Development of BWR ex-core nuclear instrumentation system. Experimental verification of ex-core local power monitoring method by using neutron streaming pipes

We study an ex-core nuclear instrumentation system for a next generation Boiling Water Reactor (BWR). For an example of existing BWRs, in an Advanced BWR (ABWR), about 210 in-core sensors are installed. About sixty penetrations of Reactor Pressure Vessel (RPV) are built for these signal cables. The core monitoring with ex-core sensors instead of in-core sensors reduces the RPV penetrations and extends the sensor life. To achieve this, we created a local power monitoring method with Neutron Streaming Pipes (NSPs), where local power is directly determined from ex-core neutrons through NSPs inserted into the core. In the previous work, applicability of this method for the ABWR was analytically shown. The most critical parameter in this evaluation was the ex-core neutron flux through the NSP. Hence, we experimentally verified validity of the previous analytical values of the flux in a research reactor, Toshiba Nuclear Critical Assembly. The verification was performed in two respects. Firstly, fundamental characteristics of the ex-core neutron flux through a NSP that do not depend on the reactor types were confirmed by measuring ex-core neutron count rates. (1) The ex-core neutron flux φ does not depend on the NSP wall thickness, when the thickness is on the order of millimeters (the measured values: independent within experimental error of 3.0% at 1 σ). (2) The ex-core neutron streaming rate (= φ x the inner cross-sectional area of the NSP) is proportional to the fourth power of the NSP inner diameter (the measured values: the 3.7 and 3.8 power). Secondly, reliability of the neutron streaming calculation was verified by comparing two independent calculations of a number of the incident ex-core neutrons at the detector. One was a neutron streaming calculation with in-core gold foil activation method. The other was a detector response calculation with measured ex-core neutron count rates. They were in good agreement within experimental and analytical error of 7.8% at 1 σ. These results showed that the analytical values of the flux in the ABWR are valid. (author)