US2726339A

US2726339A - Concrete radiation shielding means

Info

Publication number: US2726339A
Application number: US79522A
Authority: US
Inventors: Lyle B Borst
Original assignee: Individual
Current assignee: Individual
Priority date: 1949-03-03
Filing date: 1949-03-03
Publication date: 1955-12-06
Anticipated expiration: 1972-12-06

Description

nitid tes Patent '0 CONCRETE RADIATION SHIELDING MEANS Lyle B. Borst, Center Moriclles', N. Y., assi'giiorto the United States of Americaa's represented by the United States Atomic Energy Commission No Drawing. Application March 3, 1949, Serial No; 79,522

19 Claims. or. 250-103 The present invention relates in general to an improved shielding medium for providing protection from the emanations of radioactive substances, and more particularly to such a shielding niediu'mcompr'ising a concrete composition in which there is included certain heavy elements.

While radioactive emanations have long been known and used and have served many useful purposes, it has been appreciated that there are considerable physiological and biological haZards associated with the handling and use of radioactive materials'. Furthermore, apparatus and equipment often suffer deleterious effects when exposed to the various types of radiation. In recent years, with the greatly increased utiliia'tion of radiation and radioactive materials, and with the development of sources of radiation of exceedingl high intensities, the problem of protection from radiation effects has become extremely important.

The conventional method of affording protection to personnel and equipment against radiation is the provision, between them and the source of the radiation, of a shield or barrier substantially opaque to the radioactive emanations. In the past, when radioactivity was utilized on a small scale and the intensities of the emanations were of a comparatively low y'alue', radiation shielding was satisfactorily effected by providing a barrier of a suflicient quantity of any relatively dense material, it being well known that a sulllcien't amount of matter of any kind will absorb almost every type of radiation. However, with the advent of the cyclotron, the nuclear reactor, and other devices and processes wherein radiation in enormous quantity and of great energy may be produced, the problem of shielding became a much more complicated matter. n such cases practical considerations make desirable the use of shields of minimum weight, volume, and cost. This is especially true in applications in which neutronie reactors are to be employed for the generation of motive power in airplanes, ships, and the like, wherein the magnitudes of the weight and bulk of the shielding will often be the critical'f'actors in determining the practicability of the power unit. It is therefore no longer satisfactory to indiscriminately employ any type of relatively dense matter for shielding.

The efficacy of a material for shielding purposes is determined primarily by its efficiency per unit thickness in radiation attenuation. In considering thatthe weight and volume of a shield enveloping aradioactiv'e system increase approximately as the cube of the'shielding thickmess, with a consequent like exponential increase of other dependent factors such as size of supporting structures and foundation, total cost, afidthe like, thepa'rtic'ular irn portance of employing shielding of superior efficiency per unit thickness is readily comprehended. Furthermore,

various operations and processes occurring within shielded regions must be effected and controlled by operators and equipment stationed onthe safe side of the shielding; since the difiieulty in performing remote control manip- 2,726,339 7 l atented Dec. 6, 1955 2 ulations generally becomes greater with increase in distance from the operation being performed, thinness of shielding again is a criterion. It has become greatly desirable, therefore, that new shielding media of greater efficiency per unit thickness be provided.

The provision of such media of enhanced efficiency is especially important in the shielding of neutronic reactors, in view of the increasing present magnitudes of reactor size and radiation fluX. In the past, ordinary concrete was commonly used for this purpose, but with reactors of current design, exceedingly thick shields are required if this material is employed. However, success in providing a more efii'ci'ent reactor shielding medium is no simple matter. During the operation of a neutronic reactor, various types of radiation are emitted with various energy ranges, depending upon the composition and configuration of the reactor and upon its previous operation history. These radiations include alpha particles, protons, neutrons, positrons, beta rays, and gamma rays. Not only do each of these species of radiation react with matter in a substantially difierent manner, but radiation of a single species may react differently with the same matter depending on the energy level of the radiation. To constitute an improved reactor shielding medium, then, it is consequently necessary that a material be capable of performing, in a thinner section, the complex function of satisfactorily attenuating simultaneously each of the species in a reactors composite radiation spectrum.

