Optimisation of Water Chemistry Technologies and Management to Ensure Reliable Fuel Performance at High Burnup and in Ageing Plants

Corrosion of fuel rod cladding and primary circuit component materials has for many years remained a serious concern for water cooled nuclear power reactors, especially those operated at high burnup and thermal rates. As a response, the IAEA has run continuously, since 1981, a series of Co-ordinated Research Projects aimed at better understanding of the cladding corrosion processes (CCI, 1981-86), water coolant technology improvement (WACOLIN, 1987-91), development and implementation of advanced on-line water chemistry and corrosion monitoring techniques/sensors in nuclear power plants (WACOL, 1995-2000) and development and implementation of data processing technologies for water chemistry and corrosion control" (DAWAC, 2001-2005). The DAWAC CRP evaluated an optimal ratio of on-line/grab sampling chemistry monitoring techniques as function of plant design, staffing and availability of external support, demonstrated increased preference of on-line monitoring over grab sampling for important chemistry parameters. Regarding the best methodology approach to the architecture of the expert water chemistry system, combination of different methods - fuzzy logic, neural networks, analytic models, is being believed will give best results. Recommendations were given on the proper balance between R & D activities in water chemistry monitoring area and actual plant needs.

It may be concluded from the DAWAC project results, that with continuous progress of the 1st step (Plant data collection and history, future data, operational diagnosis) and the 2nd step (research, data analyses, experiments, models validation), the 3rd step (update of water chemistry specifications) needs to be also developed correspondingly. It corresponds to the need for updating the chemistry specification based on high-level experts with sufficient background to take into account materials and design evolution, plant experience feedback, operation of mixed cores and plant ageing. Recent corrosion/deposition related fuel failures (2002-2004), especially in BWRs, were attributed to uncoordinated modifications of water chemistry regimes and fuel rod/assembly structural materials. Integration of water chemistry specification and regime - materials corrosion - materials characteristics and structure should help to find optimal combination of coolant and materials characteristics.