IL26064A - Encapsulated radioactive material and method of preparing it - Google Patents

Description

o ENCAPSULATED RADIOACTIVE AND METHOD OF PREPARING I T The prfesent invention relates to a means and method of providing heat and ionizing radiation sources for energy generation and material irradiations and more particularly relates to a means and method of providing radioactive heat sources which may used as energy ting as product and food and as plastic and detergent There h s been an increasing demand for radioactive heat sources for energy purposes which are included in such devices as thermionic and attempts have been made to provide such heat sources by the fabrication of radioisotope capsules or wafers that are porated into such devices as energy Great has been encountered in the suitable manufacture of such components in view of their radioactive Prior to the development of the present concept of cold sources of and that are active were hot Because of the inherent dangers in the handling and encapsulation of these radioactive elaborate facilities are necessary at substantial cost to provide adequate radiation These elaborate facilities greatly reduce the flexibility in design and fabrication of such heat sources result in greater production and handling There has also been an increasing demand for radiation sources to reduce the bacteria content in food and some edible products and to change the properties of certain materials such as plastics and costly high energy isotope sources that require considerable shielding have been used for such In some accelerators have also been used for the same But both high energy isotopes and accelerators require considerable space and the use of cold encapsulated relatively low energy sources permit the relatively inexpensive preparation of a source that needs a relatively small amount of Space ments and shielding weight is so low that the unit can be readily moved from place to It an object of the present invention to provide a means and method of fabricating wafers or discs that can be made radioactive for use as heat sources energy generation or for other applications requiring ionizing tion It is also an object of the present invention to provide a means and method for fabricating heat ionizing radiation sources that reduce hazards normally associated with preparing radioisotope The present invention also eliminates the necessity for elaborate nuclear or radiation facilities with a consequent reduction in cost for the preparation of radioactive A further object of the present invention is to provide a method of encapsulating a stable source of material in an encapsulating material that is also stable Another object of this invention is to provide for the cold encapsulation of a stable material having a tively high thermal neutron cross section in a stable lating material having a relatively low thermal neutron cross section a relatively short half With the cold encapsulation the radioactively inert or stable material may be subjected thereafter to a neutron flux of a specific level to induce sufficient activity into the for its use as a heat source or a materials The present inventio also provides a means and method by which stable material suitable for exposure to neutron flux for use as a radiation source may be fabricated and stored prior to exposure to neutron flux for indefinite periods of In the present invention there is provided a means and method i which a stable material which has a relatively high thermal neutron section and a relatively long half life may be encapsulated prior to neutron exposure such a manner that the encapsulation containing the stable material is essentially inactive and safe for This stable material after fabrication and be subjected to a neutron As this means and method does not contain radioactively material but rather inert or it is called a These and other objects of the present invention will be more clearly understood when considered in conjunction with accompanying drawings in 1 is a cross section of a stable compressed sintered compound prior to irradiation 2 is a cross section of a encapsulation formed in accordance with the present is a schematic illustration of a plurality of encapsulations during 4 is a graphical illustration of a decay scheme for 5 is a graphic representation of power density of thulium oxide versus parametric in neutron 6 schematically illustrates a group of packaged 7 is a graphical illustration of a decay scheme for and 8 is a graphic representation of power density of thulium oxide versus parametric in neutron This invention is directed primarily toward a tion a compound or isotope which is compressed and sintered and which is made adaptable for irradiation into an active source in its encapsulated As the source is fabricated and encapsulated before the isotope is subject to irradiation it may be referred to as a The means and methods described may be used in connection with several different although certain isotopes have properties which make their use particularly as practical heat The material should have a thermal neutron cross section which is preferably in excess of 5 barns and a half life preferably in excess of 100 The material also should have no significant gaseous daughter ducts formed during For certain notably for use thermionic the material should have a high melting preferably in excess of although for other applications this is not a It has been found that prepared as thulium oxide is the most practical and technically feasible material for use in cold This is due in part because it is the stable isotope of thulium and has a thermal neutron cross of 118 barns which assures a sufficient activation to or with neutron fluxes in the range of 2 to 5 10 suitable desirable quantities of can be induced into the thu oxide encapsulated in the form of thulium oxide prepared by any suitable commercial Thulium oxide is used because pure as a solid metal may react with the material forming the casing of the lation and would fuse to the encasing The thulium oxide is compressed and sintered under heat and pressure