US2990357A - Method and apparatus for controlling neutron density - Google Patents

Description

June 27, 1961 E. P. WIGNER ET AL 2,990,357

METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 1946 5 Sheets-Sheet 1 June 27, 1961 WIGNER ET AL 2,990,357

METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 1946 5 Sheets-Sheet 3 FIE-3- June 27, 1961 E. P. WIGNER ET AL 2,990,357

METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 1946 5 Sheets-Sheet 4 June 27, 1961 E. P. WIGNER ETAL METHOD AND APPARATUS FOR CONTROLLING NEUTRON DENSITY Filed May 14, 1946 5 Sheets-Sheet 5 QEFL 1: erze FMjnreI" ale \fyoav y wflw azvzeggl:

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2,990,357 Patented June 27, 1961 sion Filed May 14, 1946, Ser. No. 669,525

1 Claim. (Cl. 204-1932) The present invention relates to a device of primary use for the production of the transuranic element 94 by neutrons released during a self-sustaining nuclear chain reaction through fission of uranium with slow neutrons. Such a device, which is usually called a neutronic'reactor is more fully described in the copending application of Enrico Fermi and Leo Szilard, Serial No. 568,904, filed December 19, 1944, now Patent No. 2,708,- 656. Natural uranium may be used in the reaction and contains the isotopes 92 and 92 in the ratio of ap proximately 139 to 1. Hereinafter in the specification and the claim the term uranium is to be understood as referring to uranium and its chemical compositions of normal isotopic content, unless otherwise indicated by the context.

In a self-sustaining chain reaction of uranium with slow neutrons, 92 is converted by neutron capture to the isotope 92 The latter is converted bybeta decay to 93 and this 93 in turn is converted bybeta decay to the transuranic element 94 By thermal neutron capture, 92 on the other hand, undergoes nuclear fission to release energy appearing as heat, gamma and beta radiation, together with the formation of fission fragments appearing as radioactive isotopes of elements of lower mass numbers, and with the release of secondary neutrons.

The secondary neutrons thus produced by the fissioning of the 92 nuclei have a high average energy, and must be slowed down to thermal energies in order to be in condition to cause slow neutron fission in other 92 nuclei. While some of the secondary neutrons are absorbed by the uranium isotope 92 leading to the production of 94 and by other materials, enough can remain to sustain the chain reaction. u

Under these conditions, the chain reaction will supply not only the neutrons necessary for maintaining the neutronic reaction, but also will supply the neutrons for capture by the isotope 92 leading to the production of 239 As 94 is a transuranic element, it can be separated from the unconverted uranium by chemical methods, and as it is fissionable in a manner similar to the isotope 92 it is valuable for enriching natural uranium for use in other chain reacting systems of smaller overall size. The fission fragments are also valuable as sources of radioactivity.

The ratio of the number of secondary neutrons.pro duced by the fissions to the original number of primary neutrons producing the fissions in a chain reacting system of infinite size using specific materials is called the reproduction factor of the system and is denoted by the symbol K. When K is made sufficiently greater than continue indefinitely if not controlled at a density corresponding to a desired power output. 7

