Chemical aspects of fission product transport in the primary circuit of an LWR

The transport and deposition of fission products in the primary circuit of an LWR following a severe accident would be governed by a combination of chemical and thermal-hydraulic effects. Experiments have been undertaken at the UKAEA Atomic Energy Establishment Winfrith to study specific chemical effects in terms of the physico-chemical behaviour of simulant fission products at high temperatures, using resistance furnaces and matrix isolation techniques. The chemical forms of fission product vapours released from simulant fuel mixtures were determined by matrix isolation-infrared and quadrupole mass spectrometry. Reactions of these species with structural materials were assessed in various atmospheres over the temperature range 500 to 1000 deg. C, and any resulting chemical interactions were quantified in terms of a vapour deposition velocity representing the rate constant for the reaction. Various surface analysis techniques have also been developed to study the reaction products. The experimental work on simulant fission products has included studies of caesium iodide, caesium hydroxide, tellurium, strontium oxide and barium oxide, and their reactions with 304 stainless steel, Zircaloy and boric acid. Caesium iodide decomposed in the presence of boric acid at temperatures as low as 100 deg. C to generate hydrogen iodide; caesium hydroxide vapour reacted with 304 stainless steel, and caesium cations were detected in the subsurface layer of chromium oxide adjacent to the metal with no evidence of compound formation; the interaction of tellurium vapour with Zircaloy was characterized in terms of the fractional oxidation of the Zircaloy; and the volatilities of strontium and barium oxides were enhanced in the presence of steam. (author)