CA1102102A - Non-combustible x-ray and nuclear radiation shields with high hydrogen content - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to compositions, methods of production and uses of non-combustible nuclear radiation shields, with particular emphasis on those containing a high concentration of hydrogen atoms, especially effective for moderating neutron energy by elastic scatter, dispersed as a discontinuous phase in a continuous phase of a fire resistant matrix.

Description

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Problems of nuclear radiation ~hielding ~or both per~onnel and certain instruments arise in a wide variety of acti~
ities such as power reactor~, industrial proce3sing and non-destructive testing. The problem also i9 found in such field~ as medicine, space technolugy, oceanography and in fundamental stud-ies involving~basic experimental research.
Shielding materials are required to mee~ increasinqly more diverse performance criteria as the applications of nuclear radiatlon expand to new areas and make signlficant improvemelts i old areas. These applications, as a group, include all types of radiations but, individually, each has it~ own special shielding requirement.
A large number of radiation shielding problems involv~
neutrons and, in thls area especially, the hydrogen a~om as a com~
onent of the shield has a particular value, because the hydrogen atom is the most effective of all atoms in moderating neutrons by elastic scatter. This proces~ occurs without the production of gamma photons, which are produced by neutron energy lost by inela~
tic scatter when a neutron strikes a massiv~ atom with a high atomic number. For this reason, hydrogerlous materials, e~pecially ~' ~oz~a~
"''`' ' those with a high hydrogen atom concent~ tion have been widely em-ployed to attenuate neutrons from many sources, especially those . produced in nuclear xeactor~, particle accelerators, and certain radioisotopic materials.
With only a few exceptions, materials containing a high concentration of hydrogen atoms are combustible in air. Non-i, combustible hydrogenou3 materials generally contain a much lower concentration of hydrogen atoms, and consequently do not provide the desired efficiency of neutron attenuation. Water iY the ob-vious exception, but this substance ha~ the great disadvantage of not being self supporting under usual opera~ting conditions of temp-erature and pressure, and complications arise from container prob-~ S
~lems when ~h~ ~ re used.

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In the prior art, nuclear shields with high hydroqen content have consisted of blocks of paraffin, polyethylene and other synthetic organic plastics, natural and pressed wood, bonded ceIlulosic products, epoxy cement mixtures and other combustible carbonaceous materials. All of these substances are highly flam-able to varying de~rees, and their use pre.sents a recoqnized fire and radiation hazard, which haretofore un~ortunately has been ac-cepted reluctantly by the nuclear industry in many situàtions. En masse, these materials form a continuous phase that supports com-bustion and adds fuel to a fire generated either internally or ex-ternally to the shield itself that,in turn, c~use~ the fire to spread and intensify.

Another definite disadvantage of this type of ~hield is that it has no supplementary supporting structure in the form of a non-cbmbu~tible continuous phase. The en~ire structure of these shields is lo~t as soon as the temperature reaches the soft-ening or decomposition point o~ the combustible, hydro~enous material, which intensifies seriously the radiation hazard.

Some o~ these or~anic materials with high hydrogen content have been forti~ied with other elements useful for special radiation problems involving a mLxed flux but, even in theso , ' ~7L

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admixtures, the carbonaceous inqredient remains as the continuous phase that cements the t~al mass together. It is this continuous phase of combustible material that oharacterizes the entire shield as combustible and renders the composite material unsafe for use S as a shield.
A detailed search of the literature has failed to dis-close a satisfactory solution for the fire and radiation hazards associated with the use of suitable hydrogenous materials for nu~lear radiation shield~. Many references are made to composi-tions useful for certain purpo~es in various industries, such as the building industry. Here, various materials have been added to cements to achieve many desired characteristics such as low densit improved workabilit~ of mortar, low cost and better acoustical properties, but the description~ are vague as well as incidental and peripheral to the problems involved in the high hydrogen-fire risk prob~lem. No disclosures have been focused directly on the combustible characteristic~ of nuclear radiation shielding materia:
containing the desired high concentration o hyd~ ~en atoms, or on the parameters -elated to minimizing the fire risks involved in their use. The literature is completely void concerning composi-tions and methods for optimization of the desired properties in-volving combustible hydrogenous materials.
U. S. Patent 3,207,705 (Hall) discloses radiation shielding compositions conYistin~ o an inorganic matrix to which ` ~ ~lOZl~Z

i8 added other components such as comminuted lead, charcoal and asbesto9. No reference is made to organic compounds containing hydrogen atoms, nor to any combustible~compound containing hy-drogen atoms. Hal~ made no reference whatsoever to any objective related to a non-combustible shield or to one containing a high concentration of hydrogen atoms.
i U. S. Patent 3,021,291 (Thiessen) produces a light-! weight, cellular-concrete block resistant to water penetrationfor the construction industry. This patent discloses the addi-tion of expandible polymeric materials, which expand during a subsequent heat treatment, into the concrete mixture for the purp-ose of reducing porosity and hence moisture absorption. No refer-ence is made to its nuclear radiation characteristics nor to its fire resistant characteristic. The Thi~ssen composition is one ha~ in lS the homogenous material in a continuous phase and not in a discon-tinUous phase~ Justificati~ for this releYant conclusion is in-~- cluded later in the specification~
Combu~tible~shielding material~ have been involved in several fires, and prove beyond doubt that the hazards associated with the uqe of combustible ~hields is a realistic possibility.
The widely publicized fire at the Senna Power Reactor on the ~elgi m-France border (1967), the 45 million dollar fire at the Rocky Flat ¦, - ~ 2 . .,-, . , .

Colorado plutonium processing facility operated for the Atomic Energy Commission by Dow Chemical Co. (1969), and at the Browns Ferry Reactor plant (1975~ are among the more dramatic examples supporting the conclusion that new solutions to the shielding prob-S lems must be found and implemented. All these above-mentioned fires invol~ed combustible, organic materi~l present in continuous pha~e.
In all cases, th3 fire risk problems were recognized an d the most sophisticated nuclear xadiation shield design known in the art was used, yet in spite of all this knowledye serious fires did occur. The inevitable conclusion reached from these facts is that impr~vements disclosed in the instant application were hereto-fore unknown to those skilled in the art, and that the prior art does not anticipate the results disclosed in the present applica-tion.

llQ~102 ' . ., ~, It is an object of the invention to provide improved radiaticn shielding.
It is another object of the invention to provide im proved techniques rendering nuclear shielding substantially free ! from th~ hazards of fire.
Another object is to improve shielding techniques to provide for casting at site.
A safe, non-combustible radiation shield with high hydrogen content is provided in accordance with the present inven-tion. This achievement resul~s ~rom a new concept wherein hydroger;
atoms in a suitable form are introduced as the discontinuous phase into a non-combustibIe aement that forms the continuous phase of the finished shielding product. Thus, even though the hydrogeneoue material may be combustible en masse, the present invention provid~
that the hydrogeneous material is dispersed in relatively ~mall particle~, each of which is ~rrounded by a non-combustible material These fire barriers between the individual combustible particles prevent propagation of fire throughout the entire mass. ~hase barriers al~o tend to prevent oxygen from reaching the discontinuot phase, as would be necessary to support combu~tion.

