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标题：NEUTRON DIAGNOSTICS FOR VOID MONITORING IN BOILING WATER REACTORS
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作者：J. Blomgren
期刊名称：PoS - Proceedings of Science
印刷版ISSN：1824-8039
出版年度：2006
出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：In present-day Boiling Water Reactors (BWR), the only in-core neutron diagnostics in fullpower operation (Power-Range Mode, PRM) is performed with fission-based ionisation chambers loaded with 235U. Since both the neutron flux and the neutron-induced fission cross section on 235U are the highest at thermal energies, the count rate from such PRM detectors is essentially due to the thermal neutrons. It has recently been proposed that adding PRM detectors loaded with 238U, thereby being sensitive only to fast neutrons (above about 1.5 MeV), to the BWR core could facilitate void monitoring. The neutron energy spectrum in a BWR is very sensitive to the boiling. Since the moderation of neutrons requires the presence of hydrogen, which in reality means water, the moderation gets significantly less efficient in regions of the reactor where the void fraction (the fraction of the water that is in steam phase) is high. Because the density of steam is significantly lower than when water is in liquid phase, increased void results in reduced moderation. Less moderation results in a neutron energy spectrum with a lower fraction of thermal neutrons and a correspondingly larger number of fast neutrons. Thereby, a simultaneous measurement of the fluxes of thermal and fast neutrons may be used to determine the void in a BWR. The potential of this technique will be outlined, as well as applications in realistic technical systems.