EXPERIMENTS WITH PREIRRADIATED FUEL RODS IN THE NUCLEAR SAFETY RESEARCH REACTOR. O.Horiki.S.Kobayashi,I.Takariko and K.Ishijima

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1 EXPERIMENTS WITH PREIRRADIATED FUEL RODS IN THE NUCLEAR SAFETY RESEARCH REACTOR O.Horiki.S.Kobayashi,I.Takariko and K.Ishijima Department of Fuel Safety Research Tokai Research Establishment Japan Atomic Energy Research Institute ABSTRACT In the Nuclear Safety Research Reactor (NSRR) owned and operated by Japan Atomic Energy Research Institute (JAERI), extensive experimental studies on the fuel behavior under reactivity initiated accident (RIA) conditions have been continued since the start of the test program in Accumulated experimental data were used as the fundamental data base of the Japanese safety evaluation guideline for reactivity initiated events in light water cooled nuclear power plants established by the nuclear safety commission in All of the data used to establish the guideline were,however,1imited to those derived from the tests with fresh fuel rods as test samples because of the lack of experimental facility to handle highly radioactive materials.the guide!ine,therefore,introduces the peak fuel enthalpy of 85 cal/g which was adopted from the SPERT-CDC data as a provisional failure threshhold of preirradiated fuel rod and, says that this value should be revised based on the NSRR experiments in the future. According to the above requirement, new NSRR experimental program with the preirradiated fuel rods as test samples was started in Test fuel rods are prepared by refabrication of the long-sized fuel rods preirradiated in commercial PWRs and BWRs into short segments and by preirradiation of short-sized test fuel rods in the Japan Material Testing Reactor(JMTR). For the tests with preirradiated fuel rods as test samples, the special experimental capsuls, the automatic instrumentation fitting device, the automatic capsule assembling device and the capsule loading device were newly developed. In addition, the existing hot cave was modified to mount the capsule assembling device and the other inspection tools and, a new small iron cell was established adjacent to the cave to store the instrumentation fitting device. 5-13

2 1. Introduction NSRR is a modified TRIGA-ACPR (Annular Core Pulse Reactor). It was built for studying reactor fuel behavior under Reactivity Initiated Accident (RIA) conditions and went to the initial criticality in May, As shown in Fig. 1, the reactor core is mounted on the bottom of 3.5 m wide, 4.5 m long and 9 m deep open pool and cooled by natural circulation of pool water. At the center of NSRR core, there exists large experimental cavity whose inner diameter is 22 cm where the test fuel rods contained in a capsule or a loop are subjected to a power burst. Access to the experimental cavity is provided either by the vertical loading tube or the offset loading tube. The vertical one is used for a long-sized experiment such as a loop and the offset one is selected for a short-sized capsule type experiment. Due to the offset location, no radiation shielding plug is required in the offset loading tube. This allows easy insertion and removal of the capsule into/from the experimental cavity. 2. Fuel Experiment In the NSRR, extensive experimental studies on the reactor fuel failure behavior under simulated RIA conditions have been continued since the start of the test program in The number of fuel irradiation tests performed so far reached approximately 950 runs and accumulated experimental data were used as the fundamental data base of the safety evaluation guideline for reactivity initiated events in light water cooled nuclear power plants established by the nuclear safety commission in 1984 as shown in Fig. 2. All of the data used to establish the guideline were, however, limited to those derived from the tests with fresh fuel rods as test samples because of the lack of experimental facility to handle highly radioactive materials. The guideline, therefore, introduces the peak fuel enthalpy of 85 ca1/g.uo 2 which was adopted from the SPERT-COC data as a provisional failure threshold of preirradiated fuel rod and, says that this value should be revised based on the NSRR experiments in_the future. According to the above requirement, new NSRR experimental program with preirradiated fuel rods as test samples was started in Test fuel rods are prepared by refabrication of the long-sized fuel rods preirradiated in commercial PWRs and BWRs into short segments and by preirradiation of shortsized test fuel rods in the Japan Material Testing Reactor(JMTR) as shown in Fig. 3. For the tests with the preirradiated fuel rods as test samples, the 5-14

3 special experimental capsule, the automatic instrumentation fitting device, the automatic capsule assembling device and the capsule loading device were newly developed. In addition, the existing hot cave was modified to mount the capsule assembling device and the other inspection tools and, a new small iron cell was established adjacent to the cave to store the instrumentation fitting device. Fig. 4 shows the outline of the experiment handling procedures for preirradiated fuel rod tests. 3. Experimental Facilities 3.1 Experimental Capsule The experimental capsule used in the preirradiated fuel rod tests is a newly developed double-container type one. Schematic configuration of this capsule(type X-1) is given in Fig. 5. Outer capsule is a sealed container of 130 mm in inner diameter and 1,250 mm in height. Inner capsule is a sealed pressure vessel of 72 mm in inner diameter and 680 mm in height. In the design work of the capsule, the easiness of assembling and disassembling jobs by remote handling was the major concern as well as structural strength. Type X-1 capsule was specially designed for the test with the test fuel rod preirradiatod in the JMTR and can be used in the test where energy deposition in a rod is less than 250 cal/g.uo 2. The capsule containing an instrumented test fuel rod, is filled with water of ambient temperature and atomospheric pressure, and is subjected to a pulse irradiation in the NSRR. 3.2 Capsule Handling Device The capsule handling devices were newly developed and mounted in the semi-hot cave as shown in Fig. 6. A double -container type capsule is assembled with the automatic capsule assembling device and examined'its tightness with Helium leak detecter. Then it is inserted into the experimental cavity through theoffset tube with the capsule loading device. 3.3 Instrumentation Mounting Device To attach Plutinum Plutinum-Rhodium thermocouples on a preirradiated fuel cladding tube in the hot cell, the instrumentation mounting device was developed as shown in Fig. 7. Thermocouple wire is stretched through the center of a welding electrode, pressed on cladding tube and welded by 5-15

