US3377993A - Radioisotope heat source with overheat protection - Google Patents

Description

pril i6, 1968 K. E. BUCK 3,377,993

RADIOISOTOPE HEAT SOURCE WITH OVERHET PROTECTION Fled June 28, 1966 26% NVENTOR.

KEITH E. BUCK 53% y 5-4 BYK y Qyf@ /ff-.Mf f77/M ATTORNEY United States Patent() 3,377,993 RADIOISOTOPE HEAT SOURCE WITH GVERHEAT PROTECTIGN Keith E. Buck, Alamo, Calif., assigner, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Filed June 28, 1966, Ser. No. 562,429 Claims. (Cl. 122-32) This invention relates, in general, to radioisotope heat sources, and, more particularly, to a radioisotope heat source provided with emergency overheazing pro.ection.

Certain radioisotopes of high specific activity which produce large amounts of heat from relatively compact fuel forms lare being developed in ever increasing numbers for providing energy in underwater, atmospheric and space applications. Such sources generally utilize physically stable solid forms, eg., alloys, solid dispersions, chemical compounds, ceramics and the like, containing a radioisotope encapsulated to provide what is termed a radioisotope fuel form or capsule. The fuel forms or capsules are usually disposed in a heat-conductive, Le., heavy metal nuclear radiation shield body to provide a heat source from which heat is extracted by means of heat transfer agents for use exteriorly, or the heat is applied as by conduction to thermoelectric generator elements and similar devices for use in proximity to the heat source.

T o prevent heat loss, insulation evacuated spacing with reflective surfaces and the like are provided between the heavy metal shield body and an exterior housing which is usually in Contact wi.h an exterior heat sink environment generally at ambient temperature. With such an arrangement, any failure of the heat transfer circuit to provide an ample coolant supply or interruption of the heat conduction path effectively stops the escape or extraction of heat from the source so that the temperature of the heat source rises in an uncontrolled fashion. Serious damage to the point of melt-down and dispersal of the radioiso tope can occur.

The present invention provides an arrangement in which a sensor element in thermal contact with the shield body senses the attainment of a predetermined maxiurn safe temperature level. The sensor element then actuates or otherwise responds to release a heat transfer medium into the evacuated space between the heat source and outer pressure or containment vessel to provide for convective and/or conductive heat transfer between the source and exterior vessel heat sink. The usual insulating effectiveness of the thermal radiation shields and/or reflective insulating surfaces in said evacuated space is eliminated and the thermal output of the heat source is transported to the exterior vessel heat sink to be disposed of harmlessly.

Accordingly, objects of the invention are to provide a radioactive isotope heat source with improved safety features; to provide a compact overheat protection system for a radioactive isotope heat source; and to provide a radioactive heat source protection system actuated by thermal contact with the shield portion of such a source.

Other objects and advantageous features of the invention will be apparent from the following description and accompanying drawing, in which similar reference numerals refer to similar components, of which drawings:

FIGURE 1 is a vertical cross sectional schematic view of a radioactive heat source including a protective system in accordance with the invention;

FIGURE 2 is a detailed vertical sectional view of a preferred embodiment of a radioactive heat source of the general type shown in FIGURE 1;

3,377,993 Patented Apr. 16, 1968 ice FIGURE 3 is a partial sectional view of the primary circuit for introducing operating coolant into the heat source of FIGURE 2;

FIGURE 4 is a partial sectional view of the secondary circuit for introducing operating coolant into the heat source of FIGURE 2; and

FIGURE 5 is an enlarged inset portion of FIGURE 2 showing the actuating means of the protective system.

The overheat protective system of the invention is adaptable for use with radioactive isotope heat sources corresponding to the generalized embodiment illustrated in FIGURE l of the drawing. In brief, elongated generally cylindrical fuel capsules 11 comprising a solid stable composition including a radioactive isotope enclosed in a cladding are disposed in tubular channels provided in the central portion of a generally spherical heavy metal shield 12. For fabrication purposes, the heavy metal shield 12, preferably tungsten, although depleted uranium may be used, is made in sections as shown by transverse joint lines 13, 13', and with a plug section 14 to provide access. To provide optimum thermal conductivity from capsules 11 to shield 12, a heat transfer medium, i.e., helium gas, liquid metal such as NaK (sodium-potassium eutectic) or the like (not shown) may be disposed to fill the gaps between the capsules 11 and shield 12.

