反应堆石墨中14C生成及滞留机制的分析和探讨

摘要

14C是退役石墨中最重要的核素,半衰期为5730 a,处理处置过程需要重点关注.石墨中14C生成机制复杂,主要来源于13C、14N、17O分别与中子发生的三个反应:13C(n,γ)14C、14N(n,p)14C、17O(n,α)14C,重水研究堆石墨中由14N(n,p)反应生成的14C约占14C总量的90%以上.本文通过理论计算和实验比较方法对反应堆热柱石墨中14C生成及滞留机制进行了分析和探讨,得出了石墨中N来源于杂质N和孔隙N2,后者生成的14C以热原子形式存在,并迅速与O2结合生成14CO2,发现了反应堆停闭后石墨孔隙闭孔中依然存在14CO2,并估算了以孔隙14CO2形式存在的14C约占石墨中14C总量的30%.%Background:Having a half-life of 5730 a,14C is the radionuclide of greatest concern in nuclear graphite, which arises principally through the interaction of reactor neutrons with nitrogenvia14N(n, p) reaction.Purpose: This study aims to propose a mechanism for the production and retention of14C in graphite and needs to be verified. Methods: The specific activities for14C have been calculated and experimentally determined. Nitrogen exists in heavy water research reactor (HWRR) graphite in two forms: impurity N and interstice N2. A comparison between14C specific activities in ground and unground graphite samples has been done.Results: The14C atoms produced by the interstice N2 usually exist in "hot atom" state and immediately form14CO2, part of which may resides in graphite and the other part may eventually escape to air.Conclusion:A comparison between14C specific activities in ground and unground graphite samples reveals that the interstice14CO2 accounts for 30% approximately of total14C in the thermal column graphite of heavy water research reactor.