The present invention provides such shielding medium of enhanced efiiciency. It has been found to be eminently adapted to the satisfactory shielding of neutronic reactors. Since neutronic reactors produce radiation of substantially all of the biologically and technically harmful types in quantity and intensity far in excess of that previously obtainable by any other means, it follows that the shielding medium of this invention is likewise effective for shielding any other source of radioactivity.

One object of the present invention is to provide an improved medium for shielding against radioactivity.

Another object is to provide such a medium which has a greater shielding efiiciency per unit thickness than prior shielding media.

A further object is to provide such a medium which is particularly effective against radiation containing neutron and gamma ray components of high energy or high in tensity, such as those emanating from a neutronic reactor. I

Still another object is to provide such a medium which may be compounded in a plastic condition and easily cast into various configurations. I

Other objects will appear hereinafter. v

The improved shielding medium of the present invention comprises essentially a solid, hydrogenous, concrete composition containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, with at least the major portion of the total mass of said contained elements being dispersed in the concrete in substantially the free elemental state.

The composition of the concrete shielding medium is subject to considerable variation within the scope of this invention, but is constituted basically of a 'multiplicity of inclusions of one or more of the elements specified above in the form of small masses randomly dispersed throughout a matrix, preferably compounded from a hydro-setting cement. The preferred inclusions comprise metallic elements of the above group, dispersed entirely in the form of free metal masses, or partially in the form of compounds of such metallic elements or in the form of mineral aggregates containing such coinponnds.

The class of shielding elements previously defined includes chromium, manganese, iron, cobalt, nickel, copper, zinc, columbium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, thallium, and lead. While any of the elements in this class, when included in the concrete composition, is effective in increasing the efficiency of the shielding medium, it has been found that those elements of higher atomic weight which may be added in a relatively'high density are generally more effective. For employing the medium of this invention for shielding neutronie reactors and other systems with similar radiation spectra, it has been observed that the relative effectiveness for the purposes of this invention of identical volumetric proportions of inclusions of each of the shielding elements in the previously defined class may be roughly approximated by the relationship:

where:

Z=atomic number of specific shielding element D=specific gravity of specific shielding element A=atomic weight of specific shielding element n=an exponent (of value between 2 and 3 for an ordinary reactor spectrum).

However, from the combined standpoints of 'shielding effectiveness, cost and over-all practicality, it is preferable to use iron, lead, chromium, manganese, cobalt, nickel, or copper in the shielding medium of this invention.

If, in addition to free elemental inclusions of the shielding elements, a minor portion of these elements is also included in mineral form, such as granular aggregate, the minerals to be chosen for this purpose should preferably have a relatively high content of the shielding element and a relatively high density. Examples of suitable minerals for this purpose are limonite, galena, hematite, alabandite, anglesite, argentite, arsenopyrite, cobaltite, columbite, hubnerite, litharge, magnetite, mimetite, nicollite, stolzite, and wolframite.

In addition to containing shielding elements from the specified group, it is particularly desirable that the medium of this invention have a high content of hydrogen; this may be conveniently incorporated as water of hydration of constituents of the concrete. Therefore, in compounding the matrix of the composition, it is advantageous to employ a hydro-setting cement which retains considerable water of hydration upon hardening. Portland cement has been found to be eminently suited for the purpose. In utilizing Portland cement, in applications where shielding efficiency is of paramount importance it is advantageous that the matrix consist of only cement and water. On the other hand, improved structural strength of the concrete may often be obtained, at some cost of shielding efiiciency, by including in the matrix composition, in addition a portion of porous granular mineral aggregate. While the mineral aggregate utilized in this respect may profitably be sand or gravel, as is conventional in compoundmg ordinary structural concrete, it is in most cases preferable to use a mineral containing one or more of the previously specified shielding elements. Better still, the mineral incorporated in the composition may be such a one which also has a high content of water of hydration. It has been found that limonite, a hydrated iron ore, is ideally suited for the purpose.

