Abstract
Chemical revaporisation or physical resuspension of
fission product deposits from the
primary circuit is now recognised to be a major source
term in the late phase of severe
fuel degradation in a nuclear accident. These results
come from tests carried out under
different experimental projects in the European
Commission (EC) Framework
Programmes. These include the revaporisation tests
carried out at the Transuranium
Institute (ITU), Karlsruhe under the Fourth Framework
Programme (FP4), the Phébus FP
post-test analysis (PTA) programme that examined FPT1,
FPT3 and FPT4 deposits in
separate-effect tests as well as EXSI-PC tests carried
out at VTT, Espoo.
The first tests at ITU and VTT concentrated on the
behaviour of caesium as a very
important fission product; this has helped detailed
interpretation of the integral Phébus
FP tests and has clarified some puzzling observations.
Testing with Phébus FPT1 and FPT4
deposits at ITU demonstrated that revaporisation is a
likely, rather than a possible,
phenomenon with a severely degrading bundle. They have
also shown that any changes in
temperature (substrate or gas), flow rate or atmosphere
composition or pressure can lead
to the volatilisation or removal of the deposited
caesium. Cs was particularly easy to
follow given the high activity levels of Cs in the
deposit. However further analysis of the
deposits shows that other fission products are also
subject to revaporisation. In the most
recent FPT3 test chemical analysis of the filters has
enabled examination of other fission
products and demonstrated that these can be equally
active in such conditions.
Further separate-effect tests in the EXSI-PC facility
have also given further insight as to
the chemical reactions that major fission products (e.g.
Cs, I) undergo under steam flows.
A significant fraction of iodine was observed to be
released and transported in gaseous
form at rather low circuit temperatures. Pure theoretical
approaches are also used at
IRSN to exhibit the mechanisms of interaction of iodine
and caesium vapours with
substrates that are representative of the primary circuit
under severe accident
conditions. These approaches are expected to help in
interpreting the above-mentioned
experimental evidence of vaporisation and the forthcoming
results of the ISTP/VERDON 2
experiment conducted by CEA in the eponymous facility
under mixed air and steam
conditions.
These studies will enable a much improved understanding
of the important chemical
interactions in the primary circuit and so permit a more
accurate simulation (with an
improved SOPHAEROS databank) of the primary circuit
chemistry in a severe nuclear
accident, and hence help to reduce the uncertainties in
estimating the possible source
term to the environment
Original language | English |
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Pages (from-to) | 208-223 |
Journal | Annals of Nuclear Energy |
Volume | 74 |
DOIs | |
Publication status | Published - 2013 |
MoE publication type | A1 Journal article-refereed |
Event | 6th European Review meeting on Severe Accident Research, ERMSAR-2013 - Avignon, France Duration: 2 Oct 2013 → 4 Oct 2013 Conference number: 6 |