A preferred fuel form for use in the capsules 11 is high specific activity cobalt60cobalt59 mixtures formed in disks and nickel plated, with a plurality of disks encapsulated as in tantalum liner. However, a variety of other radioisotope heat source means such as Sras a titanate; cesium 137 as polyglass, ceramic or other form; Ce-l44 as an oxide, as the more economical fuels; or any others such as Pm-l47, lio-210, Pu-238, Cnr-242 and Cm-244 in stable solid forms, could also be used, dependent on mission requirements. Nickel base alloys such as HastelloyC and Hastelloy-ZS can be used effectively to further clad the tantalum encapsulated Co-60 fuels and some others, which noble metals such Ias platinum may be required, eg., for primary cladding of cesium polyglass (borosilicate compositions) and other reactive materials.

For a long duration mission, to produce 3 'kilowatts electrical (3 kwe.) continuously for 10,000 hours at a feasible 14.2% efciency, fuel activity requirements of various radioisotopes in mega curies are tabulated as follows:

For various reasons, Co-60 is preferred, e.g., high specie activities available (over 200 euries/gram); high power density of 27 w./cc. (at 200 curies/gm.); and halflife of 5.24 yrs., suiciently long to assure sustained power output.

Shield 12 and capsules 11 comprise the primary heat generating means of the heat source with heat being delivered by conduction to the exterior shell surface 16 of shield 12. Heat from such a shell 16, which may take forms other than spherical, is generally transported in normal operation through conduction path, as through thermoelectric elements (not shown) or by some other means such as heat transfer medium means shown in FIGURE 1 and comprising a heat load. More specifically, a first tubular conduit 17 having inlet and outlet ends 1S and 19, respectively, arelwound spirally in bifilar fashion with a second conduit 21 having inlet and outlet ends 22 and 23, respectively, about shield 12. End 22 of conduit 17 may be used for introducing a liquid heat transfer agent 26, eg., Dowtherm-A, to be heated and emerge as vapor 27 from conduit end 23 for use exteriorly of the heat source. Auxiliary coolant, e.g., Dowtherm-A, liquid 26' in a pressurized condition can similarly be circulated through conduit 21 to remove excess heat not expended in equipment (not shown) connected to conduit 21, e.g., in standby operating condition, normal emergency or the like, for disposal as iiuid heat exchange medium 27 lby means of heat exchanger or the like (not shown). A closed pressure vessel shell 28, e.g., of nickelbase alloy, is disposed closely about spiral wound conduits 17 and 21 and a heat transfer medium such as NaK-29 is provided to fill the voids between shell 2S and tungsten shield surface 16. The exterior surface 31 of shell 28 may be polished and other insulating means provided as discussed hereinafter to minimize radiation heat loss from surface 31. The arrangement of cornponents enclosed within pressure vessel shell 28 may be considered to be a complete radioactive isotope heat source assembly 32 adapted to provide a heat transfer medium at elevated temperature for driving a turbine, piston engine or other mechanical or electrical power generating means or other appropriate device exterior thereto (not shown). The assembly 32 may also be adapted for variable power output by dividing the heat load supplied by conduits 17 and 21 and/or other means as deemed appropriate. For use submerged in a hydrospace environment (ocean, lake, etc.), the heat source assembly 32 may be arranged within the lower end of a double spherically ended cylindrical pressure hull 33, eg., of Hastelloy-C or other nickel-base alloy which can serve for heat rejection, i.e., as a heat sink. More specically, the generally spherical outer surface 34 of pressure shell 28 is disposed in spaced equidistant, i.e., in concentric spaced, relation to the inner surface of lower spherical vessel end 35. A bulkhead 36 is provided in the lower portion of the central cylindrical portion of hull 33 to close ott the lower spherical end 35 defining a chamber therein, with conduit ends 18, 19, 22 and 23 being conducted therethrough in sealed relation. An annular aluminum block 39 canned, eg., in Hastelloy-C, is used to fill the void between bulkhead 37 and upper portions of spherical surface 34 with a concave spherical lower surface 41 in spaced concentric relation to pressure shell 28 to provide for effective heat conduction from surface 41 to the pressure hull 33 for purposes described more fully hereinafter. In a typical operating device, power conversion systems, standby heat load trim means and the like to be energized by the heat produced or to control assembly 32 are arranged in the cylindrical center and upper spherical end portions (not shown) of hull 33, since same are not particularly relevant to and are not part of the present invention.

