w " : LIVE .. . * * "- *wili w mita " ... . ' . . 1. : 14 NES ELEKA 2V : 1 0 . 4" Pori TV ESPERA # PT S ir JA! _ .. u. ry- . 72 :! .- . 1 - UNCLASSIFIED ORNL Р. 1066 Vi , T - CLAV em VI . . : : : .. . . . Fr : 2 1 . VE . . . . '? . R I ... : ORIL-R-066 Carz-650 204-2 NI - AEC - OFFICIAL MASTER 2.1 MAR 23 1965 IN-PIIE PERFORMANCE OF HIGH-TEMPERATURE THERMOCOUPLES R. M. Carroll and P. E. Reagan Reactor Chemistry Division INTRODUCTION dan D. den Ab wand in the above, patrately own righto; or o de contentor, nr Woonployert wuh mal contractor. ployer of contractor of the Commiasto tes, or provides memo to, uy tutoration puro ma apboya or contractor of the Convalesho, of meplo hy habilities word da tutortion, appuntos, methods or pronoms decloud la person acttag a t to do we of, or lor du or analogue of we contractor, to the art that wa waplogue of contract of the contendont tached o report al mo atsAr paper, med deg tror dhe my mu- When you no nados na mperora person 30 para 'urgunnddo apolog in jo rhy. completati, or undtalana d the wonton contained t A. Maker ay murranty or promouden, a Belmo, so the Commissiou, bor any person acting a b o sud or implied, wo roapact on the 460 all of the countlesto: report, or that we were om periode menumbo po yunooo tn pandard u woda onu LEGAL NOTICE - The in-pile testing of ceramic nuclear fuel materials at high temperatures requires an accurate measurement of the temperatures. Thermocouples are the only practical method for measuring the temper- ature at a precise location on the fuel specimen while it is being irradiated. N Although we have not conducted a research program on thermocouples, we have made a number of observations concerning their performance, specifically, the performance of Pt/Pt-10% RH (Pt-Rh) and W--5% Re/W-26% Re (W-Re) thermocouples when exposed to long-term irradiation at high temperatures. We have also observed the influence of carbon, hydrogen, oxygen, and insulating materials on the life and performance of the thermocouples. para o Method of Testing Fuel Specimens In-Pile Tests A detailed account of the testing method is given in another docu- ment. In brief, ceramic fuel specimens are placed in a metal capsule which is attached to a positioning tube about 20 ft long. About eight thermocouples are placed in various positions in the capsule and are joined to lead wires about 12 in. above the capsule. The lead wires pass up the 20-ft tube. The capsule is placed in an irradiation facility at the ORR where the ceramic fuel specimens are heated by their own fission heat, which is regulated by moving the capsule into or out of the neutron flux (Fig. 2.1). The fuel temperature is regulated by air cooling the outside of the capsule. Because the heat is produced internally, there are large temperature gradients within the capsule. The highest temperature is at the center of the fuel and the lowest is at the capsule wall; the temperature difference has been as much as 600°C across a distance of 0.25 in, within the capsule. Thus, accurate positioning of the thermo- couples in the small capsule (1.0-in. diam, 2.4-in. long) becomes very important (Figs. 2.2 and 2.3). ORNI ~ AEC - OFFICIAL ?R. M. Carroll and P. E. Reagan, "Techniques for In-Pile Measure- ments of Fission-Gas Release," Nucl. Sci. Eng. 21, 141–46 (1965). ORNI - AEC - OFFICIAL PATENT CLEARANCE OBTAINED. RECU THE PUBLIC IS APPROVED. PROCEDURES ARE ON FILE IN THE RECEIVING SECRON. 2.2 , .. . .., -- . . 1 UNCLASSIFIED ORNL-LR-OWG 71560 BENCH TEST FUEL CAPSULE HYDRAULIC WITHDRAWALI MECHANISM PROCESS WATER (REVERSIBLE FLOW) PURIFICATION SYSTEM COUNTER Am STACK w FURNACE CHARCOAL TRAP ARGON HELIUM PURIFICATION SYSTEM PUBLICATION Coas sampai GAS SAMPLING STATION summer---- AIR SUPPLY ---AIR OUI ----STACK AIR OUT STACK . . + . COOLING AIR ;- : IN-PILE FUEL CAPSULE *; * Fig. 2.1. Facility for Testing Ceramic Puels. . . ORNE-AEC-OFFICIAL- UR TL . I TI . T 2.3 • · -......... ORNI - ACC - orricia ORNI - AC - OFFICIAL UNCLASSIFIED OANLLR-OWG 40238R3 SWEEP GAS IN THERMOCOUPLE LEADS AIR III 1 . . III AIR OUT IIIIIIIIIIIIIIII AIR OUT ;i. DUIT C UT and TIT D 11 IUNI 11 IS 1 IIUL III NI INI TIL O LE -. - -SWEEP GAS OUT I . POSITIONING TUBE 1 I ! " ULT INDI WWWWWWWWWWW ". V - SWEEP GAS OUT Al2O3 HOLDER- 2011 VAMMIVAMMIVAME -SPECIMEN CONTAINER INSULATED THERMOCOUPLES DI WWW REACTOR CENTER LINE TIT 2010 SPECIMENS TID LUT 1 ORNI – AEC - OFFICIAL .. ORNI ~ AEC - OFFICIAL Fig. 2.2i Capsule Containing 002 Specimens. -------------- -- --- ....... ... ........... ................. ...... 