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.
Simulation of corrosion product activity in pressurized water reactors under flow rate transients
Abstract Simulation of coolant activation due to corrosion products and impurities in a typical pressurized water reactor has been done under flow rate transients. Employing 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 99Mo show that the specific activity in primary loop approaches equilibrium value under normal operating conditions fairly rapidly. Predominant corrosion product activity is due to Mn-56. Parametric studies at full power for various ramp decreases in flow rate show initial decline in the activity and then a gradual rise to relatively higher saturation values. The minimum value and the time taken to reach the minima are strong functions of the slope of linear decrease in flow rate. In the second part flow rate coastdown was allowed to occur at different flow half-times. The reactor scram was initiated at 90% of the normal flow rate. The results show that the specific activity decreases and the rate of decrease depends on pump half time and the reactor scram conditions.
The chemical states of fission-product iodine emanating into a high temperature aqueous environment
The chemical states of iodine in an aqeous environment, at very low concentrations and at temperatures of 1000 to 1300°C, have been investigated using iodine produced by neutron fission of U, UO2 and U3O8. The results show that the final forms of iodine depend on the chemical nature of the environment, being largely elemental (I0) in a neutral or oxidizing atmosphere and largely I− in a reducing atmosphere. Only the reaction I + H2 = HI + H can account for the extent and rate of formation of HI observed when tracer-level iodine is released into steam containing hydrogen. Methyl iodide is produced by reactions between atomic iodine and impurities in the system. All experimental results can be explained on the basis of known iodine states.