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, ' This invention also has a great advantage of conven-ience of application compared with shielding materials produced with plastic binders. In one or more of its more useful forms, the product is commercially available as a dry powder that re-quires only the addition o~ water to produce, without heat or pressure, a formable mortar that can be poured or cast to any de-; sired shape at the construction site. It also can be troweled, plastered, or sprayed by techniques familiar to construction work-ers. The set material also may be drilled, tapped or shaped, if de~ired. Furthermore, use of this ~aterial eliminates the-machin-ing to close tolerances of precast material and the use of stag-gered joints to prevent radiation streaming, which are often troublesome problems encountered in the prior shielding art.
An additional advantage is the considerable flexibilit in formulation of the ahielding product to meet speci~ic shielding requirements, without the U9~ o expensive milling or mixing proc-essing Still another advantage of this invention is that the continuous phase cement with inorganic bonding usually has a higher thermal stability than does the carbonaceous continuous phase of ~he conventional high hydrogen content shield. Should a dangerously high temperature be reached inadvertently, the contin-uous phase hydrogeneous shield based on the prior art will soften, melt or be decomposed with the complete loss of structure, wherea5 .(~

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the higher thermal resistance of the ~ organic bonds of the contin l UOU9 phase into which the hydrogeneo 9~ is dispersed~used in this ; ¦ invention will retain the structure~and form of the shield, to . ~ p~ ~vi de cont i ed rad at on pro tec t ion .

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~ s noted above, the invention involves a certain type o~ discontinuous phase dispersed in a continuous phase matrix.
These phases ar~ discussed below.
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Continuous ?hase Cement S Any non-combustible natural or synthetic cement can be used. For example, these include the standard cements widely used in the building industry: Portland cement of various types, wall plaster, plaster of Paris, silica gel and clay. These cement may be modiied to meet special requirements and may be classified to xeflect their chemical composition, as for example: lime mortars, calcined ~me, calcium alumina silicates, magnesium oxy-chloride, phospate cement~, all o~ which may be modi~ied by the addition o various fillers such as ~alcium, magnesium, aluminum and silicon compounds to obtain properties such as desired setting lS time, strength, resistance to corrosion and high temperature en-vironments~ For example, asbestos fibers can be used to improve ~trength, reduce density and impart desired acoustical properties.
Cements that set by chemical reaction with water such as Portland cement and wall plaster have the added advantage o~
2~ containing hydrogen atoms in the bonding matrix i-tself. These , Ij ~ ~ Z102 .. ' '' . ' ' . ' ., ' .

cements are also i nexpensive, readily available, produce struc-tures of good stre ngth~and have a technology of application that i9 convenient and well established.
In add ition to those better known cements may be added lead powder, which sets by the addition of controlled quanti~
ties of water, as next described. Water is thoroughly mixed with finely divided lea d powder in sufficient quantity to form a sti~f, easily workable mo rtar, which may be applied by any one of the many standard mort ar techniques, ~fter expressing any excess water that may be present in the mortar, the entire mass sets to a self supporting structure in the course of time without the application of pressure or heat. If the mortar is cast in a mold of predetermi ned shape and size, use of perforated molds - assists in the re moval of excess water, Vibrating or rodding of the mortar in the mold also i6 beneficial in bleeding excess water~
for its removal, t o produce a dense set mass with a density of about 7.0 g cm 3.
Addition o~ new dry lead powder to the excess water risin~ durin g vibration to the top of,the partially filled mold is a convenie nt technique ~or handling this excess water, A
good uniform bond throughout the entire mass is produced,provided the additional dry lead powder is thoroughly mixed with the excess water, and the resulting mortar well rodded into the lower layer.
This cement is the subject of U.S. Patent No. 3,~27,9~2, )~

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l~Z~02 -The use of the lead powder-water mortar in this inven-tion has a particular advantage when used in connection with a mixed ~lux consisting of both neutrons and photons. The lead with its high density is especially ef~ective for photon (gamma or X-.ray) attenuation, and also contributes to a lesser degree . through the hydrogen atoms, from its retained water in the set : mortar, to the moderation Qf neutrons by elastic scatter. It also has special value when used with modified materials that induce ' gamma radiation within the shield itself.

Hvdroqenous Discontinuous Phase ,, Many d.ifferent types o~ hydrogenous materials can be used in the discontinuous phase and these include both inorganic a~d organ;c solid materia 19 .
Metal hydrides, hydroxides,inorganic ammonium salts and hydrates are examples of ~he inorganic subs~ances. Some of the metal hydrides are especially useful because of a combination of both high hydrogen content and good thermal stabllity such as is found, for example, in titanium hydride.
The suitable organic materials are more numerous, and examples include hydrocarbons (aliphatic and aromatic), hydrocarbo;
plastics (such as polyethylene, polypropylene and polystyrene), natural and synthetic rubber, other plastics or resins containing atoms in addition to carbon and hydrogen (such as acrylic, amino, ~ 2 phenolic, polyamide, polyester, polyurathanes and vinyl resins), carbohydrates (such as sucrose), organic ammonium salts, and a large variety of other classes of organic compounds and their de-rivatives. Actually there i9 very little limitation relative to the type of solid organic material that may be used but, in genera the high molecular weight hydrocarbons are preferred because of ~ their higher hydrogen content. The high carbon content of these I hydrocarbons also gives good neutron moderation. Hydrogenousmaterials particularly suited for this invention are those contain ing the maximum density of hydrogen atoms. Desirably, they should also possess relative characteristics a3 follows: high ~hermal stability, high melting point, good physical s~ructural propertieS
and its adaptability for division into controlled particle sizes and shapes by various standard techniques such as extrusion, cuttLng and spraying. The broad class o~ hydrogenous materials that have these preferred characteristics fall within the general claqsificati~ s usually known as plastics or resins. In this con-nection, it is in.teresti-ng to noke that polyethylene contains more hydrogen atoms~per unit volume than water, and thus it is a better neutron attenuator than water. Although i~ i9 somewhat limited in its operating temperature range, and suffers radia~ion damage with long exposure under heavy radiation intensities, it can be very useful for applications involving moderate temperatur~ and radia-tion intensities.

