Comparative study of nitrilo triacetic acid (NTA) and EDTA as formulation constituents for the chemical decontamination of primary coolant systems of nuclear power plants

Abstract

Radiation field reduction in the Primary Heat Transport (PHT) system of water cooled reactors can be achieved by `Dilute Chemical Decontamination (DCD) process' operated in regenerative mode. This process uses a mixture of dilute amino carboxylic/ carboxylic acids and reducing/ oxidizing agents to remove the oxide film and the activity deposited on the out-of-core surfaces of nuclear power reactors. Ethylene Diamine Tetra Acetic Acid (EDTA) is normally used as the main chelating agent in the chemical formulations employed. Detailed studies indicated that Nitrilo Triacetic Acid (NTA) instead of EDTA could be used as the main chelating agent. Consequently, a new chemical formulation was evolved and its oxide dissolution characteristics, corrosion compatibility of structural materials, pick-up on ion exchange resins, ion exchange capabilities, radiation stability have been evaluated and compared with the EDTA based formulation. Some studies have been carried out in heavy water as the decontamination of primary coolant system of Pressurized Heavy Water Reactors (PHWRs) is carried out in heavy water medium. From these studies, it has been concluded that NTA as a constituent of decontamination formulation has many advantages over EDTA for the chemical decontamination of primary coolant systems of PHWRs. The new formulation can also be applied to stainless steel systems such as PWRs and BWRs as a reducing formulation followed by an oxidation step.