While there is no marked criticality as to the proportlons of ingredients employed in compounding the concrete composltion, it is generally very desirable that the relative amounts of materials used afford as high as practicable an atomic concentration both of the selected shielding element or elements and of hydrogen. How ever, in the present composition, an increase in the concentration of one of these will ordinarily result in a decrease in the concentration of the other. For high shielding effectiveness, particularly for simultaneously shielding against neutrons and gamma rays, it is important that neither be excessively increased at the expense of the other. To compound efficient shielding compositions which are universally adaptable to any radiation spectrum, it is beneficial to employ atomic ratios of the shielding elements of the present invention to hydrogen within the range of 0.1:1 to 10:1. It has been found that within this range, a ratio of the order of 1.5:1 gives especially good results, particularly in the shielding of neutronic reactors. High absolute values of atomic concentrations within this range of relative concentrations may ordinarily be secured by omitting from the concrete composition aggregate materials, which are ordinarily of relatively low shielding effectiveness. Considering generally several concrete compositions of the present invention, all containing the same specific shielding element and all having the same atomic ratio of such element to hydrogen, the'denser the mass the more eflicient it is as a radiation shield. This increase in efficiency is attained in the present invention by the provision of concrete shields with atomic ratios within the preferred range, having densities ranging up to several times that of ordinary structural concrete.

It should be understood in connection with adjusting the ratio of the said atomic concentrations that the content of hydrogen in the concrete composition is generally present substantially entirely as water of hydration of the mineral components employed. In the setting or hardening of a concrete prepared in the conventional manner, the content of water of hydration per unit weight of hydratable material usually reaches a single constant value, regardless of the amount of excess water added when mixing the concrete. Therefore, the proportion of water, and the resulting hydrogen content of the concrete may eonsequently be adjusted to the desired value by employing the proper percentages of cement and other hydratable minerals in the mix. Knowing the concentration of atoms of hydrogen in the concrete to be used for a matrix for the shielding element inclusions, and the desired concentration of atoms of the shielding element, the proportions of ingredients for compounding the concrete composition may thus readily be calculated.

To cite some specific examples of the magnitude of the proportions of ingredients in the mix, it has been found that where concrete compositions are compounded of Portland cement, water, and one of the shielding elements in its free elemental form, the ratios of the dry ingredients for universally adaptable shielding compositions should preferably be the following for the specific elements listed:

Iron: 0.12 to 12.4 pounds per pound of Portland cement Lead: 0.46 to 46.0 pounds per pound of Portland cement Chromium: 0.12 to 11.5 pounds per pound of Portland cement Manganese: 0.12 to 12.1 pounds per pound of Portland cement Cobalt: 0.13 to 13.1 pounds per pound of Portland cement Nickel: 0.13 to 13.0 pounds per pound of Portland cement Copper: 0.14 to 14.1 pounds per pound of Portland cement When other shielding elements or combinations thereof are used, the preferable ingredient ratios are of the same order as those set forth above.

The sizes and shapes of the included shielding elements, in either free elemental form or in the form of compounds or minerals, are not critical as long as the sizes of the inclusions are relatively small compared to the physical dimensions of the shield. For example, in case of shields five to eight feet thick, the inclusions may satisfactorily be as large as walnuts, while with shields three to five feet thick the masses should ordinarily be about pea-size or smaller. Generally, as the thickness of the shield increases, the size of the inclusions may proportionally increase, however it is preferred that inall cases the inclusions be as small as practicable. However, while from the standpoint of radiation attenuation the inclusions should be as small as possible so that the composition approaches advantageous homogeneity, it has been found that the use of masses smaller than about shot-size, such as powders, filings, and scrapings, generally detracts from the structural and mechanical properties of the shielding. Any desired shape of the inclusions may be employed, such as lumps, balls, pellets, discs, cubes, and the like. Very good results have been obtained, particularly when employing metallic shielding elements, by the choice of configurations such as small rods, bars, nails, and the like, which serve to mechanically reinforce the concrete.

The standard of excellence of the raw material ingredients of the concrete composition should be atleast as high as that set for better quality structural concretes. The cement used should not have been subjected to detrimental treatment. For example, in the case of Portland cement, exposure to moisture and weather should have been avoided. The mineral aggregates used should preferably be comminuted to a granular consistency and should ordinarily be free of soft, friable, flaky, or laminated particles. Likewise the water used should be clean and free from deleterious foreign substances. As is the customary practice in the art, the ingredients of the concrete with the exception of water should be thoroughly mixed together, and the water subsequently slowly added until the mass has the usual pouring consistency. Then, in accordance with standard practice, it may be poured into hollow molds, or in any other suitable manner cast into the desired configuration.

