氡析出率测量若干问题研究

【作者】 谭延亮；

【导师】 肖德涛；

【作者基本信息】 南华大学 ， 核技术及应用， 2011， 博士

【摘要】 近年来我国掺渣建材的广泛使用使得室内氡平均水平有所提高,按建设部刚刚结束的典型城市室内氡调查得到年均氡浓度超100Bqm^-3（WHO最新限值）房屋比率3.7%计算,在我国可能有数百万套房屋氡浓度超标,这是一个关系到国计民生与公众健康的重大社会问题。氡是一种可预防和可控制的环境因素,建筑工程上的防护与减缓是降低室内氡的关键和低成本的技术措施。国标《民用建筑工程室内环境污染控制规范（2010年版）》-GB50325-2010明确规定对建筑材料表面和土壤表面氡析出率进行测定。对建材氡的析出率进行限定,具有重要意义,可以从源头上控制室内氡浓度增高。研究快速可靠测量氡析出率的方法,对开展土壤及建材氡析出率测量与控制研究提供了技术保障。本文对国内、外氡析出率主流测量方法的发展现状进行了分析,由于建材表面氡析出率一般只有数mBq.m^-2.s^-1,我们通过半定量计算认为开环式氡析出率测量方法探测限过高,累积法测量氡析出率更加合适;累积法测量氡析出率也有很多方法,从快速测量出发,选择能实时得到氡浓度数据的静电收集法测氡仪来测量得到集氡罩内氡浓度的变化规律,考虑泄漏和反扩散效应就可以快速得到氡析出率。氡浓度测量的准确性决定了氡析出率的准确性。2003年11月南华大学氡实验室组织的“室内氡检测的现场比对”试验的结果表明:在参与比对试验的各类连续测氡仪之中,RAD7与参考值的偏差最小,而且其重复测量结果平均值的相对标准偏差也比较小。因此我们选择静电收集法测氡仪RAD7作为我们开展本研究的主要仪器。南华大学丘寿康、刘小松老师首先提出了利用RAD7这类静电收集法测氡仪测量氡析出率必须考虑218Po浓度与氡浓度的非平衡修正,并且根据他们的测量条件和参数,给出了一个修正的经验公式。但是,该经验公式只在特定的测量条件下适用,不是普适的。我们对RAD7这一类静电收集法测氡仪的测量原理进行了近似数学分析,发现其主要适用于对环境长期氡浓度的平均值进行测量;即使对于恒定的氡浓度,其测量也有数十分钟的延迟（使用大干燥管时延迟40分钟,使用小干燥管时延迟20-30分钟）;对于变化的氡浓度,RAD7测量值也做了平滑处理。也就是说静电收集法测氡仪不适用于对变化的氡浓度进行追踪。从简化分析出发,我们只使用小干燥管来干燥空气,提出了两种修正算法来快速跟踪氡的浓度变化:（1）当泵的流率较大时,不考虑体积延迟,近似认为测量腔内的氡浓度与外界氡浓度同步变化,对测量腔内的氡浓度变化采用0阶线性近似并从放射性衰变方程出发得到²¹⁸Po非平衡修正的一般方法;（2）当泵的流率较小时,考虑体积延迟,对外界环境的氡浓度变化进行0阶近似,即外界的氡浓度在每个测量周期内是不变的常数,得到同时进行体积延迟修正和²¹⁸Po非平衡修正的一般方法。实验证明,这两种修正方法都能够较快的追踪氡浓度的变化。虽然用这两种修正方法对RAD7测量得到集氡罩内的氡浓度数据序列进行递推运算,就可以得到集氡罩内的实际氡浓度数据,然后通过对修正后的数据进行非线性拟合,就可以得到氡析出率,但是计算比较繁琐。在使用小干燥管时,我们根据集氡罩与静电收集法测氡仪的测量腔之间在泵的作用下氡的混合原理,得到了双室氡浓度变化规律,并同时考虑²¹⁸Po非平衡修正,得到了快速、可靠的氡析出率测量方法,只需要对静电收集法测氡仪测量得到的氡浓度数据序列进行一次非线性拟合就可以得到准确的氡析出率。对氡析出率测量的一般模型进行了简化,并且得到了简化的条件。通过实验证明了该氡析出率测量方法快速、准确。对理论模型上和实验中的误差来源进行了分析,并提出了改进的方向。更多还原

【Abstract】 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.更多还原