Research On Several Problems Of Radon Release Rate Measurement

In recent years, the widespread use of mixed slag timbering make the average indoor radon level increased. In China, millions of houses may be excessive radon concentration calculated by the Ministry of Construction has just concluded investigation on the typical urban annual average indoor radon concentrations over 100Bqm-3 (WHO up to the minute limit) houses ratio of 3.7%, it is a matter of people's livelihood and public health major social problems. Radon is a preventable and can be controlled environmental factor. The protection and mitigation on construction work are the key and low-cost technical measures to reducing indoor radon. National standard " Indoor environmental pollution control criterion of civil architecture engineering (2010 Edition)"-GB50325-2010 clearly stipulates the radon exhalation rate from the surface of building materials and soil surface must be measured. Research on fast and reliable method for measuring radon exhalation rate is of great significance for measuring the radon exhalation rate and control of soil and building materials and the limiting the radon exhalation rate of building materials. The increase of indoor radon concentration can be controlled from the headtream.We analyze the development status of the mainstream measurement method of radon exhalation rate at home and abroad. Generally, the radon exhalation rate of the building materials' surface is only several mBq.m^-2.s^-1. We consider that the detection limit of open-loop measurement of radon exhalation rate is too high by semi-quantitative calculation, the accumulation chamber technique for measuring radon exhalation rate is more appropriate. There are many ways in accumulation chamber technique. For fast measurement, select the radon monitor which can obtain the radon concentration data in real time base on the electrostatic collection method to measure the evolution of the radon concentration in the accumulation chamber versus the time, consider the effects of leakage and back-diffusion, thus the radon exhalation rate can be quickly obtained easily. The accuracy of radon exhalation rate is determined by the accuracy of radon concentration measurement. The intercomparison results of the“The intercomparison of indoor radon measurement”conducted by the radon laboratory of South China University in 2003 November show that: comparing various types of the continuous radon monitor, the minimum deviation is the measured value of RAD7 from the reference value and its relative standard deviation from the average value of repetition measurement is small. So, we chose RAD7 based on electrostatic collection method as our main instrument for measuring radon concentration to carry out this study. Qiu Soukan and Liu Xiaosong first proposed that the non-equilibrium correction for the ²¹⁸Po concentrations and the radon concentration must be considered when the radon monitor based on electrostatic collection method such as RAD7 was used to perform the radon exhalation rate measurement, and they gave a correction empirical formula according to their measurement conditions and parameters. However, the empirical formula can only be applied under certain measurement conditions. It is not universal.We performed the approximate mathematical analysis on the measurement principle of the radon monitors based on electrostatic collection method such as RAD7. We found that they are mainly applied to the long-term environment average radon concentration measurement; tens of minute delay are needed for measuring even for a constant radon concentration (40-minute minutes delays are needed while using a large drying tube, 20-minute delays are needed while using a small drying tube); RAD7 also made a smooth treatment for measuring the variational radon concentration. In other words, the radon monitors based on electrostatic collection method are inapplicability for tracing the changes of radon concentration. For simplifying analysis, we only use a small drying tube to dry air. Two modified algorithms were proposed for quickly tracing the changes of radon concentration.(1) When the pump flow rate is high, the volume delay is negligible, we can be similar consider that the radon concentration in the internal cell synchronize with that of environment. We use a 0th order approximation to describe the evolution of the radon concentration in the internal cell and obtain the non-equilibrium correction general method for the ²¹⁸Po concentration and the radon concentration from the radioactive decay equation;(2) When the pump flow rate is small, the volume delay cannot be ignored, we use a 0th order approximation to describe the evolution of the radon concentration in the environment, that is, the radon concentration is a constant in each measurement period. We obtain the general method for correcting the volume delay and the non-equilibrium of the ²¹⁸Po concentration and the radon concentration at the same time. Experiments show that both correction methods are able to track the rapid changes of radon concentration. Although the actual radon concentration data can be recursive by the two kind correction methods using the radon concentration data sequence measured by RAD7, and then the radon exhalation rate can be obtained by nonlinear least squares fitting. However, the calculation is more complicated.Using of small drying tube, we obtain the radon concentration variation rule of two-chamber form the radon mixing principle between the accumulation chamber and the internal cell of the radon monitor based on the electrostatic collection method under the action of the pump, taking into account the non-equilibrium correction of ²¹⁸Po, then the quickly and reliable measurement method of the radon exhalation rate is obtained. The accurately radon exhalation rate can be obtained by only once nonlinear least squares fitting the measurement data sequence. The general model for measuring radon exhalation rate is simplified, and the simplified conditions have been obtained. Experiments show that the measurement method of radon exhalation rate is fast and accurate. The error sources of the theoretical model and experimental are analyzed, and the improved direction is proposed.