Moreover, the pressure hull 33 and enclosed heat source assembly 32 can be provided as a heat source unit 50 as shown in FIGURE 2 to facilitate construction, transport and the like. Therein, the lower end 35 of the hull is provided with a flanged joint, e.g., a conventional Marman anged joint 51 in the hernispherical transverse plane of source assembly 32 to facilitate insertion of assembly 32 therein. (During fabrication, transport, etc., coolant may be circulated through said conduits to avoid overheating in the event convection cooling in air is not adequate.) To provide adequate support, Hastelloy-C spacers S2 (one shown) are disposed at spaced equisolid angle (e.g., 11) locations between the shield surface 16 and pressure shell 28. To further minimize radiative heat transfer, at least one dimpled, polished metal heat shield 53 of nickel-based alloy, gold-plated if desired, can be disposed in intermediate position in the void space 54 between the concentric facing hull and pressure shell surfaces. Proper spacing of heat shield 53 is obtained utilizing mullite, or, eg., alumina or other ceramic spacer elements 57 oriented in the same manner as spacers 52 and generally in alignment therewith,

and shield 53 is provided with one or more perforations (not shown) to provide for gas ow from one side to the other. In service, all void spaces within the lower spherical pressure hull end 35 are evacuated to a low vacuum pressure through ports or the like (not shown) whereupon heat loss between the heat source assembly 32 and pressure hull drops to a very low level, e.g., to less than 2% of the heat output of assembly 32.

In practice, conduit 17 ends 18 and 19, respectively, can be brought out of pressure shell 28 and bulkhead 36 utilizing concentric thermal sleeve shield joints 58 and 5), as shown in FIGURE 3. Standby coolant conduit 22 can be conveniently provided of concentric arrangement with the innermost open end of the inner tube ending in proximity to the closed end (not shown) of the outer tube to allow adequate circulation of coolant, and with ends 22 and 23 concentrically emerging through a thermal sleeve joint 5S and terminating in T-tting 61, permitting attachment to input-output circuits (not shown). The entire heat source in the lower end 31 of hull 33 can be made demountable by providing a Marman joint flange 62 on the exterior thereof just above or on a level with bulkhead 36.

As discussed above, the heat transfer media in conduits 17 and 21 ordinarily conveys the heat output from assembly 32 to external use or disposal equipment, thereby maintaining the assembly 32 in a safe operating temperature range. However, if an irremediable interruption of the ow or cooling of coolant or heat transfer media supply occurs to seriously curtail removal of heat, insulated assembly 32 can increase in temperature at an uncontrolled rate to the extent that meltdown of the assembly 32 and consequent destruction of valuable apparatus occurs, and there impends a possibility of escape of hazardous radioactivity from pressure hull 33. Due to the high thermal inertia of the source components, the heating does not occur instantaneously, but overheating can occur in the relatively short time of several seconds or minutes, dependent on the extent to which normal cooling is impaired. Usually, for high eficiency, the heat source is operated fairly close, e.g., F. to the maximum safe level and accordingly only a restricted increase in temperature can be tolerated.

Emergency cooling under such conditions is provided, in accordance with the invention, by providing means for releasing a coolant, gaseous or uid in nature, into the evacuated interconnected void spaces, particularly space 54 between the exterior surface 34 of pressure shell 28 and concentric surfaces of hull end 35 and annular block 39 on both sides of the insulating medium, i.e., shield 53, by means actuated by an excessive temperature increase. Thereupon, convective and conductive heat transfer between shell 28 and the heat sink provided by hull 33, particularly the lower end of the central cylindrical portion, lower end 35 and even by some conduction along bulkhead 36.