To more fully understand the operation of a uranium neutronic reactor, the following brief explanation is given. During the interchange of neutrons in a system comprising bodies of uranium of any size disposed in a slowing medium or moderator, neutrons may be lost in four ways; by absorption in the uranium metal or compound, by absorption in the slowing down material or moderator, by absorption in impurities present in the system, and by leakage out of the system. The neutrons which are not lost by one of the above methods are available for fission of U which produces more neutrons. In general, several neutrons are produced for each fission and consequently suflicient neutrons are produced to make up for the neutrons lost and those consumed by the fission of U Natural uranium, particularly by reason of its U content, has an especially strong absorbing power for neutrons when they have been slowed down to moderate energies. The absorption in uranium at these energies is termed the uranium resonance absorption or capture. It is caused by the isotope U and does not result in fission but creates a nucleus 92 which decays as previously described. It is not to be confused with absorption or capture of neutrons by impurities, referred to later. Neutron resonance absorption in uranium may take place either onthe surface of the uranium bodies inwhich case the absorption is .known as surface resonance absorption, or it may take place further in the interior of the uranium body, in which case the absorption is known as volume resonance absorption. Volume resonance absorption is due to the fact that some neutrons make collisions inside the uranium body and may thus arrive at resonance energies therein. After successfully reaching thermal energies, about 40 percent of the neutrons are also subject to capture by U without fission, leading to the production of 94 It is possible by proper physical arrangement of the materials in the moderator to control the amount of uranium resonance absorption. By the use of a light element such as graphite, relatively few collisions are required to slow the neutrons to thermal energies, thus decreasing the probability of a neutron being at a resonance energy as it enters a uranium atom. During the moderating process, however, neutrons are diifusing through the slowing medium over random paths and distances so that the uranium is not only exposed to thermal neutrons but also to neutrons of energy varying between the energy of fission and thermal energy. Neutrons at uranium resonance energies will, if they enter uranium atthese energies, be absorbed on the surface of a uranium body whatever its size, giving rise to surface absorption. Any substantial change of overall surface of the same amount of uranium will change surface resonance absorption. Thus, the volume ratio of moderator to uranium will control resonance absorption losses of neutrons in the uranium, and this fact can be utilized to change the K factor of the reactor. The uranium may be placed in the system in the form of spaced uranium masses or bodies of substantial size, either of metal, oxide, carbide, or combinations thereof. The uranium bodies may be in the form of layers, rods or cylinders, cubes or spheres, or approximate shapes, dispersed throughout the graphite, preferably in some geometric pattern. The term geometric is used to mean any pattern or arrangement where in the uranium bodies are distributed in the graphite with at least a roughly uniform spacing or with a roughly uniform size and shape or are systematic in variations of size or shape to produce a volume pattern conforming to a generally symmetrical system. If the pattern is a repeating or rather exactly regular one, the structure may be conveniently described as a lattice. tions are obtained when natural uranium is used as metal spheres, but short cylinders are substantially equivalent.

The K factor of a mixture of fine uranium particles in graphite, assuming both of them to be theoretical-1y pure, has been calculated to be about .785. Actual K factors as high as 1.07 have been obtained using aggregations of natural uranium into bodies of substantial size dispersed in moderators in various geometries, and with as pure materials as is presently possible to obtain.

The thermal neutrons are also subject to capture by the moderator. While carbon has a relatively low capture cross-section for thermal neutrons an appreciable fraction of thermal neutrons (about 10 percent of the neutrons present in'the system under best conditions with graphite) is lost by capture in the moderator during diffusion therethrough. This means that when volume ratios are changed, the absorption in the moderator will also be changed, as the neutrons will have path lengths varying, before entering uranium, in accordance with the volumeratio used, and the longer time the neutrons remain in the graphite, the higher the probability will be that they will be captured by the moderator.

All materials present in a uranium reactor except the pure uranium and the pure moderator are classed as impurities. The materials which make up these impuri ties all absorb neutrons in varying degrees. Since any neutron absorbed by the impurities is lost to the chain reaction, any variation in the impurities present in a neutronic reactor will affect the K factor and thus by placing more or less impurities in ditferent zones of the lattice, the K factor for each zone may be adjusted.

The effect of impurities on the optimum reproduction factor K may be conveniently evaluated to a good approximation, simply by means of certain constants known as danger coefficients which are assigned to the various elements. These danger coeflicients for the impurities are each multiplied by the fraction by Weight of the corresponding impurity, and the total sum of these products gives a value known as the total danger sum. This total danger sum is subtracted from the reproduction factor K as calculated for pure materials and for the specific geometry under consideration. v

