2,8ll,487 Patented Oct. 29, 1957 2 must be held to a value so low that at least one is available tor a new fission, after losses have been de ducted, per neutron consumed in production of ñssion. 2,811,487 l , In fission of U235 and similar , more ’are NEUTRON REACTÜR HAVING A Xe135 SHELD evolved per fission than are required to produce the fission. _ Henry E. Stanton, Chicago, Ill., assigner to the United 'For example, about 2.3 neutrons are evolved per neutron States of America as represented by the ‘United States consumed in ñssion of U235, and about 2.8 neutrons are Atomic Commission evolved per neutron consumed in fission of 94239. These evolved neutrons are used up in fission of further U235 or Application December 3, 1946, Serial No. 713,653 10 94239 atoms, or are lost to the reaction. `If losses do not reduce the ratio of neutrons evolved to neutrons con 1 Claim. (Cl. 24M-193.2) sumed and Ilost below one, the will continue. ' Losses may be external, as when neutrons escape fromA the reactor, or internal. Internal losses are caused ‘by This invention relates to neutronic reactors, and more 15 absorption of neutrons by atoms which do not ñssion when particularly to a novel method and means for shielding the neutron has been absorbed. ' ‘ such reactors against the ñow of neutrons therefrom. U238 present in natural absorbs substantial A Vgeneral object of the invention is to provide a novel quantities of neutrons to produce 94239. VThis Vloss may lightweight neutron shield Iaround a neutronic reactor. be substantially reduced by use of uranium aggregate-s. Another object of the invention is -to continuously 20 Thu-s, it has lbeen found that U238 absorbs neutrons to‘an withdraw from a neutronic reactor a highly neutron appreciable degree at (resonance energies) absorbent fission product formed therewithin and to dis greater than thermal energies due to its relatively’high‘ pose said product around the reactor, externally thereof, capture cross section with respect -to that of U235 at such‘ for the purpose of absorbing neutrons emanating there resonance energies. However, this type of absorption, from. ' 25 known as resonance absorption, may be reduced Iby de Still ,another object of the invention is to provide a creasing the amount of neutrons which passV into a uranium novel composite neutron shield consisting of an 'inner body until these neutrons have‘been slowed to thermal layer of neutron moderator `adapted to slow escaping energy. This maybe done -by reducing the ratio of sur neutrons to relatively low energy values, and an outer face area per unit weight of uranium, i. e., by using layer of Xe135 which has an extremely great capture cross 30 in the form of aggregates preferably section for the slowed neutrons emanating yfrom the inner ' having a minimum thickness of about 0.5 cm. More layer. over, this loss may be rendered negligible by use of a y‘lu neutronic reactors a neutron ñ‘ssionable , such concentrate of a iissionable isotope which contains greater as U223, U235, or 94239, or mixtures thereof is subjected than natural concentration of fissionable material. “ to fission by absorption of neutrons, and a self-sustaining 35 Neutron moderators also absorb neutrons. Generally chain reaction is established by the neutrons evolved by speaking, it is desirable to use as a moderator an element the fission. In general, `such reactors comprise bodies of (or compound thereof) of low atomic weightV and .low compositions containing such ñssionable material, for ex cross section. The ability to slow down ample, natural uranium, disposed in `a neutron slowing neutrons may be expressed by what is known as the ‘scat material which slows the neutrons to thermal energies. 40 tering cross section, whereas the ability to absorb or Such a slowing material is termed a neutron moderator. capture neutrons may be expressed as the capture cross , , and D20 (heavy ) are typical ’ section. The ratio of absorption cross section to scatter moderators suitable for such use. Heat is evolved dur ing cross section of various materials are' approximately ing the reaction and is removed by passage of a as follows: through the reactor in heat exchange relationship there 45 with. Specific details of the theory and essential char ight water (H2O) ______0.00478 acteristics of such reactors are set forth in the co-pending ' Diphenyl ______0.00453 application of and Leo Szila-rd, ‘Serial No. Beryllium ______0.00127 ' 568,904,V filed December 19, 1944, now Patent No. __ _ 0.000726 2,708,656. 