The shielding medium of this invention affords marked improvement over ordinary structural concrete in shield ing against any and all types of harmful radiation, especially in providing protection against gamma radiation of all energies, and against neutrons, particularly those with energies in excess of about 1 million electron volts. In general, of the various species of radiation, gamma rays and neutrons present the most diflicult shielding problem. This is especially true in the case of the usual spectrum of radiation emanating from a neutronic reactor. With an ordinary reactor spectrum, if sufficient shielding to satisfactorily attenuate the gamma and neutron emanation is provided, the attenuation of other species of radia tion, i. e., alpha particles, beta particles, protons, and positrons, will normally be much more than is necessary. A- pha particles, beta particles, protons, and positrons, being charged particles, are stopped, even when at high energies, in attempting to traverse thicknesses of matter of the order of a few inches. The thickness of material required for charged particle shielding is roughly inversely proportional to the density of the material. Since concrete compositions of this invention are more dense than ordinary concrete, many of them having densities several times that of ordinary concrete, they are consequently the more efiicient in shielding against charged particles.

The attenuation of gamma and neutron radiation is not so simple as that of charged particles. As is known, gamma rays, not being true particles, react with atoms of matter by special reactions such as Compton scattering, photoelectric absorption, and pair production, and hentrons are commonly attenuated by inelastic scattering, elastic scattering, and capture b'y n'uclei. However, the materials which are significantly effective in inelastic scattering and capture are, on the whole, very poor elastic scatterers, and furthermore ordinarily most deleteriously emit secondary gamma radiation upon capturing a neutron. While these shielding materials are often also good gamma attenuators, it is generally the case in neutron shielding that until the transient neutron flux is reduced to insignificance, the individual increments of shielding generate more gammas than they remove from the transitory radiation. For example, it has been shown that if it were attempted to shield an ordinary neutronic reactor with a reasonable thickness of iron or lead, the gamma flux leaving the shield would be greater than the entering gamma flux. It had therefore become the customary practice for shielding systems producing high-intensity neutron or neutron and gamma radiation to employ materials,'such as concrete, which, while being relatively poor in gamma attenuation, inelastic scattering, and neutron capture, do not emit secondary gammas to any especially adverse degree. Now, although the medium of the present invention contains materials which are particularly effective in all of the mentioned mechanisms of neutron and gamma attenuation, and so is subject to much secondary gamma generation, yet by its peculiar inate characteristics it is, able to attenuate these secondary gammas in reasonable shield thicknesses. Therefore, by successfully alleviating the secondary gamma problem, the medium of present invention provides a means which takes full advantage of all of the mentioned neutron and gamma attenuating mechanisms, and is thus able to shield with its exceptional efiiciency against radiation having neutron and gamma components.

In shielding a neutronic reactor with the medium of this invention it happens that the magnitude of secondary gamma radiation generated in the shielding is considerably in excess of that of the primary gamma radiation being emanated from the reactor. It has been found that for applications wherein this troublesome secondary gamma radiation is especially high, the compositions of this invention may be further improved by the additional incorporation therein of minor amounts of boron. Boron has an exceptionally great propensity for absorbing slow neutrons and upon doing so it does not ordinarily adversely emit gamma radiation. Thus the presence of a small quantity of boron at any point in the shield serves to reduce the flux, not only of neutrons, but of gamma rays as well. The boron may be added, as are the specified shielding elements, in the form of free metal inclusions or in combined forms such as compounds or minerals containing boron. Colemanite, a boron-containing mineral, when finely divided, serves as an ideal form of boron for dispersion in the concrete composition. It is preferable, when any boron is included, that it be in small amounts, such as below 5% by weight in the composition; concentrations of about 1% by weight have been found to be entirely sufficient.

Besides being superior in radiation attenuation to ordinary structural concrete, the concrete compositions of the present invention possess substantially all of the inci dental attributes of that material as a shielding medium. The present composition is easily prepared, and may be simply cast into virtually any desired monolithic configuration. It is inexpensive and readily handled in conventional industrial concrete apparatus. Shields fabricated from the present medium are able, in addition to providing radiation protection, to serve as mechanical structures for housing and supporting the shielded systems they envelop. Small shielded containers fabricated from the material can be produced so cheaply as to make them very well suited as discardable shipping containers for radioactive materials.

