Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
FTL-1 FEASIBILITY TEST LOOP DESIGN AND CONSTRUCTION
Thomas J. Godfroy, J. Boise Pearson, Kurt A. Polzin, Robert S. Reid, Kenneth L. Webster NASA/MSFC Huntsville, AL 35812 Tel:256.544.1104 , Fax:256.544.5926 , Email:
[email protected] Abstract – A series of small Feasibility Test Loops (FTL) are being constructed at MSFC by the Early Flight Fission – Test Facility that will attempt to obtain a hardware based measurement of pumped loop material transport effects (not mechanisms) given representative temperatures, temperature differences, flow velocities, and L/D ratios. The first in a series of desired tests, FTL1 will be used to provide specific test data as well as to address experiment design and implementation. This testing will be used to provide experimental data regarding the corrosion of stainless steel by NaK at specific operating conditions anticipated for a “best estimate” reactor. This paper discusses the design, components, and configuration of the FTL-1. higher temperature alkali metal systems the issue of corrosion is a major concern. Often it is difficult to assess or quantify the rates of corrosion because of a confluence of transport, corrosion, and chemical mechanism and interactions. Of specific interest at this time is the rate of corrosion of 316L stainless steel and NaK at the “best estimate” operational conditions for a FSP system.
I. INTRODUCTION The Early Flight Fission – Test Facility (EFF-TF) was established by the Marshall Space Flight Center Propulsion Research Center Nuclear Propulsion Group to provide a capability for performing hardware-directed activities relevant to multiple nuclear power reactor concepts using non-nuclear test methodology. This includes fabrication and testing at both the module/component level and near prototypic hardware configuration allowing for realistic thermal hydraulic evaluations of systems. The EFF-TF is currently working toward the potential development of an affordable fission surface power (FSP) system that could be deployed on the Lunar surface. Through a strong partnership with engineers at the Los Alamos National Laboratory (LANL), facets of the conceptual reactor designs are translated into hardware experiments for nonnuclear testing at NASA MSFC. A liquid metal cooled reactor was selected for further reactor design and test activities. This design was derived from the only fission system that the United States has deployed for space operation, the Systems for Nuclear Auxiliary Power (SNAP) 10a reactor, which was launched in 1965.1 An important aspect of hardware development is the knowledge and information gained from early hardware testing with both relevant materials and environment which can often deliver valuable insights with a confidence that is not always available or attainable with a paper study.
A series of small Feasibility Test Loops (FTLs) are planned for construction whose purpose is to assess and resolve potential issues with Fission Surface Power (FSP) systems that are cooled by Na or NaK. Potential issues include freeze/thaw, on-line coolant purification, loop material transport, coolant expansion/gas generation, and others. The FTLs are designed to provide needed information in a cost-effective manner. Feasibility Test Loop One (FTL-1) is being constructed and will attempt to obtain a hardware based measurement of pumped loop material transport effects (not mechanisms) given representative temperatures, temperature differences, flow velocities, and L/D ratios. The first in a series of desired FTL tests, it will be used to provide specific test data as well as to address the issues related to experiment design and implementation. Follow on FTL’s may evolve as the experiments are refined. II. FTL DESIGN PARAMETERS The FTL-1 experiment is to address the rates of corrosion in flow passages of the current “best estimate” reactor design.2 The Los Alamos National Laboratory reactor design requires the use of three distinct flow passage areas, here identified as Type 1, Type 2, and Type
Early reactor studies have identified uncertainty with respect to the rates of corrosion of stainless steel and the eutectic mixture of sodium and potassium called NaK at estimated operating conditions. When working with most
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Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
TABLE II
3, through the core to maintain desired heat removal, power distribution balance, and pressure drop. The differing area passages will facilitate core power flattening to enable increased operational lifetime of the reactor by mitigating stresses in the reactor. By studying these flow areas at designed temperatures, delta temperatures, pressure, and flow rates with the prototypic materials of 316L stainless steel and NaK it is hoped to develop an understanding and quantification of the rates of corrosion. The balance of the loop is then designed around delivering these test conditions in a test section.
