream of the target is a mass flow meter, control valve for regulating flow rate, and a HEPA filter.

3 15000 h (W/m2-K) Mass flow rate (g/s) Figure 3: Heat transfer coefficient of helium as a function of mass flow rate for this geometry (mean helium pressure in target of 260 psi). Figure 4: Velocity profile in the 5 coolant channels. Figure 5: Outside of the front face at 680 µa and 17 MeV, at a helium flow rate of 17 g/s. Peak inside temperature is 983 K. Raw data showing rate of rise and steady-state temperature at 680 µa is plotted in Figure 6. Disk 3 is reaching the highest temperature, while Disk 4 is by far the coolest. A small error in the placement of thermocouples (proximity to disk center) is the probable explaination for the deviation from prediction, perticularly for Disk 4. Nontheless, a comparison between predicted an dmeasured heat transfer coefficient shows good agreement for the first 3 disks (±10%, Figure 7).

4 :35:57 17:36:40 17:37:24 Figure 6: Temperature response of each disk with nominally 680 µa of beam heating, Temperaturre scale in Celcius Disk 1 Disk 2 Disk 3 Disk Figure 7: Normalized heat transfer coefficient (measured/calculated) for the 4 disks at the flow rate of 17.2 g/s. Horizontal axis numbering is the disk number. CLOSED LOOP HELIUM FLOW SYSTEM DESIGN AND UPCOMING TEST TARGET DESIGN While the open flow system provided some rapid indication and validation of target cooling capacity, a closed loop helium flow system was necessary for further concept development and for production facility development. A roots blower (Tuthill model 3206 PD Plus) with drive motor was enclosed in a pressure vessel, modelled after a system in operation at Princeton University [2]. A photograph is shown in Figure 8. The pressure vessel is in 2 parts: A fixed flange with all required penetrations into and out of the vessel and welded to a skid plate, and an outer enclosure (bell shaped) that slide on a rail for easy opening of the vessel for access. This enclosed blower/motor then become the driver for a fully instrumented closed loop with two heat exchangers, one to remove blower heating of the gas and the second for beam heat removal from the gas. The Piping and Instrumentation Diagram and a plumbing layout are shown in Figure 9. The target for initial testing with this system is made up of 25 disks as shown in Figure 10, and mounted inside a vacuum enclosure for installing in to beamline without the necessity of beam transport through air. The target housing was designed and pressure tested as per ASME Boiler and pressure vessel code. As in the previous target cooling designs, the present design will once again use coolant flow between target disks, thus a parallel flow pattern is established. The parallel flow is also extended to the front and back window inner surfaces. The helium coolant will flow with a inlet mass flow and pressure of 120 g/s and MPa, respectively. With a gas velocity of 264 m/s through target corresponding to a Mach number Under these flow conditions the expected heat transfer coefficient will be W/m 2 -K. Testing will be conducted at the beam energies of 35 and 42 MeV. Figure 8: Helium blower and drive motor installed inside a pressure vessel. ANSYS finite element software was used to model helium flow and heat transfer in the target, assuming 293 K helium inlet temperature, 2 MPa (290 psi) inlet pressure and 1.9 MPa (275 psi) outlet pressure. This is based on previous experience with this a peak pressure and available pressure drop. The resulting velocity profile is shown in Figure 11. Figure 12 is the resulting temperature plot at the 35 MeV, 300 µa conditions, while peak disk temperature profile is shown in Figure 13.

5 Figure 9: Helium cooled target test P&ID. Figure 12: Temperature plot at the midplane with the flow conditions of Figure 11 and beam perameters of 35 MeV and 300 µa. Figure 10: Target in-beam configuration, crosssectional view.. Peak Temperature, K Peak Window and Target Disk Temperature: psi Inlet 7.00E E E E E Front Window and Target Disks Figure 13: Peak temperatures in the disks for the conditions specified in Figure 12. TARGET OPTIMIZATION Figure 11: Calculated velocity profiles at 2 MPa pressure and 1.9 MPa pressure drop across the target MCNPX studies to optimize the production of 99 Mo have resulted in significant changes to the target geometry as compared to the above mentioned tests. The primary changes are double sided target, increased length and diameter, 42 MeV optimum beam energy, and flat beam profile. A double sided beam requires a beam entry window on both ends. The larger diameter, now 29 mm, significantly increases the helium mass flow required to maintain the high Reynolds and Nusselt numbers for adequate cooling. Toward this end, the coolant gap between Mo disks has been reduced to 0.25 mm. However, with increased length to 41 mm total. Disk thickness has been reduced to 0.5 mm to manage the volumetric heating and maintain the Mo temperature in the range of 1000 C. With 82 disks at 29 mm diameter requires about 400 g/s helium mass flow. Helium pressure will be increased to 400 psi. This results in a density increase impact on heat transfer of about 15%, and the smaller hydraulic diameter of the narrower gaps has a similar positive effect on heat transfer coefficient.

6 CONCLUSION Preliminary testing of helium cooling of a segmented Mo rod target using an open system produced encouraging results. Heat transfer was as predicted and sufficient for adequate cooling of the target and the beam entry window. A closed loop system has been designed and built. Currently operational at Argonne National Lab, target cooling and isotope production experiments will be conducted in the coming months As described herein. After completion of those tests, additional experiments will be conducted on targets designed for the optimized isotope production. REFERENCES [1] K. Woloshun, et. al., Target Design for the Accelerator Driven Production of 99 Mo, AccApp 11, Knoxville, TN, July [2] M. Kalish, M. Cropper, C. Neumeyer, R. Parsells, L. Dudek, A. Klink, L. Morris, "Design of the NSTX heating and cooling system," 19th Symposium on Fusion Engineering, pp ,