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mu n. LI A24394 -*; --A~ - -*'- ^ LEAD INDUSTRIES ASSOCIATION o^rri*ft2 MADISON AVCNUK NEW YORK 17, N. Y. - fa <p^ ,.rfw October 28, i960 SUBJECT: MATERIALS FOR PORTABLE BUCLEAR REACTORS To Members of the Lead Industrie* Association: Attached la a reprint cf an article appearing Is the October i960 Issue of "Material* In resign Engineering" en titled "feterials for Portable Nuclear Reactor*," prepared by a member of the LIA staff. We believe that you vill find It both Interesting end useful. We have a limited supply of this reprint, which Is avail able to members free of charge In quantities up to 25 and for five cents apiece Is larger quantities as long as the supply lasts. Very truly yours, ^r RLZtJRH Att. Secretary ' J P-- ' uJl|fill' lift'HJ.** HI )--I l.tfII IJII 11 ff'M UA24395 9 Materials for Portable Nuclear Reactors Reactors designed for research and training purposes pose special prob lems in materials selection. Both cost and weight must be minimised. Here is a look at current practice in fuels, controls, moderators, reflectors and shielding. f 0 o by Cdward JL Mmmmi, TVMninoi Uirscior, Lead Industrie* Xim, PtC #000 tors consists / l?> 3J5 m. in dla by if m. long arranged in hexagonal paUrm and supported by in* gr*l skounf. The take* art designed ( bald J in. dia by i in. tang natural uranium slug* whose total weight it 5SOO lb. Uranium is made available to schools under AEC assistance yrtyram. Portable research and training reactors are widely used for its* structioo and orientation with nuclear physics and to explain the principles of reactor dcairn end operation. They are also especially useful for radiation de tection and health physics in vestigations, and In moderatelevel trreer research. Such reactors differ greatly In design from larger reactors. They are moderately priced and low powered. Also, shipment, installa tion, operation and maintenance are relatively simple, safe and inexpensive. Like other reactors, portable reactors involve many compro mises in materials selection. Un fortunately, nuclear materials do not always have good structural properties, and are often heavy, massive and expensive. 1. Fuel materials Fuels can he liquid or eelId The fuel materials used in port able reactors can be liquid or solid and are usually enriched with fissionable U-235 ro as to reduce size. Homogeneous reactors use liq uid or solution-type fuels, or fuel uniformly divided and dispersed la a solid moderator. Advanced reactors such as L-77 (Atomics International) and AGN 201 and AGN 211 (both Aerojet-General Nucleonics) are homogeneous re actors and provide the only prac tical experience with homogeneous systems to date. Heterogeneous reactors use sep arate furl and moderator elements and are typified by the TRR (Nu clear Development Corp. of Amer ica) and NC 2000 (Nuclear Chi* cago) reactors. Heterogeneous portable reactors commonly use a so-called "canned" fuel element which, at the present state of the art, is giving ground to more eOcient designs. Design for minimum oriticsl mass The reaction rate and flux de sired in a reactor fixes four fac tors: 1) the number of fuel atoms required per unit volume. 2) the ratio of fuel atoms to the atoms in the structure, 2) choice of cool ant and moderator, and 4) oper ating temperature. Also, nuclear physics dictates the spatial distri bution of fuel atoms within thb core and, in the case of heteroge neous reactors, provides a flrat approximation of the shape and arrangement of fuel elements. Ikimloied by tood Industries Assoriotion 222 Modison Avsnvs New York U, New Yo<h * N 1692.01 M ...- krill' Mfi ,, < ,>; ;jari*ywfiig LIA24396 K In designing a fuel system it is fuel distribution arrangement selection, reduces fuel loading. desirable to approach the minimum This approach, combined with Ju And, from a safety standpoint it critical mass with a nonuniform dicious moderator and reflector insures that any rearrangement of the fue! will decrease reactivity. Consider ths effects of fust growth Nuclear Fundamentals of Portable Reactors Solid fuels have a tendency to grow in preferential direction! Portable reactors art the average number of corres when irradiated, (Since low pow- thermal typo ponding neutrons absorbed. Non cred reactors produce low heat Nuclear reactors are dasti- leakage probability, on the other levels we can neglect the effects fted by their predominant neu hand, is a measure of the prob of heat transfer.) This growth