Calculated Coolant Channel Properties Parameter Velocity Head Reynolds Number Coolant Velocity Pressure Loss Flow Area Mass Flow
The starting point for the FTL-1 design is the Hot Test Section (HTS). The conditions set in this test section dictate the principle design of the balance of the loop. The HTS is designed to determine the rates of corrosion in the area where corrosion will be most dominant as well as where it most critical to lifetime which is in the core flow passages. This is by definition, the highest temperature region of the system and as such will experience the greatest rates of corrosion.2,3 Additionally, this area is most sensitive to corrosion as it typically has the lowest margin of safety due to desired compact nature of a space reactor. The current “best estimate” design involves three different flow passage areas resulting in three different flow conditions.
Type 1 .215 21971
Type 2 .146 13058
Type 3 .044 2960
Units psi
2.01
1.65
0.90
m/s
0.447 1.64E-5 2.435E-2
0.395 1.22E-5 1.494E-2
0.317 3.37E-6 2.24E-3
psi m2 kg/s
II.B. FTL-1 Design Parameters With the establishment of the test section, the balance of the loop properties and requirements can be determined. Table III gives the bulk loop properties for FTL-1 and Fig. 1 illustrates the design layout. TABLE III Loop Properties Parameter Coolant Max (Inlet) Temperature Min (Exit) Temperature Delta Temperature Mass Flow Nominal Loop Pressure Pump Pressure Rise Required Input Heat Energy Loop Operational Environment Oxygen Concentration in NaK
II.A. “Best Estimate” Reactor Parameters The “best estimate” design is supplied by Los Alamos National Laboratory and the parameters of interest covering the bulk core properties which will be needed for the design of FTL-1 are provided in Table I. These values provide the bulk operational constraints on FTL-1
Value NaK eutectic 880 840 40 0.042 10-20 0-5 1452 Vacuum 15-250
Units K K K kg/s psia psia W ppm
II.C. Corrosion in System
TABLE I
Mechanisms of corrosion and mass transport are fairly well documented and a detailed discussion is beyond scope here, however factors that influence corrosion are temperature, loop temperature gradients, mass flow, materials, and impurity concentration. Mass transport equations have been developed but may not be entirely applicable to this operational regime thus the need for testing. With a NaK stainless steel loop, the concentration of oxygen and carbon present in the system are the most active contributors to corrosion, influenced heavily by the temperature and temperature gradient present in the loop. Of these two, oxygen is the most dominant actor and in a flight system will need to be kept to as low a concentration
Core Average Properties Parameter Coolant Thermal Conductivity Viscosity Density Specific Heat Inlet Temperature Exit Temperature Nominal Pressure Boundary Material
Value NaK eutectic 25.95 1.599E-4 738.78 874.07 840 880 20 316L Stainless Steel
Units W/m-K kg/m-s kg/m3 J/kg-K K K psia
Table II contains design parameters for the Type 1, Type 2, and Type 3 flow channel being evaluated.
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Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
dPT PT FTR Pump CTS
CTS PI PFM
½ ” Tube
¼” Tube
HTS
FM
HTS
Fig. 1. FTL-1 layout.
as possible. Measurement of oxygen concentration can be accomplished by taking a sample in a rigorously controlled manner and sending to a laboratory for analysis or by using
By monitoring flow as a function of temperature and comparing the temperature at which flow drops to zero to an established correlation will give an oxygen concentration in the NaK. This method is preferred as the expense and procedurally crucial steps in gathering, handling, and analyzing a NaK sample are eliminated. As well, the test can be performed as frequently as desired to develop a time history of oxygen concentration. An initial and final oxygen concentration combined with the depth of corrosion is all that is needed to reasonably bound the rate of corrosion however by taking samples at intervals over time, oxygen concentration as function of time can be developed that may provide more information. In an ideal system, the oxygen concentration should constant, however in an actual system this is not always the case.
