Sept. 16, 1958 _ . I w, E, ABBOTT ETAL 2,852,460 FUEL-BREEDER FUEL ELEMENT FOR NUCLEAR REACTOR Filed Aug. 6, 1956

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_ ATTORNEY , 2,852,460 United States Patent 0 . i Patented Sept. 16, 1958

1 23 power level, and in heterogeneous reactors this has been 2,852,460 ' set by the uranium temperature in the center of the rod._ To avoid this, and yet maintain a high ?ux, fuel elements FUEL-BREEDER FUEL ELEMENT FOR comprising a cluster of relatively small diameter uranium NUCLEAR REACTOR rods have been designed. However, this only aggravates William E. Abbott, East Pittsburgh, Pa., and Ralph fabricational and decontamination costs. Balent, Tarzana, Calif., assignors, by mesne assign An object of our invention, therefore, is to provide an ments, to the United States of America as represented improved fuel-breeder fuel element. .by the United States Atomic Energy Commission Another object is to provide a fuel-breeder fuel ele ment, wherein the fertile and ?ssionable materials are Application August 6, 1956, Serial No. 602,401 separated ‘and there is a minimum of cladding. 5 Claims. . (Cl. 204-1931) Another object is to provide such a fuel element which permits fabrication and decontamination economies. Another object is to provide such a fuel element, where Our invention relates to an improved nuclear reactor 15 in a single, relatively large diameter fuel element is the fuel element, and more particularly to an improved fuel equivalent of a cluster of smaller diameter rods. breeder fuel element. . Still another object is to provide such a fuel element of For information concerning the theory, construction improved heat transfer characteristics whereby higher and operation of nuclear reactors, reference‘is made to ?ux levels can be safely reached. Glasstone, “Principles of Nuclear Reactor Engineering” 20 Yet another object is to provide such a fuel element of, (D. Van Nostrand); “The Reactor Handbook” (3 improved heat transfer characteristics, wherein the power volumes) available for sale from the Technical Informa level is not limited by the central temperature of .the fuel tion Service, Oak Ridge, Tennessee; U. S. Patents element. , 2,708,656 and 2,714,577 to Fermi et al.; and to “The A further object is to provide such a fuel element where Proceedings of the International Conference on Peaceful 25 in a single coolant may be used for both the ?ssionable Uses of Atomic‘ Energy,” August 8—20, 1955, Geneva, and fertile material. Switzerland, available for sale at the United Nations’ Book These ‘and other objectives of our invention will become Store, New York, New York. . apparent to those skilled in the art from the following ‘Unless otherwise speci?ed, conventional, accepted detailed description, the attached claims and the accom nuclear terminology will be used herein. For example,‘ 30 panying drawings. In the drawings, Figure 1 is a cross “?ssionable material” refers to thermal neutron ?ssionable section of our fuel element and Figure 2 is a longitudinal plutonium, uranium-235, and uranium-233, and ‘,‘fertile section of an embodiment for a particular, known reactor. material” refers to thorium and uranium-238, which are In accordance with our present invention, we have pro capable of transmutation to ?ssionable species by neutron vided an improved fuel element comprising a container, capture and beta decay. When uranium, thorium or plu a central core of fertile material in said container, a ?rst tonium is spoken of, this embraces both the metal and re j bonding material surrounding said core, a sheet of ?ssion fractory oxides, such as ThOZ and U02. able material immediately surrounding said ?rst bonding In the development of economically competitive nuclear material, and a second bonding material surrounding power, particular attention is being given to the conver said ?ssionable material, said seconding bonding material sion or breeding of new ?ssionable species. Since the being in contact with said container. supply of uranium is limited, widespread use of nuclear Our fuel element is notably distinct for it provides fabri power requires the conversion of relatively abundant cational and decontamination simplicity through separa thorium to uranium-233 and of uranium-238 to pluto tion of ?ssionable and fertile material, and through mini nium. The resulting uranium-233 and plutonium can mum cladding. Also contributing to fabricational and then be used for the further conversion of fertile material; decontamination simplicity is the use of a single, rela thus, the