CHAPTER 15 EQUIPMENT FOR MOLTEN-SALT REACTOR HEAT-TRANSFER SYSTEMS* The equipment required in the heat-transfer circuits of a molten-salt reactor consists of the components needed to contain, circulate, cool, heat, and control molten salts at temperatures up to 1300°F. Included in such systems are pumps, heat exchangers, piping, expansion tanks, storage vessels, valves, devices for sensing operating variables, and other auxiliary equipment. Pumps for the fuel and blanket salts differ from standard centrifugal pumps for operation at high temperatures in that provisions must be made to exclude oxidants and lubricants from the salts, to prevent uncontrolled excape of salts and gases, and to minimize heating and irradiation of the drive motors. Heat is transferred from both the fuel and the blanket salts to =~odium in shell-and-tube heat exchangers designed to maximize heat transfer per unit volume and to minimize the contained volume of salt, es- pecially the fuel salt. Seaniless piping is used, where possible, to minimize flaws. Thermal ex- pansion 1s accommodated by prestressing the pipe and by using expansion loops and joints. Heaters and thermal insulation are provided on all com- ponents that contain salt or sodium for preheating and for maintaining the circuits at temperatures above the freezing points of the liquids and to minimize heat losses. Devices are provided for sensing flow rates, pres- sures, temperatures, and liquid levels. The devices inelude venturi tubes, pressure transmitters, thermocouples, electrical probes, and floats. Inert gasex are used over free-liquid surfaces to prevent oxidation and to apply appropriate hase pressures for suppressing cavitation or moving liquid or gas from one vessel to another. The deviations from standard practice required to adapt the various componients to the molten-salt system are discussed below. The schematic diagram of a molten-salt heat-transfer system presented in Fig. 15-1 in- dicates the relative positions of the various components. For nuclear op- eration, an off-gas svstem 1s supplied, as described in Chapter 17. The vapor condensation trap indicated in Fig. 15-1 1s required only on systems that contain ZrF4 or a comparably volatile fluoride as a component of the molten salt, *By H. W, Ravage, W, F. Boudreau, 5. J. Breeding, W. G. Cobb, W. B, Mec- Donald, H. J, Metg, and L. Storto. 661 662 MOLTEN-SALT REACTOR HEAT-TRANSFER EQUIPMENT [cHAP. 15 Pump (Includes Expansion Volume) Liquid Level Vmp indicating Device 1 I 39, High Point Heat @—{Source Exchanger Coolant System Flow Measuring Device L ; P;\:' (Venturi) Pressure Measuring Devices Shut-Off Yal u alve \J? Vent Vent . T Liquid Level Fill and Drain Line Indicating Device Yapor Trap Equ-;Iizer Dump Tank Valve Pressure ~*——Regulators — Inert Gas Supply F1G. 15-1. A molten-salt heat-transfer system. 15-1. PumMprs rOrR MOLTEN SALTS Centrifugal pumps with radial or mixed-flow types of impeller have been used successfully to circulate molten-salt fuels. The units built thus far and those currently being developed have a vertical shaft which carries the impeller at its lower end. The shaft passes through a free surface of liquid to isolate the motor, the seals, and the upper bearings from direct contact with the molten salt. Uncontrolled escape of fission gases or entry of undesirable contaminants to the cover gas above the free-liquid surface in the pump are prevented either by the use of mechanical shaft seals or hermetic enclosure of the pump and, if necessary, the motor. Thermal and radiation shields or barriers are provided to assure acceptable temperature and radiation levels in the motor, seal, and bearing areas. Liquid cooling of internal pump surfaces is provided to remove heat induced by gamma and beta radiation. 15~-1} PUMPS FOR MOLTEN SALTS 663 Top Shaft Seal Assembly ~—Oil Inlet Bearing {Upper) 3 Heated Gas Vent {(Lower) | Ring XN Drain Ak_ - . : z g iy | A o - 7 / Impeller =5 i - g‘; 3 3 - LNy NN e N oy . gif _&\ mf‘&ff-’@? 772! level Indicator Float '\ < SN > 3 i-i % _y ~ i;\‘ N b oA Fuel Inlet o Fuel Discharge Fig. 15-2. Sump-type centrifugal pump developed for the Aircraft Reactor Experiment. The principles used in the design of pumps for normal liquids are applic- able to the hydraulic design of a molten-salt pump. Experiments have shown that the cavitation performance of molten-salt pumps can be pre- dicted from tests made with water at room temperature. In addition to stresses induced by normal thermal effects, stresses due to radiation must be taken into account in all phases of design. The pump shown in Fig. 15-2 was developed for 2000-hr durability at very low irradiation levels and was used in the Aircraft Reactor Experi- ment for circulating molten salts and sodium at flow rates of 50 to 150 gpm, at heads up to 250 ft, and at temperatures up to 1550°F. These pumps have been virtually trouble-free in operation, and many units in addition to those used in the Aircraft Reactor Experiment have been used in devel- opmental tests of various components of molten-salt systems. The bearings, seals, shaft, and impeller form a