A scanning gamma-ray detector located deposits of radio-labelled magnetite and sodium and bromine salt contaminants in water fed to an electrically heated serpentine tube replicating one of those in the boilers of Wylfa Nuclear Power Station. Magnetite deposited mainly in the economiser and evaporator initially near the entrance but later spreading further into the boiler. Soluble salts were found mostly in the dryout zone but also in the superheater and, under certain conditions, in parts of the evaporator. Sodium salts were carried through stoichiometrically, but with magnesium salts the anion was carried through in excess. Disodium phosphate added in suitable quantities was found to suppress internal corrosion.

The behaviour of acid, neutral and alkali feedwater contaminants, which may be present in the AGR's (Advanced Gas Cooled Reactor) boilers has been examined in a test loop operating at 14.5 MPa. It has been found that deposition occurred at all contamination levels tested (1.5 μg kg-1 to 3200 μg kg-1 sodium) and could be attributed to two processes: a chemisorption reaction occurring between the steam and the dry tube walls, and at higher contamination levels, a gross deposition mechanism in the region of final dryout which experimental evidence suggests was associated with liquid film dryout.

No abstract

The chemical, thermal and hydrodynamic conditions present on the waterside of a once-through nuclear boiler operating under steady conditions at a pressure of 14.5 MPa have been studied in a recirculating test loop operating at plant relevant conditions. By means of a highly sensitive scanning gamma-ray detector system and the use of radio-labelled sodium chloride, together with conventional hydrothermal instrumentation a comprehensive description of the waterside environment has been obtained. A previously unsuspected form of sodium hold-up in the test-section has been observed and attributed to an adsorption process. The amount of sodium adsorbed has been found to be dependent upon the dry area of the tube available, the temperature of the metal and the ratio of sodium chloride to hydrogen chloride in the working fluid.