A novel TLA application in industrial plant protection

During 1995 and 1996, there were significant eruptions from Mr. Ruapehu, a 2797 metre high volcano in the centre of the North Island of New Zealand. The surrounding countryside received a liberal coating of volcanic ash. Ruapehu is only one of several volcanoes in this area which is known as the Volcanic Plateau. The Plateau also contains the headwaters of an extensive hydroelectricity scheme involving I I power stations. The first of these is Rangipo, representing 2% of the national generating capacity. It had to be shut down for safety reasons because ash washed into the feedwater was found to be grinding through 3 mm of stainless steel within the turbines each week. The ash is two times harder than the stainless steel. Although the damage was repaired, using hardened materials, there is continuing concern as it was estimated that it would take more than 5 years to completely wash out the abrasive ash from the headwaters. We were asked to devise a warning system which could alert the station operators to the presence of ash in the intake water of the Rangipo station. Turbidity measurements were first tried, and found to be ineffective. We then turned to the technique known as Thin Layer Activation (TLA) which we have used for many years to monitor corrosion and erosion in industrial plant. Appropriate choice of particle and energy can ensure that some of the reaction products are both long-lived and decay by high energy gamma ray emission. With a known depth profile of embedded radioactivity, it is easy to calculate the loss of surface material from the corresponding decline in gamma ray activity. This can be monitored through several centimetres of steel. In the Rangipo application, a jet of the intake water is directed onto an TLA disc made of the same metal as the turbine blades. The gamma ray activity is continuously monitored from the outside of the pressure vessel containing the disc