The danger coefficients are defined in terms of theratio of the weight of impurity per unit mass of uranium and are based on the cross section for absorption ofthermal neutrons of the various elements. These values may be obtained from published literature on the subject and the danger coefficient computed by the formula:

ii a Pu A wherein Pi represents the cross section for the impurity and Pu the cross section for the uranium, A the atomic weight of the impurity and A the atomic weight for uranium. If the impurities are in the carbon, they are computed ,as their percent of the weight of the uranium of the'syste'm. v 7

Presently known values for danger coefficients for some elements that may be used in a reactor are given in the following table, wherein the elements are assumed to have their natural isotopic constitution unless otherwise indicated, and are conveniently listed according to their chemical symbols:

Elements: Danger coeflicie nt H 10 D 0.1 He ,Be 0.04 B- 2150 C 0.012 N 4.0

' ,Fe 1.5 Cd 760 Optimum condia 1 of the reactor.

reactor.

The neutronic chain reaction referred to can be made self-sustaining in a device known as a neutronic reactor wherein uranium bodies are dispersed in an eflicient neutron slowing medium or moderator, when the reactor is made to be just abovea critical size where the rate of neutron generation inside the reactor is slightly greater than the rate of neutron loss. Under these conditions, a self-sustaining nuclear chain reaction can be obtained within the reactor having any neutron density desired. However, to prevent destruction of the reactor, the heat of the reaction must be controlled, and then removed by an amount providing a stable temperature in the reactor at some predetermined and controlled operating level. As the greater the number of fissions, the greater the number of neutrons are present to produce 92 converting to 94 is accelerated by operating the reactor at high neutron density levels. A stable temperature in an uncooled neutronic reactor composed entirely of moderator and fissionable material such as, for example, graphite and uranium metal, can only be attained at a relatively low power output as the heat generated can be dissipated only by conduction out Higher' power outputs with greater production of 94 require additional heat removal such as by circulation of a fluid.

However, proper heat removal is complicated by the fact that in a neutronic reactor where the uranium bodies are in a lattice of uniform size and spacing, and where the impurities are also uniformly spaced, nuclear fission and heat generation due to the chain reaction are both greatest at the center of the reactor and least at its edges, both activities following an approximate cosine curve distribution from the center to the edge of the reactor, as will be pointed out later. Such a centrally peaked activity limits the total power at which the reactor can operate, to a power where the more central uranium bodies are operating at a maximum permissible temperature. In other words, the temperature of the uranium at the center of the reactor is a controlling factor. If aluminum tubes and jackets are used, the maximum permissible temperature of said aluminum jacket is 70 C., since aluminum corrodes quickly at temperatures above this. The total power output, under these circumstances, can therefore be only the average power developed in the reactor when the uranium at the center of the reactor has reached the maximum permissible temperature. If, however, the reactor activity curve can be flattened across the reactor, then the central peak power can still remain at the maximum permissible value and the total power output of the reactor can be increased. One method of flattening this activity curve is described in the copending application of Gale J. Young, Serial No. 552,730, filed September 5, .1944, now Patent No. 2,774,730.

It is the principal object of our invention to so design a cooled neutronic reactor that the maximum heat generation due to nuclear fission is spread out over a large volume of the reactor so that operating power can be increased without excessive corrosion.

Flattening of the reactor activity curve across the reactor is also advantageous in that the local heat generation is directly proportional to the local absorption of neutrons by U In other words 94 will eventually be formed in .the uranium bodies in accordance with the neutron density to which the bodies are exposed. Flattening the reactor activity curve across the reactor will permit a greater number of uranium bodies to be subjected to high neutron densities.

As impurity content can control the'K factor of a reactor structure, lattices having different impurity content can provide different K factors in a neutronic Ordinarily, neutronic reactors have lattices in which uranium bodies of uniform size and shape and purity are placed in the moderator with uniform spacing throughout, the bodies are generally of substantially uniform size, and uniform volumes of coolant are used.

This results, in the absence of compensating factor, in a reactor having a peaked central neutron density, and in consequence, a peaked central heat production.

