CHAPTER 23 ENGINEERING DESIGN 23-1. RracTtor Drusiay* The LMFR readily lends itself to a wide variety of designs and arrange- ments. The concepts proposed to date may be classified according to type as being internally or externally cooled and either compact or open ar- rangement of cycle. Such clagssification has been brought about in an at- tempt to present designs which minimize bismuth and uranium inventories. If we assume the cost of U235 to be $20/g and that of bismuth to be $2.25/1b, a U-Bi solution of 700 ppm uranium by weight would cost ap- proximately $6000/ft3. This high volume cost makes it very important to design the LMFR with the minimum possible holdup. In addition to the variety of cycle arrangements, several different coolants are possible, The U-Bi may be used directly to produce steam, or a secondary fluid such as NaK or sodium may be used. The LMFR has also been proposed as the heat source for a closed-eycele, gas-turbine power plant [2]. 23-1.1 Externally cooled LMFR. In an externally cooled LMFR the fuel is circulated through the core to an external heat exchanger, where the heat is removed by the secondary fluid. This type provides the simplest core design, requiring simply an assembly of graphite pierced with holes for ecirculation of liquid-metal fuel. The major problems of heat transter are essentially removed from the core design. 23-1.2 Internally cooled LMFR. The internally cooled LMFR is de- signed so that the liquid fuel remains in the reactor core. The core thus acts as a heat exchanger in which the heat is transferred to a secondary fluid flowing through it to an external heat exchanger or steam generator. The internally cooled design offers a means of substantially reducing the U—DBi inventory of the system, but considerably complicates the design of the core. The core must be designed to accommodate two fluids and suf- ficient surface for transferring heat from one to the other. The introduction - of a secondary fluid in the core requires a greater uranium concentration than in the externally cooled system, which has only U-Bi and graphite in the core. The required concentration cannot be achieved with U-DB1 so- *Based on material by T. V. Sheehan, Brookhaven National Laboratory, Upton, L. I, New York. 832 23-1] REACTOR DESIGN 833 From Recuperator To Turbine J . \ Volatile Fission Prod. ~——Pump S U-Bi n Core Heat Exchanger Graphite Reflector Dump Fic. 23-1. Externally cooled compact arrangement LMFR for closed-cycle gas turbine. lutions, since these concentrations approach the solubility limits for the temperatures presently being considered (400 to 550°C). 23-1.3 Compact arrangements. The compact arrangement may best be described as an integral or “pot type” design and may be internally or ex- ternally cooled. In such a design [1] the fluid fuel remains in the core ex- cept for small amounts which are withdrawn for reprocessing. The breeding fluid acts as a coolant by circulating through blanket and core and thence through heat exchangers which are also contained within the primary reactor vessel, Figure 23-1 shows a concept of an externally cooled compact reactor 834 ENGINEERING DESIGN [cHAP, 23 arrangement for a closed-cycle, gas-turbine power plant [2]. In this ar- rangement the fuel is circulated through the core and heat exchanger, which are contained inside the same vessel. The compact arrangement offers a means of reducing the U-Bi inventory over a particular reactor designed with an open-cycle arrangement. It does, however, increase the problems associated with design of the core, blanket, and reactor vessel. The compacting of all the equipment into a single vessel reduces the flexi- bility of mechanical design which the open arrangement allows, as well as intensifying the problems of thermal expansion. The reactor vessel not only becomes larger, but the number of openings is also increased, both of which complicate the vessel design. Nevertheless, as operating experience with materials and equipment becomes available, the compact arrange- ment may provide a means of improving the economics of the LMFR system. 