2.4 UNCLASSIFIED ORNL-LR-DWG 57280R HOLE FOR VENTILATION OF GAS (2) HOLES FOR THERMOCOUPLES LID (GRAPHITE) 1 S CAN (GRAPHITE) ** . .: SLEEVE (GRAPHITE) AYOOOT - CENTER PIN (GRAPHITE) un 7 INCH FUEI., SPHERICAL UC, COATED WITH PYROLYTICA DEPOSITED GRAPHITE Mg. 2.3. Capsule Containing Graphite-Coated UC2 Specimens. ORNI - AEC - OFFICIAL DAILINUU. 2.5 Fission gases, released from the fuel during irradiation, are carried away by a continuously moving stream of sweep gas (Fig. 2.2). The sweep gas presents an environmental problem for the thermocouples, because part- per-million concentrations of impurities in the sweep gas are continually presented to the thermocouples, and even very small concentrations of impurities in the sweep gas constitute an endless supply. This is con- trasted to a sealed capsule test, wherein the thermocouple materials react and quickly exhaust the supply of impurities. . Two general categories of fuels have been tested: thin plates of uranium dioxide (Fig. 2.2), and spherical uranium carbide coated with graphite (Fig. 2.3). Since these fuels require different atmospheres, We use an inert sweep gas, mostly helium, for the graphite-coated fuel and a reducing atmosphere (97% He-3% H2) for the 102. Sometimes argon 18 added to these sweep-gas mixtures so that we can, from the argon activation, measure the neutron flux at the fuel specimen. From these measurements we calculate the irradiation intensity and the dose received by the thermocouples.? Bench Tests After the capsule has been assembled, it is radiographed with x rays to determine that each thermocouple junction is still in its proper position. The capsule assemb.ly is then leak tested with a hellum leak detector. A small heater is placed around the capsule, and the capsule 1s heated to some temperature between 300 to 400°C and sometimes as high as 900°C. This relatively low temperature is the maximum tempera. ture that the sheath of the Inconel capsule will reach in the irradiation facility. Both Pt-Rh and W-Re thermocouples are always placed in each capsule, and if all thermocouples indicate the same temperature, the assembly is accepted. Special Problems of Irradiation Heating and Cooling Effects When the capsule is inserted into the reactor, the fuel specimen is Pission heat and somewhat by the absorption of gamma rays. The thermocouple junctions and wires will l.lso be heated by gamma-ray absorp- tion. Since the thermocouple junctions are in close contact with ceramic fuel specimens that have comparatively low thermal conductivities, the thermocouples conduct heat away from the specimens and produce relatively cooler spots. The effects of gamma-ray heating and heat conduction are very difficult to calculate, but since these are compensating effects, . ORNI - AEC - OFFICIAL ...?R.: Mi Carroll, "Argon Activation Measures Irradiation Flux Continu- ously," Nucleonics 20(2), pp. 42-43, 1963. " - 2.6 We believe that the thermocouples indicate very nearly the correct temperatures. To minimize these effects we use thermocouple wires of the smallest diameter consistent with strength (10-mil W-Re and 5-mil Pt-Rh). If unsheathed thermocouples are used, the bare wires are ed with two-hole Al2O3 insulators (high fired, high purity). No insulator that contains organic material or binder is used in the irradiation field, and materials containing silicon are not used near the Pt-Rh thermocouples. Effects oé Irradiation on Emf The effect of irradiation on various types of thermocouples at the same position in the capsule was investigated. The procedure was to place two dissimilar thermocouples, for example, & W-Re and a Pt-Rh thermocouple or a bare and a sheathed thermocouple, next to one another in & capsule. Since, in such tests, the paired thermocouples indicated the same temperature just after the irradiation was started, we conclude that an irradiation field (up to 1 X 1024 neutrons cm*2 sec ) has negligible effect on the thermocouple emf, because it is unlikely that two different thermocouples would be affected in the same way by an irradiation field. Effects of Irradiation Dose on Emf As explained in the previous section, we measure the temperature at different points in the capsule during irradiation. The fuel is heated by its own fission heat, which is directly proportional to the neutron flux. The neutron flux level in a reactor will change (at a given position) as the reactor cycle progresses, because the control rods are shifted to compensate for fuel burnup and accumulated fission product poisons. Therefore, a fuel specimen located at