More particularly, at least one and preferably a plurality of emergency coolant storage caviies or vessels 66 shown in FIGURE 1 are disposed or formed in aluminum block 39 or other region providing convenient access. A conduit 67 leads from vessel 66 to open at end 68 into void space 54 with a fusible plug 69 being used as the actuator means to close end 68 to retain coolant in vessel 66 under normal conditions. Fusible plug 69 is preferably disposed in intimate contact with pressure shell 28 to provide good thermal contact, and is made of a material, c g., metal alloy, having a reliable melting point, original and remelt at a predeterl .ined level, eg., at least about 150 F. above the normal operating temperature of heat source assembly 32. For a typical source using Dowtherm-A, an exit temperature of the heated vapor 27 may be of the order of 700 F. with some portions of source 32 being possibly at a higher temperature, eg., 750 F. For the construction materials mentioned above, -a temperature level of the order of 1000" F. would provide a safe level. Several aluminum alloys, listed below, are adequate under these conditions:

Other fusible plug materials appropriate for different operating levels may be selected similarly from standard reference tables, such as those in the Handbook of Chemistry and Physics (Chemical Rubber Pub. Co.).

Chemically stable heat transfer agents such as mercury metal, Dowtherm-A, etc., pressurized, e.g., with helium in sutlcient volume to till void space 54, may be disposed as emergency coolant in vessels 66. However, there is preferably used an inert gas heat transfer agent such as helium, argon, neon, nitrogen, CO2 or the like, in an amount sufficient to provide adequate pressure, e.g., at least one atmosphere in void space 54 to assure effective heat transfer thereacross.

In FIGURE 2, details of a preferred arrangement are shown in which a metallic bottle 72 (one shown) containing, e.g., helium gas, is slidably disposed in cylindrical channels 73.supported by an integral tubular colurn 74 attached at the lower end as by welding to pressure shell 28 wherefor there is provided relatively constant spacing of the bottle 72 and shell 28 independent of thermal dimensional changes. A conduit tube 76 leads downwardly from bottle 72 and includes a lower end portion 77 bent to be in intimate contact with and conform to the curvatur-e of pressure shell 28. Mechanical fastenings, welding or brazng (not shown) can be used to assure contact; however, spring of tube 76 or a spring arrangement (not shown) can also be used. The walls of end 77 of tube 76 may be provided with perforations (not shown) to facilitate release of gas, and the lower end 77, including said perforations, is closed with fusible plug material 78 of the type described, along a length of tube in good thermal contact with shell 28. With this arrangement, overheating of shell 28 melts material 78 and releases helium, e.g., with a pressure of 'above at least 1 atm., into void space 54 to provide emergency cooling as above.

Parameters of a typical heat source (about 24 kw. thermal) suitable for a 3 kwe. (kilowatt electrical), allowing for pumping power, losses and about 14-15% conversion efficiency, submersible radioisotope power generator, are set forth in the following:

TABLE A DEM ONSTRATION FUEL cobalt 0.800 in. ($0.001 in.), dia. by 0.040 in. (1L-0.003 in.), thick, plated .all over with nickel 0.0006 in. ($00002 in), thick.

Density 8.9 gin/cc.

Specific activity 1 200 curies/ gm.

Power density 1 27.4 w./ cc. (9 watts per disc).

Total activity 1 1.59 106 curies.

1 At the ytime of fueling.

The cobalt fuel is doubly encapsulated in a rtantalnm liner and a Hastelloy-C capsule. Helium-filled gaps are provided inside both containers to facilitate leak checking. The fuel is distributed among 19 fuel pins or eapsules, each pin containing 143 cobalt discs. The pins are placed in holes arranged in a tungsten matrix at the center of the tungsten shield. Fueling is accomplished by handling each pin separately to minimize requirements for heat dissipation. Dimensions of the capsule and `array are given in Table B. The axial clearance provides for differential thermal expansion of the fuel and shield (the thermal coefficient for HastelloyC is approximately three times that of tungsten). The cladding and liner are shrinktted at room temperature, and the capsule and hole diameters are equal at nominal operating temperature. These close fits will minimize thermal conduction resistance between the fuel and tungsten. The tungsten holes may, if necessary, be coated with inert material for a barrier between the tungsten and Hastelloy-C fuel cladding to prevent self-welding.

TABLE B.FUEL CAPSULE AND ARRAY DIMENSIONS FOR THE RISE DEMONSTRATION UNIT Total fuel volume en. in 54. 56 Fuel VOL/disc, cu. in. 0.0201 Fuel VOL/pin (19 pins), cu in 2. 871

No. discs/piu.. 143

60 F. 1,400o F.

Plated fuel diameter, in 0. 801 0.801 Tantalum liner:

Inside diameter 0.808 0.812 Outside diameter 0.848 0.852 Hastelloy-C clad, in.:

Inside diameter 0. 848 0. 858 Outside diameter 0.948 0.959 Tungsten hole diameter, in-. 0. 955 0. 959

The volume of boiler core material, volume fractions, and effective density are listed in Table B. These parameters were calculated using room temperature dimensions based on a hexagonal array and assuming that one-half the tungsten web thickness outside the outer fuel capsules should be counted as core material rather than shield material.