However, by using lattices having different amounts of impurities in different concentric zones of the reactor, and particularly by positioning impurities to give the lowest K factor in the center zone of the reactor, the reactor activity curve can be appreciably flattened across the reactor, resulting for the same total power, in lowering the relative central peak neutron density and in raising the neutron density in outer zones. In consequence, the activity is spread more uniformly throughout the reactor. Cooling becomes more efficient and when the central uranium bodies are raised to their maximum permissible operating temperature, the total power output of the reactor with a flattened activity curve across the reactor is increased for the same central uranium body temperature. The amount of uranium exposed to high neutron densities is increased, and the yield of 94 is thereby increased. By proper 'flattening the overall power of a reactor may be increased as much as 35 percent.

When a neutronic reactor is liquid cooled as by water, for example, jackets are used on the uranium bodies to prevent corrosion and confine fission products and then these jacketed bodies are placed inside coolant pipes passing through the reactor. The water is then passed through the coolant pipes, extracting heat from the uranium bodies as the water passes over the jackets. As heretofore explained, the jackets, the coolant pipes, and the coolant itself are all designated impurities and when considered as part of the lattice structure, will reduce K by the amount of absorption caused by these added impurities.

When the size of the uranium bodies, the volume ratio of the uranium and moderator, and the dimensions of the cooling system are the same throughout the reactor, the K factor of all portions of the reactor will be the same and the normally peaked central activity will result.

As increasing the amount of impurities in the central part of the reactor will reduce the K factor there, and thus provide the desired activity flattening across the reactor, increasing the amount of coolant, the thickness of coolant pipe and jacket, or both in the central part of the reactor will increase the impurities there, reduce the K factor there, accomplish the desired activity flattening, and, in addition, will provide additional resistance to corrosion of these parts.

It is, therefore, another object of our invention to flatten the neutron activity curve across a liquid cooled reactor, and at the same time provide increased cooling, increased resistance to corrosion by the coolant, or both, at central portions of the reactor.

The protective jackets on the uranium bodies perform two functions. Uranium is highly active chemically and would disintegrate rapidly if exposed directly to flowing water. The fission process also can take place on the surface of the uranium bodies and highly radioactive fission fragments could thereby be injected into the coolant stream. The jackets protect the uranium from corrosion and also prevent fission fragments from entering the coolant stream. Since all pipes, jackets and coolant contained in the reactor are treated as neutron absorbing impurities, the weight of such materials is limited, in order that reproduction factor may remain well above unity. The amount of heat that can be removed, therefore, depends (with pipes and jackets unchanged) upon the amount of coolant in the reactor and the rate at which it can be circulated. Certain reactors can operate at about 500,000 kw. continuously with coolant annulus thickness of about 2 mm. for water or about 4 mm. for diphenyl. After the point is reached where the rate of circulation can no longer be etnciently increased by annuli of fixed thickness, further increase in power leads to too high a temperature of the coolant which, has been explained, causes excessive corrosion of the aluminum pipes and jackets.

As heretofore explained, proper heat removal is complicated by the fact that in a neutronic reactor where the uranium bodies are in a lattice of uniform size and spacing, with uniform jackets and other impurities, nuclear fission and heat generation due to the chain reaction are both greatest at the center of the reactor and least at its edges, both activities following an approximate cosine curve distribution across the reactor. However, by increasing the jacket thickness of the uranium slugs in the center of the reactor, several advantages are obtained. First, the thicker jacket affords better resistance to corrosion at the hottest part of the reactor where such corrosion resistance is most needed. Second, the impurities are increased in the center of the reactor thereby flattening the activity curve. Third, it is easier to secure watertight welds on a thick aluminum jacket than it is on a thin jacket.

Other objects and advantages of our invention may be more clearly understood by reference to the following description and the attached drawings which illustrate, as an example, one form of reactor in which the invention may be used. This example of a uranium-graphite, water-cooled reactor is not to be taken as limiting, as the invention, within the scope of the appended claim, can be used in any type of neutronic reactor wherein uranium bodies or other fissionable bodies are disposed in a moderating medium.

