The chemistry of the PSGHWR heavy water moderator system

The PSGHWR heavy water moderator and helium overblanket circuits, and the heavy water purification plant are described. Details of the deuterisation and dedeuterisation of ion exchange resin charges for the purification plant are given, and a pattern of consistency over seventeen deuterisations of anion and cation beds is shown. The chemistry specification for the moderator system is given, and the changing philosophy of ion exchange plant operation to maintain this specification, from early operation with continuous flow through cation and anion clean up columns, to the current mode of minimum intermittent flow, is explained. Anionic and cationic impurities and radio nuclides found in the moderator are listed, and their sources examined. The inter-relation between the ion exchange plant as a source as well as a remover of impurities and the part played by peroxide is examined. Radiolysis data is presented and an attempt is made to explain the correlation between radiolysis rate and impurity concentrations, ion exchange column operation and other plant parameters. A soluble poison, enriched 10boron, is used for balancing excess fuel reactivity. The boron cycle and method of control of boron concentration are explained, and experience of anion exchange resin capacity for boron at different input concentrations is given. Experience covering 26 regenerations of the anion beds, mostly using deutero chemicals, and details of the method of recovery of deuteroboric acid and potassium deuteroxide by electrodialysis are given. Finally, heavy water losses and isotopic degradation of D2O during 8 years of operation are examined. (author)