US3085157A - Protective housing for radioactive sources - Google Patents

Description

April 9. 1963 SS RHERENCE l. GINSBURGH E'TAL SEEKRCH ROOM PROTECTIVE HOUSING FOR RADIOACTIVE SOURCES Filed July so, 1959 2 Sheets-Sheet 1 Fig. 1

IN V EN TORS Irwin Ginsburg/r y Herbert G. J. Myers ATTORNEY April 9, 1963 l. GINSBURGH ETAL PROTECTIVE HOUSING FOR RADIOACTIVE SOURCES Filed July 50, 1959 2 Sheets-Sheet 2 Fig. 3

INVEN TOR-3 Irwin Ginsburg/r y Herbert 61.1. Myers W/ZZ/ ATTORNEY nited Patented Apr. 9, 1963 3,085,157 PROTECTIVE HOUSING FOR RADIOACTIVE SOURCES Irwin Ginshurgh, Chicago, 111., and Herbert G. J. Myers, Hammond, Ind., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana Filed July 39, 1959, Ser. No. 830,547 1 Claim. (Cl. 250-406) This invention relates to a container or housing for radioactive materials, and more particularly provides a housing for use when such materials are employed in industrial locations.

Radioactive isotopes are in wide use in many petroleum and chemical processing industries for such purposes as level monitoring, thickness measurement, and density determinations. These isotopes are available in a variety of materials, structures, and activities, from both government and private suppliers. These suppliers have devel oped a number of containers for positioning the sources near the associated equipment, and such containers are generally quite safe with respect to affording biological protection. However, we have found that no supplier of radioactive isotope housings has recognized a problem which potentially may lead to serious consequences.

To understand this problem, it should first be recognized that many of the gamma-active radioisotopes are available in liquid or powdered form and are sealed within a cannister of aluminum or stainless steel. These cannisters are only usable at temperatures below about 1500 F. Above 1500", the silver solder used in sealing the cannisters melts and unseals the source. Radioactive material may then spill out or even vaporize and escape into the surrounding atmosphere. Even solids may oxi dize and spall in the 2300 F. heat from open hydrocarbon flames. Or, the source may actually explode the cannisters from internal pressures caused by heat. In any of these cases, powdered radioactive material can be scattered over personnel and over a wide area, rendering that area unsafe for humans for a considerable period of time. Since this escape of radioactive material is most likely to occur during accidental fires, the existence of fallout may prevent firefighting personnel from entering the premises for the purpose of combating the fire.

Notwithstanding these potential hazards, no manufacturer of radioisotope source containers offers any means for aflording fire protection for the radioactive source. Accordingly, a primary object of the present invention is to provide a container or housing for a radioactive source which overcomes the problem of damage to and leakage from the source in the event of fire.

Briefly, in accordance with the invention, we enclose the radioactive source together with its biological shield totally within a container or box made of insulating refractory material.

The invention in its various aspects will be more fully understood from the following description when read in conjunction with the attached drawings which form a part of the instant specification.

FIGURE 1 is a sectional side view of a fire-proof container embodying the principles of the present invention;

FIGURE 2 is an elevation of the view shown in FIG- URE 1;

FIGURE 3 is a top view of the container with its cover removed, showing the internals of said container;

FIGURE 4 is the detail of a bearing which may be used in the present device; and

FIGURE 5 is the detail of a special structure which integrally combines the shielding and fire-resistant functions in one body.

Turning first to FIGURE 1, the container is placed near a wall 9 of a vessel, the contents 10 of which are to be measured for thickness or density by means of transmitted or reflected radiation. The container is shown as resting on a support such as bracket 30, although it may be secured by equivalent supporting means such as brackets, straps, bolts, etc.

Essentially, the container according to the invention houses a radioactive source 7 within a shield 4 of dense material, and the assembly of source 7 and shield 4 is disposed entirely within an insulating refractory box or container made of high-melting low-thermal conductivity material 13.

Radioactive isotope 8 of source 7 may be any naturally or artificially radioactive isotope which emits gamma rays of sufficient number and energy to penetrate refractory insulation 13, wall 9, and the contents of the processing vessel. Numerous radioisotopes are Well known and are listed, for example, in the publication "Radioisotopes-Spccial Materials and Services, published by the Oak Ridge National Laboratory, Oak Ridge, Tennessee. As specific examples of radioisotopes which may be employed in conjunction with the present apparatus, there may be mentioned cobalt-6O (1.33 mev. gamma, half-life of 5.27 years), cesium-134 (0.794 mev. gamma, 2.3 years), and cerium-l44-praseodymium-144 (0.696 mev. gamma, 282 days), etc. These isotopes are available either in solid form, as powders, or as solutions of a metal salt in water or other solvent. The particular source shown in FIGURE 1 comprises an external single or double sealed container 42 which confines isotope S, and which has a threaded portion at one end and a slotted portion 11 at the other end. By this arrangement, source 7 may be removably inserted in shield 4.

