Abstract:

AIRCRAFT NUCLEAR PROPULSION PROJECT QUARTERLY PROGRESS REPORT for Period Ending September 10, 1951 (ORNL-1154)

Authors:

R. C. Briant Director, ANP Project

C. B. Ellis Coordinator, ANP Project

Edited by: W. B. Cottrell

September 10, 1951 

This quarterly progress report of the Aircraft Nuclear Propulsion Program at the Oak Ridge National Laboratory is divided into five parts: Reactor Design, Shielding Research, Materials Research, Alternative Systems, and Appendixes, each of which is discussed separately in this summary.

Part I. REACTOR DESIGN

The Aircraft Reactor Experiment (Sec. 1). The design of the high- temperature Aircraft Reactor Experiment has been modified to use the 30- in. core instead of the 36-in. core, as proposed; in the last report. All reactor components are bn order with delivery expected this coming quarter. Design of the external fluid circuit is essentially complete, and many of the major components are on order. Construction of the building facili ty is on schedule.

Experimental Reactor Engineering (Sec. 2). Recent developments have included a frozen-sodium seal for a centrifugal pump, satisfactory operation of an electromagnetic pump and of a canned-rotor pump with sodium, operation of a liquid -me tal valve at 1500°F without self-welding, and operation of a mock-up of the reactor liquid-fuel system. Techniques have been developed further for the handling and purification of sodium and sodium systems to!minimize oxygen contamination; th is has contributed appreciably to the successful operation of these systems at high temperatures.

Reactor Physics (Sec. 3). The reactor physics calculations have been devoted largely to statics and kinetics of the ARE;, although some analysis of a sodium hydroxide reactor and a full aircraft-size BeO reactor were completed. The uranium investment in the ARE is 29.2 lb, the slight increase over previous estimates being due largely to increased structural and material poisons in the completed design. As the change in reactivity for normal control is relatively insensitive to the location of the six outer control rods, they have been located to maintain the most uniform flux distribution. Calculated kinetic responses of the ARE to arbitrary changes in reactivity, as a result of possible accidents or failures, show the system to be well damped. Preliminary calculations of a sodium hydroxide reactor designed at ORNL have yielded data on leakage and absorption spectra of the core and indicate a critical mass of approximately 32 lb.

Critical Experiments (See. 4). The preliminary reactor assemblies which are being, or have been, investigated include simulations of the air-water aircraft reactor, proposed by General Electric, and a graph i te-uranium assembly to study power and fllux distributions. Measurements of this latter reactor were still being made a t the end of the quarter. The two modifications of the air-water reactor demonstrated the savings in critical mass to be gained by decreasing the thickness of the water layers.

Part II. SHIELDING RESEARCH

Bulk Shielding Reactor (Sec. 5). The measurements on the mock-up of the ideal unit shield have been completed and indicate a weight of 124,000 lb for a 3.8-ft spherical reactor. Construction of the mock-up of the divided shield is essentially complete, and preliminary shadow-shielding tests have been completed.

Lid Tank (Sec. 6). Recent measurements on the lead—borated water unit shield, in which the boron concentration has been increased to 1.3 wt %, result in a unit shield weight for a

3-ft spherical core of 101,200 lb, 6000 lb lower than that calculated by the Shielding Board in ANP-53, a year ago.

Duct Test (Sec. 7). A practical treatment for liquid-metal ducts in a reactor shield has been demonstrated by the use of an extra conical layer of water around the duct as it emerges from the reactor shield. The weight of such a patch for a single 6-in. sodium duct surrounded by a 1-in. air- filled annulus is approximately 10001b.

Shielding Calculations (Sec. 8). Analysis of Lid Tank data has yielded “removal cross-sections” for lead, boron, and water of 3.4, 2.0, and 0.8 barns, respectively. The theoretical analysis of the divided shield is again being scrutinized very carefully. It seems likely that, when some of the approximations are better defined, the weight may be somewhat greater than previously supposed, although not prohibitively so. A weight advantage of several tons by using ammonia in the divided shield in preference to water has been demonstrated. An analysis of some possible radiation hazards associated with the ARE has also been made.

Nuclear Measurements (Sec. 9). The 5-Me v Van de Graaff generator is in operation, and preliminary experiments indicate its useful range of energies to be from below 0.2 Mev up to 6 Mev. An in-pile lithium loop has yielded data on Bremsstrahlung intensities, and measurements of the (n,2n) reaction in beryllium were made. The installation of the neutron time-of-flight spectrometer at the LITR has progressed although fabrication of the assembly has been seriously delayed during the quarter.

Part III. MATERIALS RESEARCH

Investigation of the materials problems of a high-temperature reactor continues to comprise a major part of the effort of the Aircraft Nuclear Propulsion Project. In addition to the empirical research on corrosion, radiation damage, materials fabrication, and reactor chemistry, this program includes the determination of the basic thermal and physical constants associated with these materials at reactor temperatures.

Corrosion Research (Sec. 10). Corrosion studies have been expanded to include corrosion by hydroxides and possible fluoride coolants as well as corrosion by the liquid metals and fluoride fuels already under investigation for some time. The extensive corrosion tests with sodium, both static and in thermal convection loops, conclusively demonstrate that sodium causes negligible corrosion to either inconel or a number of stainless steels at 1500°F. Static corrosion tests of the pretreated fluoride fuel mixtures indicate that containing of these liquids is likewise possible under the same circumstances. Corrosion attack by such pretreated fuels averages about 1 mil on stainless steel and 2 mils on inconel after 100 hr at 1500°F. Corrosion attack encountered with hydroxides and with hydroxide- bearing materials, however, is a great deal more severe. Both commercial and specially pure sodium hydroxide are extremely corrosive to inconel and to the stainless steels. Corrosion by potassium hydroxide is somewhat less severe. Tests with barium and strontium hydroxides indicate these caustics ‘to be somewhat less corrosive than either of the alkali hydroxides, but still so severe as to preclude their use at this time. Rigid control of the purity of the material, which was necessary with the fluorides, has not yet been realized but should reduce the present hydroxide corrosion rates.

