Contact Author:
Dr. Gunnar Wikmark
wikmarg@westinghouse.com
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COLUMBIA, SC 29250
USA

Modern Fuel Cladding in Demanding Operation - ZIRLO in Full Life High Lithium PWR Coolant

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2007 International LWR Fuel Performance Meeting, September 30 – October 3, 2007, San Francisco, CA Track 2 – LWR Fuel Performance and Operational Experience Modern Fuel Cladding in Demanding Operation - ZIRLOTM in Full Life High Lithium PWR Coolant Kenneth Kargol a), Jim Stevens b), John Bosma c), Jayashri Iyer d) and Gunnar Wikmark d) a)Pacific Gas & Electric Company, Diablo Canyon Power Plant, Avila Beach, California b) TXU Power, Comanche Peak Steam Electric Station, Glen Rose, Texas c) Westinghouse Electric Company, Dallas, Texas d) Westinghouse Electric Company, Columbia, South Carolina ABSTRACT In the competitive electric power market, utilities are striving to increase the power output and minimize the time off the grid. In order to minimize the time off the grid, 18-months cycles are today standard in U.S. PWRs. Many plants are also increasing the rated thermal power of the units, which leads to a higher average rod linear heat generation rate, LHGR. The longer cycles and power up-rates result in higher boron concentration to control the additional reactivity, although some of that reactivity can also be controlled by using burnable absorbers in the fuel assemblies. The elevated boron concentration does, however, necessitate a higher lithium concentration in order to achieve a higher pH that is required to reduce the release of corrosion products from the steam generator and to minimize the crud deposits on the fuel cladding. In addition to potential plant dose rate issues, significant crud deposits could cause CIPS (crud induced power shifts), formerly called AOA, and even fuel failures by a combination of fuel crud deposits, heat-flux and water chemistry parameters [1]. The current usual approach is to aim for a constant pH during the cycle. With beginning of cycle (BOC) boron concentration in the range 1500 to 2000 ppm and with target pH at temperature in the range 7.2 to 7.4 the initial lithium concentration must be above the previous standard levels and in many cases go above 3.5 ppm. The constant pH operation and long cycles will imply that operation with an initial lithium concentration of even only 3.5 ppm will expose the rods for a lithium concentration above 2.5 ppm for approximately 6 months. Zirconium materials are sensitive to high aqueous lithium concentrations and exhibit an accelerated corrosion in such environments. Boric acid has been shown to mitigate the effect if the boron concentration is sufficiently high [2]. Still, in a sub-nucleate boiling situation the enrichment of lithium in the oxide film can be significant even in presence of boron to produce corrosion acceleration [2]. Generally, alloys with very low tin contents will have reduced corrosion resistance in presence of high lithium concentration, at least in autoclave testing [3]. The increasing need for elevated lithium environments have pushed utilities to start programs to test the fuel cladding behavior in such environments. Early tests in Millstone and Ringhals in the 1980’s have been followed by more recent tests and actual applications. Elevated lithium to 3.5 pm was used in a recent test of Zircaloy-4 and Zr-Nb-alloy fuel cladding materials in a 1300 MWe French PWR [4], which indicates that elevated lithium is of interest also in Europe. The long fuel cycles in the U.S. PWRs and the concerns for CIPS have led to more than five plants today operating with initial lithium concentrations above 3.5 ppm and hence they are extending the previous data base of elevated lithium operation. Comanche Peak Unit 2 recently completed the third cycle with elevated coolant lithium concentration operation. Some results from previous cycles have been reported earlier [5, 6]. The recent results have been compared with the results from similar fuel rods operated in Unit 1, without elevated lithium, to determine if there is a lithium effect. The results indicate that some effect may be evident in the region where sub-nucleate boiling is predicted, although part of the recorded effect could be due to crud. Yet, for fuel rods with ZIRLOTM cladding the maximum oxide is typically less than 40 microns at a rod burn-up of 43 to 48 GWd/tU. Diablo Canyon Unit 2 is a plant with zinc injection that operates with a constant pH of 7.2 The good lithium resistance of the ZIRLOTM cladding has also been confirmed by the operation in this plant with a lithium concentration up to 4.2 ppm in cycle 13. Twice burned rods showed typically less than 25 micron of oxide in the upper spacer spans, implying no lithium effect for the ZIRLOTM cladding under those operating conditions. REFERENCES 1. Yang, R. et al, Water Reactor Fuel Performance Meeting, Kyoto, Japan, Oct 2005, p. 8-14 2. Billot, P. et al.,Zirc. Nucl.. Industry, 13th Symp., ASTM STP 1423, 2002, p 169-189 3. Sabol, G. et al., Zirc. Nucl. Industry, 8th Symp., ASTM STP 1023, 1989, p. 227-245 4. Viricel, L. et al, CHIMIE 2002, Avignon, France, Apr. 2002, paper 106 5. Iyer, J. et al, Int’l Conf. Water Chem. Nucl. Reactor Syst., San Francisco, CA, Oct. 2004, paper P1/15 6. Iyer, J. et al, Int’l Conf. Water Chem. Nucl. Reactor Syst., Jeju Island, Korea, Oct.. 2006, P2.24