cartridge-type subassembly that is removable from the pump tank after opening a single, gasketed 064 MOLTEN-SALT REACTOR TEAT-TRANSFER EQUIPMENT [crar. 15 joint above the liquid level. The volute, suction, and discharge connec- tions form parts of the pump tank subassembly into which the removable cartridge is inserted. The upper portion of the shaft and a toroidal area in the lower part of the bearing housing are cooled by circulating oil. Heat losses during operation are reduced by thermal msulation. In all the units built thus far nickel-chrome alloys have been used in the construction of all the high-temperature wetted parts of the pump to minimize corrosion. The relatively low thermal conductivity and high strength of such alloys permitted close spacing of the impeller and bearings and high thermal gradients in the shaft. Thrust loads are earried at the top of the shaft by a matched pair of pre- loaded angular-contact ball bearings mounted face-to-face in order to pro- vide the flexibility required to avoid binding and to accommodate thermal distortions. Either single-row ball bearings or a journal bearing can be used successfully for the lower bearing. The upper lubricant-to-air and the lower lubricant-to-inert-gas seals are similar, rotary, mechanical face-type seals consisting of a stationary graphite member operating in contact with a hardened-steel rotating mem- ber. The seals are oil-lubricated, and the leakage of oil to the process side is approximately 1 to 5 ce/day. This oil is collected in a cateh basin and removed from the pump by gas-pressure sparging or by gravity. The accumulation of some 200,000 hr of relatively trouble-free test op- eration in the temperature range of 1200 to 1500°F with molten salts and liquid metals as the circulated fluids has proved the adequacy of this basic pump design with regard to the major problem of thermally induced dis- tortions. Four different sizes and eight models of pumps have been used to provide flows in the range of 5 to 1500 gpm. Several individual pumps have operated for periods of 6000 to 8000 hr, consecutively, without main- tenance. 15-1.1 Improvements desired for power reactor fuel pump. The basic pump described above has bearings and seals that are oil-lubricated and cooled, and in some of the pumps elastomers have been used as seals be- tween parts. The pump of this type that was used in the ARE was de- signed for a relatively low level of radiation and received an integrated dose of less than 5 X 108 r. Under these conditions both the lubricants and elastomers used proved to be entirely satisfactory. The fuel pump for a power reactor, however, must last for many years. The radiation level anticipated at the surface of the fuel is 105 to 10° r/hr. Beta- and gamma-emitting fission gases will permeate all available gas space above the fuel, and the daughter fission products will be deposited on all exposed surfaces. Under these conditions, the simple pump described above would fail within a few thousand hours. 15-1] PUMPS FOR MOLTEN SALTS 665 Considerable improvement in the resistance of the pump and motor to radiation can be achieved by relatively simple means. Lengthening the shaft between the impeller and the lower motor bearing and mserting addi- tional shielding material will reduce the radiation from the fuel to a low level at the lower motor bearing and the motor. Hollow, metal O-rings or another metal gasket arrangement can be used to replace the elastomer seals. The sliding seal just below the lower motor bearing, which prevents escape of fission-product guses or inleakage of the outside atmosphere, must be lubrieated to ensure continued operation. If oil lubrication is used, radiation may quickly cause coking. Various phenyls, or mixtures of them, are much less subject to formation of gums and cokes under radiation and could be used as lubricant for the seal and for the lower motor bearing. This bearing would be of the friction type, for radial and thrust loads. These modifications would provide a fuel pump with an expected life of the order of a vear. With suitable provisions for remote maintenance and re- pair, these =simple and relatively sure improvements would probably suffice tor power reactor operation, Three additionad improvements, now beig studied, should make pos- =ihle w fuel pump that will operate trouble-free throughout a very long life. The first of these ix a pilot bearing for operation in the fuel salt. Such a bearing, whether of hydrostatic or hydrodynamie design, would be com- pletelv unattected by radiation and would permit use of a long shaft so thut the motor could be well shielded. A combimed radial and thrust bearing just below the motor rotor would be the only other bearing required. The ~econd mprovement is a labyrinth type of gas seal to prevent escape of fission gases up the shaft. There are no rubbing surfaces and hence no need for Tubricants, 2o there ¢an be no radiation damage. The third inno- vition 1= o hemispherical gas-cushioned bearing to act as a combined thrust and radind bearing. Tt would have the advantage of requiring no auxiliary lubrication