Computational modeling of accelerating corrosion in primary coolants of a typical PWR
High temperature and pressure environment along with large neutron and gamma-ray flux lead to significantly large corrosion in the stainless steel used in the primary coolant circuit of pressurized water reactors (PWRs). The corrosion products become activated in the reactor core upon circulation. Corrosion rate depends on reactor power and it can be a linear as well as a nonlinear function of time. Computer simulation offers a novel approach to study the effect of corrosion acceleration on coolant activity under transient conditions. We present two related mathematical models for corrosion acceleration under high temperature and flow rate transient conditions. In the first part of the work, we have formulated a mathematical model based on a system of coupled first order ordinary differential equations governing the isotopic concentration. In the second part, flow rate transients have been forced through non-homogeneous conditions. Numerical simulations show that for Na-24, Mn-56, Fe-59, Co-60 and Mo-99, the values of specific activities approach the equilibrium value fairly rapidly under normal operating conditions. Spikes are observed in the specific activity values of the primary coolant when corrosion acceleration and flow rate transients are employed in the model. Both the height and time of these spikes has been found to depend on the value of acceleration in corrosion.
Modeling and simulation of corrosion product activity in pressurized water reactors under power perturbations
Abstract An improved methodology is presented for simulation of coolant activation due to corrosion products and impurities in a typical pressurized water reactor (PWR) under power perturbations. Using time dependent production and losses of corrosion products in the primary coolant path an approach has been developed to calculate the coolant specific activity. Results for 24 Na, 56 Mn, 59 Fe, 60 Co and 99 Mo show that the specific activity in primary loop approaches equilibrium value under normal operating conditions fairly rapidly. Predominant corrosion product activity is due to 56 Mn. Flow rate has been assumed to follow power changes and different types of power perturbations are introduced after the equilibrium activity has been achieved. In particular the effects of linear changes in reactor operating power and power peaking on the corrosion product activity of the primary coolant have been studied.
A Model for the Release of Radioactive Krypton, Xenon, and Iodine from Defective UO2 Fuel Elements
The steady-state release of active noble gas and iodine from defective fuel elements is described either in terms of a kinetic or a diffusion model. Both models assume a diffusional release in the fuel. Transport of fission products in the fuel-to-sheath gas is represented either by a first-order rate process or diffusion process, and is characterized with an escape-rate constant or diffusion coefficient, respectively. The kinetic model predicts a release dependence on the decay constant of lambda /sup -1/2/ to lambda /sup 3/2/. The diffusion model predicts a dependence of lambda /sup -1/. Observed release data from inpile loop experiments, for a wide range of defect states, confirm the predictions of the models. A fitting of the model to the measured data yields estimates of the empirical diffusion coefficient in the fuel matrix, and the scape-rate constant or diffusion coefficient in the fuel-to-sheath gap. Evaluation of the fitted parameters enables the various rate-controlling processes to be deduced as a function of the defect size.