Validation of MCBEND dosimetry predictions against measurements from Hunterston B advanced gas-cooled reactor

The core restraints of advanced gas-cooled reactors are important structural components necessary for maintaining the geometric integrity of the cores. Neutron damage and nuclear heating rates, calculated using the Monte Carlo code MCBEND, have underpinned the safety case for continued operation of four reactors. To validate these calculations, neutron activation measurements were commissioned. A neutron flux activation “stringer” was deployed in the graphite side reflector of one of the Hunterston B reactors and irradiated for a period of approximately three years. A capsule attached to the bottom end of this cable contained a range of fast and thermal neutron activation monitor wires to provide additional spectral information. Following its successful withdrawal, measurements were undertaken at SCK-CEN’s laboratory in Mol, Belgium, to provide monitor wire activities. In parallel with this, activation calculations were undertaken by Amec Foster Wheeler using a MCBEND model tailored to the state of the reactor during the irradiation period. Time-varying neutron source data were used, decay-adjusted for the half-lives of the activation monitors, in order to accommodate the effects upon the expected activities of time-varying reactor power. Adjustments were made for neutron flux attenuation within the stringer capsule and cable and, where necessary, corrections were also made for parent and activation nuclide burnout. There was no requirement for spectral adjustment. Excellent agreement between calculated and measured activities was obtained for both fast and thermal neutron responses; the overall calculated/measured ratios were 1.14 ± 0.15 and 1.11 ± 0.12, respectively. These are sufficiently close to the desired value of unity to provide confidence in the ability of the calculation route to predict neutron damage rates within the core restraint components. This successful validation supports the case for life extension of the Hunterston B and Hinkley Point B power plants.