Chemical Contamination Process

Con Decon Process[4]

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It is a dilute chemical decontamination process that avoids the draining or flushing of the system to be decontaminated. It is also not necessary to defuel the reactor due to decontamination process. This is thus a simplified process which results in cleaning of the fuel at the same time, adding a huge advantage of removing a significant amount of activated corrosion product which if allowed to remain would eventually dissolve and redeposit out-core, thus partially negating the effects of the decontamination.

Figure 1- Con Decon Process

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The following equations[4] show some of the chemical reactions involved in magnetite dissolution. The study was carried out by making an oxide covered disk the working electrode in an

Diethylthiourea, 0.1 wt%, may also be added as a corrosion inhibitor when using CITROX with carbon steel systems. Also ferric nitrate can be used as a corrosion inhibitor [34, 35]. Organic acids dissolve and complex the corrosion products and the chelating agent chemically bind the released or dissolved corrosion products. The concentration ratio of oxalic acid to dibasic ammonium citrate must be less than or equal to 0.5 to prevent secondary film formation. This is important because citrate ions complex iron and inhibit the formation of an oxalate precipitate. The formation of a precipitate film could interfere with subsequent exams. Ferric nitrate, ferric sulphate and diethylthiourea are often used as corrosion inhibitors to prevent pitting of carbon steel surfaces. CITROX is usually applied for eight hours at 80 oC.

Despite of many good results obtained with CITROX processes, a replacement for this process has been studied especially in Czech Republic and Slovakia. [41] Due to some negative effects observed in material behaviour after CITROX decontamination, the so called AP/NP- NHN process has been applied to decontaminate primary circuit parts of Russian type reactors VVER 1000/440. The abbreviation

The results in terms of dose rate reduction factor have been during RSG operations as follows:

· Hot leg dose rate reduction factor from 50 to 70,

· Cold leg dose rate reduction factor around 60.

This process is specifically adjusted for stainless steel casing in the primary cooling system. When it is used to decontaminate the steam generator channel heads, the dose rate reduction factor obtained is around 5. In this case, the stainless steel surfaces account for only 5% of the total surface to be decontaminated, the rest

(95%) consists mainly in nickel alloy (Inconel 600). The qualification tests have shown that the efficiency of the EMMAC process on nickel alloys is reduced. The EMMAC process is widely used in France because it is efficient with regards to the type of contamination encountered (hot fixed contamination) and the

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materials concerned. The chemical products used and the effluents generated are also compatible with the waste treatment systems of the nuclear plants. Westinghouse has used this process during artefact testing.

Dilute concentrate decontamination (DCD) [5]

Dilute chemical decontamination techniques use dilute solvents (i.e. concentrations of solvents <8 whereas on stainless steel