Development of an Extended Range Start-Up Monitor (ERSM) for Boiling Water Reactors

Since TMI, new requirements for reactor instrumentation for PWR and BWR reactors have been stipulated in Regulatory Guide 1.97 (Rev. 2). One of the requirements calls for the measurement of neutron flux as a key variable in monitoring the status of reactivity control. This variable has been classified as Type B, Category 1, with a neutron flux range of 10-6% to 100% full power (8 decades). In the case of PWR´s, a recent trend has been to install new wide range neutron flux channels using high sensitivity fission counters. BWR´s, however, have very low leakage flux; therefore the measurement has to be made with either existing or new in-core neutron flux monitors. The existing in-core instrumentation in a BWR currently performs important reactor safety functions. To avoid interferring with these functions, a new fixed Wide Range Flux Monitor (WRFM) has been designed to cover the range of 10-6% to 100% full power with a single assembly. Each WRFM is designed with four fission detectors axially spaced along the fuel in an assembly which is dimensionally identical to an LPRM assembly. These assemblies will be installed in existing spare LPRM locations near the periphery of the core. Installation and removal techniques are similar to those for LPRM´s. The four fission detectors within each assembly are of two types: 1. Two detectors separated axially above the core midplane are fission counters with regenerative (234U/235U) coatings for long life.