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Although signiicant differences may be expected within the general class of hydrocarbons, appreciable differences are noted within limited classification such as the grpup o~ polyethylene.
! ~or example, "linear" or "high density" polyethylene has several l advantage3 over conventional "low density" types for use in neutron shielding, including greater stiffness and structural stren~th, I
wider temperature range stability, and a greater number of hydrogen atoms per unit volume. The relatively low cost of polyethylene often is an important asset.
Plastics generally are more sensitive to radiation ef-fects than most inorganic materials. Like most other organic sub-stances, plastics consist primarily of atoms of carbon and nydroger bound together by electron-exchan~e forces, or che~ical bonds.
- These bonds, often called covalent bonds, are relatively easy to break, compared with inorganic bonds, when the molecules are sub-jected to nuclear energy. In some plastics, the noticeable effect of radiaticn is the, breaking of these bonds, which is known as "cessation~' of the main polymer chains. The result is a decrease in the average molecular weight, dcgradation of all physlcal and electrical properties and increase in solubility in various sol-vents, Xn other plastics, the first e~fect of radiation i~ the formation of new chemical bonds tha~ produce a cross-linking o~ the pol~mer cha s, The general re: lt is an increa~e in molecular ~ l~lZlUZ
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weight, hardness and ten9ile strength and,in the case o~ a thermo-plastic, an increase in the so-called melting point until the material becomes inusible or thermoset. In some polymers, these two e~fects occur simultaneously, but generally at diferent rates so that one effect predominates and determines the composite e-fect. In those materi.als in which cross-linking occurs irst, continued irradiation will eventually result in cessation of the ; main polymer chains, and final disintegration.
Compared to inorganic materials, a relatively small dose of radiation is re~uired to produce detectable damage in a plastic. This great contrast in radiation stability is one of the outstanding advantages o~ oux invention. When a radiation shield with a plastic in continuous phase ~ails under a radiation ~lux, the structure is lost and ceases to be a protéctive shield, but when the plastic in a discontinuous phase fails, the m4re stable inorganic bonds of the continuous phase remain intact and - support the plastic composition long after the plastic has lost th-ability to support itsel~. While the stability of the plastic i9 much less crucial when used in the discontinuous phase, in certain applicatibns, the highest stability attainable in this phase is desirableO
Diference in molecular structures rendex materials unstable, to various degrees, under radiation exposures, Within the broad amily of plastics a wide choice of (plastic) materials, -16- j ` ll~Zl~Z

which will t~ithstand radiation damage, is readily available. For example, the presence of a benzene ring attached to the side of the main molecular chain, as in the case of polystyrene, greatly increases the abihty of the material to absorb energy without dam-S age. Other types of polymeric molecular structure are much more vulnerable to ~he effects of absorbed energy and are more easily broken at their bond~. Although there is significant variation I from one polymer to another, radiation damage for any one material apparently i9 largely proportional to the total absorbed energy.
A relationship useful for quick, approximate comparison is that for all organic materials under neutron, gamma and x-ray radiation the rad or unit of absorbed energy, is equivalent to the rep or unit of radiated energy wlthin a factor of 1.5.
The extent to which plastics are permanently damaged by radiation depends upon severaL factors such as types of radia-tion and spectra of their energies, presence or absence of oxygen, thickness of sample, temperature and physical strain of the speci-men, and composition o~ material.
For rough approximate e~timates, compara~ive valùe~
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Radiation Doses Required to Produce Plastics . . Plastic Dose (rads) ¦ 5 Polystrene 4 x 10 . Silicons 2 x lO
~poxies 1 x 10 Melamine 1 x 10 B CR 3~ 9 x 10 Polyethylene 9 x lO
Poly carbonate 8 x lO
Urea 5 x lO
Polyester film 3 x 10 Cellulosics~ 2 x lO
K~l F 2 x 10 - Unsaturated Polyesters 9 x lO
. Acrylic mechanical l x 10 optical 5 x 106 Nylon 5 x 10 Te~lon ~ 4 x 10 , .~
A unit of energy actually absorbed, regardless o~ type or energy level of radiation, equivalent to the -absorption of 100 ergs per gram of materialO
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Fiber glass ~aminated.
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Table I shows a preference with respect to radiation ~ stability, but many other parameters must be considered to optim-¦ ize the selection of a particular plastic for a specific applica-tion. Polypropylene and polybutylene, for example, have higher B5 temperature limits than polyethylene, and ~eoprene exhibi~s still higher thermal stability. A very wide choice therefore is possib-from th~ broad class of organic compounds containing hydrogen in selecting a material for the discontinuous phase suitable for achieving a desired result.
For gamma radiation alone, urethane rubber and natura rubber show nearly the same resiqtance aq polystyrene.

Additional Additives.
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Compositions previously described may be further modi ied by the addition o~ various other substances to the mortar mi - 15 Some of these additives may contain hydrogen atoms in combina~ion with ~ther elements that produce specific beneficial attentuation efects, such a~ certain ammoniu~ salts, hydrate~ and especially metal hydrates. Other additives contain no hydroqen atoms and ma be added to meet specific shielding requirements as will be de-scribed later.

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Most elements form hydrides. Because of the variety of elements that combine with hydrogen, the hydrides are charac-terized by a wide variety of propertjies and thus are representati~
of several types of bonding. often hydrides are classfied as ~ollow~:
! 1. Saline, (or salt like) characterized by ionic ! lattices.
2. Covalent or molecular, characterized by molecular lattices made up of individual saturated covalent molecules and usually volatile.

3. Meta~ic, chara~cterized by metallic structures and resembling an alloy in most of their characteris-tics. They are in effecS interstitial materials and often lack the stoichiometry associated with true electronic bonding. They are hard, brittle, lustrous, conduct electricity and they generally are not reactive toward water unless the metals themYelves are.
The lines of demarcation between these groupings are not always sharp.
Only the metallic hydrides have application in the present invention. Specifically included in this grouping are the hydrides o~ elemenSs located in the periodic table as follows:

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Column II a: Beryllium ~nd Magnesium Columns III a: Scandium Columns IVa, Va, VIa, VIIa, VIII, Ib and IIb- ¦
i, (all members) 1, Column IIIb: Thallium ~11 Lanthanides (elements with atomic numbers 58-71 inclusive) Elements with atomic numbers of 90 and higher, includ-ing the transuranic elements.
10 Of these metallic hydrides, some are unstable and others react with waterj and consequently they are not useful in this invention. For example, hydrides of beryllium and magnesium are not sufficiently stable for use. Examples of metal hydrides . which are stable and which do not readily react with water include¦
;hydrides o titaniu~, zirconium, hafnium, tantalum and plutonium. ¦
For purposes o~ this invention, titanium hydride has special value because o~ its relatively high hydrogen content of

