IN-PILE TESTING OM THERMIONIC SPACE NUCLEAR POWER FUEL USING A TRIGA REACTOR by J. Razvi and W. L. Whittemore General Atomics San Diego, California

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1 IN-PILE TESTING OM THERMIONIC SPACE NUCLEAR POWER FUEL USING A TRIGA REACTOR by J. Razvi and W. L. Whittemore General Atomics San Diego, California 1.0 INTRODUCTION The U. S. Department of Energy is sponsoring a thermionic technology development program* at General Atomics (GA) to address the feasibility issues associated with the use of thermionic fuel elements (TFE) for space based nuclear power systems (Ref. 1). The technology program being carried out at GA during the 1980s is aimed at investigating lifetime issues of the fueled emitter and insulator materials proposed for use in thermionic reactor based space power systems, both experimentally and analytically, with a goal towards developing a TFE with a lifetime of at least seven years. The TFE development work carried on in the early 1970s was consistent with a 20,000-hour lifetime goal. The experimental investigations of TFE lifetime issues consist of performing in-pile irradiations on prototype units using the 1.5 Mw TRIGA Mark F research reactor to validate TFE emitter deformation models and to measure the amount of emitter deformation as a function of fuel burnup, temperature and emitter thickness. To this end, nine thermionic emitters, fueled with uranium oxide and contained in three irradiation capsules, have been designed and fabricated to date, and are being tested in-core in the Mark F to study emitter materials integrity under irradiation conditions. In addition, a capsule consisting of only insulator test articles has also been irradiated in the Mark F to study insulator materials integrity under irradiation and applied voltage. Emitter deformation data are obtained by performing neutron radiography at periodic intervals. By using an underwater radiography device in the U. S. Department of Energy Contract DE-AC03-86SF16298

2 Mark F reactor tank with indium, dysprosium and track-etch foils for imaging, the process can be carried out routinely and rapidly. The Mark F reactor at the GA TRIGA Reactors Facility has been operated in a continuous, 3even-day, 24-h mode since January, 1985 to permit the inpilfe testing on prototype thermionic devices. Through calendar year 1987, a total of 59,744 hours of in-pile irradiations on fueled capsules had been completed. Numerous administrative, reactor, control room and auxiliary system changes have been incorporated for these TFE in-pile tests. The major changes in operation have been (i) amendments to the Technical Specifications to allow reactor operations with up to seven thermionic fueled capsules incore and with up to 40,000 hour irradiations for each capsule, (ii) training and licensing of new operators to permit continuous, round-the-clock operations, (iii) modification of the grid plate, reactor bridge and support structure to facilitate installation of the capsules, (iv) modification of the reactor scram loop, including the installation of first scram indication and two-of-three scram circuits, (v) integration into the reactor scram loop, of safety circuits in the experiment instrumentation which will cause reactor scram from certain abnormal conditions occurring in the thermionic capsules, (vi) installation and routine use of three rhodium self power detectors incore, (vii) upgrading of all auxiliary as well as radiation monitoring equipment to permit continuous operation in this mode, and (viii) installation of capsule monitoring instrumentation and data acquisition systems in the reactor room. The operational aspects of running the reactor in a continuous mode to meet the specialized needs of the TFE in-pile irradiation program are described in this paper. 2. DESCRIPTION OF THERMIONIC IN-PILE EXPERIMENT 2.1 Thermionic Fuel Element Capsule Design The design of a typical irradiation capsule containing the thermionic converters is shown in Figure 1. Three thermionic emitters, whose outputs

3 ~24 FT i 00 ~3 FT- PRIMARY CONTAINMENT, CABLE END SEAL ELECTRICAL BUS (TYP) MANOMETER ELECTRICAL - LOAD flpb== CESIUM 3 FUELED EMITTERS SECONDARY FISSION GAS FISSION GAS ADSORPTION TRAP RESERVOIR CONTAINMENT V E N T U N E (ONE PER EMITTER) ( 0 N E p E R EMITTER) Figure 1. Fueled Emitter Capsule Undergoing In-Pile Irradiation