23-1.4 Open arrangements. The open arrangement is the type receiving the most consideration at present because of the flexibility and simplicity of design it affords the system components. Figure 23-2 shows one con- cept of an externally cooled LMFR using the open-cycle arrangement [3]. In this design both blanket and core fluids are circulated to heat ex- changers located outside the reactor vessel. This type of arrangement also allows greater freedom of design for maintenance of equipment. Means must be provided for removal and/or maintenance of system components under radioactive conditions. The open arrangement makes it easier to provide such facilities. The major disadvantage of this arrangement is the high U-Bi inventory. The open-cycle arrangement may also be employed in an internally cooled LMFR to reduce fuel inventory, but it introduces those problems peculiar to internally cooled systems. 23-1.5 Containment and safety requirements. The high negative tem- perature coefficient and low amount of excess reactivity available make the LMFR inherently stable and safe. However, any rupture of the primary system, whether by reactor excursion or otherwise, would release fission products and polonium to the surrounding atmosphere. The primary system must therefore be surrounded with a secondary vessel for contain- ment of radioactivity in case of such a failure. Since all materials in the reactor core have very low vapor pressures, the containment vessel need not be designed to withstand any appreciable pressure. The containment problem in the LMFR 1s one of containing the high-temperature liquid metal together with fission products, and such containment can be ac- complished by lining the reactor and primary circuit cells with a gastight steel membrane. This containment vessel also acts as a catch basin for recovery of U-Bi in case of leaks. 23-1] REACTOR DESIGN i ’? Saturated 600 | Steam To Super- ‘ heater | Sodium Inlet And Qutlet Box 835 - ——4"" "3+ Na To Superheater And Reheater Reheat And Superheat Liquid Metal Pumps - Water Inlet ,[ -! { ‘ and ol ) Outlet Box E!! ! S jE Gas Takeoff o — Processed Th-Bi Slurry Return Boiler Bundle 4 Slurry Suction #° Header Blanket (Th-Bi, Graphite) — Liquid Metc|t To Fuel e 4 4_=V/___ T Processing System —= = S Vo 1 Superheat + Reheat Bundle oo e Slurry Drawoff Grc:ph:te Core From Slurry Coolers To Slurry Coolers Slurry "/ Discharge Header Fig. 23-2. Externally cooled open-cycle arrangement LMFR. { I - To Fuel Th-Bi Slurry Pump Dump Liquid Fuel Pumps The arrangement of the containment vessel also depends on the heat- removal design. If an intermediate heat-transfer fluid such as sodium or Nal\ is used, the containment may be handled as above. If a direct U-Bi to steam cycle is used, a double-wall heat exchanger must be used to maintain double containment, unless the entire building is constructed to act as the second containment barrier. In the event of a leak in the system, the U~-Bi would be drained to a dump tank. This tank would be provided with adequate cooling to remove the decay heat from fission products. 23-1.6 Design methods. The vessels in an LMFR are designed in ac- cordance with the Code for Unfired Pressure Vessels [4]. Vessels would be of welded construction with all seams radiographed and stress-relieved. The design temperature used can be as high as 1100°F. For 22% Cr-1% Ao steel, this gives allowable stresses of 4200 psi for normal operating condi- tions and 9200 psi for emergency, short-duration conditions. These figures correspond to 19 creep strength for 100,000 hr and 10,000 hr, respectively. 836 ENGINEERING DESIGN fcHAP. 23 23-1.7 Maintenance and repair provisions. Provisions for maintenance and repair of the LMFR raise several problems. It is anticipated that a substantial level of activity will be induced in the system by the circulating fuel. This means that the system should be designed so that it can be main- tained despite the high radiation level. Several approaches, not mutually exclusive, to this problem are being considered: (1) If maintenance or repair to a component is required, the entire com- ponent will be removed from the system and a new one inserted. (2) All connections between components will be made in one area, fully biologically shielded from the components themselves. When a com- ponent is to be removed, its connections are shiclded from adjacent connee- tions by portable shielding if the work is to be done directly rather than re- motely. The connections are broken and the shielding is removed above the pipes leading to the component