4.04 percent by weight, and because of its commercial availability Gadolinium hydride supplies desirable hydrogen atoms and also fur-nishes gadolinium atoms with a very high capture section for ther-mal neutrons (46000 harns). The relatively very high density of plutonium atoms in plutonium hydride is particularly efective in photon attenuation, exclusive o~ the middle ener~y ran~e 1 - 2 ~leV
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Many other kinds of additives may be used to meet the requirements for particular radiation types and spectrums involveJ
The action of each individual atom is ~pecific, and there even may be marked differences between different isotopes of the same chem ical element. The problem becomes more complex when the radiation source consists of a flux of mixed ra-diation types of various ener gy levels. The situation may be further complicated when seconda3 radiation effects are induced within the shield, as a result of interaction of the initial flux with certain atoms in the shield itsel~ For example, radiations from a nuclear reactor involving fissioning of uranium initially includes only fast neutrons, gamma rays and fissicn fra~ments. Except for gamma rays, which are at-tenuated by adequate mass of matter, a long series of interaction~
- involving several simultaneous side reactions must occur before total attenuation is complete. Each situation requires special study for thR most satis~actory solution for the problem.
- The efectiveness of atoms in arresting thermal neutrc is measured by their comparative neutron cross section, usually measured in barns. Table 2 shows the comparative values for a number of t~pical elements, and shows the great differences between them.
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: TABLE _2 . Thermal Neutrons Cross_Sections of T~pical Elements Element ~tomic .Absorptive Cross (In natu.ral abund~nce) Number _ _ _Section (barns) Mydrogen 1 332 ~ 2 Lithium 3 , 71 - 1.0 ~eryllium 4 ~0.01 - 0.001 Boron 5 750 _ 10 . . Carbon 6 0.0032 - 0.000 Aluminum . 13 0.32 - 0.005 .~. Titanium 22 5.6 _ 0.4 : . Iron 26 2.53 + 0.6 Copper 29 3.69 + 0.12 Zirconium 40 . 185 - 4 Silver 47 63 Cadmium 48 . 2500 - 100 Tin . 50 625 - 15 Samarium 62 5600 - 200 Europium 63 -4300 - 100 Gadolinium 64 46000 - 2000 ` Dysprosium 66 950 - 50 . Erbium 68 173 _ 17 . . Thulium 69 llS - 15 . Hafnium 72 127 ~ 4 Tungsten 74 19.2 - 1.0 '.' ~z~

The very high cross section for gadolinium is special ly noteworthy as are three other members of the lanthanides:
europium, dysprosium and samarium. C~ m,~tin and boron are also relatively high, and lithium is sufficiently high for prac-tical use, especially when the half MeV gamma from neutron capture in boron need be avoided. on the other hand, carbon is a good moderator for high energy neutrons for reducing their energies to a thermal state where they are more easily captured, but it is very poor for neutron capture as shown in Table 2. Selection of a suitable material involves a consideration of these secondary rad-iation effects in additian to cro~s section values.
More effective nuclear radiation shield~ result from the use of enriched isotopes o many of thé elements. The isotop~
of lithium effective for capture of thermal neutrons i5 lithium 6, which~has a reaction cross section o~ 945 barns. ~ithium 7 is nearly worthless for this purpose. ~ithium 6 has a natural abun-dance of only 7.52 per cent, and hence the mixture of the two lithium isotopes in natural abundances averages only 71 barns.
Similarly, natural boron consists of 19~8-per cent boron lO with a cross ~ection of 3850, and 80.2 per ce*t boron ll with nearly zero cross section. Boron in natural abundance has a composite cross section of app~ ximately 750 barns. Samarium 14 with a valu o~ approximately 40,800, and present in 15.8 per cent contributes significantly to the 5600 barns composite value for samarium in ?r, natural abundance. The improvement in shielding characteristics for thermal neutrons i9 directly proportional to the degree of ,'', ' -~ 2 : ~

isotopic enrichment.
The modifying additives usually can be added directly I to the mortar mixtur~ to finally become an additional discontinu-ous phase of the finished shield.
'5 An alternate effective means of adding such an ; 'additive' to the shield is to disperse it initially in the de-sired particle size, into a thermal plastic to be used as the hydrogenous ingredient. This mixture, consisting of a discontin-uo~s phase o the additive in a continuous phase of the plastic, may then be reduced to the desired particle size by any suitable method, such a3 grindiny or cutting. This resulting mixture, ncw available in desired particle size, may in turn be dispersed in thé matrix. This procedure eliminates, or greatly minimizes, the direct contact of the new modifier with the continuous phase matri c lS in the ~inal shield, which is particularly beneficial in certain cases where the new additive retards the setting time or reduces the ultimate strength of the matrix.
For example, a boron loaded polyethylene may be pre-pared by adding boron in a suitable form to poIyethylene in a mix-ing operation using standard roll milling or kneading equipment.
Boron carbide, because of its high boron content, may be used to i produce a boron content of thirty or more per cent by weight of ~ boron in t finished prodact. Epoxy-boron carbide products ll~ZlOZ

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con,taining 45 or more per cent by weight boron may be achieved, These products are particularly effective for applications desiring i high attenuation of thermal neutrons. Lithium atoms can be added ' ~r~ similarly to plastic compositions in the form of, for example, the ~ f l~noL
-~4~ k, hydroxide or silicate. While lithium atoms are less ! efective in attenuating thermal neutrons than are boron atoms, substitution of lithium for boron avoids the one half MeV gamma from neutron capture in boron. Atoms of elements in the lanthan-ide series also can be introduced. For gamma shielding, in the energy range below about 1 MeV and above about a MeV atoms of elements with high atomic numbers, such as lead and tungsten, may be added to the plastics, The wide choice of plastics available, and the multitude of combinations with desired atoms in various concentrations, permit excellent flexibility in prodocing material~ ;
lS to meet special situations, . ~
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~ mong the variables that affect satisfactory physical structure in the set finished structure prepared from the formable mortar is the quantity of the discontinuous phase present. In the first place, it is desirable that the continuous phase fill all voids created by the discontinuous phase. Usually physical 2~
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' . ' strength is improved when this quantity of continuous phase ex-ceeds this minimum and provides a thicker wall separating the dis-, . ,, persed particles. This situation changes from system to system, depending upon the cohesive forces within such phase, and the adhesive forces acting between all phases present. ~nother imp-ortant factor relates to the compatibility of all phases in the , sy~tem, especially in connection with all bonding actions in-volved in the setting process. This action often is related to interference of crystalline growth of the cootinuous phase by either physical or chemical mechanisms. Sometimes it may consist of sorption of very fine additive articles in knitting crystalline surfaces, especially if the additive particles are in the colloid-al size range. For this r~ason, it is usually advantageous to I use additives with sizes larger by several orders of magnitude than those classified as colloids.
: Particle size and shape distribution are very impor~
tant factors in determining void volume of the discontinuous phase and hence the limits of volume ratios between phases~ visualiza-tion of an idealized, hypothetical case will help clarify the im-portance of these variables.
Space may be divided into imaginary cubes packed sym-metrically in such a manner that together they occupy the entire ~ Z~
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sp~ce under consideration. Packing of equal size spheres, having diameters equal to the edges o~ the cubes and with centers of the spheres coinciding with the centers of cubes, presents a pattern of one sphere inscribed in each cube. Such a configuration of spheres then would give the theoretical space occupied equal to ¦the ratio of the volume of the sphere to the volume of the circum-scribed cube or nd /6 to d - n/6 = 52.4 per cent of space oc-cupied or 48 per cent voids. In this type o packing, each sphere is in contact with six other spheres, and the void per cent in the total mass remains constant and is independent of the diameter of the sphere Void space in the total mass will be reduced if the sphere arranyement is changed so that one sphere in each group of four restS symmetrically on three others in the form of a tetra hedron. The extension of this unit will result in each sphere touching twelve adjacent spheres, and the void space is reduced to approxim..~ed 26 per cent~ In actual practice, where the spheres are dumped into a container and vibrated, an average void space of : approximately 38 pPr cent results, indicating a mixed packing pat-: tern of the two described.
Further reduction o~ void spacè is achieved if smaller spheres are added to fill void space produced in the single sized sphere packing and still further reduction may be realized if smaller spheres are added to fill voids remaining in the inter-mediate sized spheres. Theoretical geometric calculations reveal .