4 can be individually monitored, were fabricated into thermionic converters and assembled into a sealed capsule for insertion into an in-core location in the Mark F. The design of each thermionic converter undergoing testing is shown schematically in Figure 2. Containment of the fission products produced from the UO^ fuel in each capsule undergoing in-pile testing is achieved by a double enclosure design. The primary containment is a hermetically sealed, stainless steel tube housing the three fueled emitters, and is itself housed inside a stainless steel secondary containment. The hard seal separating the two enclosures is penetrated by tubes leading to the fission gas reservoirs and associated pressure gauges as well as all other instrumentation leads for obtaining the test data. In addition to the converters, the primary containment contains individual cesium reservoirs with temperature control heaters; a low cesium vapor pressure is maintained in the interelectrode gap in each converter to obtain the proper electron emission characteristics for each emitter. The converters and their cesium reservoirs operate in a controlled helium environment to facilitate heat transfer. The secondary containment encloses all components of the primary section. Electrical penetrations from the secondary containment are through an extensive soft seal; the void volume between the hard and soft seals is filled with an inert gas at a pressure that is positive with respect to the helium pressure in the primary. A mixture of inert gases is used to vary the thermal conductance across the gap between the primary and secondary containment tubes. The four locations in the C- and D-rings of the reactor core which accommodate the 5.8-cm (2.3-in.) capsules presently under test are shown in Figure 3. To accommodate these capsules, cutouts in the top reactor grid plate were made by removing three TRIGA fuel elements from each of the capsule locations. Since the capsule is a cylinder which in inserted into a triangular array created by removing three 3.8-cm (1.5-in) nominal diameter fuel elements, water channels would be created around the device. To eliminate the effects of such a flux trap and to reduce power peaking in both the

5 INTERELECTRODE INSULATOR SEAL TUNGSTEN EMITTER CESIUM... j.1 NI ICLEI TR FU EL I I NIOBIUM COLLECTOR INSULATOR NIOBIUM SHEATH SHEATH INSULATOR Figure 2. Schematic of Fueled Thermionic Converter Test Article

6 LEGEND SELF POWERED DETECTORS (3) CONTROL ROD LOCATIONS (5) FUEL ELEMENT LOCATIONS Figure 3. Reactor Core Configuration for Thermionic In-Pile Irradiations

7 emitter and adjacent TRIGA fuel elements, aluminum filler pieces were placed in these water channels. The next generation of thermionic converters to be tested in-pile later in the Thermionics Irradiations Program are being designed to fit into standard TRIGA fuel locations, and thus eliminate any concern for power peaking from the surrounding water channels. 2.2 Thermionic Test Facility Requirements and Operating Conditions The TRIGA Mark F reactor provides an uninterrupted, nearly constant neutron source for in-pile testing of the thermionic converters; the constant and uninterrupted neutron source being provided by the long core life of the current FLIP fueled core. Thus, the capsules are subjected to an unperturbed neutron flux, since the reactor can be operated with very little control rod movement for extended durations and without refueling during the tests. There were other features of the TRIGA reactor which has allowed it to be used as an essentially dedicated facility for the irradiation of TFEs to experimentally validate material lifetime issues. These other features included: Minimum required reactor downtime, giving a high reactor availability Accommodation within the reactor license of up to four fueled 6-cm diameter capsules in-core Accommodation of capsules in the reactor with an above core diameter of ~13-cm, with associated instrumentation leads. 2.2.A Neutron radiography capability for obtaining periodic emitter deformation data without removal of the capsules from the reactor pool, and the capability to safely and repeatedly move capsules from their in-core location to the neutron radiography station Simulation of fast neutron environments expected in thermionic space power systems.

