JOYO, THE IRRADIATION AND DEMONSTRATION TEST FACILITY OF FBR DEVELOPMENT

JOYO, THE IRRADIATION AND DEMONSTRATION TEST FACILITY OF FBR DEVELOPMENT Aoyama T. 1, Sekine T. 1, Nakai S. 1 and Suzuki S. 1 1 O-arai Research and Development Center, Japan Atomic Energy Agency, Ibaraki, JAPAN 1. Introduction The experimental fast reactor JOYO at the O-arai Research and Development Center of the Japan Atomic Energy Agency (JAEA) is the first liquid sodium-cooled fast reactor in Japan. The major objectives of constructing JOYO was to obtain technical information about the liquid metal fast breeder reactor (LMFBR) through experience with its design, construction and operation, and to use the reactor as a fast neutron irradiation facility for the development of fuels, materials, and other components required for the LMFBR program. Through design, construction, testing, operation and maintenance experience, JOYO has contributed much to the LMFBR development program. In addition to providing operating experience, many kinds of irradiation tests have been conducted for the development of fuels and materials. JOYO has recently been upgraded to the MK-III core to provide a more robust and capable irradiation test bed. This paper provides a review of JOYO major achievements, an outline of the MK-III upgrading program, and a utilization plan. 2. Plant description of JOYO JOYO is a sodium-cooled fast reactor with mixed oxide fuel. JOYO attained initial criticality in 1977 with the MK-I breeder core. Through the MK-I operations, the basic characteristics of LMFBR were studied as the first LMFBR in Japan. As an irradiation bed, the MK-II core achieved the maximum design output of 1MW in 1983 and by 2 completed 35 operational duty cycles and several special tests. JOYO has now been upgraded to the MK-III high performance core. The JOYO specifications are shown in Table 1. 3. Major achievements of JOYO 3.1 Core management A breeding ratio of 1.3 was confirmed during operation of the JOYO MK-I breeder core. On the other hand, operation of the JOYO MK-II irradiation core confirmed a Pu conversion ratio of.28. This demonstrates that both breeding and consumption of Pu are feasible in the same LMFBR, with replacement of the core. A core management code system was developed to predict the core parameters for conducting safe and efficient operation and making refueling plans within the design and operation limits. The results from core physics tests in each operational cycle and Post Irradiation Examination (PIE) have been used to confirm the accuracy of these predictions. The accumulated data were compiled into a database and were recorded on CD-ROM for user convenience. 3.2 Demonstration of Pu fuel recycle Pu fuel recycle was demonstrated in 1984. Two MK-I driver fuel subassemblies unloaded in 1981 were transferred to the Fuel Monitoring Facility (FMF) adjacent to JOYO. The subassemblies were dismantled in FMF to remove the fuel pins, and ten fuel pins were transported to the Chemical Processing Facility (CPF) which is located at JAEA Tokai Research and Development Center for the FBR fuel reprocessing tests in 1982. About 6g of Pu was extracted in the CPF and 25g of Pu was used for manufacturing MOX fuel pellets at the Plutonium Fuel Production Facility (PFPF) of Tokai Research and Development Center. A fuel pin was fabricated at PFFF using these fuel pellets. This pin was transported to the Irradiation Rig Assembling Facility (IRAF), adjacent to JOYO, where an irradiation test subassembly was assembled using this pin. This test subassembly was irradiated in the MK-II core from 1984 to 1986, and

integrity of the fuel was confirmed by PIE. Thus, small scale FBR nuclear fuel cycle was realized by JOYO and other fuel facilities. 