Takafumi Aoyama

DEVELOPMENT OF PASSIVE SHUTDOWN SYSTEM FOR SFR

A sodium-cooled fast reactor (SFR) self-actuated shutdown system (SASS) is a passive safety feature by which control rods are inserted by gravity force and the rods would be detached by a rise in coolant temperature under anticipated transient without scram (ATWS) conditions. Various out-of-pile tests have already been carried out to investigate the basic characteristics of SASS, and a demonstration test of the holding stability under reactor operation conditions has been performed, where a function test of the driving system to reconnect and pull out the control rod has been done in the experimental reactor JOYO. Element irradiation tests have also been conducted to confirm that there is no impact from irradiation. The effectiveness of SASS for the reference core design of the Japan SFR (JSFR) has been evaluated through all ATWS types. As a result, it is ensured that JSFR will have a reliable passive shutdown system.

Demonstration of control rod holding stability of the self actuated shutdown system in joyo for enhancement of fast reactor inherent safety

Self actuated shutdown system (SASS) with a Curie point electromagnet (CPEM) has been developed for use in a large-scale liquid metal cooled fast breeder reactor (LMFBR) in order to establish the 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 material elements. 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 the 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 operational cycles of the experimental fast reactor Joyo MK-III. The rod-holding stability and the rod-recovering functions of the driving system to re-connect and pull out the separated control rod were fully confirmed. The results also indicate there is no essential problem for the practical use of SASS about its operational trouble involving the unexpected drop during reactor operation.

Distinguished Achievements of a Quarter-Century Operation and a Promising Project Named MK-III in JOYO

The experimental fast reactor JOYO at the O-arai Engineering Center of the Japan Nuclear Cycle Development Institute is the first liquid sodium fast reactor in Japan. The purpose of constructing JOYO was to obtain technical information about liquid-metal fast breeder reactors (LMFBRs) through experience with their 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 under the conditions of higher fast neutron flux and temperature than those in light water reactors. JOYO has been operated successfully for a quarter-century without any serious problem, and this operation demonstrated the safety and reliability of the sodium-cooled fast reactor. The reactor has just been upgraded to the MK-III core to increase irradiation capability for playing a greater role in providing an irradiation field as a fast reactor. Given the worldwide trend of fast reactor shutdowns, JOYO is an increasingly valuable world resource for current and future reactor development.

CHARACTERIZATION OF NEUTRON FIELDS IN THE EXPERIMENTAL FAST REACTOR JOYO MK-III CORE

In 2003, Joyo MK-III core was upgraded to increase the irradiation testing capability. This paper describes the details of distributions of neutron flux and reaction rate in the MK-III core that was measured by characterization tests during the first two operating cycles. The calculation accuracy of the core management codes HESTIA, TORT and MCNP, was also evaluated by the measured data. The calculated fission rates of U by HESTIA agreed well with the measured one within approximately 4% in the fuel region. MCNP could simulate within 6% in the central non-fuel irradiation test subassembly and the radial reflector region, while large discrepancies were obtained in TORT results. Hence, the precise geometry model was effective in evaluating the neutron spectrum and the flux at such locations.

Lumped Group Constants of FP Nuclides for Fast Reactor Shielding Calculation Based on JENDL-3.2

Fission Products were not considered in conventional fast shielding analyses that were predominantly developed in clean core experiments. However, in power reactors with high burn-up, the accumulation of FP nuclides affects the neutron balance mainly due to the absorption reaction and it cannot be neglected when calculating the neutron flux of a high burn-up reactor. In this study, the lumped group constants of FP nuclides used for a fast reactor shielding calculation were computed with the JENDL-3.2 library and compiled to the JSDJ2/JFTJ2 set. The effect of considering FP nuclides on the neutron flux calculations was evaluated in the JOYO experimental fast reactor. These tests showed conventional calculations that ignored FP nuclides overestimated neutron flux by about 2%. The effect on reaction rate calculation of the spent fuel in IVS (in-vessel storage rack) is a maximum of 10%.