it tron energy spectrum and fall into three categories: thermal, intermediate and fast. Portable research and training reactors are thermal-type reactors and incorporate a moderator and re flector. The moderator slows ability that neutrons will not leak out but wiU remain until absorbed to cause fission. Some neutron leakage is de sirable for research and train ing purposes. Extent of leakage depends on two factors: The caused by elemental changes in the fue) as well as changes in chemical and structural properties. As bum-up proceeds, each fission ing atom is replaced by two fis sion product atoma (some exist neutrons released in the fission first is the dimensions of the ing as gases) which generally reaction to thermal energies reactor. The second Is the dis have a greater volume than th* where the probability for fis tance a neutron travels frem its parent material sion is great. As ite ncjae im plies, a reflector returns to the core neutrons that would other wise leak out of the system. Ths reflector permits a reduction la fuel requirements, thus reducing the coat of both fuel and con struction materials. birth as a fission neutron to its ultimate absorption in an ex periment. This distance Is re lated to a quantity called the mgratioa length. Thermal utilization should bo high For every neutron absorbed The above effects tend to cause expansions and stresses in tho fuel element and have to be ac commodated lest they cause dis tortion. Distortion can affect cool ant flow or the distribution of the moderator within the core matrix. Subcritlcal va critical by a fuel atom, 204 neutrons And the resulting hot spots and Depending on their sustain are produced. Of these, one must disrupted flux patterns can result i-.- ing features, portable reactors be captured by another fuel in shutdowns and costly repairs. Cr~ are either sebcritica! or critical. atom to sustain the reaction. Under such circumstances it is An important factor in this Thus, at the most only 1.01 neu prudent to approach core design 1 relation la the effective nratti- trons are available for experi with the utmost feeling for safety. E plicatKOi X_:Ur, 1 the ratio of the average number of neutrons mentation. Also, some wiU be captured is the moderator and Special liquid fuel problems produced by fission In each gen structural materials In the fuel In principle, homogeneous re eration to the corresponding region. The ratio of the number actors are based on the operation number of neutrons absorbed In of captures in fuel to tho num al characteristics of the KEWB the fuel, moderator, or ber of captures in the fuel-plus- (Kinetic Experiment cn Water leaking out moderator-and-strocture is the Boilers) design. Unlike solid fuels, If k s'l, the chain reaction thermal utilisation, /. liquid fuels can be changed or re > is stationary and eelf-sustain- Ing, or critical If k < 1 the chain is not self-sustaining Since one of the 2.04 neu trons is captured in a fuel atom, 1/f represents the total number moved easily and are free of the effects of anisotropy. Naturally, and is subcriticai (without as captured In the fuel region and the materials retaining the fuels extraneous neutron source the 104--1/f the number that leak must resist corrosion and radib- chain reaction cannot be main out of the fuel region. Thus, if lytic decomposition. Because of ita tained). If k > 1, the chain is f is 1 there will be 1.08 neutron* proper combination of structural, divergent and the system In per U" capture available for nuclear and corrosion properties, supercritical. experimentation. This is equiv stainless steel (a generally used The k values for portable re alent to a maximum vaiue of in contact with aqueuus uranyl ! search and training reactor* am IS i 10* neutrons per sec ab sulfate fuel suspensions. Such sus equal to or less than 1. Flua la confined to 10* to 10>* a per aq cm k < to avoid danger of over* sorbed In experiments for each megawatt of thermal power. This maximum figure is a con pensions should be completely freo of harmful chlorides. ospoiure and needless expense. stant independent of reactor de The problem of radiolysis with Effective multiplication la sign. Thus, to make a research the water moderator in homoge mads up of two factors: the in and training reactor efficient neous reactors Is only slight, sines r finite multiplication factor k and tho nonleakage probability. the thermal utilisation in ths fuel region should be high and the free radicals that form tend to * nbine and establish equi cThe k factor Is ths ratJo of experimental facilities should be librium. The L-77 reactor, for average number of neutron* produced la each generation U properly placed to receive new* Uvns leaking out example, contains a recombiner unit containing platinised ala- tf* *''* * *****' ' IW1^1*.!.. MU.'