TABLE C.-BOILER CORE The 1.17 and 1.33 mev. gamma rays emitted by cohalt-60 must be shielded to prevent radiolysis of the Dowtherm-A working fluid and to meet radiation protection criteria. Approximately 3 in. of tungsten would reduce the gamma dose rate to a level Where Dowtherm-A radiolysis will be insignificant over a 10,000-hr. operating period; but approximately 7 in. of tungsten are required for biological safety. The Dowtherm-A tubing could, therefore, be located between split shielding; however, this conguration would increase both shield weight and fabrication problems, and it is expedient, therefore, to locate the Dowtherm-A boiling coil outside the biological shield.

Although tungsten is a less etlicient gamma ray shield than uranium per unit weight, tungsten was chosen as the reference shield material because of superior thermal conductivity (roughly three times greater than that of uranium). The temperature difference between the boiler core and the Dowtherm-A coil is therefore roughly onethird as great with tungsten as with uranium.

Large gamma ray sources absorb a high percentage of the gamma ray energy produced in the source. This effect in enhanced in this heat source because of the relativery large volume fraction of tungsten in the lfuel yregion. Calculations indicate that 74% of Ithe total power is generated in the fuel region, while 26% is generated in the shield, primarily in the 2 in. closest to the fuel. Roughly 5% of the thermal power is due to the low energy beta radiation which is entirely absorbed within the core.

In the boiler tube 17, 336 lb./hr. of Dowtherm-A is preheated to the saturation temperature (697 F.), is vaporized, and then superheated 3 F. This is Iaecomplished in a 78-ft. long, 0.625-in. OD, 0.575-in. ID Hastelloy-C tube that is wound around the exterior of the shield. There are three modes of heat transfer in the tube: forced convection to a liquid in the preheating region; two-phase boiling in the low quality vaporizing region; and forced convection to a vapor in the superheating and high quality vaporizing regions. For this analysis, it was assumed that the transition between two-phase boiling and forced convection to vapor occurs at 70% Dowtherm-A quality.

To prevent pyrolysis of the Dowtherm-A at the tube wall, it is desirable to maintain the lowest possible tube wall temperature. The 78-ft. boiler tube length was selected because it provides a reasonable pressure drop of 6% p.s.i. and a maximum tube wall temperature of only 735 F.

TABLE D 60 F. Operating Temp.

Inner Vessel (28):

Inner radius, in 10. G30 10. 091 Outer radius, in 10. 930 10. 993 Thickness, in 0. 300 0. 302 Standotls (11):

Area, sq. in 0. 442 0.442 Length, in 0. 750 O. 750 Outer Vessel (33):

Inner radius, in 11. O88 11. 093 Outer' radius, in 11.928 11. 933 Thickness, in O. 840 0. 840 Spacers (11):

Aren, sq. in 0.283 0. 288 Length, in 0.200 0. 200 Vacuum Space Thickness, u1.1 0.158 0.100 Tungsten shield (spherical radius), in 9. 88 Shield plug (diameter), in 7.00

1 May be made larger if necessary to accommodate heat sheld 53.

TABLE E Operating Component: temperature, F. Fuel-cladding interface 1660 Cladding-shield interface 1620 Dowtherm-A coolant tubing:

In 562 Out 740 Shield-periphery 800 Inner pressure vessel 800 Outer pressure vessel 1 30 to 85 F.

1 Ocean temperature.

The outer pressure hull 33 consists of a 24-in. diameter cylinder having a length of 88 in. and a Wall thickness of 0.75 in. with hemispheres enclosing each end of the cylinder. Internal rib stilfeners are in the form of standard I-beams, 3 by 2% in. equally spaced along the length of the cylinder. The hemispherical ends are 0.84-in. thick and are welded onto the cylindrical shell to form the lprimary containment vessel.

