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Outline of FFDL system in the Japanege fast breeder reaetor and the requirernents to it9 funetion i9 deseribed. Research and development on FFDL system, whieh have been implemented sinee 1969, are' introducted and reviewed from the viewpoint of availability of JOYO and MONJU. These inelude basic experiments of fission product behavior, eomponent development and testing, feasibility stud~ of detection system response, and design consideration of several kinds of loeation methods. The diseussion extends to the role of FFDL in other surveillanee and monitoring instruments for core anomaly detection in LMFBR plant. One of major eoneerns on the development of FFDL is now eoneentrated espeeiallyon gas tagging and in-and ex-vessel sipping. INTRODUCTION Failed fuel detectlon (FFD) system In the Japanese experImental tast reactor, JOYO now operating at 75~"t, i9 eomprised of a preeipitator and a delayed neutron Ebnitor. The FFO syst~~, fortunately, has indieated no fuel failure in the reaetor ever sinee the first eritleality of JOYO in 1977, and no fission produet radioaetivity is observed in the primary eooling system by radiologieal analysis of sodium and cover gas samples, whieh are periodically extracted for examination of FP radioactivity contamination In the system. The FFO system has been well operated(l) without signifieant trouble for four years. Hcwever, It is necessary to make the system modifieation for its sensitivity, and to be prepared an interpret model to the signal indieation in relation with mode and size of fuel failure possibly encountered in LHFBR. D"velopment works to in-vessel loeation system are also under way. FFDL system in JOVO and MONJU, and their function are described in Table 1 and Fig.l and 2. Requirements of minimum deteetable sensitivity of delayed neutron monitor whieh are shown in Table 2, eome from a safety eonsideratlons for preventing pin failure propagation ~ithin one subassembly. L"",," • ..., I Dha I

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Fuel failure detection and location in LMFBRs. Proceedings of an International Atomic Energy Agency specialists' meeting, international working group on fast reactors, Karlsruhe, Federal Republic of Germany, May 11-14, 1981