50 (D20) ______0.00017 The ratio of the fast neutrons produced in one genera tion by the fissions to the original number of fast neutrons For natural uranium it is preferred to use materials producing the fission in a system of infinite size from wherein the above ratio is below about 0.004. However, which there can be no escape is called the multiplication with compositions containing more constant and is denoted by the symbol k. In a system of 55 than naturalamounts of U235, a greater latitude is per finite size, the ratio of the number of neutrons in one missible. .Using carbon or Ydeuterium oxide as moderators l generationto the number of neutrons in theV preceding and natural uranium as the fissionable composition, only generation is called the reproduction ra-tio of thesystem, about 1.1 or 1.3, respectively, neutrons are obtained per and is a constant designated by the symbol r. Usually this neutron consumed due to neutron losses in the U238 and constant is expressed without .regard to localized neutron 60 the moderator. Since the external neutron losses may be absorbers such as control or limiting rods, which are no-t substantial, other internal neutron losses should be held uniformly dispersed throughout the entire system. The sufficiently 10W to prevent these losses from rising so neutron reproduction ratio r is an actual value for a high as to prevent the reaction. finite system, and differs from k by a factor due to loss Other components of the reactor including the coolant, of neutrons .through leakage and through absorption by 65 impurities in the uranium or other portions of the localized neutron absorbers. To maintain a chain re system, moderator, control or limiting rods, fission «frag action, r must be at least equal to one. As pointed out ments, restraining barrier, etc. absorb neutrons in varying in the above-mentioned Permi-‘Szi‘lard application, it is amounts depending upon their neutron capture cross preferably maintained below about 1.01 during operation section. . The eñect of these impurities or absorbers in a reactor' of the reactor. ’ ’ » 70 `In order that a self-'sustaining neutronic chain reaction containing natural uranium as the fissionable component can be established and maintained, the losses of neutrons has beenv approximately evaluated for each element aS, 2,811,487 3 4 a danger coefficient. This coefficient is computed accord As an example of the use of this table, a shield thick ing to the formula ness of 0.0033 cm of Xe135 in the form of a gas at normal and pressure will transmit thermal neutrons the same as 1.0 cm. of iron. In other words, the fraction T., A, of neutrons absorbed in 0.0033 cm. of Xe135 is the same where T1 represents the cross-section for absorption of as in a 1.0 em. thickness of iron. thermal neutrons of the impurity, Tu represents the cross For purposes of shielding, the total mass of material is section for absorption of thermal neutrons of the uranium, important. If the total mass of an absorber is designated Ai represents the atomic weight of the impurity of neutron by M, it follows from (2) that absorber, and Au represents the atomic weight of uranium. 10 The following table gives presently known Values for various elements having their natural isotopic content. M1 K2 (3) for the same transmission. Substitution into this equation Element Danger Element Danger from the table gives the following table: Coefficient Coefficient 15 Element M2 10 1. 0 0. 01 2 0 50 310 .2 0. 04 18 2, 150 870 0. 012 54. 2 ‘L 0 0. 18 0.002 1. 6 0. O2 1 3 1. 6 sa .s As an example, if l0 tons of iron is required to shield a o. 3o o. 3o 25 pile, the same shielding could be accomplished with six 0. 26 2. 4 Vo. a _ 2. 4 grams of Xe135. y 31 17 In view of the fact that Xe135 decays with a half life of 0.8 1, 430 . 2. 1 435 9.5 hours, it is necessary to replenish it at frequent in 0.37 6, 320 tervals to supply losses from decay, and also losses due to 7 ' 20 30 3. 8 200 destruction by capture of the escaping neutrons. Under 4 10 2 ‘40 certain circumstances, these losses may be supplied by the 7. 5 20 reactor itself. Between 3% and 5% of all fissions lead 1. 5 10 to the formation of Xe135, and it is possible to recover a substantial portion of this gas in certain types of reactors. 1. s «i 6 35 0. 61 2. 7 On the other hand, each fission produces approximately 1 18 two neutrons, one of which must be used, on the average, 2 1. 7 6. 3 >70 to maintain‘the chain reaction. The excess neutrons must 2. 5 2. 5 either be absorbed in the interior of the reactor or even tually leak out of the reactor. In consequence, the Xe135 0. ß4 a 40 0. 57 0. 5 in the shield vcan only absorb less than 5% of the neutrons 0. 4 0. 03 0.13 0. 0025 generated by the tissions in order to regenerate the shield 0. 4 1. 