JOYO

The CREDO data base contains data from The Fast Flux Test Facility in Richland, Washington, The Experimental Breeder Reactor - II in Idaho Falls, Idaho, The test loops of the Energy Technology Engineering Center (ETEC) in Canoga Park, California, The JOYO Liquid Metal Fast Breeder Reactor at the 0-Arai Engineering Center (OEC) in Japan, and the test loops of OEC. 

Valdes - Veo algo ent e... entre cruz y espodo que es my dorado, muy... tiene muchas joyos (Vedo anche qualcosa tra. .. tra una croce e una spada che e tutta coperta d oro, completamente...ci son tanti gioielli.)... 

Mixed oxide (MOX) fuel, 21.0-wt% Pu in 18.0-wt% enriched uranium, was irradiated from the 16 to 35 cycle at the 3" row in the experimental fast reactor JOYO. Tire effective full power days were 1019.33, and the peak bumup was 143.8 GWd/t. Atotal of 1560 days have passed from the reactor shut down to analysis. 

In Japan, fast reactor development program symbolises national nuclear fuel recycling program, as it is stated in the national long-term plan [2.3]. The experimental fast reactor Joyo has shown excellent performance for more than 20 years. The prototype reactor Monju (280 MW(e)) was stopped temporarily due to the leak in the non-radioactive secondary circuit in 1995. The design of demonstration fast reactor (DFBR-600 MW(e)) is in progress [2.4, 2.5]. 

Exchange detailed technical information on fast reactor operation and/or decommissioning experience with DFR, PFR (UK) KNK-II (Germany) Rapsodie, Phenix, Superphenix (France) BR-10, BOR-60, BN-600 (Russian Federation) BN-350 (Kazakhstan) SEFOR, EBR-II, Fermi, FFTF (USA) FTBR (India) JOYO, MONJU (Japan) ... 

OPERATIONAL EXPERIENCE AND UPGRADING PROGRAM OF THE EXPERIMENTAL FAST REACTOR JOYO... 

Twenty years of sueeessful operations at the experimental fast reactor JOYO provide a wealth of experience eovering eore management, chemical analysis of sodium and cover gas for impurity control, natural convection tests, upgrade of fuel failure detection system, corrosion product measurement, development of operation and maintenance support system, and replacement of major components in the cooUng systems. Some of the data obtained is stored in a database to preserve the related knowledge. This experience and accumulated data will be useful for the design of future fast reactors. 

The experimental fast reactor JOYO at the Japan Nuclear Cycle Development Institute s Oarai Engineering Center attained initial criticality in April 1977 and was the first liquid metal cooled fast reactor in Japan. From 1983 to 2000, JOYO operated with the MK-II core as an irradiation test bed to develop the fuels and materials for future Japanese fast reactors. Thirty-five duty cycle operations and thirteen special tests with the MK-II core were completed by June 2000 without any fuel pin failures or serious plant trouble. The reactor is currently being upgraded to the MK-III core. This paper provides a review of the operational experiences obtained through the JOYO s operation. 

SPECIFICATIONS, PLANT DESCRIPTION AND OPERATION HISTORY OF JOYO... 

JOYO is a sodium cooled fast reactor with mixed oxide (MOX) fuel. The main reactor parameters of the MK-II irradiation bed core are shown in Table 1, which compares the MK-II with the future MK-III core. 

Six-control rod subassemblies made of 90% enriched B4C were used in JOYO MK-II and were located symmetrically in the third row. In 1994, one control rod was moved to the fifth row to provide a position for irradiation test assemblies with on-line instrumentation. Since then, the control rod subassemblies have been loaded asymmetrically. The JOYO cooling system has two primary sodium loops, two secondary loops and an auxiliary cooling system. The cooling system uses approximately 200 tons of sodium. In the MK-II core, sodium enters the core at 370°C at a flow rate of 1 100 tons/h/loop and exits the reactor vessel at 500°C. The maximum outlet temperature of a fuel subassembly is about 570 C. An intermediate heat exchanger (IHX) separates radioactive sodium in the primary system from non-radioactive... 

The operating data and history of the JOYO MK-II core are shown in Table 2 and Fig. 3. The reactor operated for 48 000 hours and the integrated power generated was 4 400GWh. During the MK-II operation, 382 driver fuel subassemblies and approximately 47 000 fuel pins were irradiated. A peak burnup value of 86.0 GWd/t was attained for the MK-II driver fuel without any fuel pin failures. 

A core management code system has been developed to predict the core parameters for operation and refueling plans within the design limitations. The nuclear calculation is based on diffusion theory and corrected with a bias method. Results from core physics tests and Post Irradiation Examinations (PIE) have been used to confirm the accuracy of these predictions. These verifications are also important to conduct various irradiation tests accurately. This section describes the method and verification for core and fuel management used with the JOYO MK-II core. 