In view of the variations which may be effected in the choice of particular constituents and the proportions thereof, within the limitations pointed out herein, in compounding the shielding medium of the present invention, further understanding of this invention may be realized by consideration of the following specific examples.

EXAMPLE I 7 Pounds Portland cement; 2660 Steel punchings 5000 (Steel discs A" to /1" thick by to l" diameter;

7 While continuing the mixing, water was 'slowly sprayed into the mass until the concrete was of the consistency conventional for pouring, whence it was poured into prepared molds, rodded, and allowed to harden. The total volume of the hardened concrete was 32 cubic feet, and its density was found to be 256 pounds per cubic foot. The average concentration of iron in the mass was 156 pounds per cubic foot. It was calculated that the concrete retained 532 pounds of combined water, with a resulting concentration of hydrogen atoms of 1.85 pounds per cubic foot. The iron to hydrogen ratio was therefore 84 lbszl lb., or on the basis of atomic ratios, 1511.

For purposes of comparison, a batch of ordinary structural concrete was prepared by mixing 1 part by volume of portland cement, 2 parts by volume of sand 95 SiOz), and 3 parts by volume of rock 95% CaCOs), adding water, pouring, rodding, and permitting to harden. The density of the hardened concrete was found to be 150 pounds per cubic foot.

The samples of each of the two concretes were consecutively substituted for a section of shielding surrounding a neutronic reactor and thus exposed to a neutron flux density of the order of neutrons per square centimeter'per second, with neutron energies ranging from fission energy down to that of thermal equilibrium with the system, and an accompanying gamma flux of about 12,000 roentgens per hour. The substituted shieldsection was centrally located in a fiat planar concrete wall which shielded one face of a cubical reactor, and was small with respect to the shield face; the radiation incident upon the samples was consequently substantially equivalent to that which would be emanated from a infinite-plane radiation source situated adjacent the inner face of the shield. The concrete containing iron had been poured in four equally sized blocks, each 16 inches thick; the blocks were positioned one behind the other, the locus of centers perpendicular to the shield face, with four-inch spaces between the blocks to permit the insertion therebetween of radiation-measuring instrumentalities. For each type concrete, the relaxation lengths, that is, the distances, measured perpendicularly away from the source, in which a particular flux decreases by a factor of 2.718, were determined for neutrons and for gamma rays, and are presented in tabular form below.

Neutor; (relaxatitzin lengizh s cm. average over 6 Gamma sections) l gl eng s Section (cm) erage) Inner 2 3 Outer Concrete containlngiron 5. 59 6.93 7.33 l 7.33 9.2 Ordinary concrete 1 The slight increase in relaxation length is presumed due to a corresponding decrease in the proportion of inelastic scattering to total scattering occurring, conforming to a. decreasing percentage ofhigh-cncrgied neutrons in the neutron flux as it traverses the shield.

1 Average of 10.

From these data, the fractions of such incident neutron and gamma flux which will succeed in traversing 4 feet and 6 feet thicknesses of shielding of each type of concrete were calculated and are tabulated below.

a thickness of 6 feet of theor'dinary concrete. In'other words, to obtain the same shielding protection, a thickness of ordinary concrete of over of that of the composition of this invention is required.

As a measure of the relative mechanical properties of the two types of concretes, the seven-day strength in compression of each was determined; the results are tabulated below.

V P. s. i. Concrete with iron 3850 Ordinary concrete 3320 The composition containing iron is thus considerably superior to ordinary structural concrete in this regard also. I

g EXAMPLE II A shielding sample having a higher iron to hydrogen ratio than that in Example I was prepared by mixing the following dry ingredients in the proportions specified.

Percent by weight Portland cement 13.6 Limonite (Hard chunks, A" to 1") Natural 2Fe2O3-3H2O 27.1

Analysis:

Fe: 50 by weight. Bound H2O: 8-12% Mn: 0.21.0% Steel punchings 59.3

Water was added and the batch was cast in the usual manner. The resulting concrete had a density of 270 pounds per cubic foot, a concentration of iron in the composition of about pounds per cubic foot, and an atomic ratio of iron to hydrogen of 2.2:1. When tested as in Example I, the neutron relaxation length averaged about 6.3 centimeters and the gamma relaxation length was found to be between 9 and 10 centimeters. The seven-day strength in compression of the sample was found to be 3500 p. s. i.