a fixed point in the reactor will have a temperature that changes with the change of flux, but all temperatures in the capsule will not change by the same amount, for reasons to be explained. These different rates of change have sometimes been mistaken for a drift of the thermocouple emf. We can detect changes of neutron flux by argon activation measurements and compensate for the changes by moving the specimen into the desired neutron flux. During irradiation, the temperature differential of the fuel specimen is proportional to the amount of generated heat and inversely propor- tional to the thermal conductivity. A change of neutron flux will change the heat generation rate and, therefore, change the temperature differential as well as the temperature. On the other hand, if we maintain a constant neutron flux and all the measured temperatures remain constant over a period of time, the temperature differential also remaining the same, then the response of none of the thermocouples changes. A compensating change of thermal conductivity is unlikely, because different fuel materials show the same result. ORNI - AEC - OFFICIAL NI -AEC - OFFICIAL 2.7 To date, 40 capsules containing a total of about 300 thermo- couples have been irradiated for periods of a few days to about 8 months. Irradiation temperatures of the fuel specimens ranged from 400 to 1450°C. The maximum neutron dose was 3.6 X 1020 nvt thermal and 2.8 X 1020 nvt fast. During all these tests, we had not observed an effect on thermo- couple emf which we could attribute to irradiation damage. Since we maintained the neutron flux constant within 15%, we could not detect a change less than this in the thermocouple emf as burnup progressed. Baumanns measured the effect of neutron irradiation on the emf of Pt/Pt-10% Rh thermocouples and found that the error is less than 5% up to 6 X 102.0 nvt exposure. Effects of Trace Oxygen In some tests performed at about 1000°C, sweep gases of the purest helium available were used but which contained about 1 ppm oxygen. The Pt-Rh thermocouples performed very well, but after several weeks the W-Re thermocouples began to show abrupt shifts of emf and finally failed. Postirradiation examination showed that oxidation had destroyed the W-Re wires in the region of the fuel specimen. We believe that the abrupt shifts of emf occurred when the wires fractured but maintai contact. Effects of Hydrogen A sweep gas containing 3% hydrogen was used as a reducing atmosphere for some fuel specimens. The thermocouples were made with bare W-Re wire, bare Pt-Rh wire, and Inconel-sheathed Pt-Rh wire, hingsten-rhenium was used to measure the highest capsule temperatures, i.e., as high as 1450°C. The W-Re thermocouples performed very well in the reducing atmos- phere. No drifts of emf were detected, and failures almost never occurred. Abrupt changes of position and changes of temperature were common for these experiments. The lack of failures indicated that the W-he thermocouples did not become brittle. Postirradiation examination showed the W-Re thermocouples to be bright and strong, appearing to be somewhat better than before irradiation, On the other hand, bare-wire Pt-Rh thermocouples became embrittled rather quickly and failed within a couple of weeks. These thermocouples failed with abrupt shifts of emf and eventual loss of continuity. The 3M. J. Kelly, W. W. Johnston, Jr., and C. D. Baumann, "The Effect of Nuclear Radiation on Thermocouples," pp. 265-69 in Temperature - Its Measurement and Control in Science and Industry, collective vol 3, part 2, ed. by A. I. Dahl, Rheinhold, New York, 1962. 2.8 Pt-Rh thermocouples sheathed in Inconel lasted for several months in locations where the capsule temperatures were lower. Undoubtedly, the time required for the hydrogen to diffuse through the sheath accounts for the longer life of the sheathed thermocouple. Effects of Carbon Some capsules were made with a graphite container for the specimens (Fig. 2.3), and inert sweep gas was always used with these capsules. Thermocouples in contact with the graphite became embrittled and failed at temperatures above 900°C owing to carburization of the thermocouple materials, which was characterized by a gradual reduction of. emf. The mf is a particularly insidious type failure, because it might result in the specimen being raised to higher and higher temperatures while the experimenter believes that he is maintaining a constant temperature. Usually, after the emf drifted downward for several days, the thermocouple broke, often during a