Conceptual Mechanical Design for a Fast Energy Spectrum Nuclear Rocket Engine

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Mm^tVTY: DAOC QAOC MOD 3. OOWTAINS MO DOE CLASStFCO INFO. CONCEPTUAL MECHANICAL DF.STGN FOR A FAST ENERGY SPECTRUM NUCLEAR RClcKiaaS^ffGM®! CANCE; 710 REACTOR MECHANICAL SYSTJIMS For The At NUCLEAR SYSTEMS ENGINEERINC American Institute of Aeronautics an Propulsion Joint Specialist Conference Colorado Springs, Colo. ADVANCED TECHNOLOGY SERVICES

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Westlnghouse Electric Corp. Astronuclear Laboratory, Pittsburgh H. Hodgin, Martin Marietta Corp, (2) Edwards Air Force Base, California Rocket Propulsion Laboratory This document is a working paper expressing the views of the author or authors. It does not necessEirily reflect the position of the General Electric Company, and no representation is made concerning the completeness, accuracy, or usability of the information

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United States Atomic Energy Commission Contract No. AT |4Q-1)-2847 Tliis report was prepared as on account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of theG>mmission: A. Mokes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any infoiMOtion, apparatus, moteriol, method, or process disclosed in this report may net infringe privately owned rights; or B. As'iumes ony liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, material, method, or process disclosed in this report. As used in the above, "person acting en behalf of the G>mmission" includes any employee or contractor of the G>mmission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission or his employment with such

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

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Conceptual Mechanical Design for a FastEn e ^ y Spectrum Nuclear Rocket Manager, Mechanical Systems Engineering, and J. W. ZWICK, Senior Physicist, Nuclear Systems Engineering, Nuclear Materials and Propulsion Operation, General Electric Company, Cincinnati 15, Ohio. Results of a mechanical design study of nuclear rocket engines based on the 710 Fast - Spectrum, Refractory-Metal Reactor over a wide thrust range a represented. Both bleed and topping cycle approaches a r e included. Long operating life, multiple restarts, and part - load capability are design requirements along with high specific performance. The reactor is based on a tungsten-uranium dioxide fuel element presently under development. Nuclear analyses and optimization studies en comp as - sed the effects of reflector variations, radial power flattening, fuel and structural variations, and reactivity changes on the size and weight of the reactor assemblies. Investigations included component arrangements ; structural materials ; and conceptual design of the pressure shell, radial reflector, control drum actuators, shield, fuel element support, thrust structure, nozzle and flow system components. Numer - ous structural materials were investigated in selecting the primary components of the reactor - shield assembly. Screening was based on the thermal strain and radia - tion damage resistance of the materials. Overall study indicated that a mechanical configuration could be evolved within the present materials state of the technology. This work sponsored by U. S. Air Force Systems Command under Contract AF This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. DISTRIBUTION OF THIS DOCUMENT IS UNLIMITE

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As part of a parametric study, reported elsewhere in this meeting, conceptual mechanical designs were generated for fast energy spectrum nuclear rocket engines with thrust levels of 30, 000, 50, 000, 100, 000 and 150, 000 pounds, Basic ground rules for the mechanical studies were: The reactor design was to be based on the 710 fast-neutron-spectrunn, ref rac - tory - metal reactor concept. Consideration was to be given to potential application of the engines to a manned, space-maneuvering, recoverable and reusable weapons system. Engine capabilities of interest included lifetimes of 10 to 100 hours of operation, throttling and r e s t a r t for 100 to 2^00 cycles and maximum thrust-to-weight attainThe operating conditions of the engine, i.e.,chamber - pressure and temperature, reactor pressure drop and maximum fuel element surface temperatures were determined as a result of initial parametric study and are shown in Table 1. Studies were to be directed primarily towards the bleed cycle engine with limited design effort towards the topping cycle system. A general schematic of the bleed-cycle engine is shown in Figure 1, and illustrates the use of H2 propellant as coolant for the nozzle and primary engine structure. Figure 2 shows the overall arrangement of the 30, 000 pound-thrust bleed-cycle engine which is representative for all thrust sizes investigated. The major components of the engine are the reactor assembly, including the core, reflector and shield, the propellant feed system which is integral with the thrust