conditions into a wafer or disc form as shown in The specific dimensions are determined at least in part by the particular ultimate power or radiation level for which the unit is flux depression during irradiation may be lessened by making the wafer relatively w ile still maintaining the structural integrity of the unit for maximum A typical wafer for have a thickness of 2 to mm and a diameter of to The closer the thulium oxide approaches its cal the will be the power output efficiency since maximum power per unit dimension is a function of In order to achieve maximum power it is desirable to compress the thulium oxide to a density of at least 80 of its theoretical maximum density and most to a range of of The thulium oxide wafer is formed utilizing conventional equipment at an elevated temperature of just below the melting point of thulium oxide is i the range of approximately to Sintering the compressed wafer may be conducted in vacuum or in an inert thulium wafer 2 in is placed in and secured to a casing 3 of The material of which the casing 3 is formed must a high oint which is at least in excess of the melting point of the material contained within In the case of the preferred the ing point must be in excess of that of thulium The casing material should not be conducive to significant vation it should have a very cross section for absorption of thermal neutrons and a short half the casing should be of material having a thermal neutron cross section less than barns and a half life of less than 3 The casing material must not react with the fuel material or isotope which forms the wafer and also must capable of bein to form a casing with fuel material contained within It has been found that molybdenum is the most preferred material for the casing other materials such as airconium and tungsten might also be is preferred primarily because of its high melting relatively short half and other suitable The casing 3 should have relatively thin with the walls having a thickness in the range of 5 millimeters or haps By making the casing as thin as without weakening its structural flux degression during irradiation may be minimized neutron tion of the fuel material may be and the power density The sidewalls and bottom 5 be integrally formed and may be joined to the cover with all portions of the casing 3 joined to the fuel material 2 by ventional Conventional means for prise electron beam welding in a It is most desirable to join the casing to the fuel in this case thulium to assure an appropriate contact between the fuel wafer and the so as to conduct heat effectively from the fuel material to the container for greater The encapsulation 7 illustrated in formed is not may be termed a cold This afer requires no shielding and may be handled as an inactive Inactivated capsules 7 may be placed in a reactor as schematically illustrated in for activation and held in the reactor until just prior to Fo most are placed in a reactor a period of not less than 35 days and not more than 150 days and are exposed to a flux equal to or greater than 10 for production of thulium For production of thulium 171 the capsules are 15 2 exposed to a flux in the of 10 for a period of 30 to 90 In order to minimize flux depression a spacing in the order of at least 5 to 1 between capsules in the reactor is in the preferred the capsules 7 having thickness of 2 millimeters are spaced a distance apart of approximately eight The eight millimeter space 9 between wafers should be occupied by a material which is capable moderating the neutrons and cooling the areas so that the neutrons will be down to a speed which they will more effectively be absorbed by the fuel Any suitable cage may be used to separate and support the capsules therreacto for a cage of molybdenum or The moderating material between the should be material such as In preferred the flu depression is expected to be in the order of In actually determining the flux not only must the spacing of the wafers within the but also the thickness of the Wafer is a controlling 5 the power density of thulium oxide versus n neutron flux that may be This graph is based the assumption that the flux exists at the target As is evident from an examination of thulium oxide has a reasonabl specific activity that can be achieved at reactor fluxes which are generally available today with the radiation times within When exposed to the neutron the the form of thulium oxide absorbs neutrons which convert the stable thulium oxide to active The reaction that takes place by irradiating in a neutron flux to produce 169 1 170 0 170 n Tm e Yb 69 o 69 70 The decays to a stable illustrated in has a cross section of 150 barns which indicates some of the is converted into according to the equations 170 1 171 0 171 Tm n Tm e 69 o 69 70 The and which are formed by subjecting the wafer to a neutron flux appear to be the most practical and technically feasible material for use as a heat particularly in connection with thermionic This is due at least in part to the attainable power densities which may be achieved with fuel and the longer half life of and the radiation safety The activated encapsulated wafers 7 may be stacked and contained within an outer casing as illustrated in The encapsulated wafers 2 of the preferred embodiment which may have a width of 2 millimeters are stacked to various The outer casing 15 within which the wafers 2 are tained should preferably be formed of the same material as the casings material In the thickness of this outer casing should be substantially the same thickness as the thickness of the casings 3 and may be sealed in a similar way to casings While the only stable isotope of is a preferred stable material for use in the present other materials have also been can also be produced by the use of stable erbium or enriched i the following 170 1 171 0 171 Er n Er e 68 o 68 69 The above process a chemical separation of from the erbium isotopes or from the encapsulation is ultimately accomplished radioactively Since the energ emitted per tegra ion from is very the radiation safety blems very insufficientOCRQuality

