Chemistry Measures at Gundremmingen BWRs for Dose Reduction from the Perspective of Safe Fuel Operation

The Gundremmingen nuclear power station, is a 2 units BWR designed by Siemens/KWU and located in Germany. It consists of two BWR units B and C. Units B and C are twin units and they are rated at 1344 MW electrical power. The unit C is still in operation and is operated with normal wáter chemistry (NWC) according to the VGB Water Chemistry Guideline. No noble metal chemistry is applied. The plant observed in the primary circuits of blocks B and C a relevant activity increase. This increase was caused by CRUD deposits on the fuel elements, which are activated and mobilized. The CRUD is mainly formed by iron oxides from corrosion products generated in the water-steam cycle, during power operation In order to reduce the CRUD mobilization and to clean up the reactor water thoroughly prior to water level increase and flood up for outage works the temperature gradient during cool down of the reactors to refueling outages was decreased from 30 K/h to 10 K/h. For mitigation of activity carryover to steam piping, steam wetness was reduced. Details of this work are presented in [1]. The water chemistry and dose rate data of the two units from the past 20 years were reviewed to implement corrective actions for radiation field control. The recommendation was to establish a low level of zinc in reactor water with the objective of binding activation products on the fuel rod surfaces. In the past, both units had brass tubes in the turbine condensers which resulted in a zinc concentration level of about 1 ppb in reactor water. After their replacement by stainless steel tuve bundles, the reactor water became deficient in zinc. For the correction of zinc deficiency, it was decided to dosify small amounts of zinc to the feedwater. The operation experience gathered with the two units in the past with brass-tubed condensers was positive and therefore used as a basis to set the operation margin and the monitoring and control values for the currently applied low-level zinc addition. Zinc addition is a well-known and proven measure for dose rate reduction in boiling water reactors. Besides its positive effects on radiation fields, application of the method bears a potential risk for the formation of thermally insulating layers on fuel rods which can cause overheating and enhanced corrosion. This risk had to be excluded. This paper covers measures performed to ensure safe and reliable reactor and fuel operation during low-level zinc injection at the Gundremmingen power station mainly from a water chemistry perspective, i.e. a. Elaboration of a plant-specific water chemistry surveillance program. Further measures have been implemented to ensure safe and reliable fuel operation, namely: b. Characterization of fuel deposits by SEM/EDX and ICP-MS analysis c. Sampling of mobilized activation products in reactor water during shutdown operation Those measures have been recently presented in detail in [7]. The program was elaborated and performed in close cooperation between Framatome GmbH, Kernkraftwerk Gundremmingen GmbH and RWE Power AG. Low-level zinc addition has been successfully applied in Gundremmingen unit B since December 2014 and in unit C since August 2015 leading together with improved plant operation procedures to main steam line dose rate levels and outage doses as low as in the first few outages of the two units in the eighties. The program results enable both units to operate with zinc addition at a minimum surveillance effort until their final shut-down.