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ENGINE OPERATING CONDITIONS AND REACTOR ASSEMBLY DIMENSIONS Volume fraction, structure (TaVolume fraction, matrix Volume fraction, void Aff, free flow area, in.^ Number in core Thickness of outer cladding, in Number of tubes per element Hydraulic diameter, in. Tube cladding thickness, Pump discharge, psia Reactor inlet, psia Average in fuel matrix, Btu/sec-

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Auxiliary nozzle (2) Tank pressurization \ and control I ine Tank shutoff valve Nozzle inlet manifold Turbine inlet line Nozzle feed line (3) Cooldown line and valve Reflector module (10) Uncooled skirt from

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structure and the nozzle.''' The turbopump is attached directly to the engine thrust cone by a structural cone. The pump inlet is mounted on the engine centerline and is of bellows c o n s t r u c - tion. Pump discharge flow enters a torus for distribution to the nozzle through three propellant feed lines. Bleed gas for turbine power is extracted from the chamber into a manifold on the nozzle, mixed with cold propellant, and ducted to the turbine. The turbine exit gas is exhausted through auxiliary nozzles. The nozzle consists of a 45-degree half-angle convergent cone and an 80 percent parabolic bell expansion section. It is regeneratively cooled from the 40:1 expansion ratio forward. An uncooled skirt extends from the 40:1 expansion ratio to an expansion ratio of 100:1. Transmission of the engine thrust to the vehicle is accomplished through an engine thrust cone. A gimbal assembly mounted on the forward end of the shield allows thrust alignment and vector control. Figure 3 shows a longitudinal and transverse cross section of the 30, 000 pound-thrust engine reactor assembly. The structural advantages available when using the fast energy spectrum reactor is illustrated by the design of primary reactor pressure vessel which is an extension of the nozzle tubes and backup shell. Located between the core and reflector it forms a continuous path for propellant flow with no internal seals required and the high temperature,structural materials used which * The Rocketdyne Division of the North American Aviation Company conducted the nozzle and turbopump investigations under Subcontract No. A F - 301,Prime General Electric Contract AF 04(611)-9068.

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Shown rotated 90 degrees Module coolant tubes Fig. 3 - 30,000-pound-thrust engine with a single pressure shell and module outside

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have relatively high thermal neutron absorption cross sections would be prohibitive with thermal and intermediate reactors. Hydrogen from the pressure vessel coolant tubes enters the control drum module ahead of the shield, flows axially aft, r e v e r s e s direction and p a s s e s through the control drum shaft, and exits through a flexible tube to the inner forward shield plenum. From this point the propellant passes through the shield, enters the reactor core and discharges through the nozzle. The reactor pressure shell, of Inconel X, is designed for internal pressure loads and thermal stresses. Table 2summarizes the pressure shell analysis and illustrates the arrangements considered. Arrangement 1 indicates a single shell with coolant tubes brazed to the inner surface and is adequate for the 30, 000 and 50, 000 poundthrust engines. Arrangement 2 shows a split shell with axial coolant flow between the skins needed for the larger diameter and greater thermal loads of the 100, 000 pound-thrust engine. Dividing the material equally reduces the therinal stresses by a factor of 4. The third arrangement has both a dual skin plus the coolant tubes brazed to the inner skin and is required for the 150, 000 pound-thrust engine. When dual skins are employed, additional structure or ties must be provided to assure that both skins act together to contain the internal pressures. The reflector module and the shield segment from which it is supported is an integral package containing structure, reflector material,a rotating control drum, an actuator drive, s c r a m spring, control and coolant lines, and filler blocks. The reflector structure of Inconel X is a cylindrical pressure shell which encloses the reflector and is supported from the forward shield. Small coolant tubes, b razed to the inside surface, insure nearly uniform skin temperatures c irc um fer en - tially around the structure.

