Neutron Flux Measurements in the Side-Core Region of Hunterston B Advanced Gas-Cooled Reactor

Abstract

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.

A History of Dosimetry for the Advanced Gas-cooled Reactors

“…• Measurements from capsules of flux monitoring materials suspended in key locations (these directly informed ongoing measurements in the Hinkley Point B and Hunterston B AGRs [2]). …”

“…This allowed the NDCF to be applied to dpa assessments with effective graphite penetration within the data validation limit of 71.6 cm; where results showed a greater penetration of graphite, bounding results from nearby locations within the validation limit were given. Note that the first analysis of measurements currently being conducted in reactors at Hinkley Point B and Hunterston B are due in early 2015 and will improve the validation data for core restraint doses and may, in the future, extend the validation data beyond 71.6 cm [2]. Additionally, multiplying the sensitivities by estimates of material density variations (i.e.…”