A.L. Rufus

Chemistry aspects pertaining to the application of steam generator chemical cleaning formulation based on ethylene diamine tetra acetic acid

During the course of reactor operation, corrosion products and impurities from the make-up water and condenser leak, accumulates on the tubes and tube sheets of the steam generators. These deposits cause several problems such as loss in heat transfer efficiency, under deposit attack of the tubes leading to tube failure etc. Though, various techniques are being employed to remove these deposits, cleaning by chemical methods has been shown to be the most efficient means of solving this problem. In this paper, the work carried out to understand the various chemistry aspects involved in the application of Ethylene Diamine Tetra Acetic acid (EDTA) based chemical formulations, for cleaning the steam generators of nuclear power reactors, has been brought out. The preparation of highly concentrated solution of EDTA and the variation in the solubility with the nature of the base used for pH adjustment has been studied. Identification of the various chemical components present in the sludge of a nuclear steam generator especially the portion that remains insoluble in chemical formulations has been carried out. The steam generator sludge was subjected to a detailed study so as to understand the role played by various additives such as pH adjusting, reducing and oxidizing agents, in affecting the kinetics of dissolution of various phases present in the sludge. Observations made in the sludge dissolution study have been corroborated by the studies carried out with constituent oxides such as magnetite, oxides of copper and copper metal. The importance of the presence of reducing agent in dissolving the iron oxides and the effect of oxidizing agents like air and hydrogen peroxide in dissolving the metallic copper and copper oxides have been discussed. Various bases like ammonia, sodium hydroxide and ethylene diamine (EDA) have also been evaluated for their effectiveness as pH adjusting agents in dissolving the sludge. In addition, the thermodynamic quantities such as heats of neutralization, dilution and the various decomposition reactions have been calorimetrically determined. It was observed that the presence of certain impurities and temperature conditions of the formulation containing hydrogen peroxide could initiate a fast exothermic reaction resulting in the total decomposition of the formulation constituents. The role of various metal ions in the mutual decomposition of hydrogen peroxide and EDTA and the catalytic effect of iron ions in the decomposition of the hydrogen peroxide has been explained.

Ion-exchange considerations in dilute chemical decontamination processes operated in the regenerative mode

Dilute chemical decontamination processes use ion-exchange resins for collecting the metal ions, radio-active contaminants, and formulation chemicals. In decontamination processes operated in the regenerative mode, the ion-exchange resin is also used for regenerating the spent formulation. Normally, the cation exchange resin is used during the regeneration stage of the process. During decontamination, the chemical formulation dissolves the contaminated metal oxide film from the system surfaces. The complexants present in the formulation form complexes with the metal ions thus released and keep them in solution. An investigation has been carried out to study the ion-exchange reaction among the complexants, the metal complexes of interest to decontamination, and the cation exchange resin. Sorption behavior of ethylenediaminetetraacetic acid on the cation exchange resin in a heavy water medium as a function of pH was studied, and the observed sorption values were compared with normal water sorption and explanations offered to account for the difference. Simultaneous pickup of different metal ions on the cation exchange resin may result in elution of one or more metal ions by another ion. Results of elution experiments are discussed. An attempt to correlate the stability of the various metal complex species formed in solution and the apparent capacity of the cation exchange resin to the metal ion is made. The effect of pH, temperature, concentrations of metal ion, and the complexants in controlling the metal ion pickup on the cation exchange resin is explained. The use of strong- and weak-base anion exchange resins in decontamination is explained.

Corrosion of carbon steel and Monel-400 in EDTA based steam generator cleaning formulations