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that for achieving minimum voids, t~ size ratio of successive sphere si.;es must be large. If each sphere size added is assumed to occupy thirty~eight (38) per cent o~ the remaining void space then, for a system containing two mono-sized spheres in proper proportion, the void space is 14.4 per cent of the total and ~or system with three different mono~ized spheres, the final void ; space is approximately S.~ per cent. To obtain this maximum pack-~! ing, using three sphere sizes with lar~e diameter ratios, calcula-tions indicate that the overall mixture contains by volume 62.0, 23,6 and 9,0 per cent of large, intermediate and small spheres, respectively, leaving 5.4 per cent voids.
It is impossible to attain in practice this dense packing largely because the large size pa~ticles be~ me trapped ir the spaces between the large particles and are not fxee to move.
The nearest experimental approach to this theoretical value was achieved with a mixture containing by volume 70 per cent coarse, 20 per cen~ medium and 10 per cent fine, where ~he diameter ratio~
of the 6pheres were 50:8:1. The resulting bulk-volume was 1.202 from which was calcuiated a void volume of 16.8 per cent. Furthe~
more, in the practice of thiq invention, desirable additives for the discontinuous phase are not available in the form of mono-sized sphere~. They are irregular in shape and comprise a multip3e size distribution range. The theoretical calculations, however, are very useful as general guidelines in determining the 110216~Z

parameters and limitations involved, and the basis for optimizing desired compositions.
Another method of reducing void space is by addition o liquid solutions containing atoms of materials that contribute, at the same time,desirable shielding properties to the mixture.
These solutions move more freely throughout the mass than do solid particles, and may be drawn into very small voids by capillary action. Upon removal of the solvent by evaporatiOn or other me ns the solute crystallizes, often with the formation of a felted mass o~ interlocking crystals that contribu~e to the strength of physic ~1 structure. This procedure sometimes is the most convenient method Eor introducing certain desired atoms into the finished product.
The addition of dry sucrose to the dry powder mixtuxe illustrates the application of this technique Water added to the dry powder to produce a ormable mortar dissolves sucrose and car-- ries it into existing voids. It adds to the workability of the mortar and often adds strength and higher density to ths final sel supporting structure. Sucrose adds to the hydrogen content de-sired to attenuate neutrons by elastic scatter, and also adds car-bon atom~ to modcrat~ neutrons to lower energy levels.
In the inorganic field, u~e of boric acid H3B03 may be cited as an example, in spite of its limited solubility in "r-.~, z~oz water, relative to sucrose. In this case, the hydrogen content of the mixture is increased, and boron atoms are introduced that are effective in the capture of neutrons.
, In general, particle size of all ingredients in a ~S satisfactory radiation shield should be relatively small to insure good homog~ ity, with the absence of holes that permit transmissio~
of unde9irable radiation. The particle size limitation should be determined in the cOntext of the total situations. It makes littl~
difference whether a particular atom is added to a composite shielc 10 in the form of a massive, monolithic shield or distributed through the ma~s in a relatively finely divided powder. Of consequence is the total number o~ atoms of this particular element in the path of undesirable radiation, whether it is compacted together or dis-persed, or whether it is in the chemical form of the pure element lS or whether it is chemically bound as a compound.
Particle shape, size and size distribution are closely related to shield thickness, in a properly formed shield. In a thicker shield, we may use larger particle size and still have satisfactory attenuation. The important factor is that the densit~
20 of the atoms of the particular element be uniform as projected to n area at right angles to the path of the radiation. For a given weight concentr~tion of addltive, the area o~ the cross section, ,.

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and the volume vary as the second and third power of the diameter respectively. Thus, particle si~e limitations depend upon speci~ic situations.
A summary o test data from a broad practical view-point relative to size and amou~ of ~iscontinuous phase material pre~erred in the radiation shield indicates that no more than ; about l~/o pass through a 100 mesh screen and that the quantity range falls within about 2 to 60% volume per cent.
~e~ .
Many examples may ~e cited to illustrate the multiplic ity of compositions ~nd application of this invention. The follo~
ing specific examples only illustrate the breadth and scope of the parameters involved and are not intended to indicate limitations placed upon ~he formulation of the shield.
1. A series of tests were made with Portland Cement-polethylene powdex mixtures, with volume ratios of 1 part Portland Cement to 1, 2, 3 and 4 parts, respectively of polyethylene. Wate was added to the dry powder mixture in sufficient volume to produc a workable mortar, which was cast in molds. All mor~ars set at room temperatures to rigid structures after several hours and the set material gradually became harder as the curing time increased.
Comparative strengths of the set material decreased as the ratio o lortland Cement to polyethylene powder decreased.
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' '' ' 2. One volume of commercial building plaster was thoroughly mixed with 2 volumes of polyethylene powder and water was added to give a workable mortar which could be plastered on a metal lath anchor, or poured in a m~ld of the desired si~e and S form. The mortar set to form a self supporting structure. Similar structural results were obtained when 5 per cent of lithium compounds, carbonate, fLuoride, hydroxide and ortho silicate, respectively, were added to give improved thermal neutron capture.
Similar concentration of anhydrous lithium borate ~i2B407) also proved satisfactory structurally. A boron atom is much more effec-tive than a lithium atom in thermal neutron capture. The lithium boarate forms a hydrate ~Li2B207,5~I20) during the setting process which adds additional hydrogen.
3. A mixture of titanium hydride and Portland Cement was prepared in the ratio of 1 part to 3 parts by volume and water ~.
~as added to give a workable mortar which could be molded and set to any desired shape. A specimen cured at lahoratory temperature showed good physical structure. -This sample showed significantly ~etter thermal resistance than did a similar sample in which poly-; 20 ~thylene replaced the titanium.
4. A mixture of dry powders consisting by volume of parts Portland Cement, 10 parts natural Colemanite ore powder ontaining 35 wt % of B203, 2-1/2 parts of-charcoal passin~ a 20 me ,, -~3-~z~

screen, and 1 part granular sucrose were mixed with water to pro-duce a stiff mortar, which was cast in a mold. The mortar set to good structure on standing for 48 hours. Colemanite is a mineral containing varying concentrations of calcium borate (Ca2B601l.5H20) and thus furnishes boron atoms for effective thermaL neutron capture, and al50 supplies additional hydrogen atoms from its water of hydration. Sucrose further increases the hydrogen content, and the charcoal supplies additional carbon atoms for neutron moderation. Proportions of the components may be varied significantly to meet speci~ic requirements for mixed radiation fluxes.