Claims (2)

1. HAVING NOW particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is: ■ 1, A method of preparing a radioactive encapsulation comprising, preparing a wafer of stable thulium oxide for encapsulation , · placing said wafer in a capsule of molybdenum, e ning ' eald wa ur 0 the walla of dalfl capoulo, sealing said capsule, and, thereafter subjecting said non-radioactive thulium oxide to a neutron flux of at least 14 2 substantially 10 n/cm /sec for a period of between approximately 35 and 150 days.

2. The method claimed in Claim 1, characterised thereby that the said wafer is prepared by compressing and sintering particles of thulium oxide, 5. The method claimed in Claim 2, characterised thereby a tempera tur that the said wafer is prepared under influence of .hoa of about «300-2600°C 4. A method as set forth in Claim 1 iherei said nonradioactive thulium oxide is subjected to neutron flux of at least 10 n/cm /sec for a period of between 30 and 90 days for production of thulium 171. 5* A method of preparing a radioactive encapsulation as claimed in Claim 1, characterised by placing ¾said wafe in a capsule formed of material which has a melting point in excess of 2300°C , is not chemically reactive with the said thulium oxide, is capable of being joined to said thulium oxide, has a cross section of less than .2 barns to thermal neutrons and half life of less than 3 days, radioactive thulium oxide to a neutron flux of at least substantially 10 n/cm /sec for a period of between approximately 35 and 150 days. 6. A method as set forth in Claim 5 wherein the specific activity of said thulium oxide afte radiation is in the order of 5 to 15 wati^from Tm-170^ t5». 7 » A method as . set forth in Claim 5 wherein said thulium oxide is subjected to a neutron flux of at least 1015 n/cm2/sec for a period of between 30 and 90 days for production of thulium 171. 8, A method as set fort in Claim 7 wherein the specific activity of said thulium oxide after radiation is in the order of .5 to 1.5 9. A method as set forth in Claim 6 wherein said capsule is formed of material selected from a group consisting of molybdenum, zirconium and tungsten, 10. A method as set forth in Claim 9 wherein said thulium oxide is compressed to a density in excess of 80^ of theoretical maximum density. 11. A method of preparing a radioactive encapsulation as claimed in Claim 1, characterised by preparing a wafer of stable material having a cross section of greater than 5 barns to thermal neutrons, placing said wafer in a capsule formed of material which is nonreactive with said wafer material , is capable of being joined to said wafer material, has a cross section of less than .2 barns to thermal neutrons and a -half life of less than 3 "days , ' sealing said capsule, and • thereafter subjecting said non-radioactive mat^irial to a sufficient neutron flux for a period whereby the specific activity of said material after radiation is in excess of 0.5 watts per cc of said material. 12. A non-radioactive encapsulation adapted to be converted by exposure to neutron flux to an isotope fueled heat source , comprising, a wafer o a stable compressed sintered isotope having a cross section of greater than 5 barns to thermal neutrons and a half life in excess of 100 days, said wafer entirely contained and enclosed in a sealed capsule of material which has a melting point in excess of 2300°C, is nonreactive with said compound, is capable of being joined to said isotope, has a cross section of less than' .2 bams to thermal neutrons, and a half life of less than 3 days. 13 · An encapsulation as set orth in Claim 12 wherein said wafer is joined to the inner walls of said capsule. 1 . An encapsulation as set forth in Claim 13 wherein said wafer is formed of thulium oxide. 15. An encapsulation as set forth in Claim 14 wherein said wafer is formed of thulium oxide and said capsile material is selected from a group consisting of molybdenum, zirconium and tungsten. 16. An encapsulation as set forth in Claim 15 wherein a plurality of wafers are contained within said capsule. 17. A method of preparing a radiaactive encapsulation, substantially as hereinbefore described. 18. An encapsulation, substantially as hereinbefore described with reference to annexed drawings. DATED THIS 27th day of June , 1966 COHEN ZEDEE & SFISBACH ' P.O. BOX 1169, TEL-AVIV Attorneys for Applicants