Remote plastic optical fibre sensing for an aqueous nuclear environment

Abstract

In the nuclear industry, active waste is stored under one of three conditions: dry, wet, and wet with rust inhibitor (air, water and a high pH solution environment). The monitoring of pH, temperature, turbidity, magnesium, oxygen and carbonates levels have been highlighted. Such a challenging environment requires robust sensors to survive in both a high pH and radiation environment while navigating small/ limited access and avoiding interference with other ongoing operations, such as retrievals. This thesis focuses on the monitoring of such measurands in aqueous nuclear environments via Optical Fibre Sensing (OFS) using Plastic Optical Fibre (POF). Investigation showed POF fibre, Mitsubishi ESKA CK40 would be suitable for the basis of a sensor for nuclear storage environments, however the effects of the environment on sensitising coatings could not be removed. Fibres were monitored to check their sensitivity to gamma irradiation in all three storage environments at a dose rate of approx. 40 Gy.hr-1 (corresponding to that of intermediate level waste). Radiation induced attenuation (RIA) did occur and after 95 hours, reached a maximum of 3.5 dB.m-1 at 468 nm. However, the effects were minimal, suggesting the RIA could be subtracted from the signal by using a reference fibre. Sensor designs were investigated: an evanescent based pH sensor, and an extrinsic turbidity sensor. The pH sensor monitored pH 7- 10 for 4 hrs with a response of -328.2 ± 22 photon counts per second per 0.1 pH units, but was affected by radiation. Initial turbidity tests distinguished high concentrations but were affected by temperatures (13-18 °C) of outdoor storage ponds. Therefore, although CK40 fibre has been shown to be suitable for use in nuclear storage facilities, further work is required to define suitable coatings and sensitising materials for use with the fibre.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Bernard Treves Brown (Supervisor), Philip Martin (Supervisor) & Patricia Scully (Supervisor)