potential exists for actually ‘increasing the tively large diameter rod in place of a cluster of smaller amount of ?ssionable material while, at the same time, diameter rods. With the ?ssionable material on the out extracting useful power. Furthermore, successful breed side of the rod, the power level is not limited by a central ing, in providing a by-product of great value, signi?cantly temperature. Concentrating the ?ssionable material reduces the unit cost of generating nuclear power.v nearer the coolant provides for efficient heat transfer, and Fertile material is generally employed in reactors the same coolant isused for both the fertile and ?ssionable known as “breeder-type” reactors, wherein the breeding is material. The conversion ‘factor is surprisingly quite conducted along with the consumption of ?ssionable ma high; contrary to what might be expected, the ?ssionable terial. The manner and form in which the fertile material material surrounding the fertile material does not act as a is employed is of considerable‘ importance, for low fabri ?lter or neutron sink. With conversion occurring right cation and recovery costs are essential to the economical in the fuel element, new ?ssionable material is created, operation of such reactors. .In the past, fertile material which, in turn, can be used to sustain the chain ?ssion re has been employed in various ways. For example, pluto action. Having such marked advantages, our fuel element nium has been bred from uranium-238 in large, graphite has great promise of contributing to the economical de moderated, air or water-cooled, natural uranium reactors. 60 velopment of nuclear power. Thorium may be used in uranium-thorium alloys and as Referring now to Figure l, fertile material 1, of a blanket material in two-region reactors. In such two thorium or uranium-238, occupies the central core of the region reactors, the thorium has been employed in various element. Bonding agent 2 surrounds and is in contact forms, such as in solid rods, thoriasuspensions and nitrate ' with the core 1 and may be composed of a suitable molten solutions. Where the ?ssionable and fertile materials inorganic medium, such as sodium, sodium-potassium, have been mixed, they must be separated inreprocessing, ‘bismuth, bismuth-tin and fused ?uoride salts, for instance, thereby raising costs. Assembly costs have also been alkali ?uorides. A thin cylinder containing ?ssionable high, due, in many cases, to the use of cladding materials; material 3, such as uranium or plutonium, is in contact little or no cladding is ideal. Physical separation allows with bonding material 2. When uranium is used, it is for easier reprocessing, since most of the ?ssion products 70 preferably highly. enriched (e. g., at least 90%) in a ?s will remain in the fuel. Heat transfer problems have also sionable species, such as U-223 or U-235. Surrounding been severe. Heat removal ability usually de?nes the the ?ssionable material is a second bonding agent 4 of 2,852,460 3 4 composition similar to bonding agent 2. Bonding agent C. Operating data: 4 is in contact with the ?ssionable material and with the ‘Maximum temperature of ?s interior surface of container 5. The container may be sionable material ______1200° F. composed of any metal of suitable nuclear and metal Maximum temperature of fer lurgical properties, for instance, zirconium or stainless tile material ______1800° F. (Th). steel. - The bonding agents provide thermal ‘contact for Number of rods ______31. ef?cient heat transfer; and they also permit small dimen Power level ______20 Mw. sional changes in the solid ‘material due to radiation and thermal cycling, without rupture of the fuel element. While the above example shows the particular suit An example of'our invention'will now beggiven showing ability and adaptability of our invention for the SRE, it its adaptability to a particular reactor. This reactor is is understood that this is only by way of illustration and is not restrictive. the Sodium Reactor “Experiment reactor (SRE), a of our fuel-breeder Employingassembly, suitablethe fundamental modi?cation features. may 1 : i graphite-moderated, sodium cooled reactor which is fully and completely described ‘in a paper delivered by be made for its use in reactors of other design. There-' W. E. Parkins at the Geneva Conference ‘on Peaceful fore, our invention should be limited only as is indicated Uses ‘of Atomic Energy, entitled “The Sodium Reactor by the appended claims. Experiment.” This paper is available for sale. at the Having thus described our invention, we