A Practical Approach to Characterise and Assess a Site Drainage System in Support of Site Restoration

Abstract

Part of the nuclear site restoration and delicensing process involves the characterisation and assessment of below-ground drainage systems. Site restoration is currently underway at Harwell in Oxfordshire where there is a complex drainage system that has developed over more than 70 years. Drainage decommissioning involves visual inspections, jet-washing and radiological surveys prior to final grouting. Prior to decommissioning, the structural and radiological condition of the network was unknown and now requires characterisation and assessment against defined clean-up criteria before the land can be released for future use. This paper outlines the application of an innovative Geographic Information System (GIS) and data management methodology to assess the structural and radiological condition of the drainage network at Harwell. The approach demonstrates the importance of efficient data collection and storage, implemented using UKAEA’s IMAGES land quality data management system. It also details several GIS techniques that can be utilised to accurately position below-ground surveys and record pipe material and diameter for surveyed drain sections. The aim of the monitoring process is to identify any activity which exceeds the delicensing criteria set by the Nuclear Installations Inspectorate (NII). The relevant criteria are the residual activity and the risk to a future site occupier. The maximum missable activity for each pipe class was determined by considering instrumentation characteristics and natural background radiation levels. The equipment used (a caesium iodide scintillation probe connected to a ratemeter), was limited by the small diameter of some drains. The hypothesis chosen was that residual activity was most likely to lie along the base of the pipe, given that the drains generally ran with only a limited depth. The response was measured for 137Cs gamma radiation and then used to calculate the net count rate for a variety of pipe configurations. Variability of the data was investigated in relation to static probe response and variations in counts per second along a pipe length. Analysis of the data using GIS showed clear spatial patterns, indicating systematic variability within each pipe material and diameter class. Clearance criteria were derived from this analysis and will be used to establish whether the specific sections of drain can remain in situ or require removal. This approach improves safety and cost performance of the decommissioning process and ensures that a transparent and auditable record of the status of the drainage system remains for the future re-use of the land as the Harwell Science and Innovation Campus.