气载核裂变产物131I的准确测量方法研究

As an important signal radionuclide for detecting nuclear events and one of the major radiation exposure sources in nuclear accidents, the airborne nuclear fission product 131I monitor is an important content of radioactive nuclide monitor surrounding the nuclear facilities. However, there are still two problems need to be solved in the existing measurement method for airborne 131I: 1) the distribution of 131I in the airborne 131I sample is uneven, which brings great difficulty to accurate measurement; 2) The existing calibration method using 133Ba as substitute could bring relative high system measuring error, therefore new calibration method using 131I itself as reference nuclide should be developed. To address the two problems, this research will be conducted in two directions: 1) to study the accurate measuring method for uneven airborne 131I sample using HPGe, with the Monte Carlo simulation method and the deconvolution algorithm; 2) to study the method of preparing three forms of airborne 131I standard samples, with serials of chemical experiments to optimize the reaction conditions. Compare with the existing studies, this research will solve the problem caused by the complex distribution of 131I in the sample, and provide calibration standard for the other measuring method, to get rid of the calibration method using 133Ba as substitute, which will be helpful for ensuring unity of the measuring results between different methods. The expected results of this study will have practical value and a wide range of application prospects especially under the background of rapid development of nuclear energy in our country.

气载核裂变产物131I是核事故的重要警戒核素，也是核事故情况下辐射剂量的主要来源，是核设施周围环境重点监测对象。然而，气载131I的测量方法目前还存在两大问题：1）气载131I采样样品内131I分布不均匀，给准确测量带来较大困难；2）现存的133Ba刻度方法会带来较大测量偏差，需建立基于131I标准样品的刻度方法。本项目针对上述两个问题，主要开展两个方向的研究：1）研究低本底高纯锗γ谱仪对非均匀气载131I采样样品的准确测量方法，主要利用蒙特卡洛模拟与反卷积算法；2）研究三种形态气载131I采样标准样品制备方法，主要利用系列的化学实验优化反应条件。与现有131I测量研究相比，本研究将直接解决复杂分布下的131I测量问题，并为其它测量方法提供刻度标准，帮助其摆脱对133Ba刻度的依赖，有助于保证气载131I的量值统一。其预期成果在我国核能迅猛发展背景下，具有很强的实用价值和广泛的应用前景。

UNSCEAR指出，气载核裂变产物131I是切尔诺贝利核事故与福岛核事故中，公众辐射暴露的主要来源之一。气载131I的准确测量技术对于预警核事故、评估核事故等级、估算公众及相关工作人员的辐射暴露量至关重要。.项目专注于两个主题：1）三种形态气载131I（元素碘、甲基碘、微粒碘）及其采样样品的制备技术；2）非均匀分布气载131I 采样样品的准确测量方法。本项目研制了一种放射性气态元素碘的制备技术，在水溶液中用Fe2(SO4)3氧化Na131I。采用田口方法优化条件参数，5分钟产率可达83%，约20分钟到达产率平台93%，相较原有技术有较大突破。基于同位素交换原理，实现了甲基碘制备技术，并推导了其理论产率的理论模型。根据理论模型，设计了理论产率达100%的甲基碘制备方案。基于隔室模型，解释气载放射性碘在碘室内的行为规律，并据此建立了三种采样样品的制备方法，可根据规划制备对应活度的样品。针对非均匀分布气载131I 采样样品的测量，首次提出利用双面测量法解决其非均匀分布带来的测量问题。并引入蒙特卡罗模拟、数值求解算法、三次Hermite插值法等技术求解双面测量法建立的超越方程。验证实验结果表明，双面测量法测量结果准确可靠，当双面计数率统计涨落小于1%时，测量结果的标准不确定度在3%以内。.双面测量法满足了分布非均匀的气载131I 采样样品准确测量的需要。气载131I采样样品可作为气载放射性碘测量装置的刻度和实验室间比对所需的量值传递标准，在本研究前没有合格的量值传递标准。三种形态气载131I制备能力与碘室是一个基础科研工具，可为很多气载131I研究提供测试平台。基于项目研究成果，依托单位已经建立了全国首个关于气载放射性碘监测仪的社会公用计量标准，并对外提供了仪器校准服务，表明该项目成果已有较好的应用。