claim: United Nations’ Book Store, New York, New York. 1. A reactor fuel element comprising a thimble, a cen Unless otherwise indicated, the assembly features of our tral rod in said thimble, said thimble and said rod having . fuel rod in the reactor are the same as that shown in the relatively low thermal neutronabsorption cross sections,‘ paper, except that a single fuel rod replaces the seven rod a core of a fertile material selected from the group con fuel cluster in each fuel chamber. sisting of uranium-238 and thorium immediately sur- j _ Figure 2 is a longitudinal cross-section ‘of our fuel ele rounding saidrod, said core‘surrounded by a ?rst thermal; ‘ ment in a fuel channel of the SRE. Hanger rod 10 sup bonding material of a molten inorganic medium of .rela- , ports a column of separate, 6" length fuel rods. Sur- . tively low thermal neutron absorption cross-section se-' rounding rod 10 is fertile material 1. Around this is bond . lected from the group consisting-of fused ?uoride salts, ing agent 2, bonding and providing thermal contact be sodium, sodium-potassium, bismuth, and bismuth-tin, a“, tween'fertile material 1 and ?ssionable material 3. Bond ?rstsheet bonding of ‘?ssionable material, material and aimmediately second bonding surrounding material said of ing agent 4, which ‘is not con?ned to a single fuel slug, but rather runs the entire length of the fuel rod, provides 30 said molten inorganic medium surrounding‘ said ?ssion thermal- contact between ?ssionable material 3 and zir able material and in contact with the interior surface; it _ conium thimble 5, which contains the 6' column of of said thimble. 6" fuel slugs. Molten sodium coolant ?ows in coolant 2. A reactor fuel element comprising a cylindrical ., channel 6. Zirconium sheet'7 cans graphite moderator 8, thimble, a central rod in said thimble, said thimble and providing protection from the molten coolant. 35 said rod'having relatively low thermal neutron absorption 'Table 1, below shows the particular design data for the cross'sections, a plurality of discrete fuel slugs axially‘ fuel assembly of Figure 2 in the SRE: mounted on said rod, each of said slugs comprising a central, cylindrical core of 'fertile‘thorium immediately TABLE I surrounding said central rod, a concentric icylinderrof Design data 40 ?ssionable material surrounding said thorium, and a A. Materials: molten inorganic bonding medium of relatively low ther ‘Coolant channel ______Zr. mal neutron absorption cross-section selected from the Coolant ______Na. group consisting of fused ?uoride salts, sodium, sodiumf Fuel thimble ______Zr. potassium, bismuth and bismuth-tin disposed in the an Bond material ______NaK. 45 nulus between said thorium and ?ssionable material, and l ' Fissionable material____. U¢235. a second said molten inorganic bonding medium disposed Enrichment (uranium)_a 93.5%. in the annulus between said fuel slugs and said thimble - Total fuel ______60 kg. (?ssionable and’between each of said fuel slugs. ‘ material). 3. The assembly of claim2, wherein said thimble is" ‘ Fertile material ______Th. zirconium, each of said bonding agents is sodium potas-~ ’ . Hanger rod ______304L Stainless Steel. ' sium, said ?ssionable material is uraniumenriched -in ‘_ M. B. Dimensions: uranium-235. I Coolant channel— 4. A reactor fuel elementcomprising a container hav 0. D ______2.875”. ing a relatively low thermal neutron absorption cross" Wall thickness ______.035". 55 section, a central core of fertile thoriumin said container, Fuel thimble—~ a ‘molten inorganic bonding medium;of relatively lowv , , O. D ______2.165". thermal neutron absorption cross-section selected from, . ‘ Wall thickness ______L035”. the group consisting of fused ?uoride salts, sodium, so-» Bond material, radial thick diurn-potassium, bismuth and'bismuth-tin immediately ness ______.020". 60 surrounding said core, a layer of uranium immediately Fissionable material, radial surrounding said bonding medium,-and a second region thickness ______~ .010”. of said bonding medium disposed'between said uranium Bond material, radial thick and said container. - ness ______._ .005". 5. The fuel element of claim~4, wherein said/bonding ' ‘ Fertile material— 65 medium is- sodium-potassium. 0. D ______2.025”. I. D; ______.500". References Citedin the ?lepf-this Ipatenti Hanger rod, 0. D ______.495". Length of fuel slug ______6". International Conference on ‘the Peaceful Uses of i Length of fuel rod ______6'. 70 Atomic Energy, 1956, United Nations, N.‘Y.,‘ vol. '9, pp. Number of slugs in one rod__ 12. 179-185. ' '