SCC of SG tubing and stainless steel (SS) pipes and welds (PWRs) - 1

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1 International Conference on Water Chemistry of Nuclear Reactor Systems, October 2004, San Francisco, EPRI in co-operation with the IAEA Participation: more than 200 experts from 23 countries; IAEA supported participation of water chemistry experts from Bulgaria, Czech Republic, Hungary, Lithuania, India, Russian Federation, Slovak Republic and Ukraine. Three corrosion-related major issues identified by NEI in May 2003: -SCC of SG tubing and stainless steel (SS) pipes and welds (PWRs), -SCC of Reactor Internals (BWRs), -Fuel Cladding Integrity (PWRs and BWRs) (crud-induced fuel failures in some BWRs and Crud-Induced Power Shifts in some PWRs) 1 International Atomic Energy Agency SCC of SG tubing and stainless steel (SS) pipes and welds (PWRs) - 1 SCC of the SG tubes on the primary side (PWSCC) is mitigated by replacement of initial tubing material (Alloy 600) to Alloy 690 that is more corrosion resistant, by elimination of stress concentrators (design and weld technology optimization) and by zinc injection into primary water (proven fact that zinc inhibits crack initiation). The addition of zinc, which partially replaces Co in ferrite oxides on fuel rod claddings, is also an interesting and promising option for decreasing activity in the primary coolant. 2 International Atomic Energy Agency

2 SCC of SG tubing and stainless steel (SS) pipes and welds (PWRs) - 2 SCC on the secondary site has occurred in crevices, especially in the SG hotter regions where boiling leads to the impurity concentrations in crevices that are many orders of magnitude higher than in the bulk secondary water. Because of the complexity of these concentration effects, the nature of the phenomenon has not yet been sufficiently investigated and understood. Elimination of crevices by design improvement does not eliminate, unfortunately, sludge pipe formation. The use of dispersants is currently being explored as a countermeasure. The second crucial point for lowering costs associated with maintenance is a control of SG fouling by selection of the most appropriate secondary water treatment, such as ammonia at ph 10 when acceptable for the rest of the system or by alternate amines which allow a higher ph at operating temperature in the various parts of the system, thus decreasing generation, transport and deposition of corrosion products. 3 International Atomic Energy Agency SCC of Reactor Internals (BWRs) Water chemistry in BWRs plays a critical role in determining the economics reducing number of repairs due to SCC of piping and internals. BWR plants are in a transition from normal (oxygenated) water chemistry (NWC) to hydrogen water chemistry (HWC) to mitigate stress corrosion cracking. Almost all plants using natural zinc to control radiation fields have switched to depleted zinc to minimize radioactive zinc-65 formation. Studies showed that the presence of noble metals on structural materials significantly reduces the hydrogen concentration required to achieve the IGSCC protection potential of -230mV SHE. Noble metal chemical addition (NMCA) was introduced at Duane Arnold BWR as an in situ method of reducing the amount of hydrogen required to lower the ECP on material surfaces, which would also mitigate the effects on operating radiation fields. Following this successful plant demonstration, NMCA has been applied at many other plants, also in Europe. An interesting benefit is that shutdown radiation fields are reduced following NMCA, which leads to the possibility of reducing depleted zinc concentrations after the oxide films have stabilized. 4 International Atomic Energy Agency

3 BWR Fuel Rod Failure Root Cause Trend (p.60) 5 International Atomic Energy Agency PWR Fuel Rod Failure Root Cause Trend (p. 61) 6 International Atomic Energy Agency

4 Reduction of AOA by Elevated-Constant ph at Comanche Peak Steam Electric Station 7 International Atomic Energy Agency Modelling Axial Offset Anomaly 8 International Atomic Energy Agency

5 Fuel Performance-Water Chemistry Taking into account recent fuel failures, the Industry Initiative on Management of Materials Issues was organized and approved under the NEI umbrella (NEI 03-08) in May 2003 (with invitation of also non-u.s. fuel utilities and vendors). Its scope includes industry-funded materials degradation R&D programmes to pressure boundary components and most programmes on degradation of fuel cladding. This is because industry leaders felt that the potential impact on fuel performance of chemistry and corrosion control strategies aimed at pressure boundary components needed to be investigated in an integral manner. Revision 0 of this strategic plan was completed earlier in 2004 and identifies three corrosion issues as being of particularly high priority at present (see slide No. 1). 9 International Atomic Energy Agency