in question. The component is removed with the overhead crane and a new one set in place. The shielding is re- placed, and the connections are remade. The connections are accessible and pipes do not overlay each other so as to prevent removal of any disconnected component. Unfortunately, placing all connections in one channel increases the fuel inventory since the piping for this arrangement is somewhat longer than that required for & more conventional arrangement. (3) Both mechanical and welded connections are being studied, with a view toward the ease with which connections can be made and broken both directly and remotely. (4) Remote methods of performing maintenance tasks (welding and cut- ting pipe, making and breaking flanged joints and closures) are being studied, since direct maintenance will not be possible in some areas. (5) Fluidized powders, shot, and liquids are being studied as possible portable shielding media. 23-2. HeaT TRANSFER® In the open-cycle externally cooled, two-fluid LMFR, the bismuth- uranium solution serves as the primary coolant as well as the fuel. In the reactor itself, there is no actual heat transfer. Instead, the solution acts as a transporter of heat to an external heat exchanger. In evaluating bismuth as a primary coolant, it is helpful to make a comparison between it and three other coolants: sodium, a typical alkali metal coolant; LiCI-KCl cutectic, a typical alkali halide salt mixture; and water. (The salt eutectic used here would not be a suitable primary coolant for a thermal reactor. Its heat transfer properties, however, are typical of salt coolants.) *Based on material by O. E. Dwyer, Brookhaven National Laboratory. 23-2] HEAT TRANSFER 837 The ideal primary coolant for a nuclear power reactor should have the following characteristics: (1) High heat-transfer rates. (2) Good gamma, absorptivity. (3) Low pumping power requirements. (4) Low melting point. (5) Low vapor pressure. (6) Low corrosion rate. (7) Low chemical reactivity. (8) Low neutron absorption. (9) Low induced radioactivity. (10) Low cost. In order to have the above characteristics, the coolant should have the following physical properties in either a high or low amount: (1) Density (high): affects pumping power requirements, heat-transfer characteristics, and gamma shielding requirements. (2) Thermal conductivity (high): affects heat-transfer characteristics. (31 Specific heat (high): affects heat-transfer characteristics and coolant flow rate. (4) Viscosity (low): affects pumping power requirements and heat- transfer characteristics. (5) Melting point (low): affects auxiliary heating requirements. () Vapor pressure (low): affects mechanical design of reactor and system components. (71 Volume change on fusion (low): affects startup and shutdown pro- cedures. (81 Coefficient of volumetric expansion (high): affects thermal pumping capacity and, where primary coolant is also the fuel, reactor reactivity. (91 Electrical resistivity (low): affects applicability of electromagnetic pumps. Table 23-1 summarizes the physical properties of bismuth which are relevant to nuclear reactor design and in the temperature range of practical interest from the standpoint of electrical power generation [5,6]. 23-2.1 Nuclear aspects of coolants. From the nuclear standpoint, five important characteristics of primary reactor coolants are their capacities for (1) absorbing thermal neutrons, (2) slowing down neutrons to the thermal energy level, (3) absorbing gamma radiation, (4) developing in- duced radioactivity, and (5) resisting radiation damage. In Table 23-2 the thermal neutron absorption cross section and neutron- slowing-down power of Bi are compared with those of Na and H20. Bis- 338 ENGINEERING DESIGN [cHAP. 23 TasLE 23-1 PaYsicalL ProprerTIES oF BIsMUTH Atomic weight 209 Melting point 271.0°C (520°F) Boiling point 1477°C (2691°F) Volume change on fusion —3.329, Temperature, °C 300 400 500 600 Temperature, °F 572 752 932 1112 Vapor pressure, mm Hg 1079® 3.5X107%*12.3x1074)6.3 x 1074 Density, g/cm? 10.03 9.91 9.79 9.66 Specific heat, cal/(gm)(°C) 0.0343 0.0354 0.0365 0.0376 Viscosity, centipoises 1.66 1.37 1.16 1.00 Thermal conduetivity, Btu/(hr)(ft)(°F) 9.9 9.0 9.0 9.0 Electrical resistivity, ohms 128.9 134 .2 139.8 145.2 U solubility, ppm 480 1850 5100 13000 *Extrapolated results. muth with a