Corrosion products and the scale forming impurities foul the steam generators of Pressurized Water Reactors (PWR) and Pressurized Heavy Water Reactors (PHWR). The impurities from the make-up water, the condenser leaks and the corrosion product oxides leached from the feed train structural materials are carried along with the feed water and concentrate in the steam generators. These impurities deposit/precipitate over the steam generator surfaces and cause several problems such as reduction in heat transfer efficiency, under deposit attack, tube failures and production loss. In order to solve this problem, the power utilities are resorting increasingly to chemical cleaning of the steam generators. The chemical formulation selected for cleaning should be able to dissolve the accumulated corrosion products/sludge and at the same time should be compatible with the steam generator structural materials. Of the various chemical-cleaning formulations, the EDTA based formulations are more popular and have been used in the chemical cleaning of many nuclear steam generators. Its corrosion compatibility with the structural-materials of the steam generators needs a thorough evaluation. In this work, the effect of various additives added to the EDTA such as pH additive, reducing agent, oxidizing agent and corrosion inhibitors have been investigated. Increase in the concentration of hydrazine has been found to increase the corrosion rate of carbon steel. The variation in corrosion release from carbon steel with time was found to fit a second order equation. The variation in corrosion rate with time was attributed entirely to the increase in surface area caused by roughening of the surface. Presence of dissolved oxygen in EDTA based iron formulation enhanced the corrosion rate of carbon steel and pitted the Monel-400. The study indicated the need for strict oxygen control and the necessity of using a suitable corrosion inhibitor during the iron removal step. The role of copper in the corrosion of carbon steel during chemical cleaning has also been studied. Problems encountered in the use of oxygen as an oxidant for copper removal in steam generators having Monel-400 as tube material have been investigated. The results of experiments to investigate the corrosion behavior of Monel-400 tubes, that has seen ten years of operation and has already undergone intergranular attack, in EDTA based formulation has also been discussed.

Dissolution of synthetic uranium dibutyl phosphate deposits in oxidizing and reducing chemical formulations

Permanganate and nitrilotriacetic acid (NTA) based dilute chemical formulations were evaluated for the dissolution of uranium dibutyl phosphate (U-DBP), a compound that deposits over the surfaces of nuclear reprocessing plants and waste storage tanks. A combination of an acidic, oxidizing treatment (nitric acid with permanganate) followed by reducing treatment (NTA based formulation) efficiently dissolved the U-DBP deposits. The dissolution isotherm of U-DBP in its as precipitated form followed a logarithmic fit. The same chemical treatment was also effective in dissolving U-DBP coated on the surface of 304-stainless steel, while resulting in minimal corrosion of the stainless steel substrate material. Investigation of uranium recovery from the resulting decontamination solutions by ion exchange with a bed of mixed anion and cation resins showed quantitative removal of uranium.

Evaluation of Corrosion Inhibitors for Carbon Steel and Type 304 Stainless Steel in Nitrilotriacetic Acid Medium at High Temperature

Abstract At elevated temperatures, the corrosion of carbon steel and Type 304 stainless steel is high in media containing polycarboxylic acids such as nitrilotriacetic acid, which is a reagent used for the decontamination of nuclear reactor coolant systems. Hence, three commercial corrosion inhibitors (Philmplus 5K655, Prosel PC-2116, and Ferroquest LP7203) were evaluated for high-temperature applications. Preliminary screening of the inhibitors was done by electrochemical techniques, namely, polarization and impedance spectroscopy. Philmplus showed maximum corrosion inhibition efficiency and hence was used for high-temperature investigations. A concentration of 500 mg/L was found to be optimum. The high-temperature dissolution of corrosion product oxides such as magnetite and nickel ferrite that are relevant to nuclear reactors was also carried out in the presence of Philmplus. During the decontamination process, which involves the dissolution of corrosion product oxides, it is desirable to use an inhibitor that will alleviate the corrosion of the underlying base metal without compromising on the dissolution of the oxides present over it. Investigations were also carried out to evaluate hydrazine as a corrosion inhibitor for high-temperature applications; the results obtained were comparable to those of Philmplus.

Removal of gadolinium, a neutron poison from the moderator system of nuclear reactors

Gadolinium as gadolinium nitrate is used as neutron poison in the moderator system for regulating and controlling the power generation of Pressurized Heavy Water Reactors (PHWR) and proposed to be used in Advanced Heavy Water Reactors (AHWR) owing to its high neutron absorption cross section. Removal of the added gadolinium nitrate (Gd3+ and NO3-) from the system after its intended use is done using ion exchange resins. In the present investigation, attempts have been made to optimize the ion exchange process for generation of low radioactive waste and maximize utilization of the ion exchange resins by employing different types of resins and different modes of operation. The investigations revealed that use of mixed bed (MB) resin column consisting of Strong Acid Cation (SAC) resin and Strong Base Anion (SBA) resin followed by SAC resin column is efficient in removing the Gd3+ and NO3- from the system besides maintaining the pH of the moderator system in the desirable regime, where gadolinium does not get precipitated as its hydroxide.