The inner pressure shell 28 is spherical to conform to the geometry of the tungsten shield. Its main purposes are to contain the N aK heat transfer uid, `form one surface of the vacuum insulation gap and to provide secondary (the outer pressure containment vessel is primary) protection for the fuel, Dowtherm-A coil, shield and NaK against the destructive effects of the high pressure seawater environment should the outer pressure vessel fail. The tungsten shield, containing the fuel, is positioned in the center of the inner pressure vessel by Hastelloy-C standoffs. These standoffs, which are designed to withstand a 6 g. shock load, maintain clearance between the pressure vessel and tungsten for the DoWtherm-A coils and provide radial support for the inner pressure vessel if the outer pressure hull ruptures.

The thickness of the pressure vessel wall is based upon the stress for pressure of 1000 p.s.i.a., but not for buckling. The result is a wall that will not resist the full force of 1000 p.s.i.a. external pressure without the probability of buckling instability; however, Hastelloy-C is sutiiciently ductile to collapse upon the tungsten shield without rupturing.

The inner pressure vessel is perforated in three places for passage of the Dowtherm-A lines from the boiler to the power conversion equipment and for the standby cooling system lines. The Dowtherm-A lines are joined to the pressure vessel by thermal sleeves to minimize the temperature gradient between the tubes and pressure vessel Wall. An evacuated region is provided to separate the inner pressure vessel, the outer pressure vessel and Athe bulkhead which lies between the boiler and the PCS. The vacuum limits the heat loss to the seawater during normal conditions.

The vacuum gap is maintained by ceramic (mullite) spacers between `the inner and outer pressure vessel. The vacuum gap is designed to ensure a 0.100-in. gap at operating conditions, and includes provision for 0.0l8-in. Icontraction of the outer pressure vessel as a result of the 1000 p.s.i.a. external pressure and for 0.047-in. thermal expansion of the inner pressure vessel. Penetrations through the vacuum gap by the Dowtherm lines are seal-welded at each pressure vessel to assure leak tightness.

The Dowtherm-A lines are sufficiently strong to contain the internal pressure of the Dowtherm-A at operating temperatures. If the primary pressure vessel ruptures, the tubes are expected to resist collapse that might result from 1000 p.s.i.a. seawater pressure. Calculations show that the tubes will resist buckling if the tubing is not oval where bends are used.

The bulkhead separating the boiler from the power conversion compartment consists of a 0.54-in. thick flat plate having radial rib-type stieners to limit the deiiection of the plate and the attached Dowtherm-A lines during shock loading. Thermal sleeves are also provided for the Dowtherm-A lines where they penetrate the bulkhead en route from the boiler to the power conversion unit.

While one fusible plug or link actuator means has been described in detail, it may be noted that a rupture disc can be disposed in contact with and supported by the fusible plug 78 in the conduit 76 to insure against inadvertent leakage of emergency coolant into void space 54. Other types of thermally-actuated devices, eg., thermal expansion, bimetallic, thermostatic disc element controlled or driven puncture means, substituted lfor said fusible plug, could be used to rupture a diaphragm or rupture disc used to close conduit 76. Likewise, a puncturing device driven by a fusible plug could be used. Valves actuated by thermoelectric sensors or the like could also be used; however, bulky components disposed exterior to the spherical end of hull 33 are generally not desirable. For use in environments other than water, blown air, radiator tins, contact heat exchangers and the like may be used to dissipate excess emergency heat from hull 33.

Other modifications will be .apparent to those skilled in the art, and it is intended to cover all such together with those specifically disclosed in the appended claims.

What is claimed is:

1. In a source for supplying heat to a load, the combination comprising:

(a) heat generating means comprising a massive body of heat conductive material including a non-interruptable means for continuously generating heat therein, said body being provided with means for transferring heat generated therein to an external heat load;

(b) heat sink means including a surface in spaced relation to exterior surface portions of said massive body .and defining therewith a void space evacuated to minimize convective and conductive heat transfer thereacross under normal operating conditions wherefor overheating of said massive body can occur in interruption of said heat transfer to an external load; and

(c) means including a pressurized emergency coolant source, a conduit Vleading from said source to communicate with said void space, .and closure means normally retaining said coolant in said source, said closure means arranged in thermal communication with said massive body to sense overheating of said body and actuate release of said coolant into said void space so that the heat generated therein is transferred harmlessly across said void space to said heat sink 2. Apparatus as defined in claim 1, wherein said massive body is a heavy metal radiation shield and said heat generating means comprises an encapsulated radioisotope fuel form disposed centrally in said shield.