This specific composition was found to have such advantageous characteristics from the standpoint of overall practicality that it has been employed with excellent results as the enveloping shield of one of the largest neutronic reactors in existence.

EXAMPLE III A sample of a shielding composition of the present invention containing boron was prepared by mixing the following dry ingredients:

Percent by weight Portland cement 32.1 Iron shot (SAE 330; approx. ,5 diam; 98%

sorbent, white powder) 3.47

then adding water and pouring into molds. The sample was tested in the same manner as in the preceding examples. The empirical data are tabulated below.

Atomic ratio, ironzhydrogen 1.5:1

Density 224'lbs./cu. ft. Boron content 0.5% by weight Neutron relaxation length, average area-s39 and component ratios in compounding'concrete compositions of the present invention are the mixes detailed below. For simplicity the data are presented in tabular form, the method of preparation and the radiation spectrum to which the relaxation lengths relate being the same as in the preceding examples.

Mix A Dry ingredients:

Portland cement 32% by weight. Copper discs /2 diam. 9 10 thick) 68% by weight. Data:

Density 276 lbs/cu. ft. Atomic ratio, copper:hydrogen 1.5:1. Relaxation lengths (average):

Neutrons 6.1 cm. Gamma rays 8.5 cm.

Mix B Dry ingredients:

Portland cement 31.0% by weight. Nickel balls 31.8% by weight. Chromium pellets 37.2% by weight. Data:

Density 267 lbs/cu. ft. Atomic ratio (nickel-j-chromium):

hydrogen 1.84:1. Relaxation lengths (average):

Neutrons 8.0 cm. Gamma rays 11.5 cm.

EXAMPLE V A. concrete composition containing lead may be prepared by mixing, while dry:

I Percent by weight Portland cement 27.1 Lead shot 72.9

adding water, and casting in the usual manner. The composition will have approximately the following physi- There is strong indication that for the radiation spectrum considered in the other examples, the neutron relaxation length of this composition would be considerably less than 7 cm., and the gamma relaxation length would be less than 9 cm.

For furtherdetails concerning the theory, design, construction and operations of neutronic reactors, reference is made to the following United States Patent which has issued upon an earlier-filed copending application of the common assignee: U. S. 2,708,656, May 17, 1955, E. Fermi et al., Neutronic Reactor, application Serial No. 568,904, filed December 19, 1944.

It is to be understood that the above examples are illustrative only and do not limit the scope of the present invention as it is intended to claim the invention as broadly as possible in view of the prior art.

What is claimed is:

1. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from a hydro-setting cement and water, and containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, said contained elements being collectively present in an amount corresponding to an atomic ratio in the composition of such elements to hydrogen within the range of 0.1:1 to 10:1 and the major portion of the total mass thereof being dispersed randomly in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

2. In a shield for restraining the egress of emanations 75 to hydrogen of substantially 1.5 :1, with at least the major portion of the iron being dispersed randomly in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

3. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from Portland cement and water, and containing iron, in an amount corresponding to an atomic ratio in the composition of iron to hydrogen within the range of 0.1:1 to 10:1, with at least the major portion of the iron being dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses, there being also boron dispersed throughout the concrete in a proportion of not more than 5% of the weight of the composition in the form of a multiplicity of masses.

4. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from a hydro-setting cement and water, and containing iron in an amount corresponding to an atomic ratio in the composition of iron to hydrogen within the range of 0.1:1 to 10:1, with at least the major portion of the iron being dispersed randomly in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

5. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid hydrogenous concrete composition containing iron, the major portion of the said iron being randomly dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses, and a minor portion of the said iron being dispersed in the concrete as a multiplicity of masses of a mineral of which iron is a constituent.

6. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid, hydrogenous, concrete composition constituted by the hardened admixture of portland cement, a multiplicity of masses containing iron in substantially the free elemental state, a multiplicity of masses of limonite, and water, with the ratio by weight of the said free elemental iron to limonite being substantially 2.221.

7. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid, hydrogenous, concrete composition constituted of the hardened admixture of portland cement, a multiplicity of masses containing iron in substantially the free elemental state, a multiplicity of masses of limonite, and water, with the relative proportions by weight of the said cement, free elemental iron and limonite being substantially 3:13 :6, respectively.

8. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from portland cement and water, containing iron, at least the major portion of the said iron being dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses, and also containing boron, in a proportion of not more than 5% by the weight of the composition, dispersed as a multiplicity of masses of colemanite.

9. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from portland cement, water, and copper, with at least the major portion of the said copper being dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

10. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from portland cement, water, and lead, with at least the major portion of the said lead being dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

11. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid, hydrogenons, concrete composition containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, at least the major portion of the total mass of said contained elements being dispersed randomly in substantially the free elemental state.

12. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid, hydrogenous, concrete composition containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, at least the major portion of the total mass of said contained elements being dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses, there being also dispersed throughout the concrete a minor proportion of boron.

13. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor,

improved shield means comprising a mass of solid concrete composition compounded from a hydrosetting cement, and containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, at least the major portion of the total mass of said contained elements being dispersed in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

14. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of solid concrete composition compounded from a hydrosetting cement, and containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, at least the major portion of the total mass of said contained elements being substantially homogeneously distributed in the concrete in substantially the free elemental state.

15. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of solid concrete composition from a hydrosetting cement, and containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, the major portion of the total mass of said contained elements being dispersed randomly in the concrete in substantially the free elemental state in the form of a multiplicity of masses, and a minor portion of the total mass of said contained elements being dispersed in the concrete in mineral form as a multiplicity of masses.

16. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from a hydrosetting cement and water, and containing nickel, with at least the major portion of the nickel being dispersed randomly in the concrete 'in substantially the free elemental state in the form of a multiplicity of masses.

17. In a shield for restraining the egress of emanations of radioactivity from a neutronic nuclear fission reactor, improved shield means comprising a mass of a solid concrete composition compounded from a hydrosetting cement and Water, and containing chromium, with at least the major portion of the chromium being dispersed randomly in the concrete in substantially the free elemental state in the form of a multiplicity of masses.

18. In a shield for restraining the passage of emanations of radioactivity, improved shield means universally adaptable to providing protection against all types of harmful radioactivity comprising a mass of a solid, hydrogenous, concrete composition containing at least one element which in the free elemental state is normally solid and has a specific gravity at least as great as 6.5, at least the major portion of the total mass of said contained elements being dispersed randomly in substantially the free elemental state.

19. The shield means of claim 18 wherein said concrete composition is compounded from Portland cement, and wherein at least one of said contained elements is non.

References Cited in the file of this patent UNITED STATES PATENTS 1,390,327 Bassett Sept. 13, 1921 1,780,107 Barry Oct. 28, 1930 2,304,391 Zimmerman Dec. 8, 1942 2,416,701 Kocher Mar. 4, 1947 FOREIGN PATENTS 4,137 Great Britain of 1876 24,618 Great Britain of 1904 233,011 Switzerland Oct. 2, 1944 114,150 Australia May 2, 1940 861,390 France Oct. 28, 1940 OTHER REFERENCES Goldbergerz. The Shielding of Nuclear Reactors, MDDC-806, U. S. Atomic Energy Commission, March 24, 1947, 9 pages. (Copy in Patent Oflice Library.)

Claims

1. IN A SHIELD FOR RESTRAINING THE EGRESS OF EMANATIONS OF RADIOACTIVITY FROM A NEUTRONIC NUCLEAR FISSION REACTOR, IMPROVED SHIELD MEANS COMPRISING A MASS OF A SOLID CONCRETE COMPOSITION COMPOUNDED FROM A HYDRO-SETTING CEMENT AND WATER, AND CONTAINING AT LEAST ONE ELEMENT WHICH IN THE FREE ELEMENTAL STATE IS NORMALLY SOLID AND HAS A SPECIFIC GRAVITY AT LEAST AS GREAT AS 6.5, SAID CONTAINED ELEMENTS BEING COLLECTIVELY PRESENT IN AN AMOUNT CORRESPONDING TO AN ATOMIC RATIO IN THE COMPOSITION OF SUCH ELEMENTS TO HYDROGEN WITHIN THE RANGE OF 0.1:1 TO 10:1 AND THE MAJOR PORTION OF THE TOTAL MASS THEREOF BEING DISPERSD RANDOMLY IN THE CONCRETE IN SUBSTANTIALLY THE FREE ELEMENTAL STATE IN THE FORM OF A MULTIPLICITY OF MASSES.