macroscopic cross section of 9.0 X 107% em~! at 450°C has the lowest neutron absorption characteristic of any practical coolant, with the exception of D20 and certain gases. Its “reactor poisoning’ effect is at least an order of magnitude below those of sodium and water. The slowing-down power of Bi i1s very low, however, which means that when it is used as the primary coolant in a thermal reactor it contributes very little moderating capacity. The term £No, in Table 23-2 represents the decrease in the natural logarithm of the neutron energy per centimeter of travel through coolant. The gamma absorption coefficient, u, is defined by the equation dl = —uldx (23-1) and has the units of em™! Values of u for 450°C Bi at several gamma energies are shown in Table 23-3, along with those for Na and H:O. Bismuth, because of its high density, 1s an excellent absorber of gamma radiation, which means that it provides considerable internal shielding. The values presented in Table 23-3 are estimates based on the theoretical calculations of Davission and Evans [8]. 23-2] HEAT TRANSFER 839 TaBLE 23-2 SomME NUCLEAR PROPERTIES OF VARIOUS REAcTOR COOLANTS Thermal Macero- Thermal q . . Slowing- neutron scople scattering " £ down Coolant Temp.,| cross Cross Cross | o ot one power °C |section [7],} section [7],; Density| section, 1 o : ess (No, Ua) ?\ aO', :O) Us? _.1! barns em ! g/cm3 | barns em Bi 450 0.032 0.00090 | 9.82 9 0.0095 | 0.0024 Na 450 0.505 0.011 0.841 4.0 0.084 0.0074 H.0 250 — 0.018 0.802 — 1.23 *Average decrease in the natural logarithm of the neutron energy per collision. TABLE 23-3 VALUES oF u, THE GaMMA ABSORPTION COEFFICIENT, FOoR VARIOUS REacTOR Co0OLANTS A8 A FuncTioN oF ENERGY Energy, Mev . Temp., Coolant °C 0.5 1.0 1.5 2.0 3.0 | Bi 450 1.57 0.70 0.52 0.44 0.41 ; Na 450 0.070 0.051 0.042 0.036 0.029 -0 250 0.078 0.057 (.046 0.039 (.032 Regarding the tendency for developing induced radioactivity, Bi has a serioux disadvantage, owing to the formation of Po?!Y a very energetic alpha emitter with a 138.3-day half-life. Its formation and decay can be represented as follows: Bir(19mb) —_y. 3j210 ;6, Po2l0 — % Pp206 5d 138.3d 0?7 is one of the most poisonous materials known, the maximum allow- able concentration in air being 7 X 107 ue/ml or 3.75 X 1078 ppm. Another troublesome feature of Po2!% is its tendency to scatter throughout 840 ENGINEERING DESIGN [cHAP. 23 any accessible volume, due to recoil from its high-energy alpha emission. Thus, spillage of solutions containing Po?!? constitutes a most serious phys- 1ological hazard. Inthe LMFR, however, it is not believed that the presence of Po?!0 in the fuel stream creates a more serious radioactivity problem than already exists as a result of the fission products. Sodium is not free of the radioactivity problem either, but as a primary coolant it is not as bad in this respect as Bi. Water is comparatively free of induced radioactivity after short holdup times. For the same flux conditions, Na will give over 20,000 times as much radioactivity, on a roentgen basis, as HoO. Liquid metals, because of their simple atomic structure, suffer no radia- tion damage. 23-2.2 Pumping-power requirements. An important criterion for as- sessing the relative merits of different coolants is the amount of pumping power required for a fixed rate of heat removal in a given application. The three main pressure drops in the primary coolant circuit are those in the reactor, the external heat exchanger, and the interconnecting piping. A comparison of the four different types of coolants will now be made on the basis of their relative pumping-power requirements, with respect to the interconnecting piping and the heat exchangers. The physical properties of the coolants are listed in Table 23-4. The properties of the first three are evaluated at 450°C, as a typical average primary coolant temperature for such coolants, and those for water at 250°C. TAaBLE 234 PuaysicaL ProrERTIES oF SoME Tyrical Rracror CooLANTS Property Bi Na KCHLiCl H.0 450°C 450°C 450°C 250°C Melting point, °F 520 208 664 32 Boiling point, °F 2691 1621 — 212 Density, 1b/ft3 615 52.5 103 o0.0 Specific heat, Btu/(1b)(°F) 0.036 0.304 0.31 1.16 Heat capacity, Btu/(ft3)(°F) 22.1 15.95 31.9 57.8 Thermal conductivity, Btu/(hr) (ft)(°F) 8.95 39.5 1.47 0.357 Viscosity, ep 1.28 0.245 3.4 0.110 Prandtl number, C'p, u/k 