3. Apparatus as defined in claim 2, wherein said radioisotope fuel form includes a material selected from the group consisting of Co-60, Sr-90, Cs-137 and Ce-114.

4. Apparatus as defined in claim 3, wherein said massive body of said heat generating means is a generally spherical tungsten metal shield, said radioisotope fuel form therein is an encapsulated cobalt-60 metallic mixture with cobalt-59, and said heat sink comprises a metallic hull having spherical surfaces generally concentric to the surfaces of Said tungsten shield and spaced therefrom across said void space.

5. Apparatus as defined in claim 4, wherein said means for transferring heat from said heat generating means comprises a boiler pressure vessel shell `spaced inwardly across said void space with respect to said metallic hull spherical surfaces, encompassing said tungsten metal shield in concentric spaced relation, said actuation closure means is in thermal communication with said boiler shell, means for introducing and circulating at least one heat transfer medium into said boiler vessel to be heated, thenceforth to be conducted exteriorly to deliver the heated medium to said external heat load.

6. Apparatus as defined in claim 5, wherein said emergency coolant source closure means comprises a fusible material disposed to plug a portion of said conduit in thermal communication with said boiler vessel shell, said material having a melting point at a predetermined safe level above the normal operating level of said heat generating means.

7. Apparatus as defined in claim 6, wherein means for introducing and circulating a heat transfer medium into said shell comprises a continuous conduit spirally wound in the space between said boiler shell and shield, and a heat transfer agent is disposed in the interstitial spaces within said boiler vessel shell.

8. Apparatus yas defined in claim 7, wherein a dimpled, poli-shed metallic radiant heat shield is disposed in said void space intermediate between said concentric shell and heat sink hull surfaces.

9. Apparatus as defined in claim 8, wherein said emergency coolant is an inert gas heat transfer agent.

10. Apparatus as defined in claim 9, wherein the heat transfer agent disposed in the interstitial spaces within said boiler shell is a liquid metal, and the heat transfer agent circulated through said conduit is a high temperature, radiation resistant organic heat transfer medium.

References Cited UNITED STATES PATENTS ll/l96l Fraas et al. 176

OTHER REFERENCES CHARLES J. MYI-IRE, Primary Examiner.

Claims (1)

1. IN A SOURCE FOR SUPPLYING HEAT TO A LOAD, THE COMBINATION COMPRISING: (A) HEAT GENERATING MEANS COMPRISING A MASSIVE BODY OF HEAT CONDUCTIVE MATERIAL INCLUDING A NON-INTERRUPTABLE MEANS FOR CONTINUOUSLY GENERATING HEAT THEREIN, SAID BODY BEING PROVIDED WITH MEANS FOR TRANSFERRING HEAT GENERATED THEREIN TO AN EXTERNAL HEAT LOAD; (B) HEAT SINK MEANS INCLUDING A SURFACE IN SPACED RELATION TO EXTERIOR SURFACE PORTIONS OF SAID MASSIVE BODY AND DEFINING THEREWITH A VOID SPACE EVACUATED TO MINIMIZE CONVECTIVE AND CONDUCTIVE HEAT TRANSFER THEREACROSS UNDER NORMAL OPERATING CONDITIONS WHEREFOR OVERHEATING OF SAID MASSIVE BODY CAN OCCUR IN INTERRUPTION OF SAID HEAT TRANSFER TO AN EXTERNAL LOAD; AND (C) MEANS INCLUDING A PRESSURIZED EMERGENCY COOLANT SOURCE, A CONDUIT LEADING FROM SAID SOURCE TO COMMUNICATE WITH SAID VOID SPACE, AND CLOSURE MEANS NORMALLY RETAINING SAID COOLANT IN SAID SOURCE, SAID CLOSURE MEANS ARRANGED IN THERMAL COMMUNICATION WITH SAID MASSIVE BODY TO SENSE OVERHEATING OF SAID BODY AND ACTUATE RELEASE OF SAID COOLANT INTO SAID VOID SPACE SO THAT THE HEAT GENERATED THEREIN IS TRANSFERRED HARMLESSLY ACROSS SAID VOID SPACE TO SAID HEAT SINK.

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