Do . . • I OFT ORNL P 2177 . i .. . Lo * . ... . . .. . . - . . . . - encome-owne... veg. .. . mm 3 . .. . . . . . . . : 5 6 PEETE SIE i · MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS - 1963 entre cerce.com Orroro - 1277 2197 Conf-660511-4 ho MASTER i COSTI PRE:5 ** JUN 22 066 . - 11.05. Leto; lin 50 H.C. IRRADIATION CAPSULE TEMPERATURE CONTROL* J. A. Conlin RELEASED FOR ANNOUNCEMENT IN NUCLEAR SCIENCE ABSTRACTS Oak Ridge National Laboratory Oak Ridge, Tennessee LEGAL NOTICE This roport was prepared as an account of Government sponsored work. Neither the United States, por the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the lniormation contained in this report, or that the use of any Information, apparatus, malbod, or process disc'h.ned in this report may not Infringe privatoly owned rights; or B. Assunod any liabiliuos wiüs isomea! In tha wo có, or for damagos reswung from the use of any infurauation, apparatus, method, or process disclosed in this com ! As used in the abovo, "person acting op beball of the Commission" includes any om- ployoo or contractor of the Commission, or employse of such contractor, to the extent that such employoo or contractor of the Comission, or employee of such contractor proparos, dienomlantes, or provides accos to, way information pursuant to die omployment or contract with the Commission, or his employment with such contractor. A Prepared for Presentation at the International Symposium on Capsule Irradiation Experiments. Pleasanton, California May 3, 4, and 5, 1966 *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. riana IRRADIATION CAPSULE TEMPERATURE CONTROL . .. One important phase of irradiation capsule design is that of attain- ment and control of temperature. I would like to present a method of look- ing at the problem that I have found helpful. Many methods have been em- ployed for temperature control; however, two of the most common are the use of a radial gas gap and or supplemental electrical heat. Both methods have their limits and it is the limits and methods of extending some of the limits that I would like to discuss. Figure 1 is an idealized plot showing the range of usefulness for two methods of control: gas gaps and electrical heat. These limits are not exact, will vary from design to design, and give only an approxima te rela- tive picture for electrical heat vs gas gaps. Even for a given design it may be hard to draw an exact curve; the limits will depend in some degree on the optimism of the designer in predicting performance. The abscissa is the test specimen heat generation rate per unit length. One could use heat flux per unit area of the specimen but the curves would be more sensitive to a given capsule configuration. The ordinate: is the surface temperature of the test element assuming water as the coolant of the outer capsule wall. For any capsule configuration there is a minimum specimen temperature for any givan specimen heat rate. This results from the thermal resistance of the materials of the capsule's construction. This limit is represented by the lower line which puts ε, floor on the use range of any means of tem- perature control. The electrical heater is most useful in the range of low nuclear power and moderate temperature and may be used in conjunction with gas gaps. It has an upper temperature limit which depends on the heater design and ma- . terials. Heaters have been developed to exceed 1200°C for special cases. However, high performance heater development can be costly and if you are not careful the irradiation program degenerates into a series of electrical heater performance tests. The electrical heater has a practical upper limit for temperature control based on the heat input per unit length that can be crowded into a capsule.. Unless the heater is equal to some appreciable fraction, perhaps 25% of the nuclear heat, its::contribution to temperature :-....... - 13 - .. bort the sither within ORAL-OWO 66-3680 15006 °C 3.2 lime w/ It GAS GAP CONTROL LIMITS . 