temperature change (indicating a brittle failure). Protective Coatings for Thermocouples Near Graphite Carburization of thermocouples could be minirized by preventing the thermocouple from being in contact with the graphite. When the thermocouple tip was wrapped with platinum foil or the sheathed thermo- couple was plated with a platinum layer, the thermocouples often lasted as long as 2 months at temperatures as high as, 1350 °C. However ir test capsules containing Inconel-sheathed Pt/Pt-10% Rh thermocouples protected by platinum foil, we believe that the platinum alloyed with nickel from the Inconel. This alloy is very destructive to pyrolytic- carbon-coated fuel specimens. In other capsules containing carbon- coated specimens and Inconel-sheathed Pt/Pt-10% Rh thermocouples (without a platinum wrapper), the specimens were also damaged by, what We believe, is a Pt-Ni alloy. Based on their emf output, these thermo- couples never exceeded 1250°C, but metallography showed that the Inconel sheath had melted. The formation of a Pt-Ni alloy when Inconel and platinum are in contact at 1400°C may be accelerated by irradiation, but preliminary investigation of a platinum-wrapped, Inconel.sheathed thermocouple, heated in graphite at 1400°C, indicates that this alloy is also formed in bench tests. Tantalum-sheathed thermocouples were tested in two experiments to study the prevention of carburization. Although the sheath protected the thermocouple, during irradiation the tantalum combined with probably either carbon or oxygen to form a very fine dust-like compound that distributed itself throughout the inside of the sweep system piping and became a radiation hazard. (During irradiation, tantalum absorbs neutrons to forma 182Ta, which decays with a 115-day half life and emits high-energy garmma rays.) The tantalum compound adhered tenaciously to the walls of the sweep system but had little cohesion; thus, in places 2.9 ORNL - AIC - OFFICIA where more than a monomolecular layer existed, any disturbance would spread the tantalum compound to other surfaces. Even though the amount spread throughout the system was small, considerable decontamination was required. In recent tests rhenium foil was wrapped around W-Re thermocouples (in Beo insulators) to form a carburization barrier. One capsule was tested for about 6 weeks and another for 10 days at 1400°C with excellent results. CONCLUSIONS Platinum Pt-10% Rh and W-5% Re/W-26% Re thermocouples are not . affected to a significant extent by neutron irradiation flux up to 1 X 1024 neutrons cm? seca? or a dose of 3.6 X 102° neutrons/cm2 thermal or 2.8 X 1020 neutrons/cm2 fast. .. . . Oxygen and hydrogen cause failures in W-Re and Pt-Rh thermo- couples, respectively. Failure is characterized by abrupt shifts of emf rather than by a gradual drift. Tungsten-rhenium thermocouples perform very well in an atmosphere containing some hydrogen. Platinum- rhenium thermocouples should not be used for temperatures above 1100°C, because the platinum reacts with materials in the capsule (102, graphite, Inconel, etc.). .. . .. . ... . .... .. . . . . . - Thermocouple insulators of high-purity, high-fired Al2O3 will with- stand all.conditions listed previously (for temperatures up to 1400°C), if carburization barriers are used. . . . . . * , . . . . . . Direct contact with graphite will cause both Pt-Rh and W-Re thermo- couples to fail, as evidenced by a decreasing emt as carbon attack progresses. Any method that prevents direct contact with carbon will slow or stop this attack. Platinum and tantalum cause deleterious side effects and should not be used as protective barriers. Rhenium appears to be a good barrier material, but our experience is too limited to state a conclusion on this point. . . . We believe that higher temperatures (< 1400°C) will require different insulators and thermocouple protection. A W-5% Re/W-26% Re thermocouple, insulated with Bed and sheathed in either tungsten or rhenium, with an outside diameter of not more than 0.050 in., would appear to meet our needs. . ORNI - AC - OFFICIAL pose 26 A DATE FILMED 4/27 165 IMAI. . MES RY 12 Son miri 2 . Ver LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor 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 information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission” includes any em- ployee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, di88.9minates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor. . .. w END * . - , ! . ' ... . .. . . I TU . . . IS!. .?". . . ... . .. ." . : :: .. ::