Monte Carlo fixed source calculations to support Bruce Power start-up instrumentation preliminary design

For the initial fuel loading at a CANDU reactor, start-up instrumentation (SUI) is temporarily installed within the Viewport (VP) to provide the operator with a continuous indication of the neutron flux level and rate of change of neutron flux. Recently, Bruce Power has proposed to use retractable Fission Chambers in available spare in-core Vertical Flux Detector (VFD) assemblies. The VFD SUI is expected to provide comprehensive start-up instrumentation for power monitoring during the initial fuel loading and initial approach to criticality, as well as for starting-up after a normal outage that exceeds ∼20 days. Due to the dimensional constraint with the inner diameter of the VFD unit, preliminary engineering effort was required to identify a suitable detector that provides sufficient sensitivity to detect a change in neutron population postrefurbishment during fuelling. Physics analyses using the MCNP fixed source methodology were performed to determine the detector neutron sensitivity required to observe a change in the neutron flux in the first stage of the fresh fuel loading. The approach considered various possible photo-neutron backgrounds and a variety of fuel channel configurations to help define the first stage of the fuelling. (author)