Ex-Core Local Power Monitoring Using Neutron Streaming Pipes

To simplify in-core instrumentation in a next-generation boiling water reactor (BWR), we study an ex-core nuclear instrumentation system. As a first step of this study, we focused on ex-core local power monitoring, which is especially difficult because neutrons inside a core cannot fly out of a reactor pressure vessel (RPV) due to shielding of fuel, water, etc., except when they are generated in the outer edges of the core. To resolve this, we created a local power monitoring method with neutron streaming pipes (NSPs). An NSP is a gas-filled pipe of size comparable to an instrumentation tube of an existing BWR. NSPs are axially inserted into the core. In-core neutrons are transported to the RPV through NSPs. The neutrons transmitted through the RPV are monitored with ex-core neutron sensors. We analytically evaluated the applicability of this NSP method for an advanced BWR (ABWR) with a three-dimensional BWR core simulator and the MCNP5 code. The ex-core neutron flux through the NSP was highly proportional to local power (1.0% of the residual standard deviation). The flux amount and the linearity gave feasible specifications for the ex-core neutron sensor in typical operation modes (pulse, Campbell, and current modes). Therefore, the NSP method is applicable to an ABWR.