3.3 Irradiation tests of fuels and materials In JOYO, 478 driver fuel subassemblies were irradiated from the MK-I core (17 driver fuel subassemblies) through the 35 cycles of the MK-II core (371 driver fuel subassemblies). Approximately 58, fuel pins have been loaded in JOYO, and they have attained a peak burn-up of 84,6MWd/t without any fuel pin failures. The statistical irradiation data of the driver fuels were obtained from PIE conducted on 65 driver fuel subassemblies. The data were used in the evaluation of the design and operation of JOYO and were used in developing the design for the prototype reactor MONJU and future FBR fuel. It is necessary for the commercialization of the FBR to realize an economic efficiency that can compete with the light water reactor (LWR). From the view point of fuel development, an extended fuel burn-up and a higher linear heat rate are the major points. Continuous efforts have been conducted to improve MOX fuel performance for the large FBR based on these points. There have been approximately 3 irradiations of advanced fuels in irradiation test rigs. The material irradiation test is important because the irradiation characteristics of materials influence the reactor performance and lifetime. Over 4 test pieces, core material such as cladding material and wrapper tube, structural material such as reactor vessel and core support plate, and absorber material, have been irradiated in JOYO. 4. MK-III Upgrading program 4.1 Upgrading program JOYO is expected to play a greater role in providing an irradiation field for irradiation tests to develop FBRs and for various other materials irradiation tests. An upgrading project named MK-III was initiated to satisfy those needs. The main objectives of this project are the increase of neutron flux, the increase of irradiation periods, and the upgrade of irradiation technology. An outline of this project is shown in Figure 1. The maximum fast neutron flux was increased by 3%, and the reactor thermal output was increased by 4%, which necessitated the increase of the cooling system heat removal capacity. Outlines of the modification of the MK-III core and the cooling system are shown Figures 2 and 3, respectively. All of the modifications of the core and the cooling system were completed successfully. 4.2 Performance test results The performance tests were carried out from June 23 as the last phase of MK-III upgrading program. The performance test items included core characteristics, plant characteristics, radiation dose distribution, and monitoring of abnormal conditions such as fuel failure and cooling system vibration. JOYO attained initial criticality with the MK-III core on July 2, 23. After that, the reactor power was raised step by step, while confirming the nuclear and thermal characteristics of the MK-III core and the heat removal capability of the cooling system. All performance tests were successfully carried out and it was confirmed that performance of the JOYO MK-III plant satisfied the design requirements. MK-III rated-power operation started in 24 with greatly increased irradiation capability. 5. Irradiation and safety tests in the MK-III core 5.1 Demonstration of self actuated shutdown system Self actuated shutdown system (SASS) with a Curie point electromagnet (CPEM) has been developed for use in a large-scale LMFBR in order to establish passive shutdown capability against anticipated transient without scram (ATWS) events. The basic characteristics of SASS have already been investigated

by various out-of-pile tests for basic components. As the final stage of the development, the stability of SASS needs to be confirmed under the actual reactor-operational environment with high temperature, high neutron flux, and flowing sodium to ensure a high plant availability factor. For this purpose, the demonstration test of holding stability using the reduced-scale experimental equipment of SASS was conducted in the 1st and 2nd cycles of the MK-III core. Figure 4 shows the test results of the holding stability test. The control rod holding stability under the actual reactor-operational environment was successfully confirmed. The results also indicate there are no fundamental impediments to the practical use of SASS that might arise from operational trouble involving an unexpected drop during reactor operation. The element irradiation test, which provides basic magnetic characteristics data under irradiation, is to be conducted during the 3rd to 6th cycles of MK-III from 26 to 27. 