. ii.aw.1. LIA2^397 mint catalyst pellets which recom bine circulating hydrogen and oxy gen gases to form water. The equilibrium of the water depends on impurities and the fuel solute which can create an excess of either peroxide or hydrogen. An excess of hydrogen tends to sup press decomposition almost com pletely. Conversely, an excess of peroxide tends to increase decom paction. lhe L-77 reactor also contains an overflow chamber to offset pos sible fuel solution expansion caused by bubble formation. If expansion is great enough it will force some of the fuel solution into the overflow chamber, thus making the reactor subcritlcal (k becomes less than 1) and pro ducing an automatic shutdown. Hot gases can bo boAoflclal The gases generated in a re actor can be beneficial For ex ample, in a liquid fuel reactor such as the L-77, "hot" gas can be transported to remote experi mental apparatus giving the unit great versatility. About 10 curies (15 to 20% of the total) of vola tile fission product activity is present in the L-77 under equilib rium conditions. Unlike gases in the L-77 design, however, the gases generated in the AGN 211 reactor are not bene ficial and cannot be used for other gainful purposes. In the AGN 211 core there are about 45 curies of radioactive gases present st steady-state operation. About onethird of the gas is harmful Iodine131. (Maximum permissible con centration for iodine-131 is 5 * 10'* curies per ml or S ^ 10-* curies per cumof air.) However, thia quantity of activity should cause no trouble. In order to be hazardous the iodine must first diffuse out of the oranium oxide particles, through the polyethy lene element and Ita coating, and then escape from the water. This likelihood la extremely small Ufa of polyethylene core materials Polyethylene is somewhat more radiation resistant than water sad tends to trap any decomposi tion products or gases occurring in the matrix. Radiation damage manifests itself in reduced den sity and loss of hydrogen from the polyethylene after exposures of about one week at a flux of 1011 a per cm*-sec. A flux time (nvt) extrapolation indicates that the core life of the material would be more than 10,000 br based on an average continuous power of 100 w. 2. Control element materials Reactors are controlled by chang sired low value of 0.75% k. poisons in the core region. Such ing the nuclear reactivity of the The AGN 211 reactor ia even changes are typically accomplished core. Change ia positive reactiv more flexible. Aside from certain in the AGN 211 reactor by two ity can be made by moving the disadvantages inherent in swim Boral safety rods: an aluminum- fuel or by changing the modera ming pool-t>pe reactors, this re clad cadmium rod for coarse con tor density or core leakage. actor has unrivaled accessibility trol and a stainless steel rod for In the case of s reactor like and adaptability in its class. It ia fine control TRR, simply changing the deute purportedly the only reactor in In a subcritlcal heterogeneous rium oxide level of the heavy which the core mater ials and crit reactor using natural uranium and water moderator permits fine ical geometry can be varied in light water moderator (such as "tuning" of the excess reactivity three dimension! without fabri NC 9000), no control system U available. Fuel consumption can cating new fuel dementi for each needed to prevent it from going then be compensated for by add variation. critical accidentally. Natural ura ing small amounts of deuterium Changes In negative reactivity nium can go critical only when R oxide, and the excess reactivity are made by changing the concen Is used in combination with heavy present need never exceed the de tration of neutron absorber! or water. 