Oxygen solubility in NaK 10000
Data 5th Order Poly Fit
ppm Oxygen
1000
Exponential Fit
100
10
III. FTL-1 DESIGN Using the design parameters from the “best estimate” design from LANL, the test section for the FTL-1 was designed. The balance of the FTL-1 is to deliver these test conditions to the test section.
1 0
200
400
600
800
Temperature (°C)
Fig. 2. Oxide concentration as a function of temperature.
a plugging indicator. The concentration of oxides dissolved in the NaK is well characterized as a function of temperature. Refer to Fig. 2. As the temperature of the NaK is reduced, the oxides will precipitate and accumulate so that small orifice, such as that found in a plugging meter, will eventually become plugged, preventing flow.
III.A. System Layout Fig. 1 shows a three dimensional rendering of the FTL-1 layout inside a 3’dia x 6’long vacuum chamber. The loop will be insulated (not shown) using multiple layers of
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Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
foil (MLI) to prevent undesirable radiative heat loss. A high purity argon/vacuum system will be connected to the Flow Through Reservoir (FTR) to enable system bake-out, system fill, and general operations as well as supply the 510 psia nominal pressure head to the system. The pump housing and tubing close to the pump will be pinned and the remaining elements of the system will be allowed to float and move as needed to accommodate thermal expansion and contraction. Testing will take place under vacuum better than 10-4 torr for a duration of approximately 3000 hours.
parallel, further reducing the required pumping pressure. This test coupon design is an evolution of a design used for testing corrosion samples in the 1960s for the Atomic Energy Commision.4 The pressure drop or loss across the section will not be corroborated empirically, however water flow tests are planned to characterize the pressure drop. Information about the pressure drop will not affect the corrosion assessment.
Area Passages
The plugging loop uses smaller ¼” diameter tubing than the ½” tubing of the main loop as do the legs to the pressure transducers. One absolute value pressure transducer (PT) and one differential pressure transducer (dP) will be located on the NaK filled tubing and an additional PT will be located on the FTR. The plugging leg contains a plugging indicator (PI) and a plugging flow meter (PFM). From the pump, the NaK flow continues to the flow meter (FM) through the heater (HTR) to the hot test section (HTS). From here it enters the heat exchanger (HX) when it is cooled and enters the cold test section (CTS) continuing along through the FTR and finally back to the pump. The approximate dimensions of the loop are 18” wide by 60” long. The plugging operation, as well as all other operations performed at temperature, are operated remotely. The vacuum chamber provides an additional barrier of protection between the hot NaK and personnel. The loop is designed to be operated for a year continuously.
Fig. 4, FTL-1 hot test section coupon area passages.
Fig. 3 shows the HTS design and Fig. 4 highlights the differing test coupon flow passages or areas. The test coupon is housed in a precision machined 316L stainless steel tube. To maintain positive location of the coupon, a spacer and a retaining ring are utilized. VCR type sealing fittings have been used with success on earlier experiments5 and are used here to facilitate ease of assembly, disassembly, and fabrication of FTL-1. From the comleted HTS design the balance of the requirements for the system were derived. III.B. Pump
III.B. Hot Test Section (HTS)
A DC electromagnetic (EM) pump for this system was designed by MSFC specifically for use in this application. It is designed to be cavitation free with a maximum developed pressure of 5 psi at a minimum flow rate of 56 g NaK/s. It requires an input voltage of ~1 V at 50-100A DC current. The pump design is illustrated in Fig 5.
VCR Fitting
Retaining Clip Spacer Ring
Tri-Area Passage Test
HTS
Fluxtrol
Magnet
Electrode
Fig. 3. FTL-1 hot test section design.