5. A rnixture by volume of 9 parts Colemanite and 1 part sucrose to which water was added, set to a self supporting structure~ although somewhat more slowly than the previous mixture cited in.example 4.
G. Compositions were prepared to produce lead-water mortars, which set as the bonding matrix, witll polyethylene in the discontinuous phase. The lead was used as a powder, screen ed through a standard 200 mc~h ~ieve. The polyethylene was a high density t0.96 g.cm ), hi~h molecular weight plastic with a melt index of 0~5 tg/l min.) and a particle size of between 60 and 100 mesh. Three different volumc lead/plastic ratios tl:l, 1:2 and 1:3 /ere tested. water was added in suficierlt q~lentity to ¦

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produce a stif~ mortar , which was then p~a ced in a mold and allowed to set. Excess water tended to cause a separation of the ingredients and ~hould be avoided. Even with relati~ely dry mortar mixes, all possible expressed water should be removed to acilitate setting. ~11 three mixes, with the xatios specified, set satisfactorily to self supporting structures in 48 hours.
Strength of all structures produced continued to improve grad-I ually for a period of several weeks. Satisfactory results also were obtained using 100 and 300 mesh powdered lead as well as with polyethylene powder'varied from 35 to 100 mesh.' Structure improved as particle size of lead decreased and particle size of polyethylene increased. Particle size does not appear criti-cal within these size limits and from the observations it is reasonable to expect good results well beyond these limits.
Additives were added to modify the nuclear shielding characteristics of the preceding mixtures containing a 1:2 ratio of lead/polyethylene. Based on the dry powder, additions were made respectively of: (1) 2~4 per cent by weight of technical grade boron carbide containing 7~/O boron, and passing a 60 mesh standard sieve; (2) 4.5 per cent by weight of boxon nitride;
and (3) 5 per cent by weight of granular sucrose. All mixtures set to self supporting structures, although more slowly than did the' mixtures without th" additives.

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, 7. A series o tests were made to compare various plastics as the discontinuous phase using a non-combustible i matrix as ~he continuous phase.

~ Compositions of Dry Powder Mixtures (Parts by Volume) .
5Lead Powder Portland Cement Plastic Powder 1 ~ Polypropylene -- 1 1- Polystyrene 1 -- 1- Polybutadiene 1 -- 1- Polymethylmethacryl __ 1 1- Polyvinylchloride 1 -- 1- Nylon -- 1 1- Neoprene ~

1 -- 1- Polyethylene glycol terepthalate - 15 1 - 1 1- Crepe rubber All mortars set to form self support ~g structures.
8. A polyethylene slab containing 5 weight per cent boron was pulverized to pass a 60 mesh screen. Two parts by volume of this powder were mixcd with one part by volumc of lead I powder passing a 300 mesh screen and water was added to form a stiff mortar. When placed in a mold and the excess water ex-pressed, the mortar set to ~orm a self supporting structure.

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Similar results were obtained when the boron content in the poly-ethylene was increased to 30 weight per cent. Satisfactory resul also were achieved when powdered ep`oxyboron carbide loaded plas-tic, containing 45 weight per cent of boron was substituted for the boron-loaded polyethylene plastic used in the previous ex-ample. In a second series of tests, with the same metal-loaded plastics with Portland Cement replacing lead powder as the contin uous phase in the set product, satisfactory structures were ob-tained. These structures produced from metal loaded plastics were significantly superior in strength compared with thosa ob-tained when equivalent quantities of boron in various forms were added directly to the continuou~ phase matrix.
9. A series of tests were made to illustrate furthe the wide v~r~ety of aombinations possible for introducing addi-15 ~ tional hydrogen and important elements as additive~ into the ; discontinuous phase.
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,'' ~' Compositions o Dry Powder Mixtures _ _ (Parts by Volume) ' . ' : Lead Portland Polyethylene Powder Cement _ Powder Other Material ; __ 20 0 ~n8 um borate, (NH4)2B10 (a) : 20 ~~ 10 l-Lithium hydroxide, Li(oH) (b) i 10. 10 10 l-boric-acid, H3BO3 (c) 20__ 10 I-calcium sulfate, CaSO4 (d) 2010 10 l-gadolinium oxide, Gd203 (e) , __20 10 l-rare earth concentrate (f) 20~~ 10 1 SUcro9e, C12E~220 (g) . _ 20 10 l-sucrose, C12H22 11 (h) 20 _ 10 l-Sorbitol t ( CH2OH(CHOH)2)2 (i) -- 20 10 l-Urea, H2NCCNH2 ~j) . 20~~ 10 l-anthracene, C~H4(CH)2C6H4 (k) 1010 10 l-paraffin C H (average) (pellet9) 25 52 . (1) .1010 .. 10 l-hydroxyethyl cellulose (m) 0 20~~ 10 l-titanium hydride, TiH2 (n, - 20 -- 10 l-berylliùm hydroxide, Be(OH)2 (o) -- 20 10 l-beryl, natural ore contain-. , ing.3BeO.A1203.6Sio2 (p) . All dry mixtures formed workable mortars with water and set to form ~elf supporting structures.

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Notes (a) Additional hydrogen WA3 introduced from both the amonium radical and the water of hydration. Boron gave good thermal neutron capture.
(b) Additional hydrogen was introduced from the (OH) group, and also from the fact that a hydrate ~ i(oHJ.H2o) was formed during setting.
(c) Borlc acid furnished additional hydrogen and a 190 boron.
(d) Calcium sulfate conver~ed in presence of added ; watér to CaS04.2H2d, and s;upplied additional hydrogen. Setting was slow but good structure was assisted by the formation of in-terlocking crystals (e) Gadoliniu~ has a ~ery high cross section for thermal neutrons capture (46000 barns).
(f) Analysis of the rare earth concentration showed the following comppsitioni calculated as weight per cent oxide:
Sm o -44 3% ~d2o3-15.o~/O~ Nd203-18 7%~ Pr6ll 2.6%~ L 2 3 EW23--0-02%~ C'~02-2~5%, Y203-5.23%, Tb407-1.0%~ H20-6.5%.
(g?, (h~, (i), (j) AlL of these added hydrogen as water soluble substances, that as~isted in filling small void~, and upon crystallization contributed to a varying degree of bond-ing action.
(k), (1) Illustrate~ use of aromatic and aliphatic hydrocarbon to the discontinuous phase. In general,these hydro-carbon~ are infcrior to polymerized hydrocarbons.

