CHAPTER 17 CONCEPTUAL DESIGN OF A POWER REACTOR* The design of a homogeneous molten-salt reactor of the type discussed in the preceding chapters is described below. The choice of the power level for this design is arbitrary, since the 8-ft-diameter reactor core, chosen from nuclear considerations, is capable of operating at power levels up to 1900 Mw (thermal) without excessive power densities in the core. An electrical generator of 275-Mw capacity was chosen, since this is in the size range that a number of power companies have used in recent years. It is estimated that about 69 of the power would be used in the station, and thus the net power to the system would be about 260 M. Two sodium circuits in series were chosen as the heat-transfer system between the fuel salt and the steam. Delayed neutrons from the circulating fuel will activate the primary heat exchangers and the sodium passing through them. A secondary heat-exchanger system in which the heat will transfer from the radioactive sodium to nonradioactive sodium will serve to prevent radioactivity at the steam generators, superheaters, and re- heaters. The fuel flow from the core 1s distributed among four primary heat exchangers which serve as the first elements of the four parallel paths for heat transfer to the steam. A single primary heat exchanger and path 18 provided for the blanket circuit. Plan and elevation views of the reactor plant are shown in Figs. 17-1 and 17-2, and an isometric drawing showing the piping of the heat-transfer sy=tems 1s shown in Fig. 17-3. The reactor and the primary heat exchangers are contained in a large rectangular reactor cell, sealed to contain any leakage of fission-product gases. All operations in the cell must be earried out remotely after the reactor has operated at power. The principal char- acteristics of the plant are listed in Table 17-1. 17-1. FUEL AND BLANKET SYSTEMS 17-1.1 Reactor vessel. The reactor vessel and the fuel and blanket pumps are 2 closely coupled assembly (I'ig. 17-1) which is suspended from a flange on the fuel pump barrel. The vessel itself has two regions— one for the fuel and one for the blanket salt. The fuel region consists of the reactor core surmounted by an expansion chamber, which contains the single fuel pump. The blanket region completely surrounds the fuel region, and the blanket salt cools the walls of the expansion chamber gas space and shields the pump motor. The floor of the expansion chamber is *By 1. G. Alexander, B. W, Kinyon, M. E. Lackey, H. G, MacPherson, L. A, Mann, J. T. Roberts, F. C. VonderLage, GG. D. Whitman, and J. Zasler. 681 682 CONCEPTUAL DESIGN OF A POWER REACTOR [crAP. 17 Primary Sodium Pump (1 of 5) Reactor . Intermediate Heat Primary Sodium To Secondary Exchanger Cell Sodium Heat Exchanger (1 of 5) Blanket Pump Primary Secondary Air Lock Shield Sech?nlcilflfy Sodium Circuits Fuel Pump Shie Boiler Hot Maintenance Area / (1 of 5) \ T A N e - " N : ¢! Maintenance | ; \NCX'.N’"'F ’ 1.' ¥ e .‘:A‘r::a et 'v. .1._- Turbo- {, Heater | X ‘\ ' J I Ir ] Generator Removal ) -, g J M _:;' Q= / / | A '//‘i—' TITE B & L ,g ,r" b i y Je 1|1|I o= |t | Chemical " - ) ‘ o - Reheater Processing L ud "W Ry {1 of 4) ) Contral # ""II it . 2 Fuel Drain- Tank [ o = Blanket Superheater Fuel Chemical Enricher Blanket Heat (1 of 5) Processing ~ Transter Circuit ! FAUEI Expansion Secondary Sodium Blanket Drain Enricher bump {1 of 5) Blanket BlanketTo-Sodium Chemical Heat Exchanger _ Processing Fuel-To-Sodium Heat Exchanger (1 of 4) Fic. 17-1. Plan view of molten salt power reactor plant, a flat disk, 3/8 In. thick, which serves as a diaphragm to absorb differential thermal expansion between the core and the outer shells. 17-1.2 Fuel pump. The fuel pump is of the type illustrated in Chap- ter 15 (Fig. 15-3) and is designed to have a capacity of 24,000 gpm. It is driven by a 1000-hp motor with a shaft speed of 700 rpm. This pump incorporates three major advanced features that are being developed, but which are not present in any molten-salt pump operated to date. These are a hydrostatic lower bearing to be operated in the molten salt, a laby- rinth type of gas seal to prevent escape of fission-product gases up the shaft, and a hemispherical gas-cushioned upper bearing to act as a com- bined thrust and radial bearing. These advanced features are intended to provide a pump with greater resistance to radiation damage and less complex auxiliary equipment than necessary for pumps presently used for molten salts. 17-1.3 System for removal of fission-product gases. About 3.59; of the fuel passing through the fuel pump is diverted from the main stream, 17-1] FUEL AND BLANKET SYSTEMS 683 Primary Sodium To Secondary Sodium Heat Exchanger (1 of 5) Fuel To Sodium Heat Exchanger {1 of 4) 5 h . Jet Pump uperheater p Fuel Pump S:g}‘:;‘y (1 of 5} {1 of 5] Manipulator Pump Removable Boiler (1 of 5) Air Lock (1 of 5) Concrete Slabs Hot Maintenance Ared Turbine-Generator O Maintenance Area Steam Heater Header Removal Fuel Drat Tank * i Reheater Blanket To Sodium (1 of 5) Heat Exchanger Blanket Pump Secondary Sodium Pump Boiler Feed (1 of 5) Water Pump Primary Sodium Secondary Sodium {1 of 5] Drain Tank Drain Tank (1 of 5) (1 of 5) F1a. 17-2. Elevation view of molten salt power reactor plant. Fia. 17-3. Isometric view of molten salt power reactor plant. Blanket | Pump Fuel Pump Motor Section A-A Motor Blanket Expansion M Tank Siphon Drain 7 Fuel Lire To 1 - Heat Exchanger g L-—-J Fuel Expansion Tank Breeding Blanket 01 2 3 4 5 . dw o dow dboa ] Scale—Feet Fuel Return 3 Rorket —~- Return Fi1a. 17-4. Reactor vessel and pump assembly. 