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Analysis of the revaporisation behaviour of radioactive deposits of fission products in non-stationary thermal conditions and constant atmosphere
K. Knebel, P.D.W. Bottomley, V.V. Rondinella, A. Auvinen and J. Jokiniemi

In case of a severe accident in a nuclear reactor, the revaporisation of fission products can have a major effect on the late phase release source term. An experimental revaporisation facility was built in the Institute for Transuranium Elements to study this phenomenon with sample of fission product deposits originating from the upper vertical line of the Phébus FP experimental reactor. The PHEBUS reactor core was equipped with a test loop which allowed investigating fuel rod bundle degradation and melting behaviour. Such tests promoted the release of fission products, particularly 137Cs, and their transport/deposition through the primary circuit of the loop.

The revaporisation facility consists of a compact high temperature furnace in which the deposit samples can be heated up to 1273 K under controlled atmosphere conditions simulating the late phase of a severe accident. The revaporisation behaviour of the 137Cs deposited on the sample is scrutinized by means of an on-line gamma spectroscopy system.

In the current experimental campaign, three samples from the Phébus FPT3 experiment were investigated and first heated up by 10 K/min to 573 K in a nitrogen atmosphere. In a second step, each sample was analysed with a specific carrier gas (i.e. hydrogen, steam or air) and heated further by 2 K/min to 1273 K. For the last experiment in air, measures were also adopted to collect samples of the FPs after revaporisation and their subsequent recondensation as aerosols for further analysis via Transmission Electron Microscopy. Revaporisation results deduced from the on-line measurement showed very similar behaviour in terms of starting temperature and kinetics for either steam or air. This indicates that the variation in oxygen potential between steam and air does not affect so much the reaction kinetics (and the active Cs species is not changed). The experiment with hydrogen showed a far higher starting temperature and slower kinetics. This suggests that in a nuclear accident fission products under reducing conditions would form deposits up to rather high temperatures; the deposits may be remobilized at lower temperatures under oxidising conditions.

Keywords: Revaporisation, fission product, release source term, caesium, nuclear aerosols, phébus FP, severe accident, gamma-ray spectroscopy

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