0170°C 2 kwift me. Si ani SPECIMEN TEMPERATURE 820°C 10 kw/ft MC 760°C 20 kw/lt ELECTRIC HEATW CONTROL LIMIT . •A 650°C 30 kw/ft - Gas an NE PLEASE LOWER LIMIT FROM CAPSULE STRUCTURE WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWALL memandandi HEAT GENERATION RATE PER UNIT LENGTH Limits of capsule Terraperoture Control vs Heat Generation. -- - . .. control is not worth its complication. Since most heaters have a higher heat capability at lower temperatures, I have shown the temperature limit for electric heat as decreasing with increasing power. . The second set of curves applies to gas gaps. The gaps may employ a fixed gas composition, usually "e, Neon or argon, or may use an adjustable mixture of two of these gases. The former requires that the designer be more sure of performance or have the the option of adjusting power. The latter requires more complicated controls but provides a means of adjust- ing temperature on stream. At low heat fluxes the upper gas gap boundary is limited by how large a gas gap one can use. If che heat flux is too low it may be necessary to supplement it with electric or other heat sources to drive the effective operating point into the region of gas gap control. As the temperature in- creases the upper limit levels off as radiant heat transfer across the gap becomes predominant. Once in this region changing the gap or the gas com- position has little effect on the temperature required to drive the heat across the gap. The lower gas gap limit is somewhat above the limit imposed by the thermal resistance of the capsule structure. There is some finite gas gap below which it is not practical to go. The differ unce between the two lines represents the resistanre of this minimum gap. Even if the gap were reduced to a shrink fit some resistance would be present, but this could hardly be called a temperature control gap. The lower limit for gas gaps will tend to approach the line for the structure at very high temperatures because here radiant heat transfer again predominates. I have plotted some representative points on the curve. These, with the exception of point "A" where we only investigated the problem, are cap- sules' we have run. Note these are only representative curves and may vary considerably for different capsule configurations. I do not consider it impossible to build capsule A, however it would require different designs than we have employed. The region to the left and above the upper gas gap curve can be handled by devices such as thermal radiation baffles which will raise the limit or structural thermal resistance which in effect drive the design point down into the region of gas gap control. it " !!! The 10 kw/ft 820°C capsule was designed to operate well within the range of gas gap control. It consisted of a fuel pin in a Nak bath, a stainless steel primary wall, a gas gap and then an outer water-cooled wall. The 20 kw,' ft 760°C case is on the edge of the lower gas gap control line. The design of this capsule was similar to the 10 kw/ft case. Sare- guards dictated double containment; hence, we were forced to use two walls. Initially these were a shrink fit with no gas gap and we attained the neces- sary overall thermal resistance by adding a Zircaloy sleeve of the required thickness in the NaK annulus. The 3.2 kw/ft 1500°C capsule was a materials compatibility test achiev- ing its temperature from y heat only. It operated to the left and above the gas gap control curve. Iwo 3-mil thick molybdenum foil reflector baf- fles were inserted in a large gas gap to shift the curve into a region of control. Without the baffles the maximun temperature we could achieve with the same gap was 1170°C. Table 1 shows some of the requirements of the capsule for point A, which was to the right and below the lower limit from the thermal resist- ance of the capsule structure, that is high heat flux and moderate tempera- tire. In order not to exceed a given heat flux to the water the capsule Table 1. Proposed Capsule – Thermal Design Evaluation Requirements Specimen surface temperature, °C Heat rate, kw/ft Design – stagnant pool cooling Allowable heat flux to water, Btu/hr. °F.ft2 Minimum capsule On, in. Required minimum mean capsule structure conductivity, Btu/hr•°F.ft Design – forced convection cooling : Capsule OD, in. Required minimum mean capsule structure conductivity, Btu/hr• °F.ft 35 1.5 26 wetted surface area must not be less than some minimum. Without fins this fixes the minimum diameter and hence minimum distance from the fuel speci- men to the heat sink. The requirements were 30 kw/ft at 650°C. We assumed simple cylindrical geomeiry, and a maximum heat flux to the water which was a stagnant pool of 190,000 Btu/hr.fta. This gave an overall diameter of 2.2 in. with a required