0.0125 0.00454 1.7 0.863 23-2] HEAT TRANSFER 841 The pumping power required to circulate the coolant through the piping system per unit rate of heat transport for a fixed temperature rise in the coolant has been shown [9] to be : 02 bp = % = (a constant) ;#GF. (23-2) The quantity u%2/p2C,28, represented here by the symbol X, is an index of the pumping power required to circulate a coolant through a fixed piping system, for a given heat load. Table 23-5 gives relative values of X for the four typical coolants mentioned above. The units and values of the physical properties used in evaluating X are the same as those given in Table 23—4. TaAaBLE 23-5 RELATIVE VALUES oF X vorR VARriorus COOLANTS Frowing THrROUGH A FIXED PIPING SYSTEM Coolant Temp., °C X x 104 Bi 450 308 Na 450 77 LiCHKCI 450 32 eutectic H.0 250 1.7 The very large spread in pumping-power requirements is striking. Bis- muth has about four times the pumping-power requirements of sodium and both have manifold greater requirements than that of water, which has the least of any known liquid. The tremendous superiority of water as a heat-transport medium is due to its low viscosity and very high volumetric heat capacity. 23-2.3 Heat transfer for LMFR. So far as is known, no heat-transfer data have been obtained for liquid bismuth. However, several investigators [10-14] have published experimental heat-transfer results on the bismuth lead eutectic and on mercury. For these results the Lubarski and Koffman equation [15] expresses the results most closely: "D 0.625(DV,Ca/R0 (23-3) 842 ENGINEERING DESIGN [cHAP. 23 This equation may be used for turbulent flow in round tubes or for turbulent flow outside round tubes. In obtaining the heat-transfer coefficients for comparison with bismuth, the sodium coeflicients were calculated from the Martinelli-Lyon relation- ship. The coeflicients for molten salt and water were calculated from the conventional Dittus-Boelter equation. Using the above relationships and assuming (1) total fixed heat load, (2) fixed diameter of tubes, (3) fixed inlet and outlet temperatures, (4) av- erage bulk temperature of coolants same as in Table 23-4, and (5) combined heat-transfer resistance of tube wall and second fluid equals 0.001, a typical value for 1-in. ID alloy steel tubes with 0.1 in. wall the values in Tables 23-6 and 23-7 were calculated. Although the heat-transfer characteristics of bismuth are slightly inferior to those for sodium, it is clear from these two sets of calculations that all four coolants hehave similarly. The heat-transport capability of bismuth are simply related to its volu- metric heat capacity. The values of this property are given in Table 23-4. Bismuth is definitely superior to sodium but inferior to the fused salt and water. To achieve good thermal contact between bismuth and a solid metal surface, the surface must be cleaned to a high polish, the bismuth must be free of oxide and dissolved gases, and the system must be filled under a high vacuum. Guses or oxides on the heat-transfer surface can greatly reduce the heat-transfer coefficient for bismuth. Bismuth hags a less stable oxide than the oxides of iron, chromium, and nickel which may be present on the tube surfaces. Hence the bismuth would have a tendency to non- wet the walls. Good wetting of alloyed steels by bismuth may be achieved by adding small amounts of alkali or alkaline earth metals, by heating to high tem- TaBLE 236 CoMPARISON OF CooLANTS IN HEAT-EXCcHANGER DESIGN WHEN NuMBER oF TUBES IN PARALLEL 18 FIXED Coolant \o?lgf)? noo U, Relative size of ft/sec’ Btu/(hr)(ft)2(°F) | Btu/(hr)(ft)2(°F) | heat exchangers Bi1 15 2700 730 1.00 Na 20.8 10230 910 : LiCI-KCI 10.4 2400 706 1.12 eutectic H,0 5.73 2360 703 1.12 23-3] COMPONENT DESIGN 843 peratures (above 1200°F), or by both. FFor good heat transfer with bismuth extreme care must be taken to ensure oxide- and gas-free systems. 23-2.4 Heat-exchanger design. In a commerecial liquid-metal fuel sys- tem, the primary bismuth coolant would probably exchange heat with a secondary metal coolant before generating steam. Typical conditions for a 5-Mw countercurrent bismuth-sodium heat exchanger are given in Table 23-8. 