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Successful Completion of the International Cooperative MEGAPIE Experiment, the First Irradiation of a Lead-Bismuth Liquid Metal Spallation Target of the MegaWatt Class - A Great Advance towards Realization of Transmutation of Long-lived Nuclear Wastes -Feb. 7, 2007The first irradiation of a lead-bismuth liquid metal spallation target of the megawatt class has been successfully completed at Paul Scherrer Institute(PSI) in Switzerland through the international cooperative MEGAPIE(MEGAwatt PIlot Experiment) program which Japan Atomic Energy Agency (JAEA) participates in as one of members. The long-lived minor actinides (neptunium, americium and curium) are the main contributors to the radio-toxicity of nuclear wastes. Their transmutation in short lived or stable elements is theoretically achievable in a Sub-critical Accelerator Driven System (ADS), equipped with an internal neutron source driven by a high power proton beam. The development of a powerful spallation target, made of a heavy liquid metal (lead-bismuth eutectic) able to produce the targeted intense neutron flux level, was identified as one of the key experimentation for the progress of ADS. Therefore, in the year 2000, nine research institutions in Europe (CEA, CNRS, ENEA, FZK, PSI, SCK•CEN), Japan (JAEA), Korea (KAERI) and USA (USDOE) set up a collaboration in order to design and built a 1 MW beam power target and license and operate it in the spallation neutron facility SINQ of the PSI in Switzerland. This facility is served by an accelerator cyclotron delivering a continuous proton beam current of up to 1.4 mA at 575 MeV energy. The experiment named MEGAPIE had the objectives of demonstrating the feasibility, the licensibility and the longer term operation under conditions representative of the operation of a high power spallation target, which shall later provide the high energy neutrons to drive a sub-critical core where the minor actinides can be fissionned. After an intense study of the basics of such an experiment in the year 2000 and 2001, the target and the ancillary systems were designed and manufactured in France, Italy, Latvia and Switzerland. The target material consists of 920 kg of liquid lead-bismuth eutectic (LBE), which is contained in a steel casing. The impinging proton beam of 800kW deposits about 580 kW of heat in the target, which is removed by circulating the LBE in forced convection through a heat exchanger. The proton beam penetrating the LBE generates 1017 high energy spallation neutrons per second. Parallel to the development, the licensing process was engaged with the competent Swiss Safety authorities lead by the Swiss Federal Office of Public Health. The license was obtained in 2004 and the go-ahead for the irradiation in August 2006. Prior to irradiation, all the systems and particularly those related to the control and safety systems were intensively tested out-off beam. Regular operation of the target at nominal power levels started from August 21st, 2006 and continued according to predictions until December 21st, 2006 when the experiment was stopped according to plan. The number of unscheduled beam shutdowns triggered by the target was small. Its availability reached 95 %, which is a very good value for an experimental target. The accumulated proton charge amounts to 2.8 Ah. The heat produced by spallation was extracted very satisfactorily, and the neutron yield confirmed the calculations. After stop of the beam, the LBE in the target has been solidified. After extraction from the facility, the target will be stored for about one and a half years. It will then be opened and the status of the internal components and structural materials will be analyzed. JAEA has been conducting ADS research for the purpose of transmuting long-lived nuclear wastes such as minor actinides. The present success of the MEGAPIE experiment leads to a great progress in development of the spallation target peculiar to the ADS. Furthermore, it is a useful result for the transmutation experimental facility which is planned as the phase II facility of J-PARC. The successful behavior of the target during irradiation and the results still to come about irradiated materials, will provide the ADS community with a unique relevant design and operational feedback, paving the way for the development of high power spallation targets for future powerful ADS, which would have the industrial capability of transmuting large amounts of nuclear waste. |