1 at a sutlicient rate to maintain it. An alternative procedure would be to generate Xe135 The exceedingly large thermal neutron capture cross by the bombardment with neutrons of Xe134, a stable iso section of Xe135 suggests the possibility of its use as a 45 tope with an abundance of about 10%, but this is econom light weight shield for a neutronic reactor. If this possi ically vnot feasible at present. bility can be realized, great economies in weight and cost l An operative structure ’hereinafter described comprises can be effected in the shielding problem through the a homogeneous ‘liquid pile so loaded with absorbers (as elimination of massiveconcrete, etc. ) that the neutronimultiplication constant for the It is well known that the `thickness of a neutron absorb 50 inñnite «pile would not Vexceed about 1.03. The Xe135 ing material for agiven transmission (i. e. the ratio of the generated'is taken 0E continuously by sweeping it out emerging neutron ñux tothe rimpinging ) varies in with the other gases, or other means, and subsequently versely with the product of the density of the absorber led into the shield space. The whole assembly is pref and its danger coefñcient. Consequently, if x represents erably surrounded by lead or other suitable material to the thickness of an absorber, p and K its density and 55 shield against fy and other . The Xe135 shield danger coeñicients respectively, surrounds the reflector which'adequately slows the fast 1 neutrons generated'in the active portion. x~pK (l) ­The ­foregoing and other/objects and advantages of the invention will be readily apparent from a consideration of Two different absorbers, designated by subscripts 1 and 60 the following specification and the accompanying draw 2, may then be compared for the same transmissionv by ing which comprises a iìow diagram illustrating the in the equation, vention as'applied to a system wherein the reactive com position and the neutron absorber are in >fluid form. . ¿U2 P1K1 The reactor, diagrammatically illustrated in the draw Usingpthis relation, the following table may be con 65 ing, comprises a suspension such as a solution or slurry structed, (subscript 1 referring to Xe135). of a'ïñssionable material in a liquid moderator. For example, a solution of uranylsulphate in Water is chain Element p, glee. K 'x1/xn reacting where the solution fills a­ spherical reactor 12 inches in‘diameter and surrounded with an eñicient neu 2. 5 2, 150 .1.:49 70 tron ­»reflector and 'where the reactor contains at least 7. 9 .1. 5 . 0033 8. 65 870 2. 09 575-600.*grams of U235 »asa uranium concentrate con 7. 7 1, 430 2.78 tainingabout 15%­ U235 based upon the total uranium. 7. 7 6, 320 13. 5 13. 6 '8 . 31 Higher amounts of U235 for example 600-700 or more . 006 2.5 X 10ß l grams are required where the reñector'contains a neutron 75 ­absorber as 'in the. 'present instance. 'Moreoven other 2,81 1,48? »

solutions such as solutions of uranyl ñuoride, uranyl condensed D20v or H2O being conveyed to the before nitrate, plutonyl sulphate (PuOzSO-t) may be used where mentioned purifier tank 46. is conveyed from the-iissionable isotope content of the or uranium the condenser 56 by a line 57, preferably including a 1s above abolut 5-15% of the uranium or equivalent com pump or blower 58, to a separator tank 60 for removal position. In addition, natural uranium compounds of gaseous fission products and thence to the helium reser (uranyl ñuoride U02 or UaOa) may be dispersed or dis voir Si). solved in heavy water (D20) to establish a chain re An emergency dump line 62 is connected to the tank action. 2 and the reservoir 18, said line including a dump valve The shape of the reactor may be cylindrical as shown 64 adapted to be opened under emergency conditions to by the drawings or may be spherical or other form. A 10 reduce the body of composition 4 within the tank 2 to a slender elongated cylinder is particularly advantageous size smaller than that at which a chain reaction may be where maximum leakage is desired in order to secure a sustained, maximum of neutrons for bombardment of the neutron It may be noted that the system, thus far disclosed, is, absorbent or `target isotope or isotopes. purely illustrative and such systems are more fully de In the ñgure, a reactor tank or chamber 2 of low neu 15 scribed in co-pending application, Serial No. 613,356, tron absorbing material, for example stainless steel or tiled August 29, 1945, in the United States Patent Otïìce aluminum, is provided. This tank is sufficiently thin (for by Eugene P. Wigner, Leo A. Ohlinger, Gale J. Young and example 1A, inch or less) to permit passage of neutrons Harcourt C. kVernon and also in an application of Rob therethrough without substantial absorption