Decay Heat of Fast Reactor Spent Fuel

Decay heat from JOYO Mk-II spent fuel subassemblies was measured to obtain experimental data and to improve the accuracy of related calculations. The measurement was taken in the JOYO spent fuel storage pond. The fuel burn-up was approximately 66 GWd/t and the cooling time was between 40 and 385 days. The decay heat was calculated with the ORIGEN2 code using the JENDL-3.2 cross section library and the JNDC-V2 decay data and fission yield data library. The fuel power used as an input to ORIGEN2 was calculated by the MAGI core management code system. The ratios between calculated and experimental values were between 0.94 and 0.89 and decreased with a longer cooling time. This systematic discrepancy is not fully understood, but the change with cooling time appears to be due to the actinide decay heat uncertainty. This indicated that cross sections of actinides are important to evaluate decay heat accurately.

Core Modification to Improve Irradiation Efficiency of the Experimental Fast Reactor Joyo

Core modification has been investigated to further increase the core burnup and to improve the irradiation efficiency of the experimental fast reactor Joyo. This modification enables the core to accommodate more irradiation test subassemblies that have lower fissile material contents compared with the driver fuel. The design calculations showed that the replacement of the radial reflector elements made of stainless steel with those made of zirconium alloy or nickel-based alloy is effective in improving neutron efficiency. The irradiation test capacity can be increased by reducing the number of control rods based on a reevaluation of the design margin in the control rod worth calculation. The design calculation results show that these modifications, without any change in fuel specification, will be useful for conserving driver fuels and enhancing the irradiation capability of Joyo.

Upgrade of the Resonance Ionization Mass Spectrometer for Precise Identification of Failed Fuel in a Fast Reactor

Isotopic analysis of krypton (Kr) and xenon (Xe) by resonance ionization mass spectrometry (RIMS) is an effective tool for identification of failed fuel in fast reactors to achieve their safety operation and high plant availability. Reliability of the failed fuel detection and location (FFDL) system depends on the precise determination of {sup 78}Kr/{sup 80}Kr, {sup 82}Kr/{sup 80}Kr and {sup 126}Xe/{sup 129}Xe isotopic ratios, which is mainly hampered by statistical errors for detection of the corresponding isotopes except {sup 82}Kr generated in large amounts during operation of fast reactors. In this paper, we report on improvements of the laser optical system of our spectrometer to increase the resonance ionization efficiency of Kr and Xe atoms, focusing on (i) utilization of the uniform YAG laser beam to improve the wavelength conversion efficiency of sum frequency generation and (ii) reflection of the ultraviolet light by a concave mirror to increase the photon density. The results indicate that our upgraded resonance ionization mass spectrometer has enough performance for isotopic analysis of Kr and Xe required in the Monju FFDL system.

Development of Sodium Leak Detection Technology Using Laser Resonance Ionization Mass Spectrometry

In a sodium-cooled fast reactor, highly sensitive technology is required to detect a sodium leak from the cooling system piping or components. Conventional sodium leak detectors have difficulty in measuring small amounts of sodium leak because of the presence of salinity in the atmosphere. In order to overcome this problem, an innovative technology has been developed to selectively detect the radioactive sodium of the primary cooling system using Laser Resonance Ionization Mass Spectrometry (RIMS). This research and development program consists of investigating the detection process of sodium aerosol by RIMS, manufacturing the prototype sodium detection system, and testing its function using an actual radioactive sodium sample, which will be taken from the experimental fast reactor Joyo. The schedule and the results obtained from the investigation of the sodium leakage detection process are shown in this paper.

Student Training Course Using the Experimental Fast Reactor Joyo and Related Facilities

The student training courses using the experimental fast reactor Joyo and related facilities of the Japan Atomic Energy Agency (JAEA) have been initiated to utilize the nuclear facilities and their engineering staffs for the education purpose. The development of the student training course was also strongly supported by the faculty of nuclear engineering of domestic universities whose curriculum has recently been reduced. The program covers the reactor physics test analysis of Joyo core or experiments using the Joyo full-scope training simulator, neutron dosimetry, trace amount of noble gas measurement and chemical analysis of sodium, and the program has started after check and review by the specialists in university education. It is expected to promote the human resource development for the younger generation in nuclear industry, and to strengthen the relation between JAEA and universities in research area.Copyright