AN ONLINE WATER CONTAMINATION MONITORING-SYSTEM

A nuclear power station was recently built in the People's Republic of China, some 50 km from a reservoir which supplies fresh water to Hong Kong. An on;line water contamination monitoring system, built to the specification of the Royal Observatory, was installed at a pumping station at the border to monitor continuously the activity concentration of gamma-emitting radionuclides present in the raw water supplied to Hong Kong. The contamination monitoring system comprises a sodium iodide detector, a stainless steel water chamber, a lead shield, and a microprocessor-based spectrum analyser. Fresh water is allowed to now through the water chamber while the activity concentrations of K-40, Ru-103, I-131 and Cs-137 in the water are measured. An alarm level can be set for one of the four radionuclides to alert the operator of abnormal activity concentrations. Remote display of the data and alarm status are also available. The system is calibrated one radionuclide at a time. In reality, more than one radionuclide may be present in the contaminated fresh water, and interfere with one another in the measurement. The problem of interference has been evaluated, and it was found that the presence of I-131 gave rise to a positive bias in the reported activity concentration of Cs-137. A simple rule for correcting the bias was drawn up.