Oxygen Addition to the Annulus Gas Systems of Pickering NGS A, Units 3 and 4

Abstract

An Oxygen (O₂) addition system has been designed and installed in conjunction with the Annulus Gas System (AGS) of Pickering NGS A, Unit 3. It has been operational since January 1989. One of the short term purposes of O₂ addition is to remove Carbon-14 (C-14) deposits on pressure tube (PT) surfaces through oxidation to carbon (14) dioxide which is subsequently removed by absorption/chemical reaction in a calcium hydroxide bed. Removal of C-14 deposits is necessary to minimize the occupational dose to workers during retubing.

Experience to date has shown satisfactory operation of the O₂ addition system for Unit 3 AGS, which has provided controlled O₂ concentrations up to 10 percent (V/V). The O₂ bottle supply system has involved significant handling requirements by the station. This will be minimized for Unit 4 where a liquid O₂ tank has been selected for the supply. The Unit 4 O₂ addition system is otherwise similar to that for Unit 3 except that lower O₂ addition rates are accommodated. This reflects the fact that O₂ addition for Unit 3 is designed for purge operation only whereas that for Unit 4 is designed for both purge and recirculation operation. The latter mode is required for maintaining an impervious oxide layer on the surface of the PTs when satisfactory C-14 decontamination has been achieved. It is considered that this minimizes D₂ ingress aid hence the potential for PT hydriding. Unit 4 is scheduled to shut down for retubing in May 1991.

Throughout the design it has been ensured that the leak detection capability of the AGS is not impaired by O₂ addition.