23-3. CompoNENT DEsign*® This section discusses the design and development experience obtained on components required in LMFR systems. Besides the requirements for these systems, considerable component development is needed in the re- search und development program for experimental apparatus. Both kinds of components are treated here in detail and by case histories, 23-3.1 Pumps. In the case of liquid-metal pumps, which can be classified as mechonieal or electromagnetie, a good deal of preliminary development work hax been done by the Iairchild Iingine and Airplane Corporation Nuclear Fuoerev for Propulsion of Aireraft Division (NEPA), the Allis- Cliliners Co., the Babeock & Wilcox Co., and the Government Labora- tories, WAPL, ORNL and ANL [19]. TasLe 23-7 CovMrarisoN oF CooranTts 1N Hear-Excraxcer DEsIGN AT FIXEDp LiNEAR VELOCITY OF 13 FT, SEC o Conlait of tube% Temp., h, U, size of e | °C Btu/ (W) (i0)2C°F) | Btw/ () (f)2(°F) | heat parallel exchangers I3 n 450 2770 730 1.00 N ©1.38n 450 8810 897 0.88 LiC-IKCT 0.6Y9n 450 3200 762 1.03 ertertic H.0 (¢ 42n 250 5150 837 (.94 *Busceil on a contribution by C. Raseman, H. Susskind, and C. Waide, Brook- hiaven Nutional Laboratory. 844 ENGINEERING DESIGN [cHAP. 23 TABLE 23-8 TypicaL ConNpITIONS IN A COUNTERCURRENT, B1-NaA HeaT EXCHANGER Tube material Low Cr-Steel Thermal conductivity of tube, Btu/(hr}(ft) (°F) 15.8 Tube inside diameter, in. (.70 Tubhe thickness, in. 0.100 Tube spacing (triangular), in, 0.250 Bi temperature (bulk), °If 850 Bi velocity (outside tubes), ft/sce 15.0 Bi heat transfer coeflicient, Btu/(hr) ({t)2(°I") 3,390 Na temperature (bulk), °F 750 Na veloeity (inside tubes), ft/sec 25.5 Na heat transfer coefficient, Btu/(hr)(ft)2(°F) 12,300 Over-all heat transfer coefficient, Btu/(hr)({t)2(°T) 1,015 Fraction of resistance offered by tube wall 0.60 Heat flux (outside tube surface), Btu/(hr}(ft)?2 101,500 Power density, Btu/(hr)(ft)3 510,000 By, ft3/mw heat 0.56 Na inventory, {t3/mw heat 0.45 Lleetromagnetic pumps. In the early days of the LMI'R project, a mag- netic pump for Bi was deseribed by B. 'eld and L. Szilard [20,21]. The Fuel Processing Group of Brookhaven National Laboratory required pilot- plant pumps that would circulate uranium-hismuth fuel with absolutely no leakage. The U-DBi fuel was eventually to be circulated through an experimental hole in the Brookhaven reactor where fission products and polonium would be generated. Since a small flow rate of approximately 1 gpm was desired and efliciency was of little concern, it was decided to use an electromagnetic pump. An experimental loop [22] was set up to circulate nonradioactive U-Bi by means of a General Eleetric Model G-3 ac (IParaday) electromagnetic pump. This loop ran continuously for 2400 hr. During the first 160 hr the rig was operated isothermally at a temperature of 645°F; during the remainder of the time, the loop was run isothermally at 840°F. The U-Bi solution was circulated for most of this period at a rate of 1 gpm. There was no sign of plugging or flow restriction. The General Electric G=3 ac pump was calibrated (Figs. 23-3 and 23-4) in another AISI type-347 stainless steel liquid bismuth loop at 930°I° [22]. It was operated continuously for over 13,000 hr. 23-3] COMPONENT DESIGN 845 0.08 — — Efficiency, % 0.06 }— — 0.04 — —] | l | 1 | | i | 0 0.2 04 06 08 1.0 1.2 1.4 16 18 Flow, GPM Fig.23-3. AC electromagnetic pump efficiency. Molten bismuth in AISI type-347 stainless steel cell. (Manufactured by General Electric Co.) 119} I | I I [ l 200 Vaolts 8 — — I — vy = 150 Volts - o @ I 4 | — 2 100 Volts - 50 Vaolts { | . | | L l 1 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Flow, GPM Frg, 23-4. AC electromagnetic pump characteristics. Molten bismuth in AISI tvne-347 stainless steel cell. (Manufactured by General Electric Co.) The =ame pump was used to circulate bismuth at 930°17 in a 239, Cr-19, Mo steel loop. - The efheiency and characteristic curves were somewhat lower than those obtalned in a stainless steel loop. This is probably due 1o =hort-circuiting of magnetie flux in the ferritic steel walls. A theoretical study [23] was prepared by the Atomic Energy Research Israblishment at Harwell, Iingland, for linear-induction pumping of bismunth, The report indicates the feasibility of using this type of pump. Linear-imduction pumps have been built and successfully used at Ames 846 ENGINEERING DESIGN [cHAP. 23 Laboratory to circulate Mg-Th eutectic (37 w/0o Th) and Bi-U alloy (dbw/oU) in an Inconel-enclosed