3. Moderator materials Neutrons released during fission have very large kinetic energies-- as high as 2 mev (million electron volts). Moderation changes this kinetic energy to thermal energy by forcing the neutrons to collide with the nuclides of construction materials. The best moderator materials have low atomic weight, high scattering cross section. Urge energy loss per coltision, and low absorption cross section for ther mal neutrona. Water, beryllium and graphite is advantageous since it enables Hava good properties the U-235 fuel to compete more In this discussion we can ignore favorably with water for neutrons aqueous homogeneous reactors In the fuel region. Thus, thermal since their moderator-fuel make utilisation may be almost the up is Urgely Axed. Although rela same whether heavy water or tively expensive, heavy water, light water le used. beryllium and graphite are emi However, apart from the above, nently suited as moderator mate the designer must transport, with rials and provide lower fuel load-) . minimum losses, the maximum Ings than other materials such aa , * number of excess neutron to where light water. However, extra fuel . they can be experimentally used. loading in a light water reactor Low absorption material* provide i f ! -a* LIA24398 an efficient **pipinf .rilem.w Deu terium dioxide in particular deliver* more neutron* per gram of uranium consumed than an/ other --it* efficiency of delivery it al most 100?&, Organic materials ara Inexpensive Less expensive moderator ma terials are also available with properties approaching those of heavy water, beryllium cod graph ite. In addition to light water, organic materials such aa the poly phenyls, paraffin and polyethylene can be used. These organics are particularly interesting because of their; 1. High hydrogen-to-carbon atom ratio. 2. Compatibility with fuel, can and other structural material*, resulting in low corrosion rates. 3. Low induced radioactivity. 4. Elimination of hazardous chemical reaction*. 5. Fair resistance to radiation damage (.ee table on p 113). Organic moderator materials help to reduce cost since they can be used with inexpensive carbon and low alloy structural steels. They also permit weight reduc tions because of their compatibil ity with light, low-crow-iectlon materials like aluminum. Organics plus motel* offer good combination The fair resistance of the or ganics to radiation damage can be improved by mixing them with appropriate reflector, gamma shield or fuel materials. The com posites perform dual roles, Im proving both the durability of the organic material and the effective ness of the metal. Combination with lead or lead-lithium pellets, for example, reduces energy ab sorption by the organic material This technique is used in the multi-region primary of the L77 reader. The relatively poor heat tranafer properties of the organics can also be offset by combining them with other material!. For ex ample, intimately mixing hydro genous organics with lead or other metals increase* thermal conductivity and specific heat, thus allowing higher operating temperatures, eliminating hot spot*, etc. The AGN 211 reactor use* fuel elements made of 20% enriched ur&nium dioxide particle* em bedded in a radiation-stabilised polyethylene. This system has so inherent safety advantage due to the negligible time delay in the transfer of heat from the fuel particles to the polyethylene mod erator. It something should go wrong the prompt heating of the moderator would increase the thermal energy of the neutrons sod tend to reduce fission-toeapture probability. Furthermore, the prompt heat expansion in the fuel-rooderator elements causes a decrease in density and iocreaaes leakage from the core, resulting (n immediate shutdown. e i i 4. Reflector materials The purpose of a reflector is to return to the core leakage neu trons that are not intended for experimental use. The size of the core (fuel and moderator), it* critical mass, and its cost are sub stantially affected by the presence of a reflector. Important selection factor* Like the best moderators, re flector materials are characterized by a high scattering cross section and low neutron absorption. They should be chosen so aa to: 1. Minimise costa. 2. Provide a large negative temperature and void coefficient (these quantities tend to increase with a decrease in reflector thick ness). 