In the drawings:

FIG. 1 is a diagrammatic vertical cross sectional view, partly in elevation, of the major portions of a liquid cooled neutronic reactor;

FIG. 2 is a diagrammatic vertical cross-sectional View, partly in elevation, taken on the line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary perspective view, partially in cross-section of a portion of the lattice showing one of the coolant tubes containing the jacketed uranium bodies or slugs;

FIG. 4 is an enlarged fragmentary cross-sectional view of a portion of the lattice showing the geometrical arrangement of the uranium bodies in the graphite moderator;

FIGS. 5, 6 and 7 are enlarged cross-sectional views of different jacketed uranium bodies in coolant tubes showing three size slugs and jackets; and

FIG. 8 is a graph showing the relative neutron densi ties across the diameter of the reactor.

Referring to the drawings the invention will be described as embodied in a water cooled, graphite moderated uranium reactor in which the uranium is in the form of aluminum jacketed rod segments positioned in horizontal coolant carrying'channels in the graphite moderator.

Such a reactor embodying liquid cooling for high power outputs, up to 100,000 kilowatts, for example, is shown in FIGS. 1 and 2. Specific features of this reactor are more fully described, and claimed in the application of Edward C. Creutz et al., Serial No. 574,153, filed January 23, 1945 now Patent No. 2,910,418.

The reactor proper 50 comprises a cylindrical structure built of graphite blocks. The reactor is surrounded with a graphite reflector 51 forming an extension of the moderator and is enclosed by a fluid tight steel casing 52, supported on I beams 54 within a concrete tank 55, erected on foundation 53. Tank 55 is preferably filled with water 56 to act as a shield for neutrons and gamma radiation.

The encased reactor is surrounded on all sides except one by the water 56, and the side not surrounded, which is the charging face 57 of the reactor is provided with an inner shield tank 58 filled, for example, with lead shot and water.

Coolant tubes 59 extend through an outer shield tank 60, through the inner shield tank 58, and through the graphite moderator block 50 to an outlet face 62" of casing 52 to empty into water 56 in tank 55. Only a few tubes 59 are shown in FIG. 1 for sake of clarity of illustration. A backing wall 64 is placed in tank 55 spaced from outlet face 62. Coolant tubes 59 are preferably of aluminum.

On the outside of tank 55 where the coolant tubes enter the reactor, the ends of coolant tubes 59 are removably capped, and are supplied with coolant under pressure from conveniently positioned manifolds. Thus water can be passed through tubes 59 to be discharged at outlet face 62 into tank 55. Water, after having passed through the reactor is removed through an outlet pipe 65.

The coolant tubes 59 may be charged with aluminum jacketed uranium slugs 72 (FIGS. 3-5), by uncapping the tube to be loaded and pushing slugs into the tubes in end to end relationship. The reactor can then be loaded with sufircient uranium to make the reactor operative to produce high neutron densities, the heat being dissipated by the coolant circulation. This coolant may be water, for example, from a source such as a river, passed once through the reactor, and then discarded, or, the water may be cooled and recirculated in a closed system. If diphenyl is used a closed system is required. Loading and unloading and control of the reactor are described in the Creutz et a1. patent heretofore mentioned.

Referring to FIG. 3, which shows the relation of a moderator coolant tube 59 and a uranium rod, it will be seen that slugs 72 forming the rod are positioned in the coolant tube 59 on longitudinal ribs'73 providing a uniform annulus of coolant around the slugs.

In this case, the jackets, the coolant itself and the pipes introduce parasitic losses which, for one specific example of a liquid cooled uranium-graphite reactor have been evaluated for a water cooled reactor capable of continuous operation at about 100.000 kilowatts.

For such a reactor employing uranium rods disposed in graphite in accordance with near optimum geometry conditions and utilizing uranium metal and graphite of presently obtainable purity, the value of K would be about 1.07. The value of K for the structure is determined as follows:

K for uranium rods in graphite (including residual impurities) -1.07 K reduction due to aluminum jackets and pipes 0.013 K reduction due to coolant 0.023

Total K reduction for cooling system-" 0.036 0.036

The value of K for the structure 1.034

The principal dimensions of the reactor are as follows, using the K constant set forth above:

Axial length of active cylinder of reactor 7 meters. Radius of active cylinder of reactor 4.94 meters. Total weight of uranium metal in rods 200 metric tons.