8 2.2.6 Facilities to allow on-line, real time testing of each capsule undergoing in-pile irradiation. This meant that dedicated space for instrumentation needed to be provided, as well as unlimited access by thermionics personnel to the reactor room during normal operations. The reactor is operated at or near 1.5 Mw(t) with 93 fuel elements and five fuel follower control rods. This core configuration maintained an excess reactivity of less than $0.50 for long periods of operation to conform to the thermionic design requirements and to ensure that all control rods are above the level of the upper thermionic cells. This eliminates skewing of the axial power distribution from the cell. Operation at 1.5 Mw(t) also simulates closely the fast neutron environment in actual space based thermionic systems. During 1987, the reactor was operated for the thermionics experiment with an availability of about 98.5%. The few reactor related scrams that were experienced were primarily due to faulty instrumentation, and did not cause extended shutdowns. Generally, startup and full power were achieved within 24 hours after a reactor scram. 2.3 Irradiation Testing Data Summary Table 1 shows the irradiation exposure history on the thermionic capsules achieved on the Mark F as of the end of calendar year In addition to the fueled capsules summarized in Table 1, an in-core capsule consisting of sheath insulator material test articles was also irradiated; this capsule was removed after 6455 hours of irradiation testing (fast neutron fluence of ~6 x 10 n/cm.) FACILITY OPERATIONAL ENVIRONMENT FOR THERMIONIC TESTING The TRIGA Mark F was extensively modified to permit it^s use for thermionics fuel testing in an essentially dedicated manner - that is, startup, shutdown, operating power, power stability and neutron radiography procedures which are all defined by thermionic requirements. Four core

9 locations (1, 2, 3, and 4 in Figure 3) have been used to date for thermionic test capsules; however, up to seven capsules are permitted by the reactor license for simultaneous in-core testing. TABLE 1: TRIGA IRRADIATION EXPERIMENT STATUS Capsule 1 1 Capsule 2 Capsule 3 T = 19,426 hrs T» 22,146 hrs T = 18,172 hrs Fluence Burnup Fluence Burnup Fluence Burnup (x 10" 21 ) Atom (x 10~ 21 ) Atom (x 10~ 21 ) Atom E >0.1 MeV Percent E >0.1 MeV Percent j E >0.1 MeV j Percent Cell Cell Cell Administrative Requirements Conduct of the thermionic experiment and all associated facility modifications were approved by the reactor safety committee under the application of 10 CFR Certain license amendments had to be obtained however, to permit thermionic capsules approaching the seven-year design lifetime to be tested in this program. Specifically, the following amendments to the Mark F Technical Specifications were already in the GA license or were specially applied for and approved by the Nuclear Regulatory Commission: o Insertion and testing of up to seven thermionic capsules simultaneously for in-core testing, with each device containing up to 100 g U-235; o Irradiation of each device for up to 40,000 hours in-core. o Inclusion of minimum experimental safety systems to provide (a) automatic reactor scrams in case of abnormal thermionic capsule operation, and

10 (b) personnel warning systems to initiate evacuation of the reactor room if certain capsule abnormally high radiation levels from the capsules are detected, indicating fission product leaks from the secondary containment. During the three years of operation, seven senior reactor operators and four reactor operators have been added to the staff to form a complement of 19 licensed operators at GA which permits operation for 168 hours per week. Of the 19, seven serve in a "reserve" capacity and are not required to function full time as reactor operators. 3.2 Reactor Room and Auxiliary System Modifications Numerous reactor room modifications as well as upgrades of auxiliary systems were undertaken to prepare the Mark F for operation in a continuous mode. These major modifications and upgrades were made in part prior to startup of the program and partly during the program Modification of the reactor bridge and control rod support structure to allow insertion and removal of the capsules. The control rod drive mounting plate was raised ~168-cm. (66-in.) [Figure 4] from it's original height to facilitate installation of the 24 ft. long capsule assembly in the core. This necessitated extending the connecting rods by a corresponding length for each control rod Since the reactor would operate only in the steady state mode during thermionics testing, the pulsing capability was removed, and a fifth standard fuel follower control rod and rod drive were installed Installation of a 5 kw uninterruptable power supply (UPS) to facilitate continued operation in the case of a momentary power loss. This power source is used for the reactor console instrumentation as well as the thermionic capsule safety systems. The purpose of the UPS is mainly to make the system independent of "noise" on the AC lines, most notably that due to periodic switching by the power company and that due to lightning in the surrounding region.

