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Aviation History
1956
1956 - 1613.PDF
9 November 1956 771 NUCLEAR POWER FOR AIRCRAFT A Lecture by the Chief Executive of the Hawker Siddeley Nuclear Power Co. ON Tuesday, October 30, a forward-looking talk with theabove title was given to the Graduates and StudentsSection of the Royal Aeronautical Society by Mr. E. P. Hawthorne, B.A., A.F.R.Ae.S., director and chief executive ofHawker Siddeley Nuclear Power Co., Ltd. Mr. Hawthorne began by briefly describing the principles of the thermal reactorand the fast reactor. The former depended on the neutrons arising from fission being slowed by a moderator, after which the slowneutrons were captured by further uranium-235 atoms, thus sustaining the chain reaction. This type of reactor required amuch lower proportion of the fissile uranium-235 isotope than did the fast reactor, but was considerably more bulky. A NordiAmerican Aviation proposal for a 75 MW reactor was approxi- mately 17ft in diameter and 14ft high.A fast reactor used the fast neutrons to produce further fission without the slowing-down action of the moderator. This type ofreactor required a much higher proportion of U-235 fissile isotope in the fuel. It did, however, have the great advantage of smallbulk, the reactor core for a 60 MW unit being only 2ft in diameter and 2ft long. This small bulk gave rise to the need for very highheat-extraction efficiency and also to more critical control-charac- teristics. The Argonne Laboratory in America had proposed afast reactor with a power output of 28 MW/cu ft core. The reactor and all its associated controls and pumps were, for safetyreasons, contained in a tank 10ft in diameter and 13ft high. Heat could be extracted by air, gas or liquid metal. For thefast reactor the very high design-efficiency made a liquid-metal system necessary.In aircraft propulsion the main problem was that of providing adequate shielding. Gamma-particles could be absorbed only byinteraction with electrons, which demanded a material with a high mass such as lead or iron. Conversely, neutron capture wasbest achieved by a material of relatively low mass, such as boron or lithium. Aircraft shielding would possibly consist of a con-siderable thickness of lead followed by layers of the lighter material interspersed with iron. Important weight-reductions could beobtained by using partial shielding, i.e., by shielding only that part of the reactor which faced the crew compartment. The lecturer thought that it should be possible to conceive anaircraft powered by a fast reactor with solid fuel elements. For an aircraft with a desired performance of Mach 2.5 at 60,000ft,250 MW (335,000 h.p.) was required, obtained from a reactor of 20in core diameter with a rating of 50 MW/cu ft core. Theheat-exchange would be by liquid sodium (with an intermediate heat-exchange to restrict active sodium to shielded areas) whichwould be passed through a flat-plate heat-exchanger (2ft long) in the "combustion chamber" of an otherwise normal turbojet (seeillustrations on this page and page 743). The outlet temperature of the reactor heat-exchange fluid would be around 750-800 deg C,compared with about 500 deg C for present reactors. This rise in Schematic arrangement of a metal-cooled reactor, and engine, in an aircraft—an illustration from Mr. Hawthorne's paper. A, primary heat-exchanger; B, secondary coolant; C, laminated boron steel shield; D, fuel element; E, lead shield; F, salety rod; G, primary coolant return pipe; H, reflector control; J, primary coolant; K, secondary coolant heat-exchanger. 1 IJiffiitrT l5tJ F G H temperature gave a more efficient cycle with a decrease in reactorsize and aircraft drag. An aircraft of this size would weigh a minimum of 150,000 lband might well weigh twice as much; its payload would be of the order of 10,000 1b. The reactor weight would be between30,000 and 35,000 lb and the heat-exchanger would weigh about 1,000 lb per 1,000 lb of mrust. Because of the great weight andmass of the reactor it would probably have to be mounted on or near the e.g. At take-off considerably higher thrust was required than incruising flight. If the reactor were designed for the cruise con- dition, the take-off would require either a temperature overload(which would be difficult to achieve) or the provision of auxiliary thrust. It seemed that use of the reactor was the best solution,but this in turn brought up the problem of the accident case. Reactors had an immediate residual energy level of six per cent ofthe operating level prior to cut-off. If an atomic-powered aircraft suffered a take-off accident when the reactor was operating atmaximum power, the residual energy level might prove a serious hazard. Speaking of the accident problem in general, Mr. Hawthornesaid that the result of a reactor going out of control was the vaporization of the liquids and solids it contained. If these weredischarged to atmosphere a large part of the surrounding country would be seriously contaminated. Similar considerations appliedin the crash case, when rupture of the reactor would result in wide-scale dispersal of highly radioactive fission products. Evenif rupture of the reactor could be avoided there remained the problem of the six per cent residual energy level, for which therewas no obvious solution at present. One suggested way of avoiding these risks was to use atomic-powered aircraft as remotely controlled tugs for conventionally powered machines. These tugs would always remain at highaltitude, the towed aircraft taking off and landing under their own power and joining and leaving the tug near the departure anddestination airfields. Another suggested application in the aircraft field was the "nuclear rocket" engine. This envisaged a reactormade from porous ceramic, which would have one of the greatest surface/volume ratios obtainable, operating at temperatures around3,000 deg C. The fuel, probably hydrogen, would be pumped over the hot reactor surface. A disadvantage was that a fuel hadstill to be carried. Summing up, the speaker concluded that the advantage of thenuclear-powered aircraft was unlimited range, and that if it was decided that such an aircraft was required he thought that itcould be built. The magnitude of the task, however, could perhaps be judged from the film of the building of Calder Hallpower station which he was about to show. This power station had a rating of only one-third of the proposed aircraft powerplant;he would leave it to the meeting to decide how complex were the engineering problems involved. In the subsequent discussion many interesting points were made.The lecturer considered that the safety aspects prevented nuclear power being used in any but military aircraft at the present time.The fuel economy of a nuclear-powered aircraft appeared to be better than one using kerosine. In a different vein, he thoughtthat a closed-cycle propeller-turbine was also quite practicable; it depended on what one wanted the aircraft to do. Another proposal was to put the fuel in the "combustion cham-bers" diemselves, which Mr. Hawthorne considered feasible pro- vided there was sufficient heat-exchange efficiency. At the presenttime there appeared to be only one forseeable way of using the energy from nuclear fission and that was by heat-exchange; theprospects of using nuclear power in conjunction with such devices as electro-static lift were too remote to be considered. Theresponse to rapid throttle-movements was a tricky subject and might have to be solved by using a "heat dump." On the subject of the design effort necessary to construct anuclear-powered aircraft, Mr. Hawthorne believed that this would be very large indeed. Maintenance requirements would prob-ably be less than on conventional aircraft, since there were no moving parts. There might be some problem in keeping thesodium liquid between flights, but this was not too difficult. Finally, aircraft structural designers were relieved to hear thatthe level of radioactivity would not be high enough to cause appreciable material damage during the expected life of the air-craft. On the other hand, the proposal that the powerplant life would exceed the airframe life and that the "nuclear engine" wouldbe removed from an old airframe design and installed in a new design was one of the many revolutionary ideas put forwardduring a most stimulating evening.
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