As the total K-l available for uranium-graphite reactors is only about 0.1 it is obvious that the amount of coolant cannot be greatly increased over the values given above, as the K constant would be so reduced as to preclude the construction of a reactor of practical size.

It thus follows, since the coolant and the circulating elements required to be placed inside the reactor are considered as parasitic impurities, that by increasing the weight of impurities in the center of the reactor over the normal weight of impurities necessary for operation, the neutron density curve may be flattened.

A portion of the moderator showing one type of lattice in which the uranium is positioned in triangular spacing is shown in FIG. 4. This figure shows all tubes as standard; all having the same jacket thickness and same coolant annulus. I

In FIG. 5, is disclosed a coolant tube 59 in which is positioned a standard slug 72. For the example given the uranium metal rod 20 is 1.7 centimeters in diameter, the aluminum jacket 21 is 0.5 millimeter inthickness.

In FIGS. 6 and 7 are disclosed non-standard slugs which positioned in a standard coolant tube 59 will introduce more impurities at that position than would a standard slug 72. Thus, FIG. 6 discloses a slug 172 which contains a uranium rod 120 having a smaller diameter than the standard uranium rod 20. The outer diameter of the aluminum jacket 121 is standard and therefore the jacket is thicker than standard. This thicker jacket'means an increase in impurities which absorb neutrons and the reduction in uranium content means that less neutrons are given oif from fission and, therefore, neutron density is reduced for these two'reasons. At the same time the increased jacket thickness increases the resistance to corrosion.

In FIG. 7, a second type of nonstandard slug 272 is shown. This slug contains a uranium rod 220 that is smaller in diameter than the standard. The aluminum jacket 22.]. is of standard thickness. Therefore, the coolant annulus is thicker than the 2.2 millimeters which has been designated standard. This slug 272 will also reduce the neutron density when positioned in a coolant tube 59. As in the slug 172 the quantity of uranium is less than standard and therefore the neutrons produced by fission are reduced. Furthermore, because the coolant annulus is increased in thickness the quantity of impurities is increased since water is an impurity. An added benefit of this type of slug is the fact that the increased coolant annulus provides better cooling than with the standard slug 72. Therefore, if this slug 272 is used near the center of the reactor increased cooling is provided at the hottest portion of the reactor. If desired, compromises of slugs 172 and 272 may be used having a thicker jacket than standard but not as thick as 172 thus providing a thicker water annulus than standard but not as thick as 272. Note that all these non-standard slugs may be used in a standard coolant tube 59, so that the invention may be practiced on a reactor already built. It is not necessary that the reactor be originally designed for this type of neutron density curve flattening. For this reason ribs 222 are welded to jacket 221 so that the slug is centered in the tube 59. However, these ribs 222 may be eliminated, if non-standard tubes with larger ribs 73 are used.

If the geometry of the disposition of the uranium in the moderator of the active portion of the reactor is uniform throughout in the loading above described and standard s ugs are used throughout, it follows that the K factor throughout the entire reactor is also uniform leading to a reactor activity across the reactor having a distribution curve generally resembling a cosine curve as indicated by curve A in FIG. 8, which is a diagram in which the ratios of local neutron density to the average neutron density are plotted for different radial distances from the center of the reactor.

Curve A is a centrally peaked curve, indicating that when the maximum water temperature is set, for instance at C., the total power at which the reactor can operate is limited by the central reactor activity required to bring the central slugs up to the temperature that will heat the cooling water to this point. Proper cooling, and that this temperature is attained at a total power output of 500 kw. are assumed. It can also be assumed that the uniform geometry used provides a K factor of about 1.06 throughout the reactor to give a self-sustaining chain.

reaction in the reactor at the size described, Unless the temperature of the central uranium is permitted to rise, assuming maximum cooling no more power can be obtained from the reactor under these conditions.

