Method for protecting components of a primary system of a boiling water reactor in particular from stress corrosion cracking

Abstract

A method for protecting the components of the primary system of a boiling water reactor, especially against stress cracking corrosion, includes feeding a reducing agent into the primary coolant in order to reduce the number of substances having a oxidizing effect or in order to modify the electrochemical potential of the component surfaces covered with an oxide layer to negative values. An alcohol that can be oxidized in the conditions of a reactor is fed in as a reducing agent, preferably in a liquid form, into the primary coolant. The component surfaces are provided with a blank coating or a single intrinsic oxide coating.

Description

[0001] This is a continuation of U.S. patent Ser. No. 10 / 715,069, filed Nov. 17, 2003, which is a continuation of International Application No. PCT / EP02 / 05274, filed May 14, 2002.BACKGOUND [0002] The invention lies in the boiling water reactor technology field. More specifically, the invention relates to a method for protecting the components of the primary system of a boiling water reactor in particular from stress corrosion. In a boiling water reactor, the coolant which comes into contact with the reactor core is known as primary coolant, and the lines and components which are exposed to the primary coolant are known as the primary system. In addition to the reactor pressure vessel, the primary system of a boiling water reactor includes systems of lines as well as various internal fittings and pumps. The components generally consist of stainless steel, for example of a CrNi steel, or an Ni-base alloy, such as Inconel® 600 (Inco Alloys International, Inc.). Radiolysis of the primar...

AI Technical Summary

Benefits of technology

[0008] An object of the present invention is to provide a method for protecting the components of the primary system of a boiling water reactor which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensures efficient protection against corrosion with little outlay on materials and time.

[0012] In other words, the objects of the invention are achieved by a method in which an alcohol that can be oxidized under the conditions prevailing in the reactor system, preferably in liquid phase, is fed into the primary coolant instead of hydrogen, with the component surfaces being bright or being covered only by a native oxide layer. In this context, a native oxide layer is to be understood as meaning an oxide layer which forms as a result of corrosion to the component material, if appropriate with the intercalation of foreign metals or foreign metal oxides, during reactor operation or during an oxidizing pretreatment. It has been found that the metering-in of an alcohol of the above type as the only measure is sufficient to reduce the corrosion potential of the component surfaces to values of lower than −230 mV, and it is possible to dispense with complex coatings in particular comprising substances with a photocatalytic action.

[0013] The advantage of an alcohol over hydrogen as reducing agent is firstly that it can be metered in in liquid form or as a solution. A liquid is more easy to feed into the primary coolant than a gaseous substance in terms of the apparatus required. Furthermore, the compounds mentioned offer advantages in terms of handling and storage. Finally, they are less expensive than hydrogen, with the result that the plant operating costs can also be reduced.

[0014] In accordance with a concomitant feature of the invention, the component surfaces are doped with precious metal, for example with Pt, with the result that a lower concentration of alcohol is required in the primary coolant.

Problems solved by technology

The oxidizing conditions which result from the excess of oxidizing agents promote corrosion, in particular stress corrosion cracking, of the components.

A drawback of the conventional method is that relatively large quantities of hydrogen are required to ensure sufficient protection against corrosion.

The high demand for hydrogen, which entails corresponding costs, is attributable not least to the fact that the electrochemical oxidation of the hydrogen on the component surfaces which are covered with an oxide layer is subject to considerable reaction inhibition, and this has to be compensated for by increased hydrogen concentrations.

A further drawback is the outlay on apparatus for metering the gaseous hydrogen.

A problem associated with very high alcohol concentration is that a relatively large portion of the alcohol remains unused, i.e., it is not oxidized by radiolysis oxygen or decomposed by the radiolysis, it subsequently passes through a phase change into the vapor phase and then reaches the steam turbine and the condenser downstream of the steam turbine.

An alcohol component in the vapor phase could, on the one hand, disturb the recombination.

High alcohol contents, furthermore, lead to radiolysis in the reactor due to the high radiation density, which results in products such as CO2, formaldehyde, and formic acid.

These products, of course, are undesirable in the reactor pressure vessel itself and in the downstream vapor carrying systems such as the condenser.

Besides an increase in the conductivity of the primary coolant, they can lead to a decrease in the pH which has a negative effect on the component corrosion.

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