5.2 Fuel failure simulation test When a fuel failure occurs in a nuclear reactor, it is essential to quickly detect the event and identify the failed fuel subassembly. As JOYO has not yet experienced any natural fuel pin failures, three fuel failure simulation tests had been conducted in the MK-II core. After the MK-III modification, performance of the fuel failure detection (FFD), the failed fuel detection and location (FFDL) systems, and the plant operation procedure in case of fuel failure needed to be demonstrated. The fourth in-pile fuel failure simulation was therefore conducted in the MK-III core. A test subassembly containing two fuel pins with an artificial slit on the fuel cladding tube in the plenum gas region was used for the irradiation test. The test fuel subassembly and the test results are shown in Figure 5. When the reactor power reached 12MWt, which is 86% of the rated power, the fission gas released from test fuel pins was observed using several detectors of the FFD system. After the reactor shutdown, the subassembly which released fission gas was identified using the FFDL system. As a result, the performance of FFD and FFDL were verified and the plant operation procedure was confirmed. The test results will be also useful for preparing the future run-to-cladding- breach (RTCB) tests in JOYO. 5.3 Irradiation test for MA-MOX fuel An irradiation test for mixed oxide fuel containing minor actinides (MA-MOX) is performed in JOYO to investigate the irradiation behaviour of this fuel. An irradiation subassembly consists of 3type fuel pins, test pins containing 3% and 5% Americium (Am-MOX), fuel pins containing 2% Americium and 2% Neptunium (Np/Am-MOX) and reference MOX pin. Am-MOX pins were fabricated in the hot-cell of the Alpha Gamma Facility (AGF) in Oarai Research and Development Center, Np/Am-MOX and reference MOX pins were fabricated in the Plutonium Fuel Production Facility (PFPF) in Tokai Research and Development Center, respectively. A MA-MOX fuel irradiation test series in JOYO named B11 is being conducted. A capsule type irradiation test subassembly, which enables the irradiation test of advanced fuel, such as those containing minor actinides, is used. Each test fuel pin is enclosed in a capsule to contain any fuel and fission products in case of fuel failure. This irradiation test series includes initial structural change confirmation test (B11(1)), MA re-distribution confirmation test (B11(2)), and steady irradiation test (B11(3)). The fuel pin specifications containing MA are shown in Figure 6. The B11(1) irradiation test was successfully conducted in May 26. The power history of B11(1) test is shown in Figure 7. In the B11(1) test, the maximum linear heat rate of the test fuel pin was set by the reactor power level, and the reactor power was raised continuously, held at 12 MWt about 1 minutes, and then the reactor was manually shut-down to keep irradiated structure. The B11(2) test will be performed in the MK-III core from August, 26 and B11(3) will start form June, 28. The test results and fuel fabrication technique will be utilized for the development of low decontamination factor fuel cycle development.

5.4 Irradiation test with MARICO Material Testing Rig with Control (MARICO) was used in the MK-II core for the in-pile creep rupture test of cladding materials. MARICO could control the temperature by changing the thermal conductivity between double walled capsules through adjustments in the He and Ar gas ratio. The capsule temperature was successfully controlled within ±4deg-C during full power operation. For the MK-III core, MARICO-2 was fabricated. Figure 8 shows the outline of MARICO-2. As an additional temperature control system, an electric heater is installed in MARICO-2 to control the temperature during the start-up and shutdown period. In the JOYO MK-III core, creep rupture test specimens of oxide dispersion strengthened (ODS) ferritic steel, which is the promising candidate for fuel cladding of long life fuel, is irradiated with MARICO-2. 6. Conclusion JOYO has been operated successfully about 3 years since its criticality was first achieved in 1977 without any major trouble, and this operation has demonstrated the safety and reliability of the sodium cooled fast reactor technology. In light of the shutdown of several fast reactors around the world, the ability to make such major contributions to reactor development takes on even greater significance. Irradiation tests, steady-state and safety related operations in JOYO are also expected to contribute the operation of Monju and to promote commercial FBR. 7. References [1] Maeda Y., Aoyama T., Odo T., Nakai S., Suzuki S., Distinguished achievements of a quarter-century operation and a promising project named MK-III in