Revaporization Behavior of Cesium and Iodine Compounds from Their Deposits in the Steam−Boron Atmosphere

This paper presents our investigation on cesium
and iodine revaporization from cesium iodide (CsI) deposits on
stainless steel type 304L, which were initiated by boron and/or
steam flow. A dedicated basic experimental facility with a thermal
gradient tube (TGT) having a temperature range of 1000−400 K
was used for simulating the phenomena. In the absence of boron, it
was found that the initially deposited CsI at 850 K could be
revaporized as CsI vapor/aerosol or reacted with the carrier gas
and stainless steel (Cr2O3 layer) to form Cs2CrO4. The latter
mechanism consequently released gaseous iodine that was later
accumulated downstream. After introducing boron to the steam
flow, a severe revaporization occurred. This, in addition to the
revaporized CsI vapor/aerosol, was caused by cesium borate (Cs2B4O7 and CsB5O8) formation, which then largely released gaseous
iodine that was capable of reaching the TGT outlet (<400 K). In the case of a nuclear severe accident, our study has demonstrated
that an increase of gaseous iodine in the colder region of a reactor could occur after late release of boron or a subsequent steam flow
after refloods of the reactor, thus posing its inherent risk once leaked to the environment.