Toxicity Limitation on Radioactive Liquid Waste Discharge at OPG Nuclear Stations

Abstract

The Municipal and Industrial Strategy for Abatement (MISA) regulation, which came into effect in 1995 in Ontario (Ontario Regulation 215/95 under the Environmental Protection Act), imposed additional limitations on liquid discharges from power generating stations. The MISA regulation has divided discharges into non-event and event streams, which have to be monitored for the prescribed parameters and for toxicity. Radioactive Waste Management Systems fall into the category of non-event streams. Standard toxicity testing involves monitoring lethality of Daphnia Magna and Rainbow Trout in the effluent. The new legislation has imposed a need to address several issues: acute toxicity, complying with the specific limits prescribed by the regulation and, in the long run chronic toxicity. In the first phase, the correlation between various chemical parameters and acute toxicity was established and several investigations were initiated. The effects of microbial activities in the waste streams were the most difficult to address because many synergies between chemical toxicants and microbial toxins were not known. Due to a limited time schedule to achieve MISA compliance, a wide approach was taken. Approaches included best management practices in the power plants, waste stream segregation and a choice of treatment technology that could simultaneously address a number of potential toxins. Furthermore, the variability of the waste streams in present and in future had to be taken into account. This approach had a drawback in potentially generating significant costs of consumables and volumes of secondary solid radioactive waste. A strategy to monitor treatment technology performance and to optimize processes and costs in the long run, was therefore devised. Control of waste generation and centralizing laundry facilities from Pickering and Darlington to the Bruce site have enabled achievement of MISA compliance with simple processes and modifications at Pickering and Darlington stations (laundry is a key source of organic toxins). In parallel, more elaborate technologies were applied at Bruce A site for treatment of additional laundry wastes. Originally, Active Liquid Waste Treatment (ALW) systems were designed to remove radioactivity from the liquid waste streams. Achieving radioactivity Derived Emission Limits (DEL's) and Station Target Limits was never a problem in Ontario Power Generation stations and the AL W treatment systems were rarely required for this purpose. Modifications of these existing systems and installation of the new treatment systems, were a part of the strategy developed at OPG. At Pickering a completely new treatment system was designed and installed. The process consists of an ultraviolet biocide unit, cartridge filtration, Granulated Activated Carbon (GAC), cation resin and neutralizing filter (CaCO3) bed. This combination of processes enabled the use of adsorption for reduction of organics and cation IX resin for removal of dissolved cations. Filter optimization will have to be considered for long term operation. The Darlington ALW system was modified so that a redundant IX column was used for GAC ( later switched for Macronet resin). For long term operation, filtration, cation resin and a CaCO3 bed are being considered. The Bruce A ALW system is a complex, multi-stage treatment system consisting of pretreatment, a multi-stage reverse osmosis system (ROS), and an evaporator solidification system (ESS). The concentrates from the ROS are fed to the ESS and evaporated via the use of a thin film evaporative process. The evaporator bottoms are solidified in bitumen and the solidified residue placed in a container for storage on-site as a low-level radioactive waste. In the final polishing stage of the process, the permeate from the ROS is treated via a calcium carbonate bed for pH adjustment and this water discharged to the lake. The overall effort to achieve MISA compliance at OPG Nuclear sites was extremely challenging, in particular because of the very tight schedules. Lessons have been learned in the areas of procurement and design. Internal management processes have been established to ensure continued MISA compliance.