Shielding materials such as those employed in shield 4 and shield 12, are dense materials which attenuate radiation from source 7 and afford biological protection to personnel working in the area. Materials useful as radiation shields should have a specific gravity greater than about 4.5, and preferably greater than about 6, since the thickness of shield required to produce a specific absorption of radiation is a function of the density. Lead, either alone or in alloys with various other materials such as antimony, is most commonly employed; lead has a specific gravity of about 11.3. Iron, either as wrought iron, cast iron, steel, etc., is likewise suitable, as it has a specific gravity of about 7.8 and additionally possesses a substantially higher melting point than lead. Other metallic elements or compounds of suitable density may be substituted in whole or in part for the lead or iron in shields 4 and 12.

Shield 4 which receives source 7 is preferably a lead or iron cylinder and is rotatable within the insulated container. Details of a preferred method of effecting such rotation are best reserved for a discussion of FIGURE 3. Cylindrical shield 4 is provided with a radial recess or hole 5 which is threaded at end 6 to receive source 7.

In the position shown in solid lines of FIGURE 1, source 7 is adapted to radiate in the direction of the small arrow into the contents 10 of a processing vessel and the like. A special advantage of the embodiment shown in this FIGURE 1 is that rotatable shield 4 may be turned in the position shown in dotted lines 511 so that radiation previously transmitted into the processing vessel is now absorbed by a second shield 12, which may be contoured to permit shield 4 to rotate. This second shield 12 is desirably placed in one corner of the interior of the insulated container and made of a material which likewise is sufiiciently dense to absorb most of the radiation coming from source 7.

Source 7 and shields 4 and 12 are disposed in accordance with the invention entirely within an insulating refractory box or container.

Insulating refractory 13 surrounds all portions of source 7 and shields 4 and 12 to thereby protect these elements from destruction by fire in the event of a conflagration. Thus, should the shields melt and deform, even if the source is undamaged, there could otherwise be severe radiation exposure to nearby personnel fighting the fire.

Insulation 13 should be composed of a material which has a melting point of at least about 3,000 F. so as to be able to directly withstand temperature effects, and should have a thermal conductivity, as conventionally determined at about 100 F. of less than about 30 B.t.u./(hr.) (sq. ft.) F. per inch), and if possible should have a thermal conductivity of below about 20 B. t.u./ (hr.) (sq. ft.) F. per inch). This insulation, as previously noted, entirely surrounds the internals of the container.

In the region wherein radiation is to be transmitted through insulation 13 and wall 9, a panel of insulation 14 should be of a material which has a low specific gravity so as not to materially reduce the intensity of radiation through the walls. Thus, in this region, which is adjacent hole in shield 4, the specific gravity should be less than about 4.5, and preferably less than about 0.5, in addition to possessing the previously indicated low thermal conductivity and high melting point. Many materials having such characteristics are known, and are available commercially under such names as Thermabestos, Superex, Fireblock, etc. Elsewhere, insulation 13 may be made of either the same or of another insulating refractory. Specific examples of other suitable materials are alumina (specific gravity 4.0, thermal conductivity at 1,000 F. of 22.0), blast furnace grade carbon (2.10, 24) diaspore (3.4, 11.6) calcined dolomite (2.8, 15), fire-clay refractories (2.6, 7.7), magnesia (3.6, 27.6, silica (2.65, 9.6), asbestos, vermiculite, etc.

The specific embodiment of FIGURE 1 shows the source 7 and shields 4 and 12 placed with a perforated metal support line 3 with beam hole 41 which is then surrounded by insulation 13. The thickness of insulation is selected with a view toward the expected fire hazard and the degree of safety desired, and is preferably at least one inch, optimally about two inches, on all sides. A thin external liner 1 of carbon steel or the like in turn surrounds the insulation to provide mechanical protection for insulation 13 and to weatherproof the same.

In order to provide access into the container, at least a portion of one side is made removable. This may be the top 40, which lifts olf to expose the contents of the inventive container. Cover 2 which is on top of top may be held in place at one edge by shoulder 17 and at the other by a lock such as hinge 16, which may be closed by the insertion of pin 15.

Turning now to FIGURE 2, an elevation of the structure shown in FIGURE 1 is displayed. A crank or shaft 21 is attached so as to operate or rotate shield 4 in order to move it into a non-radiating position with respect to shield 12. This shaft 21 may be weighted by a large metal block or weight 27 which has an open and a close position. The weight of block 27 is such that gravity tends to move it in the normally closed position and thereby cause rotation of shield 4 with respect to shield 12 to intercept the radiation.

In the preferred form of the invention, shaft 21 is held in the normally open position by means of a temperature responsive element 26. Element 26 connects bracket 24 with shaft 21 and maintains the source in normally open position. However, should the atmosphere temperature reach a pre-determined level, as would occur in the event of fire, temperature-responsive element 26 would melt, permitting gravity to swing weight 27 and arm 21 to the normally closed position and thus interrupt radiation into the processing vessel. A catch 53 locks the weight in the beam-off position. Temperature-responsive materials for use in element 26 are well known, and may include low-melting point metals such as -50 tin-lead 4 solder, Woods metal, Roses metal, bismuth, and the like, in addition to bi-metallic elements and the like.