JOYO, Nuclear Technology Vol. 15, No.1, 25, pp.16-36 [2] Takamatsu M., Sekine T., Aoyama T., Uchida M., Kotake S., Demonstration of control rod holding stability of the self actuated shutdown system in JOYO for enhancement of fast reactor inherent safety, GLOBAL 25 Tsukuba, Japan, 25 [3] Ishida K., Ito C., Aoyama T., Fuel failure simulation test in the Experimental Fast Reactor JOYO, GLOBAL 25 Tsukuba, Japan, 25 Table 1 Items Reactor Thermal Output (MWt) Max. Number of Driver Fuel S/A Max. Number of Test Fuel S/A Core Diameter (cm) Core Height (cm) 235 U Enrichment (wt%) Pu Content Total (wt%) Fissile (wt%) Max. Linear Heat Rate (W/cm) Max. Neutron Flux Total (n/cm 2 s) Fast (>.1MeV) (n/cm 2 s) Max. Burn-up (Pin Average) (GWd/t) Primary Coolant System Flow Rate (t/h) R/V Inlet Temp. (deg-c) R/V Outlet Temp. (deg-c) Blanket/Reflector/Shielding JOYO Specifications MK-I 5/75 82 8 6 ~23 ~18-32 3.2 1 15 2.2 1 15 42 2,2 37 435/47 Blanket/SUS MK-II MK-lll MK-III 1 14 67 85 9 21 73 8 55 5 ~18 ~18 <3 <3 ~2 ~16/21* 4 42 4.5 1 15 5.7 1 15 3.2 1 15 4. 1 15 75 9 2,2 2,7 37 35 5 5 SUS / SUS SUS / B4C * Inner/Outer Core

astneuttronflux(115n/cm2sfast Neutron Flux Increase 3 % than MK-II Core Heat Removal Capacity Enhanced in Primary and Secondary Cooling System MK-III Core Core Replacement for High Neutron Flux Irradiation Capability Enhanced about 4 Times Number of Irradiation Test Assemblies Increased 9 21 MK-II Reference Core MK-III Reference Core Shielding Subassembly Control Rod Reflector Irradiation Rig Driver Fuel )4. Higher Plant Availability Factor Upgrading in Irradiation Techniques MK-III 3. Annual Inspection Period and Fuel Exchange Time Reduced Development of Irradiation 2. Figure 1 Outline of MK-III project Figure 2 MK-IIF1. 6 5 4 3 2 1 1 2 3 4 5 6 Row Modification of MK-III core Intermediate Heat Exchanger (IHX) Capacity Heat Transfer Area Number of Tubes Reactor Vessel 5 7 MWt 352 363 m 2 356 363 m 2 2835 288 (A Loop) 1812 288 (B Loop) Primary Pump Motor and Flow Control System Main Motor Capacity 33 33 kw Pony Motor Capacity 2.5 2.5 kw EMF M Pump Overflow Primary Column Pump IHX DHX (Air Cooler) Air M Main Blower M Secondary Pump Electromagnetic Over Flow Flow Meter (EMF) Tank Primary Cooling System MK-Ⅱ MK-Ⅲ R/V Inlet 37deg-c 35deg-c R/V Outlet 5deg-c 5deg-c Cold PL EMP Trap CT Dump Flow Rate 11 t/h 135 t/h Tank Dump Heat Exchanger (DHX) Capacity 25 35 MWt Tube path 2 4 Heat Transfer 125 24 m 2 Area Air Flow Rate Blower Motor Capacity 74 77 m 3 /min 4 71 kw Secondary Cooling System MK-Ⅱ MK-Ⅲ DHX Inlet DHX Outlet 47deg-c 47deg-c 34deg-c 3deg-c Flow Rate 11 t/h 12 t/h Secondary Main Pump Motor and Flow Control System Motor Capacity 18 22 kw Renovated Components in Cooling System Figure 3 Modification of cooling system 1.9 3 1.7 25 Current Coil 1.5 2 1.3 15 Voltage 1.1 1 23 Electromagnet.9 5 18 Weight.7 13 16 Connecting Reactor Thermal Power 8 12 Surface Armature of 3 8 Control Rod -2 4-7 24/5/2 Sensitive Alloy Current (A) Voltage (V) Weight (N) Curie point electromagnet (CPEM) Holding stability test Figure 4 Demonstration of self actuated shut down system Reactor Thermal Power (MW) 7/29 8/23 1/27 Month/Date

A Handling Head Upper Cap Monitor A Wrapper Tube Lower Cap Wrapper Tube Compartment Entrance Nozzle Test Fuel Subassembly Compartment Test Fuel Pin A-A section Spacer Wire Slit(.1mm 1mm) Plenum Spring Fuel Pellets Cladding Tube Test Fuel Pin (No slit type) Figure 5 Counting rate of FFD-CG method (cps) Counting rate of FFD-CG method(cps) 4 Counting Counting rate rate of of FFD-CG method(cps) 3 2 1 Reactor thermal power ( ) Reactor power 1 2 Elapse of the time from the nuclear reactor started Fuel failure simulation test Twice of BG counting rate Twice of BG counting rate(cps) BG counting rate(cps) Time (day) Time history of reactor power and FFD-Cover Gas signal 16 14 12 1 8 6 4 2 Reactor power (MWt) Reactor thermal power (MWt) Fuel pin containing MA specification No. of Fuel Pins Figure 6 Fuel pin containing MA specification Reactor thermal power (MWt) 12 1 8 6 4 2 26/5/25 1:36 Figure 7 12MW/h (~12MW) NIS Calibration 12MW (about 43W/cm) 1 minutes Manual shut down 26/5/26 1:2 Power history of MA-MOX fuel irradiation test (B11(1)) Sodium Outlet Line Figure 8 Gas Gap Electric Heater Specimen Thermocouple Sodium Inlet Line Outline of MARICO-2 Cable Gas Line Specimen Capsule Thermocouple