of neutrons. ert F. Cristy, Serial No. 623,363, filed October 19, 1945. The tank contains a chain reacting liquid composition 20 Surrounding the tankkZ is another tank or chamber 66 4 such as above-mentioned. within which a production area or zone containing a ñuid The reactive composition is continuously circulated composition 68 to be bombarded by neutrons emanating through a heat exchanger 6 by means of a pump 8 from the reactor 2. This tank also is constructed of a low having its suction side connected to the tank 2. The neutron absorber such as aluminum or stainless steel or discharge side of the pump is connected to the heat 25 other material having a danger coeilicient below l0 and exchanger 6 through which the reactive composition is having a thickness, e. g., ïÁs inch or less, sul‘liciently low passed in heat exchange relationship with a coolant cir to prevent substantial neutron absorption by the tank culated through the heat exchanger by inlet and outlet 66. The composition 68 may be a slurry or solution of pipes 10 and 12. The cooled composition is returned to the neutron absorbent material, such as thorium, in a the tank 2 through a return line 14. 30 neutron moderator such as heavy water, said composi The amount of reactive composition Within the tank tion being admitted to the chamber 66 through an inlet 2, as well as the concentration of uranium-containing line 70 including a conventional shut-ott valve '72, After material in the composition, is controlled by a system in the composition 68 has been bombarded for the desired cluding a reversible delivery pump 16 connected to the length of time, it is conveyed‘from the chamber 66 by an bottom of the tank 2 and to a reservoir 18 having an 35 outlet line 74 including a conventional drain valve 76 to inlet 20 to accommodate the introduction of uranium a conventional separator device '78 adapted to separate containing material into said reservoir 18. The reservoir the heavy water from the neutron absorbent material 18 is connected to the tank 2 through a line 22 having which has at this point been converted to a radioactive a conventional three-way operating valve 24 connected isotope by the capture of neutrons as above discussed. to the discharge side of :a pump 26, the suction side of 40 The radioactive material is conveyed from the separator which is connected to a moderator reservoir 30 having '78 by a line 80, and the separated moderator is con an inlet 32 through which moderator may be conveyed veyed from the device '78 by a line 82 connected to the to this reservoir. suction side of the before-mentioned pump 44 which thus The reactive composition 4 is continuously Withdrawn urges the heavy water from the line 82 to the before from the bottom of the tank 2 through an outlet line mentioned purifier tank 46. lt will be understood that 34 lconnected to the suction side of a pump 36, the dis the presently illustrated system will be used when the charge side of which is connected to a conventional same moderator is used in tanks 2 and 66 and that where separator device 38 adapted to separate the moderator ditlerent moderators are used diñcrent purifiers may be from the uranium-containing material. Such a separator required. may comprise an evaporator or settling tank `and the 50 The bombarded composition 68 is preferably circulated separated material may be conveyed from the device 38 through a heat exchanger 84 by a pump 86 having its by an outlet line 40 for recovery of 94 and ñssion products suction side connected to the chamber 66, the cooled formed as a result of the neutronic reaction where natural composition being returned to the chamber 66 through uranium or U235-U238 mixtures are used Within the tank a return pipe or line 88. 2. Separated moderator is conveyed from the separator The chambers 2 and 66 are disposed within an alu device 38 through a line 42 including a pump 44, the minum or steel tank or chamber 90 containing a heavy discharge side of which is connected to a moderator puri water neutron reñector 92 adapted to reflect escaping ñer 46 from which the purified heavy Water is conveyed neutrons back into the chamber 66 and/or reactor 2. to the before-mentioned reservoir 30. The purilication The heavy water 92 is conveyed to the tank 96 through may be effected by various means such as by distillation. 