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., . - . , ':' , . ' i (m) A nonionic water-901uble polymer illustrates another type high molecular weight that can be used for increas-ing the hydrogen content.
(n) A convenient means of introducing more hydrogen in the form of a hydride.
(o~ Both beryllium and hydrogen are added. Be i9 a good neutron moderator and reflector.
(p) Another illustration o the use of a low cost natural ore to add desirable radiation shielding characteristics.
10. A series-of tests were made on mixtures which contained a fixed ratio o~ lead powder and high den3ity poly-ethylene-350 parts by weight of lead to 60 parts by weight of the plastic. Sucrose was added to 410 parts of the said powdered ~ lead-polyethylene mixture in varying proportions from 0 to 30 par g by weight. Forty parts of water were added respectively, to each~
mixture to form the mortar. All mortars set satisfactorily in th 3 mold. ~bove about 15 parts sucrose, the setting time was signi-ficantly longer, increasing with increasing concentration of ~ucrose. ~xpo5ure to infra-red radiation decreased the setting t Lme 11. A plaster mortar was prepared by mix~ q the weil ht per cent of the followi-ng ingredients: l~ad powder- 75%; poly-ethylene powder- 14%, sucrose- 5%, carbon black- 1/~% and water-5 3/~%. The ea9ily workable plaster was applied successully on a concrete block wall, and the co~ting set in 24 hours. Hardness : .
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o~ the plaster coating improved with time. A small i~provement in application was noted when a standard concrete bonding agent I was initially applied to the wall.
Tests showed that this material could be plastered easily on sur~aces of Portland cement, concrete or cinder block, brick, plaster board, and wall tile. Initial application of a coating of standard, commercially available bonding agent assist ! the bonding qualities to varying degree.
Numerou~ tests also showed that thi3 plaster could be applied to suraces of other set shielding products described in this ~vention. These tests led to the conclusion that it is practical to produce a composite radiation shield by applying successive layers of plasters comprising the desired atoms in th desired sequence~ This procedure has great advantage when the source xadiation striXing the face of a shield is of an entirely dif~erent character in both type and energy spectrum than that of t~e radiation encountered in various othex parts of the shiel born wi~hin the.shield as a result of various interactions, whic often trigger simultaneous and sometimes successive secondary radiation effects It was ~ound that a protective coating such as an epoxy or vinyl resin could be ~plied to the survaces of these - shielding materials to improve surface hardness and appearance, and to seal its surface.

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12. Experiments were made to study the effects of various mixtures of particle sizes of hydrogenous material added as the discontinuous phase on non-combustible structures. Re-il sults were shown by a series of tests made using combinations of different sizes of polyethylene 1/8" to 100 mesh, in the lead-; water matrix continuous phase using 200 mesh lead powder, It was concluded from these studies that the best structure, with given lead power-polyethylene ratio, was obtained with the maxi-mum difference in polyethylene particle size, and with the ratio o fine to coarse particles ranging from about one part fine to 2-4 part~ of coarse. Similnr results were obtained using Portlan~
I cement as the continuous phase ma~rix. S~ill larger pellets gave~
better strength of the set product in large mass, using constant ratios of lead to polyethylene. However, this general trend is invalid when particle size exceeds about 10 per cent of the shield thic~ness. These factors arn also limited by considerations of non-homogehity with respect to radiation shinldin~ ef~ectiveness.
.A t~pical mixture of dry ingredients within the de-sired range of particle size and compo~ition consists of the fol-lowing volume ratios: lead powder t200 mesh) - 1 part, poly-ethylene pellets (1/8" diameter~-1.5 parts and polyethylene powder (lO0 mesh) - 0.5 part. However, the invention ~ operable over much wider limits.
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Additional experiments showed that additives could also be used in larger sizes as aggregates in the shield to modify various comyositions. Each additive became an additional dis-. continuous phase in the matrix. The particular aggrégates used may be varied widely to meet specific requirements and include such examples as borated graphite, lead shot, iron filings, mine tailings, tungsten, and depleted uranium pellets~ Materials with a den~ity greatex than about 7 gcm are particularly useful for the better attenuation of gamma rays in mixed radiations.
: .
Flre Resistance_Characteristlcs A 3eries of studies were made to determine the rela-tive fire resistance character of the compositions disclosed in ~ this invention.

I ~n analysis was first made relative to the important parameters involved in formulating a fire resistant shield con-tainin~ a combustible material with a high concentration of hydro _ gen atoms. The most promising means seemed to lay conceptionally in dividln~ the combustible material into relatively small parti-cles and placing them into separate compartments having non combustible walls. The fire b~rriers around each particle of combustiblc, hydro~enous material would isolate it from dircct contact with a flame, and also would seal off the air supply which i3 necessary for combustion. The basic principle is an adaptation of the use made o ire walls in building constructio ~ .
-~3--. ~ 2 and fire proof vaults, and may be expressed technically in this new application as dispersing the hydrogenous combustible mater-¦
ial as a discontinuou`s phase in a non-combustible matrix in I continuou~ pha~e, It must ~e noted that this improvement was never used nor disclosed by sophisticated shield designers who recognized the need for a solution to their fire-hazard problem.
Flame test studies directed toward a more complete understandiny of the problems showed, as expected, that a stack of polyethylene sheets burned-much more readily than did similar ¦
polyethylene sheets separated by thin layers o set Portland cement. A cast sample of a mixture of polyethylene beads and Portland cement in e~ual volume ratio, with the polyethylene in a discontinuous phase also was less combustib1e than was the sample containing alternate layers of Portland cement and polyethylene. ~ ~
- Reference was made previously (prior art) concerning the phase character of the Thiessen composition. Comparative studies were made using Example l described in the Thiessen paten' :
and one using similar ratios of matrix ~o polymeric material, using the procedure outlined earlier. The Thiessen composition was ~iven the additiona] 7 hour steam treatment as described.
These two finished samplcs were then broken and examined micro-scopically for similarities and differences, especially with ref-erence to phase charLIct2ristlcs. ~1~

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The broken sample of this invention showed that ' each bèad o expandable polystyrene was surrounded by a continu-ous phase of Portland cement rnatrix, and thus the beads were en-cased in distinctly, separate, fire resistant compartmen~s.
This discontinuous phase character of the beads was very clear along all of the broken faces of the sample. The system can be accurat~ly described as one having one solid continuous phase of Portland cement matrix and one discontinuous solid phase of polystyrene beads dispersed therein.
The Thiessen product sample showed a di~tinctly dif-ferent appearance along the broken faces. The polystyrene beads had lost their spherical shape and had flowed together irregular~
throughout the mass into a cont'inuous pha'se. This system can best be described as one having two discontinuous interlocking phase's with no discontinuous phase. The system is analogous to that o'f a sponge, where the air (gas) phase portion of the sponge - repres'ents the polyetyrene phase, and the solid portion of the sponge xepresents'a combination o~ all other ingredients in the Thiessen formulation.
This phase characterization of the Thies en product coincides with the one expected from a careful reading o U.S.
Patent No. 3,021,291. Fir~t, Thie~sen foxmulated a lightweight concrete block by beating his initial rich creamy slurry of -45~