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12 3.2.4 Installation of a new 2 MW cooling system. A system with a highefficiency plate-type heat exchanger with a secondary evaporation tower cooling system was installed to replace the existing tube-and-shell heat exchanger." The water treatment system capability was upgraded to provide a larger (12 cu.ft.) demineralizer resin bed, with on-line flush and reload capability. Conductivity in the 24,000-gallon reactor tank is constantly maintained at a 1.0 to 1.5 /lmhos/cm level with this system. 3.3 Instrumentation and Control System We needed to enhance the ability to operate in a round-the-clock mode by minimizing scrams or shutdowns caused by component failures in the safety channels. This required several modifications to be made to the older reactor scram instrumentation. In addition, the nature of the experiment dictated that abnormal conditions in the TFE capsules be detected, and that signals from these sensors be transmitted to the reactor scram loop to initiate a reactor scram Reactor Scram Loop Modifications The two major changes to the scram loop were (a) installation of a first scram indication circuit, and (b) installation of a two-of-three logic circuit which requires two of three trip signals for both reactor power and fuel temperature to the reactor scram circuit before reactor scram. The first scram indicator is a fast acting (~15 nanoseconds) circuit that latches on the first occurring scram and provides an indication of it's identity on a separate scram annunciator panel. This circuit complements the usual scram annunciator panel which does not distinguish between the order that the channels tripped. Thus, "sympathetic" channel trips, which frequently occur with an externally generated scram, such as one from a thermionic device, scrams the reactor, can be sorted out and the true cause of the reactor scram is established.

13 The two-of-three logic circuit takes advantage of the fact that the Mark F utilizes six independent safety channels, three as power safety channels and three as fuel temperature safety channels. The minimum license required safety system scrams require only two power level channels for steady state operation. We have elected to use three power levels and three temperature channels for this experiment (i.e., both power and fuel temperature scrams). Therefore, to minimize spurious scrams, advantage was taken of the additional channel available for each parameter to allow continued operation with the minimum required two independent channels for each parameter on line at all times, but allowing any one of three channels to fail without causing a reactor scram. The logic for operation of the two-of-three circuit is shown in Figure 5. The wiring of the contacts on each of the three relays is such that if two of three circuits are operational, then relay K4 is held in; if any two should trip, then K4 would open causing a reactor scram. Each trip is annunciated on the console in the order in which they occur. In order to complete necessary repairs on tripped channels and put them back on line, scram reset circuitry was also added which allows recovery from tripped channels while maintaining the reactor at power In-Core Power Monitors Three rhodium self-powered detectors (Reuter-Stokes) with active lengths of 40 cm. and 0.5-mm diameter, were installed for in-core flux profiling. To provide a constant in-core neutron flux for each of the TFE capsules, long term power stability is maintained by monitoring the output of these in-core detectors, while ensuring that the licensed 1.5 Mw(t) power level of the reactor is not exceeded as indicated on the ex-core ion chamber power safety channels. Power calibrations (calorimetric method) are also performed in a manner which ensures a constant in-core flux as the water temperature increases (Ref. 2) Thermionic Capsule Instrumentation and Safety Systems The essential experimental instrumentation for operation of the TFEs in

14 HOT O SAFETY CHANNELS K2 _ K1 =±= K3 r = K2 K3 SCRAM LOOP IN K4 An,SCRAM LOOP OUT COM Figure 5. Two-of-Three Logic Circuit Schematic