17-1] FUEL AND BLANKET SYSTEMS 685 TaBLE 17-1 REACTOR PranT CHARACTERISTICS Fuel Fuel carrier Neutron energy Moderator Primary coolant Power Electrie (net) Heat Regeneration ratio Clean Average (20 yr) Blanket salt Yefueling eycele at full power Shielding Control Plant efficiency Exit fuel temperature Steam Temperature Pressure second loop fluid Third loop fluid Structural materials Fuel circuit Secondary loop Tertiary loop Steam boiler Steam superheater Active-core dimensions I'uel equivalent diameter Blanket thickness Temperature coefficient, (Ak/k)/°F Specific power Power density Fuel inventory Initial (clean) Average (20 yr) Clean eritical mass Burnup >909, U2y 62 mole 97 LilF, 37 mole 9, BeFs, 1 mole 9, ThF, Intermediate LiF Bel'; Circulating fuel solution, 23,800 gpm 260 Mw 640 My 0.63 0.50 71 mole 9 LiF, 16 mole 9, BeF o, 13 mole 9 Thl'y Semicontinuous Concrete room walls, 9 ft thick Temperature and fuel concentration 44 .39, 1210°1" at approximately 83 psia 1000°F, with 1000°F rcheat 1800 psia Sodium Sodium INOR-8 Type-316 stainless steel 50, Cr, 197, i steel 2.59, Cr, 19, Mo steel 507 Cr, 197 Si steel 8 ft 2 ft —(3.840.04) x 1073 1000 kw /‘kg 80 kw/liter 604 ke of U235 1000 kg of U235 267 kg of U233 Unlimited 686 CONCEPTUAL DESIGN OF A POWER REACTOR [cuAP. 17 A Y Fuel 4 ~—Blanket Pump - Pump A 1.95 SCFM He . 0.65 SCFM Blanket Expansion 0.1 CFS Fuel Expansion anl;: Blanket Bypass 1210°F Tank 1.8 CFS 0.ICFS 1250°F Fuel Bypass Y Yy - - 4 235 f13 785 Ft3 &1 Ft3 16 Ft3 Cooclant Coolant Coolant Coolant Fic. 17-5. Schematic flow diagram for continuous removal of fission-product gases. mixed with helium from the pump-shaft labyrinth seal, and sprayed into the reactor expansion tank. The mixing and spraying provides a large fuel-to-purge-gas interface, which promotes the establishment of low equilibrium fission gas concentrations in the fuel. The expansion tank provides a liquid surface area of approximately 26 ft* for removal of the entrained purge and fission gas mixture. The gas removal is effected by the balance between the difference in the density of the fuel and the gas bubbles and the drag of the opposing fuel velocity. The downward surface velocity in the expansion tank is less than 1 in/sec, which should allow all bubbles larger than 0.008 in. in radius to come to the surface and escape. In the Aircraft Reactor Experiment at least 979% of the fission-product gases were continuously purged by similar techniques. With a fuel purge gas rate of 5 cfm, approximately 350 kw of beta heating from the decay of the fission-product gases and their daughters is deposited in the fuel and on metal surfaces of the fuel expansion tank. This heat is partly removed by the bypass fuel circuits and the balance is transferred through the expansion tank walls to the blanket salt. The mixture of fission-product gases, decay products, and purge helium leaves the expansion tank through the off-gas line, which is located in the top of the tank, and joins with a similar stream from the blanket expansion tank (see Fig. 17-5). The combined flow is delayed approximately 50 min in a cooled volume to allow a large fraction of the shorter-lived fission products to decay before entering the cooled activated-carbon beds. The 17-2] HEAT-TRANSFER CIRCUITS AND TURBINE GENERATOR 687 capacity of the carbon beds will hold krypton from passing through for approximately 6 days, and xenon for much longer times. The purge gases, essentially free from activity, leave the carbon beds to join the gases from the gas-lubricated bearings of the pumps. The gases are then compressed and returned to the reactor to repeat the eyele. Ap- proximutely every four days the gas stream is diverted from one set of carbon beds to the other. The inactive bed is then regenerated by warming it to expel the Kr™ and other long-lived fission products. It will probably be economical to recover some of these gases; others may be expelled to the stack. 17-2. HEaT-TransrFir CircuiTs AND TurBINE GENERATOR The primary heat exchangers are designed to have the fuel on the shell side and sodium inside the tubes. This arrangement makes full use of the superior properties of sodium as a heat-transfer fluid and appears to yield the lowest fuel volume. The heat exchangers, which are of semicircular construction, as shown in I'ig. 17-3, provide convenient piping to the top and bottom of the reactor. The thermal characteristics of the primary heat exchanger, to- gether with the characteristics of other heat exchangers of the reactor svstem, are listed m Table 17-2. The sodium in the intermediate heat-transfer system (see Fig. 17-6) is heated by the fuel in the primary heat exchanger and is pumped out of the reactor cell and through the reactor cell shield to adjacent cells, which con- tuin the secondary sodium-to-sodium heat exchangers and the pump. No control of intermediate sodium flow is required, so there are no valves and a constant speed centrifugal pump is used. To permit the sodium to be at u lower pressure than the fuel in the primary heat exchanger, the pump for the intermedinte sodium is in the higher temperature side of the circuit. The secondary heat exchangers are of the U-tube in U-shell, counterflow design, with the intermediate sodium in the tubes and the final sodium on the