This feature of temperature-responsive interception of radiation is of special value when firefighting personnel may be required to enter the processing vessel before there is. suflicient opportunity to manually deactivate radioactive source 7. In the event that the source container is dislodged from its location during the fire, the shut-01f radiation beam does not constitute a runaway radiation hazard to personnel. However, if it is unnecessary or undesirable, shaft 21 may be held in either open or closed position by suitable clamps, padlocks, or the like.

Turning now to FIGURE 3, a top view of the previously described embodiment is shown. In this view, the cover represented by top 40 and lid 2 has been removed so as to expose the contents.

It is seen that shield 4 is a cylinder which is rotatable by means of shafts 20 and 20a which are connected to the metal of shield 4 and which pivot or rotate in bearings 18 and 18a. Shaft 20a is connected via pin 23 to small diameter shaft 21, which carries weight 27. Shaft 21 passes through insulation 13 via port 22. Shaft 21 is small in diameter in order to minimize heat leakage into the housing in the event of fire. An alternative arrangernent is to mount the entire weight 27 and shaft 21 assembly inside the fire-resistant housing described herein.

On alternate procedure is to use shields 4 and 12 made of a dense material which is also a good insulating refractory. Some concretes are available which will fulfill both these requirements. The exterior housing would be omitted and the radiation is held and the fire-resistant housing is one and the same physical device. A device of this type is partly shown in FIGURE 5. Source 7 is in shield 4 which consists of a dense shielding inner concrete layer 50 and an insulating outer concrete layer 51. The second shield 12 can also be made of concrete.

FIGURE 4 shows an illustrative detail of bearing 18. This bearing is open at the top to permit manual removal of the entire shield 4 for insertion or replacement of source 7. In the embodiment shown, brackets 24 and 28 prevent inadvertently directing the radiation from source 7 upwards unless and until shield 4 is manually removed from the open portion 19 of binge 18. Two of binge 18 or equivalent may be required.

The desired thickness of the several shields and insulation 13 with respect to affording biological protection should be selected with a view to permitting personnel to work safely in the area. Current Atomic Energy Commission requirements limit personnel to 20 mrem. per day for a five-day week, and if personnel are to continuously work in the vicinity of sourse 7, then sufiicient dense material should be employed so as to ensure that they are not over-exposed.

From the foregoing description, it is seen that we have provided an especially useful and valuable container for radioactive materials. By insulating all portions of the radioactive source and shielding material with an insulating substance, damage to the source and shield as a result of fire can be reduced or eliminated entirely. Furthermore, in the event that over temperatures should be experienced within the container as a result of very long exposure to high temperatures, the additional mechanical enclosure afforded by the porous insulating refractory material 13 and the perforated metal liner 41 insures against widespread dissemination of radioactive isotopes by providing a large mechanical trapping surface. Also, such dense but low-melting materials as lead can be used as shields without undue fear of the shield melting during a fire and thereby exposing firefighting personnel to harmful radiation.

While the invention has been described with reference to particular embodiments thereof, it will be apparent that variations and modifications will be evident to those skilled in the art. Accordingly, it is intended to embrace all such modifications and variations as fall within the spirit and broad scope of the appended claim.

We claim:

An improved radioisotope source unit comprising: a radioactive source capable of emitting gamma rays and normally susceptible to destruction when exposed to fire; a rotatable radiation-attenuating shield for said source adapted to permit said source to radiate gamma rays through said shield in one direction only, said shield being made of a dense material having a specific gravity of at least 6 and of sufficient thickness to afford biological protection to personnel; a fixed radiation-attenuating shield adapted to intercept gamma rays from said source; a refractory insulating container completely surrounding said shields and said source to protect both said shields and said source from external fire, said container being made of a high-melting low-thermal-conductivity material having a melting point in excess of 3000 F. and a thermal conductivity of less than about 30 B.t.u./(hr.) (sq. ft.) F. per inch), said material in at least the region of said container through which said gamma rays are transmitted having a specific gravity of less than 4.5 so as not to materially reduce the intensity of gamma rays transmitted through said region of the container; and means for rotating said rotatable shield from a position allowing gamma rays to be transmitted through said region, to a safe position in which said gamma rays are directed into said fixed radiation-attenuating shield.

References Cited in the file of this patent UNITED STATES PATENTS 2,477,648 Piggott et a1 Aug. 2, 1949 2,512,711 Bremer June 27, 1950 2,586,839 Maples Feb. 26, 1952 2,685,984 Lisciani Aug. 10, 1954 2,744,199 Juterbock et a1. May 1, 1956 2,773,459 Dechy Dec. 8, 1956 2,796,529 Morrison June 18, 1957 2,829,608 Blackburn et a1 Apr. 8, 1958 2,876,363 Forrer et al Mar. 3, 1959 2,891,168 Goertz et a1 June 16, 1959 OTHER REFERENCES Radiation Shielding, by Price et al., Pergamon Press, 1957, page 231.

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