Development of a criterion for assessment of fuel washout during operation of WWER power units

Fuel failures may occur during operation of nuclear power plants. One of the possible and most severe consequences of a fuel failure is that fuel may be washed out from the leaking fuel rod into the coolant. Reliable detection of fuel washout is important for handling of leaking fuel assemblies after irradiation is over. Detection of fuel washout is achievable in the framework of coolant activity evaluation during reactor operation. For this purpose, 134I activity is historically used in WWER power units. However, observed 134I activity may increase during operation even if leaking fuel in the core is absent, and fuel deposits are the only source of the fission products release. The paper describes a criterion which enables to reveal the cases when the increase in 134I activity results from the fuel washout from the leaking fuel rods during operation of the WWER-type reactor. Some examples of applications at nuclear power plants are discussed.

Study of corrosion product activity due to non-linearly rising corrosion rates coupled with pH effects for long-term operating cycles in pressurized water reactors

This work is a study of changes in coolant activity due to corrosion products of extended burn-up cycles of 18-24 months duration, in a typical pressurized water reactor, under pH and boric acid variations. It deals with non-linearly changing corrosion rates coupled with pH effects. The CPAIR-P computer program was modified to accommodate for time-dependent rising corrosion and effects of coolant chemistry. These simulations suggest that the effect of an increase in pH value for an extended 24-month cycle on a specific activity, in the form of a decrease in the said activity, is smeared by the rising corrosion. The new saturation values for activity at the end-of-cycle are lower than with a reactor operated at constant low pH/natural boric acid in the coolant. For a non-linear rise in the corrosion rate coupled with a pH rise from 6.9 to 7.4 and the use of enriched boric acid (30%-40%), coolant activity first rises to a peak value during the cycle and then approaches a much smaller saturation value at the end of the cycle, when compared with the activity for the system having a constant low pH value (6.9) in the coolant. In this work, we have shown that the use of enriched boric acid as a chemical shim actually lowers primary coolant activity when higher pH values, rather than natural boric acid, are employed in the coolant. For multiple long-term operating cycles, the saturation value of corrosion product activity increases in the first two cycles and becomes constant in subsequent ones, due to the high operational pH value for enriched boric acid (40%) as a chemical shim.