Microporous Insulation
The hot test section (HTS) was designed to match the flow conditions in Table II. While the wetted perimeter profile of each passage type is not replicated, the flow area, and operational temperature are preserved. In this manner, the coolant velocity and mass flow can be matched to the design specification for each passage type. Due to the essentially incompressible nature of NaK, the operating pressure is not required to be maintained. The overall length of the test section is five inches to minimize the pumping requirements for the loop. The flow areas are in
NaK Flow Tubing
Copper Cooling Block
Fig. 5. FTL-1 DC EM pump design.
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Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
A ½” diameter stainless steel tube with a wall thickness of 0.049” is flattened to produce the channel section. Two rare earth (SmCo) magnets straddle the tube, producing a relatively uniform field in the pump channel. Current is delivered to the liquid NaK through two electrodes entering perpendicular to the applied magnetic field and the streamwise direction. The magnets are separated from the channel by a layer of Microporous insulation, which limits thermal conduction from the NaK, and a copper housing. The copper housing provides both the mechanical restraint for the magnets as well as nearly complete coverage of the magnets to prevent overheating due to radiative heat transfer. The copper housing blocks are water cooled to maintain temperature. Thermal management in a high temperature vacuum system can present challenges. Rare earth magnets are not typically used due to their lower Curie points. However, this novel design allows the use of this class of magnets, which allows for achievement of the target developed pressure at a current level that is reduced relative to other DC EM pump designs. Fluxtrol, which is characterized by its high magnetic permeability, is used to complete the magnetic circuit, concentrating the magnetic flux and further increasing the field strength in the channel.
g/s. Fig. 7 shows a cross section of the heat exchanger design. Gaseous nitrogen is used to remove the heat in an open cycle system. The NaK will enter at 880K and exit at 840K raising the temperature of the GN2 by approximately 200K. If more cooling is needed, the GN2 flow rate can be increased. Bellows are utilized to accommodate the thermal dissimilarity and resultant expansion differences between the NaK tube and the nitrogen barrier. VCR fittings (not depicted) are used to mate with other loop components. A mass flow controller is located upstream of the HX and controls the flow rate of the nitrogen gas to automatically adjust and maintain required temperature differentials. GN2
Tube Body
NaK
Machined Tube Body
Bellows
IV.D. Heater (HTR) Fig. 7. FTL-1 Heater exchanger design.
Electrical Connections
IV.F. Cold Test Section (CTS) The cold test section (CTS) is inserted in the system to attempt to evaluate the deposition of material that has been dissolved from the hotter sections of the loop. Deposition will occur in the coldest regions3,4 of the loop thus the CTS is positioned immediately following the HX. Fig. 8 depicts the CTS design. As with the HTS, a spacer and retaining ring are utilized to positively constrain the test coupon which will be removed and evaluated at the conclusion of testing. The test coupon area does not have significance other than to be sufficient large not to restrict flow and cause too great a pressure drop.
½” Tube
Spacers
Watlow Heaters
Fig. 6. FTL-1 heater design.
The heater (HTR), shown in Fig. 6, is a very simple arrangement that radiatively heats a section of the ½” stainless steel tube. The required input heat to raise the NaK temperature 40K at the prescribed flow rate is approximately 1.5 kWt. The heaters selected are manufactured by Watlow and provide radiative heating in a vacuum. These ceramic heaters can deliver a maximum of 1.1 kWt per heater with two being utilized resulting in the ability to deliver 2.2 kWt. IV.E. Heat Exchanger (HX) The heat exchanger (HX) is designed to remove 1.8 kWt of energy from the system at a GN2 mass flow of 8.6
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Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
IV.H. Plugging Indicator (PI)
CTS
Spacer Ring
The plugging indicator for the FTL-1 is a VCR seal with a 60 micron screen sintered in the center. A small heat exchanger cooled by GN2 will be used to reduce the temperature of the NaK to precipitate the oxides. These precipitated oxides will be collected by the screen and eventually flow will stop. Following the plugging indication, the indicator will be allowed to heat back to near loop temperatures where the oxides will go back into solution. At the time of writing a small amount of additional engineering is needed to determine the orifice area required.