Physical Properties .and Heat-Transfer Research (Sec. 11). Of particular interest in the design of the reactor are the measurements of the physical properties of probable reactor materials as well as the studies of heat – transfer phenomena and associated coefficients. Heat-capacity measurements have been completed for 316 stainless ; stee 1, lithium, molybdenum, and zirconium in the range 3 00 to 1000°C. The thermal conductivity and density of the flo.ori.de fuel have also been defined in the region of interest. Apparatus for the measurement of the viscosity of high – temperature liquids and. the heat- trans fer coefficients of hydroxides and fused:salts is now essentially complete.; Investigation of the free-convection mechanism within quiescent liquid-fuel elements 1s underway. :

Metallurgy and Ceramics (Sec. 12). The metallurgical processes involved in the construction and assembly of a high – temperature reactor core, including welding of tubular fuel elements, fabrication of solid fuel elements;, creep of metals, fabrication of control rods, and cladding of beryllium oxide moderator, are currently being investigated. It is now apparent that tube-to-header welds having tensile strength comparable to that of the parent metal may be readily effected with inconel by an electromagnetic “cone-arc” technique. Added to the list of techniques by which solid-fuel elements may be fabricated is that of “rubbers tatic pressing,” In all techniques the use of a screened fraction of sintered: U02 seems to be desirable. The creep laboratory;and stress-corrosion laboratory are now in operation and are emphasizing tests on inconel and stainless steel specimens. A ceramic laboratory has been set up and is being equipped and staffed.

Chemistry of High-Temperature

Liquids (Sec. 13). The chemical research on reactor fluids has been extended to include study of non- metal .1 i c liquids for use as moderators and/or heat-transfer fluids, in ad- di tion to the development of liquids for use as high – temperature reactor fuels. Nine fluoride fuel systems, both ternary and quaternary, are singled out as covering a useful range of uranium concentration end possessing satisfactory melting points.! In the development of homogeneous reactor fuels, solutions of U03 in NaOH-Na2B40? show promise, in addition to the NaGH-LiCJH system previously developed. The recent investigations for moderator coolants have indicated that several binary hydrogenous systems appear to be satisfactory as far as liquid ranges, moderating ability, and heat-transfer properties are concerned. Preparation of sodium hydroxide of greater than 99,8% NaOH by weight was required in the above and in the associated corrosion studies. The list of nonmetallic coolants now includes 11 fluoride systems of usable liquid range and low corrosiveness, although their heat-transfer properties are not sufficiently well known to permit evaluation of their usefulness.

Radiation Damage (Sec. 14). Although a number of fuel capsules have been irradiated in both the pile and cyclotron, complete data are not yet available owing to the residual activity of the capsule. From preliminary results it appears unlikely that radiation will have a significant effect on the fuel. On the other hand, the thermal conductivity of inconel appears to decrease by a factor of 2 iu less than a week of exposure in the X-10 pile. The flux dependence of this decrease has not been determined, but a temperature anneal of the effect has been demonstrated. As regards creep under irradiation, there is a definite reduction in primary creep due to irradiation with 347 stainless steel. However, after long periods of strain, 200 hr and above, irradiated specimens exhibit significantly greater elongations than control specimens. Two preliminary experiments on irradiating lithium in iron capsules show no appreciable added corrosion ascribable to radiation ef fects.

Part IV. ALTERNATIVE SYSTEMS

The major effort of the Aircraft Nuclear Propulsion Project at Oak Ridge National Laboratory is directed toward a 1500°F liquid-fuel reactor. However, research is in progress both here and at associated laboratories on several other reactor systems which show promise. Among these alternative of this moderator-coolant reactor is likewise specified as sodium hydroxide. A 2.5-ft spherical core using liquid fluoride fuel is expected to deliver 200 megawatts with a maximum wall temperature of 1500°F. An essential feature of this design is the use of annular fuel elements to attain the high ratio of heat-transfer surface to fuel volume necessary with the hydroxide coolant.

High-Temperature Power-Plant Studies (Sec. 18). North American Aviation, Inc., has conducted an investigation of high-temperature (above 1800°F) helium and sodium liqu :i d – va por power cycles with regard to their application to a Mach 1.5, 45,000-ft-altitude aircraft. They conclude that a helium- cooled reactor cannot achieve the supersonic propulsion of aircraft even if reactor temperatures as high as 33 00°F and he 1ium pressures in the range 1000 to 2000 psi should be used. On the other hand, a liquid-metal – cooled reactor operating in conjunction with a sodium liquid-vapor compressor jet system does appear feasible for the supersonic aircraft.

Part V. APPENDIXES

Analytical Chemistry (Sec. 19). Chemical analysis is required in almost every phase of the reactor program. Although some of these analyses are routine, the development of many new analytical techniques is required. In all, over 400 samples were’ submitted for analysis during the last quarter and over 1700 determinations were made. The n-butyl bromide method, which has been developed for the determination of oxygen in sodium, is extremely accurate for oxygen contaminations down to 0.015%. The oxygen content of argon and helium is now determined by a colorimetric method which gives excellent precision below 25 ppm. Methods are also being developed for the determination of metallic: corrosion products in fluoride fuels and metallic coolants and for the determination of oxygen in lithium and lead.

List of Reports Issued (Sec. 20).

The reports issued during the past quarter; include some fifty reports on all phases of the ANP program.

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