The release and deposit of corrosion products, which play a key role in activity transport, are controlled by the properties of the primary water and oxides present on component surfaces. Some of the VVER 440 type reactors have started to use hydrazine dosing to primary coolant instead of ammonia, because it has been shown to be efficient in reducing activity transport. On the other hand, some other studies have shown that there is no significant difference between new VVER units using hydrazine dosing and the ones operating with standard potassium/ammonia water chemistry. In this paper the results are presented concerning the out-of-core high temperature water chemistry and in-core redox potential measurements at Rez research reactor in Czech Republic during hydrazine/ammonia water chemistries.

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