Demonstration of the PWR axial offset anomaly in IFA-665

Many PWRs have suffered from the axial offset anomaly (AOA) since the early 1990s. AOA is a phenomenon associated with localised boron hideout in corrosion product deposits (crud) on fuel surfaces formed because of the presence of sub-cooled nucleate boiling. Since boron absorbs neutrons, the reactor experiences a shift in power output towards the bottom of the core. AOA has caused one plant to de-rate and forced other utilities to design less efficient cores to avoid the phenomenon. This study is the first phase in an experimental program to investigate whether AOA can be delayed or avoided by using enriched boric acid (EBA) in the coolant . The objectives of this first phase were to demonstrate the symptoms of the AOA: i.e. to deposit crud on fuel rods and measure the resulting flux depression. A bundle of eight tests rods was irradiated for 349 full power days under PWR water chemistry and thermal-hydraulic conditions. On -line, in-core instrumentation included a diameter gauge to detect crud deposition, and neutron detectors and coolant thermocouples to detect flux/power depressions. The test was conducted over three reactor cycles. In the fi rst cycle, the mass evaporation rate (steaming rate) of the coolant on the fuel clad was similar to that calculated for PWRs that have experienced AOA. No crud was deposited during the cycle. For the second cycle, several steps were undertaken to increase Fe and Ni levels in the loop coolant to accelerate crud deposition, including operation for periods at low pH300C (6.5) and injection of an Fe-Ni-EDTA solution. These methods did not result in crud deposition, and towards the end of the cycle both the test fuel power and coolant temperature were increased, with the result that the mass evaporation rate was doubled. These changes resulted in deposition of a crud layer of 15 - 20 μm. During the third cycle, the neutron flux in the upper section of the fuel bundle decreased with respect to both the power in the lower section of the fuel bundle and to the power in neighbouring test rigs. Corroborating evidence of entrained boron on the fuel surface came at shutdown at the end of the cycle. Significant lithium return was measured in the loop coolant, similar to measurements experienced at commercial reactors that have experienced AOA. It was concluded that these effects were due to incorporation of boron into the crud layer ; i.e. an AOA effect. (Author)