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Fig. 6-5 Power distribution monitor (PDM) for future pressurized water reactors (PWRs) with a position-sensitive fission counter as compared with the newest PDM In the newest model PWRs, reactor power distribution in the direction of the core axis is measured by using four or six neutron sensors. The PDM System studied by JAERI monitors the continuous power distribution using one position-sensitive fission counter (PSFC).

Fig. 6-6 Principle of position-sensitive fission counter A fission counter that is designed as a pulse transmission delay line with a solenoid anode structure delays output pulses from the anode. The cylindrical cathode transmits a signal pulse without delay. The position where a neutron is detected in the fission counter is determined by measuring the pulse interval between the output pulses from the cathode and the anode.

It is important to measure the nuclear power distribution in a reactor core for reactor safety and for fuel burn-up management. In the newest PWR power plants, the power distribution in the reactor core is measured by four or six neutron sensors that are installed on the outside of reactor vessel in the direction of the core axis. JAERI is going to start R&D on the power distribution monitoring system that provides continuous, real-time power distribution monitoring using one position-sensitive fission counter (PSFC). The theoretical feasibility study of the PSFC found that the PSFC has a position resolution of 10 mm. For a system with PSFC and electronics for processing fast-current pulses, the pulse counting error is less than 2% with a count-rate as high as 106 counts per second. The PSFC is also useful for measurement of neutron flux distribution in research reactors, critical assemblies, and graphite piles having neutron sources.

Reference H. Yamagishi, Theoretical Study on Position-Sensitive Fission Counters Designed with a Transmission Delay Line Structure, JAERI-M 94-010 (1994).

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