60 an inlet line 94 having a conventional shut-off valve 96 It will be understood that the water within the tank 2 and is conveyed from the tank by an outlet line 98 hav is continuously decomposed into D2 and O2 or H2 and O2, ing a conventional drain valve 16d. depending upon the type of water used, as `a result of The gaseous fission products which are separated from the neutronic reaction; and these decomposition products, the helium in the separator 69 are pumped through a as well :as gaseous fission products of the reaction in line 102 into a shield tank 164 by a pump having its dis cluding Xe135, are swept from the tank 2, and the de charge side connected to said tank. These fission prod composition products are recombined. A gas pump or ucts are expelled from the tank through an outlet line blower 4S is provided having its suction side connected 10?» which is connected to an exhaust Stack (not shown). to a helium reservoir 50, and its discharge side connected It will be understood that in a system such as above to the tank 2 above the level of the reactive composition 70 described Xelï‘)5 constitutes about 25%-33% of the gase therein. The helium passes through the tank 2 and is conveyed therefrom by an outlet line 52 connected to a ous iission products swept from the chamber 2. rlÍhus the conventional recombiner device 54 adapted to recombine shield chamber 110 formed between the tanks 90 and the isotope or isotopes and Oz into vaporized 164 must be about four times as thick as above described D20 or H2O which is conveyed to a condenser 56, the 75 to replace an equivalent thickness of iron.V In other 2,811,487 7 8 words, approximately 0.0132 cm. of gas within'theshield reflector comprising» a tank containing heavy;> water-sur."v chamber 110 is equivalent to 1 cm. thicknessof iron. rounding said. reactor, a shield tanksurroundingsaidrea It may be` noted that the reflector 92 and thev gaseous'. ñector, . a . gamma rayV shield surrounding;` said­ shieldvl tank~,­. iission products within the tank 104 constitute av com and means for conveying gaseous’?i‘ssion- products in-> posite shield in that thev neutron moderator 92y is eiîec ~ cludingv Xel35 vfrom the reactor chamber to the shieldy tive to reduce fast neutrons emanating from the neutronic tank including` ak heliurn- reservoir,c a blower for­ blowing reactor to relatively slow energy values atßwhich theyf are helium gas through said reactor chamber,.afrecombiner, readily absorbed by Xe135 within the chamber. 111|).y Thus a condenser, and a separator tank for separating helium very few neutrons escape from the tank». 104 andx these~ from the gaseous vñssion products. are preferably absorbed by a> relatively. thinshield 112.y of lead around this chamber adaptedlto--absorb game and References Cited'in theiile. of this patent other biological radiations. UNITED STATES PATENTS; Thus, it will be understood thatiLhave~designed.a\novel lightweight neutron shield which4 is capable of-.etîèctively 2,708,656v Fermi’et al; ______May 17;` 1955. shielding operating` personnel from. neutrons. emanating` 2,716,705 Zinn`> ______Aug. 3'0, 1955 from a neutronic reactor. Preferably:y this lightweight 2,743,225' Ohlinger etal ______Apr. 24, 1956 Xe135 shield withinthe chamber. 1~10Yis provided, .assabove FOREIGN PATENTS described by extracting this. isotope from: the. reactive 861,390 France ______Oct. 28, 1940>` composition undergoing av chain. reaction.. However, it. 233,011 Switzerland'- ______Oct. 2‘, 1944? will be readily understood thatzif desired1the. Xe.135 may 20 be Vproduced by bombarding Xe134 with neutrons. and-the OTHER REFERENCES chamber 110 may be iilled with. substantially purezXe135, “Atom Movements,” pub. by American Society for an arrangement which would greatly reduce the thickness Metals, 1951, Cleveland, Ohio, p. 175. of the shield within the chamber 110. Introduction to Nuclear Eng., by- Richard Stephensom, It will be understood thatthe above-described embodi 25 McGraw-Hill Book Co., New York, 1954, pp. 270-274. ment of the invention is merely by way of illustration Smyth: Ai General Account of` the Development of and not limitation inasmuch as variousv and other forms Methodsv of Using,> Atomic Energy for Military Purposes of the invention will be readily apparentto. those skilled Under theAuspicesof. the U. S; Government 1940-1945; in the art without departing from thespiritofthe inven~ p. 108, August 1945, also pp. 85, 102, 103. tion or the scope of the appended claim. 30 Business Week, Sept. 1, 1945, pp. 58-­64. What is claimed is: Katcoiî: MDDCv 293. Date of ManuscriptApril 1956; A system of the class described, comprising a neutronic date declass. Sept. 11, 1946; 14pages. reactor comprising a reactor tank containingA a substan Kelly et al.: PhysicalReview, 73, 1135-9 (1948). tially homogeneous composition of> thermal neutron lis Hackhïs- Chemical Dictionary, revised by Julius Grant, sionable material and liquid neutron moderator, a neutron 35 3rd ed., 1950, Blak-iston Co., Philadelphia, p. 916.