HTGR Knowledge Base

Conference Article: Modelling of caesium deposition in CAGR reactor circuits

Phillips, M.E. (Central Electricity Generating Board, Berkeley Nuclear Laboratories (United Kingdom))

Abstract

A laboratory rig at BNL has been used to measure the deposition of radiocaesium in a CO2 gas stream onto steel surfaces in a thermal gradient furnace. The results of these measurements show that at high temperatures the deposition can be described by a model in which caesium diffuses into and out of the oxide film on the steel surface, with an evaporation boundary condition at the gas-oxide interface. A simple version of the model gives the deposition rate (dN)/dt = ngAot1/2 where ng is the concentration of caesium in the gas and Ao, the 'absorptivity', is a property of the surface. At low temperatures (below about 300 deg. C), caesium was retained by the surface. More recently, the experimental programme has been extended to cover deposition on graphite surfaces. Results show that significant retention takes place at temperatures typical of the cooler region of a CAGR circuit, although desorption is rapid at higher temperatures. A computer programme, CAESAR3, has been written to apply this model to the circuit of a CAGR. The programme handles a flexible set of parameters and can accommodate changing temperatures, pressures, flow rates, leak rates and source terms in a circuit which may have two interconnected branches. Measurements have been made of caesium distributions in the boilers of the Windscale prototype AGR and in the Hinkley Point 'B' CAGR. The results obtained are broadly in agreement with the model, although there are a number of apparent differences. Possible reasons for these are discussed.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Specialists' meeting on fission product release and transport in gas-cooled reactors Berkeley (United Kingdom) 22-25 Oct 1985
International Atomic Energy Agency, International Working Group on Gas-Cooled Reactors, Vienna (Austria)
IWGGCR--13, pp:299-317