Retaining Clip
Passage Test
VCR
Fig. 8. FTL-1 cold test section design.
IV.I. Flow Meter (FM), Plugging Flow Meter (PFM) The FM and the PFM are designed with nearly the same design as the Pump, see Fig. 5. The FM is an exact replica with the only difference being the electrodes are used to read an induced current which correlates to a flow rate. The PFM uses either a 1/16” or 1/8” tube for the channel. Due to the significantly lower flow rate in the plugging leg, greater flow speed is advantageous. While this is not an optimum design for this size tubing it is a cost effect solution and will provide the information necessary.
IV.G. Flow Through Reservoir (FTR) A flow through type reservoir (FTR) is utilized in this application to ensure the homogeneity of the NaK since the oxygen concentration in the loop will be evaluated at various intervals. Stagnant pockets of NaK could artificially influence the total oxygen concentration of the system. The FTR is the interface point between the loop and the external environment. It serves as an expansion volume as NaK will expand greater than 15% when at temperature, provides a region for static pressurization by Argon gas, and provides access for NaK introduction and sampling. Additionally, it provides a volume for generated gas accumulation. As was discovered in prior FSP-PTC testing5 gasses can be generated in the system or accidentally introduced that can present difficulties. The FTR is positioned at the highest point in the system with all legs of the loop sloping at 3 degrees to enable any trapped or developed gasses to eventually migrate to the FTR. Also, NaK samples can be taken from the FTR following the conclusion of testing for confirmation analysis of oxygen concentration. It is constructed of 3” diameter schedule 10 316L stainless steel with end caps welded as shown in Fig. 9. To Ar/Vac System
Fill Port
IV. CONCLUSIONS Assembly of the FTL-1 is on schedule to begin May 2007 with testing flowing shortly thereafter. Following the termination of the FTL-1 experiment, the HTS and CTS will be disconnected from the loop, the test coupons removed, neutralized, and sent for analysis by the Materials Analysis Branch at MSFC. The primary area of interest will be corrosion depth although as much information as possible will be retrieved. A report of findings will be published at a later date. When working with most alkali metal systems the issue of corrosion is concern. Often it is difficult to assess or quantify the rates of corrosion because of a confluence of transport, corrosion, and chemical mechanism and reactions. Of interest at this time is the rate of corrosion of 316L stainless steel at some likely operational conditions for a surface power reactor. Feasibility Test Loop 1 is meant to understand and quantify, if possible, rates of corrosion of the stainless steel by the NaK under given operational conditions.
Sample Port
NaK Height
ACKNOWLEDGMENTS
Flow
The authors would like to thank Roger Harper, James Martin, Rick Kapernick, Mike Houts, and Stan McDonald for their valuable and important contribution of experience and knowledge to the design of FTL-1.
Fig. 9. FTL-1 flow through reservoir.
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Proceedings of Space Nuclear Conference 2007 Boston, Massachusetts, June 24-28 2007 Paper 2029
NOMENCLATURE A V W m K kg s J psia ppm C g kWt
Amperes Volts Watt meter Kelvin kilogram second Joule pounds per square inch absolute parts per million Celcius grams kilowatt thermal REFERENCES
1.
J. A. ANGELO, Space Nuclear Power, Chapter 9, Orbit Book Company, Malabar (1985).
2.
R. N. LYON, Liquid-Metals Handbook, Chapter 4, Atomic Energy Commission Publisher, Oak Ridge (1952).
3.
J. W. MAUSTELLER, Alkali Metal Handling and Systems Operating Techniques, Chapter 2, Gordon and Breach, New York (1967).
4.
R. W. LOCKHART, Sodium Mass Transfer: I, Test Loop Design, Chapter III, General Electric, San Jose (1962)
5.
A. E. GARBER and T. J. Godfroy, “Design, Fabrication, and Integration of a NaK-Cooled Circuit,” Proc. of ICAPP, 2006 International Congress on Advances in Nuclear Power Plants, Reno (2006).
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