3. Eliminate or reduce core- reflector interface flux peaking and smooth flux distribution fa the core. 4. Properly control the flow of neutrons into experiments. Lead reduces weight# east One of the most useful reflector materials for portable research and training nzetort Is lead. Important savings in weight and costs are obtained when it is used in the dual role of reflector and gamma shield. The homogeneous L-47 reactor (uow replaced by the more refined L-T7) was the first to use lead in this way. Lead acts at sn essentially In finite reflector in the L-47 design. This means that experimental apparatus used at or near the reflector surface will not perturb the neutron balance in the core. Furthermore, so hazard ( pre sented in the vicinity of the re flector when the apparatus is moved during operation. Also, since lead acta as an effec tive shield against In-core resid ual gammas, personnel have ac cess to the Interior when the secondary shield water tank Is drained. In addition, since the lead Is cast and bonded around the core vessel St helps improve the resistance of the core vessel to any sudden pressure surges that may oceur. The L-77 carried the use of multipurpose materials even fur ther. Here an intimate mixture of lead shot and diphenyl is used inside the prim*ry structure to provide neutron shielding or mod eration In combination with gamma shielding or reflection. Composition hardboard alaa low In cost Another popular reflector ma terial la composition hardboard. Hardboard has good nuclear prop erties and maintains high neutron flux density within the core and experimental region. It is rela tively free of impurities that ab sorb neutrons and has a targe entering cross section. Further more. it is inexpensive, readily available, and offers good resist ance to moisture and heaL la designing the TKR reactor several reflector materials were considered, such as graphite, heavy and light water, beryllium and wood planking. Because of its low cost and other desirable properties hardboard was chosea O I J o .' IJ .l"**"' Jl" LUJN(JSWPle"W7* wwwnwi --' lu>MI W a.,1 LIA2A399 o ---* ~| TRR MKtOf WtA cktVbfmf rwwwi h?<Ktr*m r</l*cor ujtcf on tides an4 6{tom eofwiitf of 1 ft of graphite theathed xntk 3 in. of vuuonito tchich in htrr. it covered with 4-in. layer of borated (S to 10%) polyethylene ehielding. Additional shield (net thston) consult of t-ft layer of ordinary concrete blocks. Enriched uranium fuel it centrally located in aluminum reactor tank and to cooled by natural convection of heavy water. rnent it compored e/ a homogeneous nurture o/ uraruuM dioridt in polir ethyteno, A d-in. graphite ee/Uctor it used at each end 0/ (net element and a t-in. lead shadow shield j. need at outer endo of ffraphite. to supplement graphite and heavy water reflectors. Cost of the hardboard for this spplication was |1.10 (all prices per oq ft) os eOiCpAred to $38 for grephito, $600 for heavy watei and $3000 for beryllium. Wood planklnr waa rejected because it was less homo geneous than hirdboard and more lusccptir* to warping and shrink ing. Light wit*" was rejected since it required a container that would be difficult to erect and modify. Tha hardboard la used in a 4-In. thick laminate made up of 5 layers, weighs 3.1 lb per aq ft, and la atructurally self-repporting. 5. Shielding materials Shielding fall. Into two brood tha outaide aurfaca of the reactor In addition to having proper classifications: thermo) ond bio to a tolerable level--generally nuclear properties, shielding ma logical. Thermo] .hleldinf I. tued agreed to be 7.S mlltlrema per hr. terials must be Inexpensive and to dissipate exceuivo beat from Unf vtunately, materials that readily available since they are high absorption of radiation en effectively stop neutrons art poor used la large quantities. Typical ergy. It I. not needed with (mail gamma absorbers. Up to about 3 good shielding materials are lead, o portable reactor* becauao of their mev, neutrons are attenuated best polyethylene. Iron, concrete and low power levels. Biological shleld- by low-atomle-welgbt materials. water. Numerous metals may be Ing, however, la needed to reduce Conversely, good gamma absorbers used for shielding but lead Is pre gamma and neutron radiation at should hava high atomic weight. ferred because It can be used rala- fifU'HB: -n s.i ^(i 111 "! 'W 1 squill wj urj* ti j j LIA2 4400 l I1 -.Wn Three Important Design Objectives &. Minimize radiation dump Although the dux level ia port* able research and training re actors is usual!