15 the TRIGA reactor consists of (a) driving power supplies for emitter cooling, (b) cesium reservoir heaters, (c) water cooling systems for cooling the high current bus bars which carry the electrical current from the devices, (d) a secondary containment gas exchange system, (e) reactor scram circuitry and (e) a data acquisition system for acquiring, analyzing and storing all pertinent data on a near-real-time basis. This system presently monitors, stores and displays in a user friendly manner over 100 pieces of analog data, including temperature, pressure, current and voltage as well as reactor data for the in-pile capsules. The complete instrumentation system is housed in about a dozen full height instrumentation racks located in the reactor room. Of all the measured parameters, only three measurements are required to initiate a reactor scram (through the reactor external scram relay bus). These sensors are monitored continuously and are hardwired into the scram loop. They do not rely on computer generated signals to scram the reactor. They are: 1. Excessively high collector temperatures, which trigger any one of four high temperature relays. 2. Loss of bus bar water cooling, detected by a optical sensor on the flowmeters. 3. Low electrical current flow in a capsule, which triggers a meter relay on the power supply. In addition, there is an alarm system with much broader capabilities that is controlled by the computer used for the data acquisition system. Alarms are issued for exceeding preassigned minimum and maximum values of the various parameters being monitored, to alert the operator at the reactor console of a potentially abnormal condition. 4. NEUTRON RADIOGRAPHY Neutron radiography (NR) is the primary means for obtaining periodic

16 emitter deformation data on the capsules. During 1987, NR was performed on the fueled as well as insulator capsules at periodic intervals ranging from hours of continuous irradiation. Typically, about 40 hours of reactor operation was required to obtain about a dozen stereo NR images of the nine fueled emitters. 4.1 Neutron Radiography Apparatus An underwater NR device in the Mark F tank is used for the exposures. This device uses a 1.27-cm (0.5-in.) aperture, and an L/D of 120, to expose dysprosium, indium and track etch foils. The entire device is wrapped in 1.0-mm. (0.04-ln.) thick cadmium to eliminate exposure from stray thermal neutrons. A Gd-Dy-In shutter located approximately 10-cm (4-in.) in front of the aperture allows the capsules to be positioned in the various exposure positions within the camera with the reactor at power. The images are produced on Kodak Industrex SR-5 film in vacuum cassettes, or by developing track-etch foils in a NaOH solution at the proper temperature. 4.2 Neutron radiography Procedure The exposure of each capsule consists of irradiating foils of dysprosium and indium and a sheet of track-etch film. A series of six exposures are taken on each capsule; three each at two vertical position. Three angular positions, 60 apart, are exposed at each vertical position. Following exposure of the foils in each geometry, they are placed in vacuum cassettes for exposure to the film, which is developed to obtain the NR exposure (Figure 6). The vacuum cassette has allowed a greater resolution in the images on film, making the NR more useful for dimensional measurement around sharp edges. Dimensional changes of the thermionic emitters as a function of irradiation exposure are measured on the radiograph using a traveling microscope

17 Figure 6. Typical Neutron Radiograph of Thermionic Capsule

18 fitted with a digitized lead screw, and coupled to a dedicated personal computer for data processing and storage. With this system, dimensional changes with precisions of ± mm (0.001-in.) can be obtained, if the same operator measures the exposures. 5. CONCLUSIONS The Mark F at General Atomics will continue to operate on a round-theclock basis during 1988 to provide in-pile irradiations for thermionic test capsules. More capsules with varying designs are expected to be added to the core during 1988 and The number of shutdowns required for NR of the capsules is expected to be reduced to about twice per calendar year, putting added emphasis on reliable long term operation of various reactor systems and components. We are developing programs to perform routine preventative maintenance of some items that require more frequent maintenance and calibrations to be done while the reactor is on-line. Thus, down time can be minimized for such purpose. Further, the increase in reliability of operation decreases the probability of damage to thermionic devices from thermal shock that results from frequent reactor scrams from full power. Finally, the thermionic capsule irradiation program has been a success to this point. The codes used to predict emitter deformation have shown good agreement with measured deformation obtained from NR exposures of the nine fueled devices. Curves of measured vs. predicted deformations have indicated to date that the codes based on the underlying principles of radiation damage are generally predicting emitter distortions accurately. REFERENCES 1. "Thermionics Irradiation Program," by Project Staff, GA Technologies Document GA-A18916 (July, 1987) 2. W. L. Whittemore and J. Razvi, "Power Calibrations for TRIGA Reactors," presented at the 11th TRIGA Owner/User Conference, April 1988, and published in the Proceedings.