tantalum loop [24,25]. The pump operated successtully in the Mg-Th system for 2000 hr at 1470°F with a temperature differential of 250°I°, and in Bi-U for 5250 hours at 1740°T with a temperature differential of 210°F. For ecalibration, about 1 gpm of Bi-U was pumped at 750°F against a head of 0.5 in., with an efliciency of 0.169. Mechanical pumps. Most pump development work has been aimed at pumping sodium or sodium-potassium alloys. The most serious problem relative to the design of a mechanical Liquid-metal pump appears to be that of suitable bearings and seals. Bismuth was pumped by NIEPA in 1950 [26]. The system was operated for 37 hr, the maximum flow rate measured was 2 gpm, the maximum head developed was 66 psi, and the maximum bismuth temperature reached was 1765°F. The pump was a modified Browne and Sharpe No. 206, machine- tool-coolant pump. In another experiment [27] NIEPA circulated bismuth with a 50-gpm centrifugal pump for 100 hr at a mean temperature of 1500°L" with a temperature differential of 500°F. An accumulation 1n the sump of a residue high in oxide content and dissolved elements reduced the flow and forced suspension of operation. This residue probably resulted from an impure inert atmosphere above the liquid metal. The container material selected was AISI type-:3147 stainless steel which had shown some promise in bismuth solubility tests at temperatures up to 1800°I. The California Research and Development Corporation made a survey of the various types of pumps that might be used for liquid bismuth and came to the conclusion that a centrifugal pump would best fit the need. A test unit was built that operated for 1037 hr, and a report [28] stated that the centrifugal pump proved to be a very satistactory means for cir- culating bismuth in an isothermal system at 700 to 750°L". This pump and its driver are on a common shaft, the shaft being top-suspended with all bearings in the motor chamber. Space was provided for a labyrinth to separate the pump chamber from the motor chamber, although no seal was used during operation. This pump has also been used to circulate mercury in a test loop at BNL. It has been run successfully for an accumulated time of over 4000 hr. Brookhaven has developed a totally canned overhung-impeller centrit- ugal pump. Iigure 23-5 shows the major design features of this pump. These units pump 5 to 25 gpm Bi against heads up to 30 ft while operating at 525°C. These sump-type pumps run with no bearings in the liquid metal and have proved reliable so long as sufficient internal baffling is included to stop surface splashing. There are several centrifugal pumps that have been used to circulate 23-3] COMPONENT DESIGN 847 4 i _~Induction Motor Gas & Vac. Liquid Level Cooling Probes __ Water 2 [ g LJ ™~ Flange Cooling gy Tubes Shaft Cooling Chamber ] Pump Baffles & P P j §7 Splash Can Overhung ol Impeller Shaft Liquid Level Side Arm Discharge Inlet Fig. 23-5. Canned-motor centrifugal pump developed at Brookhaven. lead-bismuth eutectic [29,30]. They are all vertically mounted sump pumps with overhung shafts and impellers. All would require a can around the motor and shaft for a hermetie seal. The University of California has used a double-volute pump which is rated at 30 gpm and a 40-ft head at 1000°F.* The lower bearing is 2 ft wbove the liquid metal. The pump utilizes a packing gland (Johns-Manville =uper ~eal No. 6) adjusted to allow helium at 2 ps:g to leak out of the ~v=tent at a rate of about 10 ft3/hr. The pump used at North American Aviation, Inc. [30] is made of cast ~teel. The lower bearing is cooled with a water jacket and a graphite seal “The vendor is Berkeley Pump Co., Berkeley, Cal. The 71-in.-diameter impeller and the pump casing are made of AISI type-410 steel. The pump is V-belt driven by a 30-hp motor. 818 ENGINEERING DESIGN [caAP. 23 minimizes gas leakage from the casing. A flow of 0.82 gpm at 400-rpm shaft speed and a temperature of 700°F was maintained until oxide dross forced shutdown of the pump after 496 hr. A completely canned, modified Series T-34 MD Duval stainless steel pump was used at the University of California to circulate mercury [31]. The packing gland was replaced with a bushing and any metal leakage was drained to a reservoir. The pump was driven by a 5-hp, 3-phase in- duction motor at a shaft speed of 1200 rpm. North American Aviation, Inc. has circulated tin with a graphite pump at 2 gpm against a head of 22.5 psi at 1830°F" [32]. The pump has a 4-in.