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ingredients to a frothy pulp. This mixture was poured into forms to set. At this stage of the process, the polystrene bead pha~e may have been present as a discontinuou~ phase in the continuous matrix phase, and the gase phase forming the froth may also have been in a discontinuous phase in the same - matrix. However, the system was then placed in an autoclave - under 15 to 20 pounds steam pressure for about 7 hours. As ! ~escribed: "The steam curing increases the compressive struc-ture and also permit~ the polystyrene beads to expand into the voids within the concrete filling them completely and sealing them against the transmission o~ water and humidity". (Column 3 lines 60-65).
A mass trans~er of polystyrene in a discontinuous phase into a second dlscontinuous phase of air bubbles in the same continuous phaie, obviously necessitates the breaking of th~
continuous phase barrier separating them. The inevitable conclu sion i5 that when all the voids (air) have been filled by the poly~tyrene phase, this phase has become substantially continuou In thi3 connection, it is important to note that Thie~sen states that the polystyrene beads "expand into the voids within the concrete filling them completely". The increased compressive strength of the concrete structure as a re3ult of the final stea~
curing step supports the observation that the polystyrene phase .~ llVZl~iZ
' .`
, has been transformed from a discontinuous phase to a continuous phase, giving a more rigid interlocking supporting structure augment that supplied by the continu~ous phase matrix.
The Thiessen product was compared ~urther with the product o this invention relative to the effects of contact with an open flame, in an assimulation o~ practical conditions.
Discs made of the two materials, (namely, product of this invention and that developed by THIESSEN) 4 inches in diame :
and 0.5 inches thick, were suspended horizontally over similar small open flames, the flame strikir)g the bottom face at th~
center points of the specimens. Examinations were made at periodic intervals up tQ 4 hours.
Results from above observation may be summarized as follows:
l. The relative physical strengths of the product of this invention was superior for all comparable times, as was determlned by hand hreaking tests. While neither sample actually collapsed under i.ts own weight, microscopic examination of the broken specimens following the flame test showed a superior integral structure for the product of this invention.
2. Temperatures at top o~ the respective discs were compared. It was observed that the sur~ace temperature of the disc made of Thiessen's product was higher than th~ temperature ~ ~ : llOZlUZ

,. ' at the corresponding point on the disc cast from material of this invention. This highèr temperature probably indicates exothermic combustion of polymeric`material in the Thiessen . product. The burning odor also was more pronounced in the : 5 Thiessen product, indicating a faster decomposition of this . ~ produce.

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.. . -48-

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method comprising attenuating nuclear radiation by intercepting the radiation with a non-combustible, self supporting system formed of a continuous phase of fire resistant cementitious material and in which is dispersed a discontinuous phase, said discontinuous phase containing hydrogen atoms in the form of a rigid, combustible organic plastic or resin with a particle size distribution such that no more than 10 percent of said dis-continuous phase passes a 150 mesh screen and with a volume per-cent range of said continuous phase between about 40 percent and about 98 percent of the total shield volume.

2. The method as defined in claim 1 wherein the dis-continuous phase is a hydrocarbon.

3. The method as defined in claim 2 wherein the hydrocarbon is selected from the group consisting of polyethylene polypropylene, polybutane and polystyrene and combinations thereof.

4. The method as defined in claim 1 wherein a metallic atom is dispersed in the organic compound prior to the mixing of the dry mortar ingredients.

5. The method as defined in claim 1 wherein the organic material is a plastic which is adapted to withstand radiation doses of less than approximately 1 x 107 rads.

6. The method as defined in claim 1 wherein the continuous phase of the fire resistant cementitious material is a matrix produced from metallic lead powder and water.

7. The method according to claim 1 in which the con-tinuous phase of the fire resistant cementitious material is selected from the group consisting of Portland cement, wall plaster, Plaster of Paris, silica gel, clay or combinations thereof, set with water.

8. The method according to claim 1 comprising dispersing a further material in the continuous phase and including a neutron attenuating substance.

9. The method as defined in claim 8 in which said further material is selected from substances containing atoms with atomic numbers below 10.

10. The method as defined in claim 9 in which said attenuation substance is carbon, boron, lithium, hydrogen or combinations thereof.

11. The method as defined in claim 1 comprising dispersing a further material in the continuous phase, said further material including hydrogenous compounds selected from the group consisting of an organic substance, a hydroxide, a hydrate and combinations thereof.

12. The method as defined in claim 1 comprising dispersing a further material in the continuous phase, the further material including a metal hydride which is stable in the presence of water.

13. The method as defined in claim 12 comprising a further material dispersed in the continuous phase and including metallic atoms selected from the group consisting of titanium, gadolinium, samarium, europium, dysprosium, plutonium and combinations thereof.

14. A self-supporting, non-combustible, nuclear radiation shielding composition comprising a continuous phase of fire resistant cementitious material and a discontinuous phase dispersed therein, said discontinuous phase containing hydrogen atoms in the form of a rigid, combustible, organic plastic or resin with a particle size distribution such that no more than 10 percent of said discontinuous phase passes a 150 mesh screen and with a volume percent range of said continuous phase between about 40 percent and about 98 percent of the total shield volume, said continuous phase being adapted to retain its shape in the presence of a temperature adapted to soften, melt or decompose the discontinuous phase, said composition further comprising a further material dispersed in the continuous phase and including a substance containing at least one enriched isotope of an element in a ratio different from that present in natural abundance.

15. The composition of claim 14 wherein said enriched isotope is selected from the group consisting of lithium 6, boron 10, cadmium 113, samariun 149 and gadolinium 157.

16. The method as defined in claim 1 comprising dispersing a further material in the continuous phase, said further material including a substance containing a heavy metallic atom that in the free elemental state has a density greater than approximately 7.0 g cm-3 at 20°C.

17. The method as defined in claim 1 comprising dispersing a further material in the continuous phase, said further material including a natural metallic ore aggregate.

18. The method as defined in claim 1 comprising dispersing a further material in the continuous phase said further material being in the form of aggregates with a multiplicity of screen sizes.

19. The method as defined in claim 18 in which the distribution of particle size of aggregates is selected to produce an aggregate mixture with less than 40 percent void volume.

20. The method as defined in claim 19 in which the setting agent is water and comprising a further material dispersed in the continuous phase and including sucrose.

21. The method as defined in claim 1 in which the weight percent hydrogen in the set material is greater than 2.9.

22. The method as defined in claim 1 wherein said dis-continuous phase contains hydrogen atoms in the form of titanium hydride.