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Engine Thrust Size, lb chamber pressure, psi Nuclear radiation heat Mechanical stress, ^ psi Thermal stress, ^ psi '^Allowable mechanical stress 80, 000 psi (80% of yield stress). Allowable mechanical plus thermal stress 160, 000 psi. ^Arrows indicate heat flo-\^ to coofarit channel.

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The drum contains beryllium neutron-reflecting material and a sector of neutron absorber material. Reactor control is maintained by rotating the drum, thus the absorber material relative to the active core. Axial holes through the beryllium permit coolant flow. The neutron absorber is located in larger holes along approximately 105 degrees of the periphery. Bearings for the control drums are located at the ends of the drums. Axial thrust is transmitted through the bearing to the structure. If found nuclearly desirable, the void between the reflector modules and the pressure shell can be reduced by providing filler blocks. These consist of a thinwall Inconel X tube containing neutron-reflecting material. Axial channels in the blocks p e r m i t coolant flow. This coolant is supplied from the module plenums. The pneumatic actuator used is an adaptation of one developed for the NERVA program. It is a piston actuator whose linear motion is converted to a ISO-degree rotary motion through a rack and pinion. Nornnally, the control actuator acts against the scram spring force. However, the actuator is also designed to assist in the s c r a m action to provide added protection in the unlikely event of a spring failure. Gas flow requirements are small, approximately 0.006 l b / sec control flow and below 0. 2lb / sec for coolant flow. The latter figure assumes that the actuator is located forward of the shield, however, coolant requirements for the actuator assembly located near the nozzle throat were also investigated. This location may be desirable to reduce radiation streaming at the forward end and either position is feasible for the four engines considered. When the actuator is located at the nozzle throat, however, some redesign must be made on the driving spline because of the higher heating r a t e s

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The forward shield is formed of circular steel plates welded together at the perimeter and eliminates the need for a separate external pressure vessel.Small holes,staggered to reduce radiation streaming,permit coolant passage. The inner portion of the shield acts as the structural backbone of the reactor assembly.The reactor and pressure shell are supported from the shield, and the engine thrust is transmitted through the shield to the thrust structure.The inner portion of the shield also supports the outer shield segments which in turn support the reflector The forward tube sheet positions the fuel elen n ents and provides their axial and radial support.The tube sheet is a plate,perforated with circular holes located on the triangular pitch of the fuel cell and counter bored to short depths to receive the fuel cartridge sealing rings.The outer periphery of the tube sheet is machined to eliminate excess material reducing weight and preventing large temperature differ - en ces across the tube sheet. A structural cylinder attached to the inner portion of the shield supports the tube sheet and permits differential radial expansion relative to the pressure shell flange.A welded connection assures sealing between flow passages. Cooling the tube sheet ligaments is by the direct passage of hydrogen between the ligament and the fuel element fastener.In con elX was chosen for the tube sheet material because of its high strength as well as its high off - design peak - temperature linait. Loads on the tube sheet are those caused by the reactor pressure drop,reactor inertia loads, and thermal loads. In designing the tube sheet,a deflection limit of 0. 032 inch was established to prevent excess radial deflection of the fuel elements. In analyses, the perforated plate was replaced by an equivalent solid plate of equal

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thickness with modified elastic constants. Ordinary solid plate equations were then applied to calculate deflections. The maximum ligament s t r e s s was then calculated by multiplying the largest principal stress in the equivalent solid plate by a factor which allows for the effect of the holes. The core is made up of fuel elements fastened to the tube sheet at the forward end and interconnected at the aft end to maintain clearance between elements.Each fuel element consists of an active core section, a nose p i e c e , and tail piece as seen The active core or fueled section contains the W-UO2 m a t r i x which is clad with a refractory - metal alloy. The nose piece at the forward end of the fuel element connects the element to the front tube sheet. A threaded fastener seals the face of the nose piece against the forward tube sheet. An integral seal ring located the fuel element in the counterbored hole of the tube sheet. Fuel element circumferential orientation is provided at the forward periphery of the …