However, the lattice in all parts of the reactor, and hence the K factor, does not need to be uniform, if the average K factor of the reactor is left sufficiently high that the critical size does not become too large or impractical. It has been found that if lattices having K factors that differ in the center of the reactor and in shells or zones surrounding the center are utilized, when the reactor is assembled with the lattice having the lowest K factor at the center, a reactor can be built wherein the reactor activity is no longer peaked, but is flattened across the reactor as shown in curves B, and C, FIG. 8. With this type of activity distribution, for the same total power output, neutron densities around the central slugs and consequently, their temperatures, will fall, and the temperatures of the slugs in the intermediate zones will more nearly approach those in the slugs in the center of the reactor. Under these conditions, the total power output of the reactor can then be raised until the more widespread central neutron density is the same as when the activity follows curve A, and the central slugs are at the maximum permissible temperature again. If it is assumed that cooling is uniform throughout the reactor, more slugs and coolant than before will then be at or near maximum temperature and the total power output can be greatly increased.

Thus, a density curve resembling either B or C of FIG. 8 may be attained by loading the central coolant channels 59 with non-standard slugs such as 172 or 272 heretofore described and loading the outer channels with standard slugs.

If slugs with thick aluminum jackets similar to slug 172 are used in the central channels, the K factor of the lattice of that portion will be reduced. Furthermore, as has been explained the corrosion resistance of the slug is increased by the thicker jacket 172 which means that these slugs may be safely operated at a higher temperature than standard slugs.

If slugs similar to 221 are used in the central channels of the reactor, the K factor of that portion will be reduced because of the decrease in the quantity of uranium as compared to the standard slug and because more impurity is introduced in the form of the larger annulus of coolant. It follows that the increased amount of coolant provides better cooling for these channels. Therefore since the limiting temperature is the temperature of the coolant in the central channels, the power of the reactor may be raised over what it would be with the standard coolant annulus.

If a 1 percent excess K factor is available in the reactor with all slugs having standard 0.5 millimeter jackets, then by changing the central slugs from a di- 10 ameter of 1.7 cm. to 1.47 and increasing the jacket thickness to 3.3 mm., it is possible to increase the overall power about 35 percent without increasing the maximum. slug temperature. The thick jacketed slugs should comprise about 26 percent of the total number of slugs in the reactor.

If the same amount of metal is loaded in 1.55 cm. radius slugs with a 2.5 mm. jacket, then the power may be increased about 33 percent without increasing the maximum slug temperature.

The limiting factor as to the number of channels in which the non-standard slugs may be used is the excess K factor available in the reactor. The K factor must be at least 1 or the reactor would not be self-sustaining chain reacting. Methods of calculating the average K factor are set forth in the Creutz et al. and Fermi et al. applications heretofore mentioned.

A method and means of improving the operation of a neutronic reactor have been described. The invention may be incorporated in the design of new reactors, but it is useful in any reactor which is capable of being loaded and unloaded. While a horizontally loaded, grapihte-uranium reactor has been described the invention is usable on other types of reactors.

Although the theory of the nuclear chain fission mechanism in uranium set forth herein is based on the best presently known experimental evidence, it is not desired to be bound thereby, as additional experimental data later discovered may modify the theory disclosed.

The above description is meant to be illustrative only and the scope of the invention is limited only by the appended claim.

What is claimed is:

A neutronic reactor comprising a moderator having uniformly sized channels parallel to one another and spaced a uniform distance apart, and composite uniformly dimensioned cylindrical bodies comprised of a core of thermal neutron fissionable material and a jacket of aluminum and positioned in each of said channels so as to be spaced from the walls of said channels, the cores and the jackets of the bodies in the central channels of said reactor being respectively thinner and thicker than the cores and jackets of the bodies in the remainder of the reactor.

References Cited in the file of this patent UNITED STATES PATENTS

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