Dimensional behavior of fuel channels. Update on the operational experience and evaluation results

Even if the underlying mechanism is not completely understood, there are enough data available to support the design of fuel channels which can withstand unexpected bow under demanding operational conditions: - Use material with a low hydrogen pickup fraction - Use material which is less sensitive to shadow corrosion - Use fuel channels with less fast neutron fluence induced growth Based on the available data, empirical thresholds for burnup and control blade exposure have been established for AREVA NP's fuel channels. These limits differ based on the fuel channel design and, more specifically, a smaller wall thickness leads to more restrictive limits. Recent efforts have been focused on integrating all the new data and observations into the models for determining fuel channel bow and predicting core cell friction. Specifically, the neutronic feedback effect and a non-linear fast neutron fluence growth model are being developed. A cell friction model has been developed based on these models allowing for a detailed approximation of the control blade to fuel channel friction contact. Based on all these new observations surveillance recommendations have been developed for the concerned C- and S-lattice BWR plants as a means of avoiding excessive control rod interference with AREVA NP BWR fuel channels. The calculations with the cell friction model help identify core cells that are susceptible to slow settle times. For these identified core cells the recommendations include selection criteria, test frequencies and the sampling extent for the control blade friction and settle time testing. Different surveillance levels are established based on the inspection results. The supply of Zry-4 fuel channels to susceptible C- and S- lattice plants is seen as a short term option by AREVA NP. In 2004 the irradiation of ss-quenched lead fuel channels was started in a German BWR. These fuel channels will complete their fourth irradiation cycle in 2008 and the results with respect to deformation are very promising so far. A material test program was started in 2005 in the same plant. Based on the results of these two programs the insertion of lead test fuel channels is possible in 2009. (orig.)