4 electric current as illustrated in Fig. B. After the thermocouple wiring is completed between cladding tube and electric signal terminal, electric overcurrent is flown on the excess thermocouple wire between the electrode and terminal to cut the wire and shape a small bulb on the end of the wire. The procedure is preset in a minicomputer and the computer controls the movement of the mounting device. Fig. 9 illustrates the instrumentation used in the preirradiated fuel rod test. The test fuel rod was equiped with 6 thermocouples (Pt/Pt-135JRh, 0.2 mm in diameter) to measure cladding surface temperature. 4. Conclusion Seventeen RIA experiments with preirradiated fuel rods have been conducted so far in the NSRR since the start of new program. The results of the SPERT, PBF and NSRR experiments are shown in Fig.10. The experimental data obtained from these experiments will contribute to update the Japanese safety evaluation guideline for reactivity initiated events in LWRs. 5-16

5 Capsule "Ho Id-Down Device Control Rop Drive Water Level "I Vertical Loading Tube Offset Loading Tube Capsule Storage Pit Neutron Detectors Neutron Radiography Room Capsule Gripping Device Fig.l Vertical cross-section of the' NSRR reactor 5-17

6 Energy Deposition (cal/g U0 2 ) Fuel Conditions No Change, Oxidation, Deformation Crack, Fragmentation I ' 230 y' Safety Criteria Non Failure Failure Pressure Pulse Generation Fig.2 Safety Criteria for Reactivity Initiated Accident in Japan

7 unit: mm No. Item Material 1 Top end fitting Zry-4 2 Spring Inconel 3 Spring holder SUS Iron core MEIF 5 Cladding tube Zry-4 6 Alumina pellet Al UO2 pellet (Nat.) U0 2 8 UO2 Pellet ( 5%) U0 2 9 U0 2 pellet (10%) U Disk SUS Spacer SUS Bottom end fitting Zry-4 Fig.3 Design of the test fuel rod preirradiated at the JMTR 5-19

8 Large Hot Laboratory (Dep.of Reactor Fuel Examination) NSRR Hot Laboratory (Dep.of Research Reactor Operation) Fuel Fabrication Dev ices -Fabrication of test fuel rods from LWR fuel - J't*Ll'l?QJ 1*11 J 1 TJ a Fuel Transportation Control Rod Drive Mechanism 111!/ Capsule Loading Device Semi Hot Q. T5 a Capsule Transportation Capsule Disassembling Device? Fuel Inspection Devices Capsule d I sassembl I ng. PIE en I O Reactor- - \ Core ^ W Fuel Capsule Instrumentation Storage Assembling Fitting Device Device H m Newly Fabricated Modified Preirradiation of test fuel rods Fig.4 Instrumentation fitting, capsule assembling, pulse irradiation Outline of the experiment handling procedures for preirradiated fuel tests.

9 Capsule cap Gas discharge valve (Outer capsule) Hanger fixing bolt Top closure (Outer capsule) Gas discharge valve (Inner capsule) Flange bolt (Outer capsule) Instrumentation plug (Outer capsule) m Hanger rod Water level (Inner capsule) Test fuel Capsule body (Inner capsule) Water level (Outer capsule) Instrumentation plug (Inner capsule) Flange bolt (Inner capsule) Top closure (Inner capsule) o m CVJ Capsule body (Outer capsule) Unit (mm) Fig.5 Schematic configuration of type X-I atmospheric presure. capsule for pre-irradiated fuel rod test 5-21

10 i,\\\\\\\\\w\\\^ jn\n\\\\^ No. Item 1 Capsule assembling device 2 Capsule stand 3 He leak test vessel Fuel storage pit AJ JJ Manipulator 6 Preriscope 7 Shielding window 8 In-cell monitor 9 Liquid waste storage tank 10 Armed hoist II Running hoist 12 TV camera 13 Sample transfer hole 14 Work table 15 Washing machine s\ww\\l\\\\y ^\\\\W\Y Unit lmm) Semi-hot cell Fig.6 CopsuJe handling devices in semi-hot cave of NSRR 5-22

11 No. Item t Test fuel 2 Capsule top closure (Inner capsule) 3 Instrumentation supporting rod 4 X-table 5 Y-table 6 Supporting rod rotating device 7 Test fuel fixing device... 8 Table 9 Electrode cartridge 10 Earth electrode I Unit (mm) Fig. 7 Schematic of instrumentation mounting device

12 Cladding tube Electric Signal Terminal'' Bulb Welding Electrode in Fig.8 Illustration of the Movement of Thermocouple Welding Electrode

13 Elongation sensor Supporting rod Test fuel T/C / Outer capsule Test fuel E o 10 CN Cn / Inner capsule ^g-9 Instrumentation I t Capsule Experiment Cap sule and Instrumentation

14 in l to CN O 400 ^> 300 o " c o 200 en O CL (D O h to 8 a Experiment PWR NSRR BWR JMTR USA No failure A O D O Failure a UJ _L FuelBurnup (MWd/kgU) 50 Fig.io Result of the SPERT, PBF and NSRR Experiments with Preirradiated Fuels