پوسیدگی آیروسل های رادیواکتیو در یک جریان محدود به دیوار
Abstract
1- Introduction
2- Methodology
3- Results and discussion
4- Conclusion
References
Abstract
This paper aims at radiation monitoring and activity inversion when radionuclides are transported in duct or a pipe. The Lagrangian trajectory model was established, and a quick numerical method was used to simulate radioactive aerosols transport and radioactive decay in a wall-bounded flow. Total 12 cases were studied in the simulation according to the particle diameter and density. The dimensionless deposition velocity has been validated by comparing with Wood’s predictions and the experienced gravitational settling velocity. The results show that turbophoresis has a significant effect on particle transport in the wall-bounded flow, leading to particles migration and concentrating near the wall or channel center. It will cause particles to move with different velocities in the streamwise direction, so that their residence times in channel will be complexly distributed. In addition, the gravity settling will enhance the disequilibrium of particle distribution. The activity losses are obtained in detail including particle deposition ratio and radioactive decay. One radionuclide with half-life 7 s was employed in simulation, and the activity error between the estimated and initial values is up to about 40%. The activity error decreases if the half-life becomes large. When the half-life is 7.5 and 0.5 times larger than the mean time spent by airflow through the channel if the gravity is considered or not respectively, the final activity error will be less than ±5% in the present simulation setup.
Introduction
The regulatory guide 1.45 published by U.S. Nuclear Regulatory Commission mentions that it is very important to continuously monitor and quantify the reactor coolant leakage for ensuring the safe operation of the facility (Regulatory guide 1.45, 2008). One of the efficient methods to consider for incorporation in the technical specifications is to monitor airborne particulate radioactivity. Some short half-lived nuclides are considered such as 19O, 16N, 13N, 18F and 3 H. During the monitoring, airborne particles are sampled and pumped in pipe lines. According to the requirements by the International Organization for Standardization (ISO), a representative sample is necessary in monitoring the activity concentrations of radioactive substances (ISO, 2889). The losses of aerosol particles in the transport lines due to particle deposition therefore needs to be determined. The depositions during particle transport in a turbulent flow include gravitational settling and sedimentations on the walls caused by Brownian diffusion, turbophoresis, thermophoresis and other factors.