/ lower than ia other reactors, radiation can till affect the strvctural integrity of materials and components. Radiation resistance depends on many variables and cannot be predicted with absolute assurance (see "Radiation Damage ia Metals," M/DC, Jan '60, p 69). Complete assemblies may aid con stituent parts or materials. Oa the other hand, the proximity of certain materials may render useleu otherwise resistant materials. Some materials may have a lower threshold value than others, but may be more resistant to higher dotages. For example, nylon shows radiation damage sooner than TFC fluorocarbon, but with increasing dosage nylon main tains fair integrity long after TFC fluoroearboa has failed. Similarly, devices such as semi conductor diodes and triodes con tinue to function with degraded performance at dosages far above the damage threshold. Ia short, extrapolated damage data should be checked by actual testa. The tables at far right show the approximate levels at which damage occurs for various reac tor materials aad components ex posed to irradiation. These levels are conservative, and ia most eases the specimens also received an equal number of gamma pho tons From the data and cooaidering power levels it is appar ent that organic materials and some electrical components are most likely to be affected by ir radiation. Selection of the most resistant material should be conaiitent with nuclear and cost re quirements. Most instrumentation prone to damage can be relegated to remote consoles and positions. However, Uua may be impossible with electrical machinery, drive mechanisms, etc. Electrical failures can be caused by insulation or lobrication break down. Formes and polyvinyl formal Inculatious for magnet wire appear to be quite resistant at medium temperatures. Nylon, polystyrene, poi>batyral, mineralfilled phenol formaldehyde, poly ester and furane plastics are all good organic insulations capable of withstanding high radiation dosage, although their physical changes most be considered in specific applications. Acrylic, TFE and CFE fluorocarbons, vinytidene chloride copolymers, casein, vinyl, epoxy and all cellulosic materials become poor insulations when exposed to high dosage. Alnico permanent magnets are not appreciably affected. Increas ing radiation resistance Is ex hibited by Ihe inorganic, inor ganic-filled and heat resistant insulation materials. The best known insulations under intense radiation are glass and quarts which deteriorate very slowly. No physical damage is observed in temperature sensitive devices such as thermocouples and resist ance thermometers below dosages at which the basic materials ars affected. The damage in these components Li primarily caused by temperature rise due to irradi ation. Radiation heating 1c a characteristic problem ia eoeTgetie-type reactors and requires the o of pressure shells and special heat dissipation shields. It is not normally encountered Us low-power `'package'* reactors. 2, Minimize corrosion Salts containing boron ars fre quently used in water shields to further attenuate neutrons and to reduce radiolytis. Testa show the following range of corrosion rates (in mils per year) of metals ia several berated solutions: Ja4. 0.1 to 0.6; mild steel, 0 to 04; mild steel-lead combinations, 04 to 0 9. These rates are negligible. Stainless sleet can be expected to have still lower rale*. Aluminum, however, despite its good corT^ aioo resistance, is susceptible to galvanic attack. An example of how corrosion can occur in portable reactors is provided by the NC 9000 reactor. Corrosion was traced to galvanic action between cor* components and the tank material. It was found that galvanic action could be stopped by replacing tap water with purified water having higher clectroylytic resistance. Corrosion can also be prevented by using suitable inhibitors Ui solution. Testa conducted with a 1630 ppm lithium chromate solu tion show that its inhibiting ac tion it effective for at least six months with carbon steel, copper and lead. tively thin, thus providing cost and weight savings. Designing primary and secondary shielding The primary nr first shielding immediately surrounds the core or core structure. It ia made of solid materials. Is invariably leakproof, and In such reactors aa AGN 201 and L-77 la made inte gral with the package. Great attention Is required (n its design since it comprises much of the overall cost and weight. By nature of their design, swimming pool reactors such as AGN 211 do not have a leakproof primary shielding. However, the induced core radiation level of this reactor is low enough for aafe fori handling while providing sufficient flux for many activation experiments. Secondary shielding <s not con sidered part of the reactor pack age since it is usually eroded on the site. Water and/or concrete blocks are frequently used as sec ondary shielding. Unlike primary shielding, secondary shielding (a not used In equal thickness around the entire reactor. Secondary shielding requirements above