Revaporisation issues: An Overview

The SARNET programme is investigating revaporisation under the Source Term topic (WP15). Revaporisation of already deposited radioactive fission products can considerably increase fission product release if the primary circuit is then breached. The Phebus Revaporisation project explained certain phenomena of the FPT1 test and showed Cs deposits can rapidly revaporise. The similarity between the behaviour of pure CsOH deposits (tested at VTT) and the mixed radioactive FPT1 deposits (tested at ITU) indicated that CsOH could be a principal species in the reactor accident case. The revaporisation of fission products deposited onto different surfaces in a hot-cell fission-product release experiment has also been investigated at AECL. The fission products were deposited on coupons of CANDU-typical materials (carbon steel, Inconel 600, Zircaloy-4, Zr-2.5Nb, and stainless steel alloys 410 and 403) at temperatures between 160°C and 600°C. Deposits were heated at 700°C for 1800 s in either Ar/H2 or air environments and revaporisation occurred for Ru, Cs & Te. UJV used the REVAP FPT1 and VTT single species data with SOPHAEROS/ASTEC to model revaporisation. The extended Material Database (a part of ASTEC, about 800 species) was mostly used. The Database was checked first using the single species VTT tests. The calculated CsOH revaporisation in the VTT experiments was too fast, that of CsI was in agreement with the experiment. Possibly, the saturated vapour pressure for CsOH is too high in the Database in ASTEC V1.3; therefore the MELCOR 1.8.5 data with lower CsOH saturation pressure will be used in the VTT and the FPT1 calculations with ASTEC V1.3. The ASTEC V1.3 version was used to analyse the Phebus FPT1 sample revaporisation. Whole experiment calculations with ASTEC indicated the non-volatile Cs2MoO4 being the main Cs-deposited species, the REVAP experiment indicated a more volatile form like CsOH or CsI. Fortum has implemented a VTT revaporisation model into the severe accident simulation code APROS SA. Simulations on the loop part of PHEBUS FP in FPT1 showed very rapid revaporisation of deposited material from the hot leg. The simulations slightly overestimated the final deposition in the SG part. In the cold leg the simulations resulted in deposited fractions that were clearly too high.