mean conductivity of all the interveniing material of 35 Btu/hr. °F•ft. As an alternate we considered forced convection with an outside diameter of 1.5 in. In this case the required mean thermal con- ductivity of the structure was 26 Btu/hr. °F.ft. We have not carried the design further; however, the double wall stainless steel capsule will not work. It is necessary to lower the line representing the limit imposed by structural thermal resistance. The outer wall could be thick and be made of aluminum Where practical, as in poolside capsules, a carefully designed, fixed thermal resistance with final tenperature control attained by adjusting the capsule position relative to the core is probably best for capsules at or below the gas gap control line. If one must have on stream temperature adjustment for such capsules without changing the flux, some other technique than gas gaps or electric hekters must be resorted to. It may be possible initially to have a sodium annulus, thermal conductivity about 38 Btu/hr: °F.fc at 900°F, to which small amounts of potassium are added at operating power to increase the overall thermal resistance until design temperature is reached. This would by no means be a simple technique. In summary, electric heaters are most useful for low power moderate temperature capsules. Gas gaps do not function well at low heat rates or at very high temperatures. The use of supplemental heat with gas gaps will alleviate the former difficulty, and devices such as radiation baffles or structural thermal resistance às. pyrolytic carbon will help in the latter case. The low to medium temperature, high power case may be the most dif- ficult to accommodate and appears to be best handled by designing for a fixed structural thermal resistance, and if possible obtain temperature control by adjusting the nuclear heat. Ans ORNL-DWG 65-4200 PURGE GAS LINES - THERMOCOUPLES * * COATED PARTICLES (MONOLAYER) * - GRAPHITE SSRSRSREBRES - ZIRCONIUM CARBIDE MINIMI11IN0MOANA PYROLYTIC-GRAPHITE INSULATOR SLEEVE kes VOI OOOOOOOOOOOOOOOOOO ZI GRAPHITE SLEEVE . . . . SASKS:- . . . . . . TEMPERATURE-CONTROL GAS ANNULUS - KZ . . . PE***** . STAINLESS STEEL: PRIMARY CONTAINMENT - CCCCESS . . . . 0 000OOD . 0 . SASSSSSSSSSSSS 0 - ZIRCALOY-2 SECONDARY CONTAINMENT 44 - POROUS-CARBON INSULATOR, BOTH ENDS ORNL-OWO 65-4210 33JJ COCCO - THERMOCOUPLES AND GAS LINES SEPTES: MOLYBDENUM REFLECTORS - MOLYBDENUM 8.0 INDICAZIN VI DT MOLYBDENUM REFLECTORS - MOLYBDENUM THERMOCOUPLE WELLS DIV U TV V viii. B&O CORE GRAPHITE SLEEVE -GRAPHITE SPECIMEN 10 3/32 ICIDIOMVOIDIOMICROID 9 PRIMARY GAS GAP -B&O SPECIMEN SECONDARY GAS GAP - FLUX MONITOR HOLDER T AV .. inicio + WWW € - E S2C2OIDDICTIVI G MOLYBDENUM REFLECTORS - MOLYBDENUM PRIMARY CONTAINER SECONDARY CONTAINER QUOMODUL www INCH ORML-LR-DMB 5544 HELM MEASU ON Nok Simic (315 poid) A CONTROL GAS IN " - auXHEAD CONTROL GAS OUT . SECTION A-A LUG FOR HEAT DAM GRUP POSITION POR RE- MOTE MAMPILATOR TS SHAMSTOCK FOR More CONVECTION FLOW DAM SSION GAS WTHORAWAL PUNCTURF. STEM . AW UD END CAPV . ALUMINUM FIIT (ON ORLE CAPSULES ONLY) - MOO SPACER AW 2 PELLETS (10, 11 OR 12) EL CAPSULE CONTROL GAS ANLUS, X SPACER WIRE : IL STAINLESS STEEL THERMOCOUPLE BANO ANO DOSIMETER 1 - THERMOCOUPLE MW:: -CONTROL GAS THERMAL BARRIER (2005-2012-in GAP) TI * . . CONTROL GAS CONTAINMENT TUBE III تم تسعه NOK VESSEL CAPSULE LOCATING PIN VO INCHES EGCR Prototype Diometer Fuel Copsule for Irradiation in ORR ond ETR. CHLOWO 65-67780 - HELIUL PRESSURE ON NOK SURFACE - 800 caig BULKHEAD --- MONTCR AND CONTROL GAS GAS OUT MONITOR AND CONTROL GAS CONVECTION CIRCULATION TUBE O 300 - EXTRACTION RING A A : GAS IN 'HELLA PRESSURE ON Mark SUA FACE 800 polo THERMOCOUPLE - SQUARE GUIDE SLEEVE (SEE SECTION 8.8 FOR "FLEX TIJBE", STANLESS STEEL WOOL TO REDUCE SEPARATION OF MIXED GASES - MONITOR AND CONTROL GAS RETURN -POSITIONER AND NOK CONVECTION FLOW DAM GAS OUT STANLESS STEEL THERMOCOUPLE BAND AND OOSIMETER "FLEX TUBE" HSR THERMOCOUPLE -- UPPER FUEL CAPSULE GAS N SQUARE GUDE FOR "FLEX TUE" 9-8 - NOK VESSEL (PRIMARY CONTANENT) STANLESS STEEL THERMOCOUPLE BAND AND DOSMETER MONITOR AND CONTROL GAZ CONTANMENT TUBE (SECONDARY CONTAMMENTI - THERMOCOUPLE LOWER FUEL CAPSULE SKSS - CORED VOZ PELLETS (11 N LOWER CAPSULE) — NATURAL VOZ INSULATOR -Al2O3 INSULATOR END CAP - CAPSULE LOCATING PIN _ IL MEN END . homeinnsaminary ON DATE FILMED 7) / 29 / 66 - - IN T.1 .. . - 1