- diameter impeller and is driven by a variable speed (20 to 2000 rpm) pc compound-wound motor mounted outside the gastight enclosure to avoid the high temperatures. A rotating Graphitar bushing on hardened steel provides the gas seal. The spindle bearings are in a cooled housing. The pump was operated for 500 hr in one run; this was followed by additional runs. To overcome differential thermal expansion, a molybdenum adapter joins the graphite shaft to the stainless steel spindle. A miniature canned centrifugal pump to circulate bismuth, ideally suited for in-pile work, has been developed by the Atomic Energy Research Establishment at Harwell England. The over-all pump dimensions are 33-in.-diameter by 24% in. long, with a 2-in.-diameter impeller. The bis- muth flow is 1.5 gpm with a head of 9 ft. The motor rating is 0.75 hp and 2800 rpm. Two gas-lubricated bearings are utilized. The material of con- struction is 23 Cr-1 Mo steel. The Allis-Chalmers Manufacturing Co. has built a canned rotor centrif- ugal pump with fluid piston-type bearings to pump bismuth at 1050°F. The pump is rated at 10 gpm and a head of 25 ft, with an efficiency of 10%,. Those parts of the pump in contact with the bismuth are made from AISI type—410 steel. The pump was used in loop GG at BNL to pump bismuth at 1020°F with a temperature differential of 300°F. After 15.5 hr the pump failed, due to scoring of the bearings and seizure of the can by the rotor. 23-3.2 Valves. The standard-stem packed gate valves used in early NEPA bismuth tests [26] proved that special valves would be required for successful liquid-bismuth operation. High leakage rates through the packing caused maintenance difficulties throughout the tests. A 14-in. Fulton-Sylphon bellows-type stainless steel valve was cycled 1000 times at the rate of 77 times/min against bismuth at a temperature of 1000°F and a pressure of 25 psig. No failure of the bellows or other valve parts occurred. NEPA also checked valves for metal-to-metal self-welding effects [33]. Tests of valve operation reached 1500°F with liquid bismuth on Standard Stellite-faced poppets and seats without indi- cation of self-welding effects. 23-3] COMPONENT DESIGN 849 The two types of valves which have seen extensive service up to 1050°F in liquid-metal fuel systems are standard Y pattern globe valves and needle valves. Due to the stringent requirements of zero gas leakage {(into or out of the metal systems), the only acceptable stem seal has been a steel bellows. Packings are unacceptable. Brookhaven National Laboratory has used both types of valves exten- sively [22,34]. The 1/2-in. IPS 150-1b Y pattern globe valves constructed from AISI type-347 stainless steel for all parts in contact with bismuth (including bellows, stem, and disk) have heen used continuously for over 8000 hr at 930°" without mishap. Similar valves with mild carbon steel disks (instead of type—347 stainless steel) have been used at 930°F for over 13,000 hr without failure or extensive corrosion. A high-velocity loop operating with bismuth at 1020°I" at BNL uses I-in. IPS 150-1b Y pattern globe valves made from 239, Cr-19, Mo steel, AIST type—130 =teel bellows and disk, and AISI type-416 steel stem. Needle valves (1 8in. IPS AISI type-347 stainless steel construction, mcluding the bellows) have been in use for intermittent service (i.e., drain valves A~ un additional safety measure, 1/2-in. IPS globe valves used in an m-pile Toop at Brookhaven National Laboratory have utilized two sets of bellows [54]. The space between the two bellows was pressurized with inert gus which wus continually monitored to detect pressure changes (thus mdieating a valve leak). None was detected. The valve drives have been modified to facilitate remote operation. The wlohe valve handwheels are replaced by gears and these are, in turn, connected to extension rods projecting through the enclosures, Extension rod~ wre welded directly to the needle valve bellows, Universal joints and richt-umgle gear drives are used for changes in direction between valve and operator. When relatively gastight enclosures are desired, as in in-pile loup~. the extension rods project through rubber-gasketed compression