Study on production and retention of ¹⁴C in thermal column graphite of HWRR

Abstract 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 nitrogen via ¹⁴N(n, p) reaction. Purpose: This study aims to propose a mechanism for the production and retention of ¹⁴C in graphite and needs to be verified. Methods: The specific activities for ¹⁴C 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 between ¹⁴C specific activities in ground and unground graphite samples has been done. Results: The ¹⁴C atoms produced by the interstice N₂ usually exist in 'hot atom' state and immediately form ¹⁴CO₂, part of which may resides in graphite and the other part may eventually escape to air. Conclusion: A comparison between ¹⁴C specific activities in ground and unground graphite samples reveals that the interstice ¹⁴CO₂ accounts for 30% approximately of total ¹⁴C in the thermal column graphite of heavy water research reactor. (authors)