It is considered that the decrease of atomic number densities of major fissile nuclides as and Pu are the dominant factor of bumup reactivity because of JOYO s small core size, which results in hard neutron spectmm and small internal conversion ratio (-0.3). Therefore, the burnup reactivity can be predicted accurately even at a high burnup. 

It was considered that the fuel thermal expansion, which is the major component of the power coefficient of JOYO, decreases at high bumup due to fuel restmcturing during irradiation. It was also observed that the power coefficients varied depending on the reactor power as shown in Fig. 6. This phenomenon appeared to be due to a combination of the core bowing effect, fuel thermal expansion and Doppler effects. These causes need further investigation. 

FIG. 7. Axial hurnup distribution of JOYO MK-II driver fuel. 

The accuracy of decay heat calculations depends on the individual heat generation rate from fission product decay nuclides and actinides, and the burnup calculation for its production and transmutation. To obtain experimental data and to improve the accuracy of related calculations, the decay heat of MK-II spent fuel subassemblies was measured at the JOYO spent fuel storage pond [7], The fuel burnup was approximately 66 GWd/t and the cooling time was between 40 and 385 days. The measured decay heat is shown in Fig. 9. 

FIG. 9. Measured and calculated decay heat of JOYO MK-U spent fuel. 

FIG 10. Adjusted neutron spectrum at JOYO MK-II core center. 

FIG. 11. Hydrogen and oxygen content in JOYO primary coolant sodium. 

The JOYO FFD system consists of both delayed neutron (DN) monitoring systems and a cover gas (CG) precipitating system. The schematic diagram of the JOYO FFD system is illustrated in Fig. 13. 

A Run-to-Cladding-Breach (RTCB) test is planned in JOYO. The RTCB test is expected to improve the FBR fuel performance. The results will increase the fuel burnup and extend the cladding life-time. As part of the preparation work, the FFD system has been upgraded to improve its accuracy and reliability and FP traps have been installed. A series of simulated fuel failure tests has been conducted [10]. 

The On-line Gamma-ray Monitor (OLGM), shown in Fig. 13, has been developed and installed in JOYO. 

Two types of FP traps have been installed in JOYO. One is a cesium trap installed in the primary coolant sodium purification system to capture cesium released from failed fuels. An open pore, foam-like glassy carbon that consists of thin struts of Reticulated Vitreous Carbon (RVC) is used as a material for collecting cesium. The capacity of this trap is designed to be 7.4E+12 Bq. The other trap is a Cover Gas Clean-up System (CGCS) to collect and store the noble fission gas released from failed fuels. Although it is planned that only one failed fuel pin will be in the core at any time, the CGCS is designed to handle the releases of up to twelve failed fuel pins. 

The sodium cooled fast reactor JOYO has been operated more than 20 years (about 5 years of effective full power years) since its initial criticality and the cumulative reactor output achieved over 1.9E+5 MWd. Since JOYO has not yet experienced any operation with breached fuels, FP radioactive contamination has not become an issue in the plant system. To reduce the radiation dose from long-lived Na, all primary coolant sodium in the main circulating loops is drained into a storage tank during annual plant inspections. Under these conditions, the spatial gamma-ray dose rate distribution is dominated by the radioactive CPs deposited on inner surfaces of the primary piping and components. This means that most personnel dose was due to these CPs. 

A Plastic Scintillation Fiber (PSF) measured the dose rate distribution in the primary cooling system of JOYO [14], Figure 20 shows the schematic diagram of the PSF system. 

The JOYO operation and maintenance support systems ensure more stable operations and improve operational reliability. Artificial intelligence techniques [15] have been applied to develop these systems. One system objective is to support intelligent decision making by the operators and maintenance engineers, and another is to conduct skill-based and rule-based operator actions automatically. With proper instructions and guidance from the support system, the JOYO operators can make better decisions and carry out necessary actions with more confidence and less mental pressure. 

The JOYO operators control the reactor power, i.e. neutron flux level, by adjusting the position of the control rod subassemblies in the core. This is a manual operation performed from the central control room. To improve operational reliability as well as to reduce the mental load on the operators, an automatic control rod operation system [16] has been developed. This system has the following capabilities ... 

In the past, the control rod drive stroke was calculated by the operators prior to manual control when approaching criticality. After the installation of the system, it calculates and displays an inverse multiplication curve and the stroke of each control rod subassembly that needs to be driven using the neutron flux and the vertical control rod position. The reliability of this system was validated during a complete operational cycle of the MK-II core. It was also demonstrated that the operation guides provided by the system were very similar to those chosen by the experienced JOYO operators. 

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