^(18)F气溶胶监测仪校准方法研究

Research on Calibration Method of^(18)F Aerosol Monitor

压水堆冷却系统压力边界的泄漏会引发严重安全事故。冷却剂在反应堆高中子通量下会产生^(18)F,在泄漏后形成的放射性气溶胶可使用^(18)F气溶胶监测仪连续监测,以确认泄漏的位置与严重程度;监测仪测量结果的准确性关系到核电站的稳定运行,但目前尚缺乏可用的现场校准方案。本文对^(18)F气溶胶监测仪进行校准方法研究,使用放射性^(18)F标准滤膜源、22 Na及68 Ge/68 Ga标准点源对国内新设计的某型号^(18)F气溶胶监测仪进行计量校准,确定新测量方案下仪器的活度响应,研究不同校准方法对活度响应的影响。利用^(18)F标准滤膜源得到的活度响应的扩展不确定度为6.0%(k=2),满足监测仪校准的准确度要求。利用22 Na及68 Ge/68 Ga标准点源进行的校准研究还表明,正电子发射能谱对该监测仪活度响应的影响最大,应选取与^(18)F能谱接近的长寿命核素作为替代,现场校准推荐使用22 Na点源或标准滤膜源。

To prevent severe safety accidents caused by primary coolant leakage in pressurized water reactors(PWRs),it is necessary to monitor the leakage rate of key parts of the reactor coolant pressure boundary(RCPB)in order to take timely countermeasures.The primary coolant of PWRs is usually high-pressure supercritical water containing boron,which can produce^(18)F in the high neutron flux environment of the reactor.The leak rate of the RCPB can be inferred by measuring the activity concentration of the radioactive^(18)F aerosol produced by the coolant leaking into the containment.A measurable sample source is usually obtained by extracting sample gas containing^(18)F aerosol from the containment and filtering it in the monitor.The measurement method used by the monitor is to screen theγsignal by using theβsignal as the gate signal,which is actually aγrelative measurement method using the coincidence method to reduce the background.The electronic design of the monitor prevents theβ-channel signal from being recorded,which makes it impossible to achieve the absolute measurement as with conventional coincidence methods.To ensure the accuracy of the monitoring results,the^(18)F aerosol monitor needs to be calibrated.In this paper the monitor was calibrated using a standard^(18)F surface source produced by the intermittent spot method on a filter paper base.By means of the decay of the^(18)F source and the automatic recording function of the monitor,the indication number of the monitor was measured at six source activities of different orders of magnitude,i.e.the count of the energy spectrum at the position of the 511 keVγ-window.The data were used to obtain the activity response and uncertainty of the monitor’s readings and to verify the feasibility of such a calibration scheme on the monitor.Two standard point sources of 22 Na and 68 Ge/68 Ga were also used to calibrate the monitor to verify the feasibility of using long-lived nuclides to make alternative sources for calibration.Sets of 511 keVγ-window count data fromγand coincidence channels were recorded to investigate the effect of theγcoincidence additive effect and differentβemission spectra on the activity response,and to determine which long-lived positron nuclides should be used in the field calibration.Calibration with a standard^(18)F surface source shows that the monitor can give stable and reliable readings of this type of standard source over different orders of magnitude within the range.The extended uncertainty(k=2)of the displayed activity response is 6.0%,which meets manufacturing requirements.And the calibration of the long-lived positron nuclide standard point sources shows that the positron spectrum has a great influence on the activity response of the monitor,and theγcoincidence additive effect is not very significant.Therefore,it is recommended to use 22 Na withβenergy close to^(18)F to make point sources or surface sources for field calibration.