Method for decontaminating surfaces

Abstract

A method chemically decontaminates a surface of a metal component of a primary circuit of a pressurized-water reactor, the surface contains an oxide layer. In a first stage, an oxidation step, a reduction step, and a first decontamination step are performed. The component is treated in the oxidation step with an aqueous solution containing an oxidizing agent, which converts trivalent chrome present in the oxide layer into hexavalent chrome. The component is treated in the reduction step with an aqueous solution containing a reducing agent for reducing excess oxidation agent from the oxidation step. The component is treated in the first decontamination step with an aqueous solution containing a decontamination acid that forms no antisoluble deposits with metal ions in the solution. The solution is fed through an ion exchanger for removing metal ions. In a second stage, the component is treated with an aqueous solution containing an oxalic acid.

1 . A method for chemically decontaminating surfaces having an oxide layer of a metallic component of a primary circuit of a pressurized water reactor, the method being divided into two process stages, which comprises the steps of:
 during a first process stage, performing at least one treatment cycle, including an oxidation step, a reduction step, and a subsequent first decontamination step, the first process stage including the steps of:
 during the oxidation step, treating the metallic component with a first aqueous solution containing an oxidizing agent for converting trivalent chromium present in the oxide layer to hexavalent chromium; 
 during the reduction step, treating the metallic component with a second aqueous solution containing a reducing agent for reducing excess oxidation agent producing during the oxidation step; 
 during the first decontamination step, treating the metallic component with a third aqueous solution which exclusively or predominantly contains at least one decontamination acid that forms no sparingly soluble deposits with metal ions present in the solution, including bivalent metal ions; 
 leading the third aqueous solution through an ion exchanger for removing the metal ions, present in the third aqueous solution, which originate from the oxide layer or a base metal of the metallic component; and 
   during a second process stage, performing at least one further treatment cycle, including a second decontamination step in which the metallic component is treated with a fourth aqueous solution which exclusively or predominantly contains oxalic acid as a decontamination acid.   
     
     
         2 . The method according to  claim 1 , which further comprises during the further treatment cycle of the second process stage, performing an oxidation step prior to the second decontamination step. 
     
     
         3 . The method according to  claim 1 , which further comprises providing an organic acid in the first decontamination step. 
     
     
         4 . The method according to  claim 3 , which further comprises providing the decontamination acid to consisting exclusively of carbon, oxygen, and hydrogen. 
     
     
         5 . The method according to  claim 1 , which further comprises providing in the first decontamination step an organic acid which does not form a complex with the metal ions. 
     
     
         6 . The method according to  claim 1 , which further comprises providing in the first decontamination step at least one decontamination acid containing a maximum of two carbon atoms in a molecule. 
     
     
         7 . The method according to  claim 6 , which further comprises using at least one of a formic acid or a glyoxylic acid. 
     
     
         8 . The method according to  claim 7 , which further comprises using the glyoxylic acid in the first decontamination step. 
     
     
         9 . The method according to  claim 1 , which further comprises neutralizing a residue of the oxidizing agent which is present in the first aqueous solution at the end of the oxidation step with a reducing agent which is added to the first aqueous solution resulting in the second aqueous solution, and the second aqueous solution is used in the subsequent first decontamination step. 
     
     
         10 . The method according to  claim 9 , which further comprises employing the decontamination acid used in the first decontamination step as the reducing agent.