or below the reactor can be mini mised by taking into account the benefits of the foundation or floor below the reactor and uninhabited air space above the reactor. In most designs access to the ir /X w io j u l *i LIA24401 l p mu u spwwrwaytoitofwfwi ii>iw(iwPTw*wsijmwwFWl*l,9WW|ywwf9wiw*99,wf9ir"i*4*^^ --a--. ' 4 - Truilfti --rill nrUS . o oo S, MhilmUs ih I|M Naturally th weight of port able reactor* Km to be kept to a minimum. Hownrer, compromise* In materials selection are inevit able, and the osuaJ practice is to use mixture* or layers of gamma ai d neutron absorber*. Simple geometry tells us that considerable weight savings are obtainable with circular polyshielda. The first drawing below shows the approximate weight difference in two shielda designed to provide i same doee con straint. As indicated, the leadwater design has a considerable weight advantage over the altwater design. S milar compartsoas for slab-type shields are shown la the second drawing. o s t ia UjOmc warn a tt*0 caapoo* - voraa wcwwf - uvoo m Csffpirsfhs weights of spheri cal KiUU mode e/ water aleno end doable leper #/ troler and lead, lit--ill wa *Tte a nu <r.*oe *- rena. o p w m- *asoo% Comparative weights / slob tMiebU modi a/ water aai (refer pfai lead. 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S * IS" faa* UiltMi'e ca lilia at ar>I. **4 1A 10" Pmm.W to (>k w # s I Mfl< JKAMATIOW *Ma.vr * lOSISIU AWO (HSTWUWOCT COrOWO*Tl CompoAMt Capacitor tautor $aiy* tsdubm-SmnppUf tutorial CftmaM ThrvPKW Pif'isft (2590. rads Pap* * at diriscthc. pUmtc * u pap* c m* 44-liW PUtbccww 43-7* 19 fUsbc cssa tod actuator 43-7 a 19 (Vfswc *ir HtsJsbOA. bnrt* fceto*. tol town, a*d pwndwA|s castftf imtoafiwi. 43-7119 1 reactor I. lateral or redial and it The AGN 211 and NC MOO re stages. First, hick energy neu ia in these directions that shield- actor. differ from othera in that trons passing tbroucb a ahietd are inf requirements and dimensions experimental acceaa to the core is slowed by elaalie or inelaoUe an greatest. That, reactors are from the top of the reactor. eattering; the capture trees Me* centrally aomewhat wider than Shieldinc for the AGN 211 la the tioa for neutron, at high energies they are hlfh. However, overall redial dlrectlona la provided by la extremely small. The second diameter ia usually kept under I 40 In. of concrete in the form of stare of capture occurs only when (t to facilitate transport. When overlapping blocks. Vertical shleld- the energy of neutrons drops t water or h im other liquid It uaed inf I. provided by ( In. of graph- low values. Large cross soctiooa o for aecondary hieldii'c the tank can double aa a structural lupport for the core and/or the primary ita, 2 in. of lead and 4V4 feet of water. Attenuating neutron. of most elements will produce quirk capture of low energy ar slowed neutrons. Therefore, the ahleldlnc. Neutron, are absorbed la two usefulness of neatroa shield aio i unjA ^ s u..im* *vn WH.. *w iini.,3J9WP>P*.1 *JiiiiUSpwgpw LIA244Q2 bWm r1 ---^ri --ri IV -ifr iTr- -Wiiiri -- ............. - ^ *v n --- ------ ------ terials depends largely on their from neutron interaction with radiation and captures a signifi ability to attenuate high energy lead. The alpha radiation pro cant portion of thermallted neu neutrons. duced has an extremely short trons, The third region, made of Elastic and inelastic scattering are produced by collisions between high energy neutrons and the range--several microns la lead-- and is comparatively harmless. Although the alloy tends to cor a mixture of lead peJIcta and paraffin, sets as a neutron-gamma shield. o shield nuclei. In Inelastic scatter rode in air and water, corrosion Mixtures of lead and paraffin ing the recoil nucleus U left in an can be prevented by incorporating should be nude as light as pos excited state and it therefore it in hydrocarbon materials. Lead sible. Since neutrons create hard absorbs a larger fraction of en is virtually Inert to nonpolar hy gammas in the carbon of the ergy than it does with elastic drocarbons tnd only slightly at paraffin, there is no need to have scattering. In general, inelastic tacked by moat polar materials. more lead than Is necessary to scattering it more important with Use of a lead-boron alloy for reduce the core gamma strength heavier nuclei which can be more gamma shielding has not proved to approximately the tame level easily raised to excited states. feasible because boron is only as the neutron-induced *ecoodaxr Elastic scattering is