PRIMARY CHEMISTRY AS AN ACTION LEVER TO REDUCE THE SOURCE TERM. REVIEW OF THE EXPERIENCE AT DOEL 3 AND 4

摘要

Dose rate reduction is of primary concern in order to make nuclear energy part of the future energy mix. This article presents the chemistry related actions undertaken at Doel 3 and 4 units in order to reduce the source term.Doel 3 and 4 are two 3 loops 1000 MW_e Pressurized Water Reactors which respectively started operating commercial in 1982 and 1985. Both units have undergone a steam generators replacement: from Inconel 600MA tubing material to Incoloy 800 in 1993 for D3 and to Inconel 690TT in 1996 for D4.Until now, very similar chemistry strategies during operation, startup and shutdown have been applied in both units. They are in good accordance with the international standards. Both units operate under a coordinated chemistry, with a target pH_(Tave)=7.2. Dissolved hydrogen concentration is maintained in the range 25-35 cc/kg STD H_2O. During shutdown, oxygenation is performed at full loop by means of hydrogen peroxide injections. However, significant differences in dose rates have been observed between the two units, with higher levels in Doel 3.In an attempt to understand these differences and to define new improvements pathways to lower the dose rates, this article assesses the chemistry strategy based on the available radiochemistry data, the RCS oxide layers characterization and the measured dose rates around the primary system.Primary chemistry strategy seems to be suited to maintain low level of radiation fields at Doel 4. However, Doel 3 is characterized by a contamination mainly driven by the Co-60. Specific actions have to be undertaken to minimize dose rates at Doel 3. Regarding the high contribution of Co-60 to dose rates, zinc injection seems to be a promising mitigation technique.