The core restraints of advanced gas-cooled reactors are important structural components that are required to maintain the geometric integrity of the cores. A review of neutron dosimetry for the sister stations Hunterston B and Hinkley Point B identified that earlier conservative assessments predicted high thermal neutron dose rates to key components of the restraint structure (the restraint rod welds), with the implication that some of them may be predicted to fail during a seismic event. A revised assessment was therefore undertaken [Thornton, D. A., Allen, D. A., Tyrrell, R. J., Meese, T. C., Huggon, A.P., Whiley, G. S., and Mossop, J. R., “A Dosimetry Assessment for the Core Restraint of an Advanced Gas Cooled Reactor,” Proceedings of the 13th International Symposium on Reactor Dosimetry (ISRD-13, May 2008), World Scientific, River Edge, NJ, 2009, W. Voorbraak, L. Debarberis, and P. D’hondt, Eds., pp. 679–687] using a detailed 3D model and a Monte Carlo radiation transport program, mcbend. This reassessment resulted in more realistic fast and thermal neutron dose recommendations, the latter in particular being much lower than had been thought previously. It is now desirable to improve confidence in these predictions by providing direct validation of the mcbend model through the use of neutron flux measurements. This paper describes the programme of work being undertaken to deploy two neutron flux measurement “stringers” within the side-core region of one of the Hunterston B reactors for the purpose of validating the mcbend model. The design of the stringers and the determination of the preferred deployment locations have been informed by the use of detailed mcbend flux calculations. These computational studies represent a rare opportunity to design a flux measurement beforehand, with the clear intention of minimising the anticipated uncertainties and obtaining measurements that are known to be representative of the neutron fields to which the vulnerable steel restraint components are exposed.

Specimens for the Sizewell B reactor pressure vessel (RPV) in-service steels surveillance program are irradiated inside eight capsules located within the reactor pressure vessel and loaded prior to commissioning. The periodic removal of these capsules and testing of their contents provides material properties data at intervals during the lifetime of the plant. Neutron activation measurements and radiation transport calculations play an essential role in assessing the neutron exposure of the specimens and RPV. Following the most recent withdrawal, seven capsules have now been removed covering nine cycles of reactor operation. This paper summarizes the dosimetry results of the Sizewell B surveillance program obtained to date. In addition to an overview of the calculational methodology it includes a review of the measurements. Finally, it describes an extension of the methodology to provide dosimetry recommendations for the core barrel and briefly discusses the results that were obtained.

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.

The core restraints of advanced gas-cooled reactors are important structural components that are required to maintain the geometric integrity of the cores. A review of neutron dosimetry for the sister stations Hunterston B and Hinkley Point B identified that earlier conservative assessments predicted high thermal neutron dose rates to key components of the restraint structure (the restraint rod welds), with the implication that some of them may be predicted to fail during a seismic event. A revised assessment was therefore undertaken [Thornton, D. A., Allen, D. A., Tyrrell, R. J., Meese, T. C., Huggon, A.P., Whiley, G. S., and Mossop, J. R., “A Dosimetry Assessment for the Core Restraint of an Advanced Gas Cooled Reactor,” Proceedings of the 13th International Symposium on Reactor Dosimetry (ISRD-13, May 2008), World Scientific, River Edge, NJ, 2009, W. Voorbraak, L. Debar-beris, and P. D'hondt, Eds., pp. 679–687] using a detailed 3D model and a Monte Carlo radiation transport program, MCBEND. This reassessment resulted in more realistic fast and thermal neutron dose recommendations, the latter in particular being much lower than had been thought previously. It is now desirable to improve confidence in these predictions by providing direct validation of the MCBEND model through the use of neutron flux measurements. This paper describes the programme of work being undertaken to deploy two neutron flux measurement “stringers” within the side-core region of one of the Hunterston B reactors for the purpose of validating the MCBEND model. The design of the stringers and the determination of the preferred deployment locations have been informed by the use of detailed MCBEND flux calculations. These computational studies represent a rare opportunity to design a flux measurement beforehand, with the clear intention of minimising the anticipated uncertainties and obtaining measurements that are known to be representative of the neutron fields to which the vulnerable steel restraint components are exposed.

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