more impor lightly soluble in lead. Similarly, gamma strength. (Boron addition! tant with light nuclei which can production of the alloy by powder can serve to reduce the secondary absorb a large amount of energy metallurgy has not worked out gamma level. Lithium on the other via recoil. because of segregation problems. hand could reduce the contribu Neutron shielding must also be However, a cementation technique tion of lead and the hydrocarbons.) designed to protect against sec developed by Battelle Memorial Thia may suggest use of lead pow ondary gamma radiation produced Institute appears promising (see der suspensions; however, these by inelastic scattering and cap U iltxima et 41). could tend to separate by gravity ture. This can be done by in corporating absorbers such as Coffovsits shielding combines advantages of single materials over r rulongrd periods at mod erate temperature*. This would boron which reduce the energy of not be the case with pellet shields. * capture gammas and remove low energy neutrons as they are pro duced. Boron can be used in the form of soluble or insoluble salts, dej*ending on the medium, or in the form of Boral, a mixture of aluminum and boron carbide. The TRR reactor uses a borated plas tic; the IrTl reactor uses borated Organic-lead mixtures are ad vantageous where it is desirable to have a hydrogenous material for neutron attenuation together with a heavy material with a high neutron inelastic cross section but low gamma ray production effi ciency as the gamma attenuator. Since carbon is a good source of According to ffoce, cylindrical pellets would be teas expensive than most spherical pellets. Also, the effective density of composite shielding can be varied without affecting the packing factor by using a cylinder of one material (f f, carbon steel) with a liquid core or core of another metal fe.g.. paraffin. Attenuating gaatens radiation The effectiveness of s material to shielding against gamma radi ation depends strongly on its spe cific gravity or atomic number, i.e^ electron density. It is useful to remember that a gamma inten sity of 2 mev is reduced by a fac tor of ten by 1? in. of water, 10 in. of concrete, 2.6 in. of iron and 1.7 in. of lead. hard gammas when bombarded by high energy neutrons, U is desir able to distribute lead throughout carbon shield*. In addition to satisfying nuclear requirements, mixtures of lead pellets and or ganic materials provide good de sign flexibility and inherent satety. Also, homogeneous mix tures are easily cast Into shapes and easier to work with than laminated layers. Spherical lead pellets have been lead). This arrangement permits uniform materials distribution-- more o than a random mixture of solid pellets of different materials. Also, by using cylindrical pellets to realize a specific packing fac tor, it is possible to obtain a ranre of gamma and neutron stopping ratios by changing pellet core size and the filling materiaL The average packing factor for a random arrangement of singlesize cylinders (with length equal Gamma shields in themselves used to advantage in the unique to diameter) Is 9.366--slightly are also a source of neutron-pro primary shield of the LT7 reac less thar. that of random-packed, duced gamma radiation or sec tor. This shield consists of three single-size spheres which have a ondary radiation resulting from concentric regions. The inner packing factor of 0.4. capture. This adds to the secon most rerion is composed of a mix dary emission of hydrogenous shields and to the attendant weight of shielding required. According to Sanjen it of, a lead-lithium alloy shows ?ood potential protec tion. Because of its thermal neu tron absorption properties, lith ium Is effective In suppressing gamma ray production resulting ture of lead pellets and diphenyl which surrounds a stainless steel sphere containing uranyl sulfate. It acts primarily to reduce neu tron leakage, moderate escaping neutrons, and attenuate gamma radiation. The centra) shield region is composed of borated paraffin which further moderates fs/WMCM B4 p,, -A* mU Pw i<i w < Sy1--l 'St, on m iiiiit. tin* ft. H, H*e4ft. K Lm4 Ifc*. ft. M-, -LmSLNUm JUW*. cWtaa.'' OILNUM4, Kt4lvfff *4 TO-4MS, oek Xu#- x ml u*. **. >. n k , o . a Ate ru KAruass. *sa WutUM. D. M. n*mtk. J. A. JtMm, K `A N Om f ti>< Sw Aim" arM rnmi re `a o Rffi/Hu J jr-'m Ma m ma m in Di 'Kj N Cmj mi*in <s 0.`"Ar !%o it<*t Runltol.l PuHidung OocrofAlkm * Vo P^rk Auntie. New York .*2. N. Y. V9* is ij i n ifiim j x T