ERMSAR-07 –FZK June07- Karlsruhe Revaporisation issues: an Overview D. Bottomley 1,.R.S. Dickson 2, T. Routamo 3, J. Dienstbier 4, A. Auvinen 5, & N. Girault 6 1 JRC-ITU Karlsruhe (DE) 2 Chalk River Laboratories, AECL (Canada) 3 Fortum Nuclear Services Ltd. (FIN) 4 UJV Rez (CZ) 5 VTT Espoo (FIN) 6 IRSN Cadarache (FR) Presented by D..Bottomley - ITU

ERMSAR-07 –FZK June07- Karlsruhe Introduction- current status Experiments on Revaporisation –single effects & Phebus (VTT, ITU & IRSN) Modelling of revaporisation – SET’s, Integral tests (UJV & Fortum) Use in severe accident analysis Advances & future outlook Outline of Talk

ERMSAR-07 –FZK June07- Karlsruhe Revaporisation –a simple illustration CsOH (g) or (CsOH) 2 (g) CsOH (ads) 1) Diffusion in oxide layer along grain boundaries and cracks during deposition Metal substrate Cracks, defects in oxide Outer Oxide 2) Diffusion out of oxide layer and cracks during revaporisation along with direct desorption Gas flow with changing T, Cp, composition, flow rate CsOH (gb) CsOH (ads) Chemical reactions with other compounds in the deposit possible

ERMSAR-07 –FZK June07- Karlsruhe Revaporisation : current status Fission products can rapidly deposit upon cooler surfaces but under various changes in conditions can cause the deposited fission products (eg. Cs & I) volatilise again Revaporisation was initially anticipated from decay heat reheating of deposits, but changes in T, flow rate, or gas composition (with changes in C p or pO 2 ) are also causes. Revaporisation was recognised as a source term factor in the late phase and an especially dominant risk with the occurrence of a by-pass (eg. late stage steam generator rupture). The Phebus REVAP project (FP5 Programme) had demonstrated the phenomenon for simple Cs deposits & for more complex deposits in steam and showed that it could be triggered with simple temperature or atmosphere variation. Also SETs from AEAT showed original deposition conditions greatly affected the extent of, & susceptibility to revaporisation. Understanding the revaporisation mechanisms both experimentally & from a modelling standpoint was important.

ERMSAR-07 –FZK June07- Karlsruhe Phebus FPT1 : Revaporisation of vertical line deposits Fig. a Revaporisation device with lead-shielded oven, diluter & filter with  -detector above glove-box Fig. b Diagram showing gas flow of Revaporisation device at ITU

ERMSAR-07 –FZK June07- Karlsruhe Phebus - Revaporisation testing of deposits from FPT1 vertical line & FPT4 shroud Revaporisation of fission product deposits (loss of Cs-137 activity at 661keV) upto 1000°C from different substrates of FPT1 (stainless steel vertical line) and FPT4 porous ThO 2 shroud. Revaporisation from steel surfaces is rapid above 550C, it is slower and at higher temperatures from porous ceramic

ERMSAR-07 –FZK June07- Karlsruhe Power level, hot leg Cs-137 activities and H 2 production during FPT2 test. Phébus FPT2 test profile: Revaporisation noted during the results evaluation Note the final decrease in hot leg activity after reactor shutdown

ERMSAR-07 –FZK June07- Karlsruhe Schematic Diagram of AECL HCE Facility HCE3 Hot Cell apparatus used for the revaporisation testing of the deposits derived from sheathed CANDU fuel segments

ERMSAR-07 –FZK June07- Karlsruhe AECL Revaporisation testing Vaporisation & Deposition six characteristic CANDU materials used: Zircaloy 4, Zr-2.5Nb, stainless steel Types 403 and 410, Inconel-600 and carbon steel, were exposed to an air, steam, or steam + Ar stream in a ss thermal gradient tube downstream of the fuel sample at °C. XPS Spectroscopy showed Sn, Cs and Mo, with lesser amounts of Rb, Te and U in all coupon deposits. Revaporisation Coupon was heated to 700°C for 1800 s in a slow gas stream Air-deposited coupons were revaporised in air. steam or steam + argon-deposited coupons were revaporized in Ar-2% H 2. Post test counting (after 80 days to reduce background levels) detected 134 Cs, 137 Cs and 129m Te, with 103 Ru and 106 Ru in air & steam tests, and 110m Ag detected in all three gas flows. Changes in isotope levels were consistent for an element. This indicates that the Cs & Ru revaporisation occurs in oxidising environments & Ag revaporisation in all atmospheres.

ERMSAR-07 –FZK June07- Karlsruhe VTT forthcoming revaporisation testing facility Objective: to determine fission product compounds formed by reactions on primary circuit piping surfaces. The sampling system will be identical to that of CHIP facility - facilitating comparison of the results Sampling is designed to avoid losses due to 1) aerosol deposition or 2) vapour condensation as the sample cools from 1000°C to 150°C. Size distribution and concentration of aerosol particles as well as chemical speciation of gaseous species are measured on-line. Compounds transported as aerosol particles at high and at low temperature can be differentiated from the vapour phase and gaseous species During 2007 the experimental facilities will be constructed and tested. Gaseous iodine in the sampling lines and in the filters will be studied by radiotracers in 1 st five experiments with the release of iodine by revaporisation of CsI sample in the 6 th (final) test of Samples containing compounds of boron, molybdenum, tellurium and uranium will be examined in the following years (2008/10).

ERMSAR-07 –FZK June07- Karlsruhe Modelling of VTT single effect tests by UJV Réz (ASTEC v1.3) a) VTT REVAP test 7 Calculated sample mass b) VTT REVAP test 7 Measured pulse rate Experimental Method CsOH solution or CsI powder labelled with 134 Cs or 137 Cs were put on type 304 stainless steel tray inside a furnace tube, dried in N2 flow at lower temperatures, then vaporized in steam flow. The temperature was gradually increased to 800C for CsI and to 1000C for CsOH. Gamma scanning was performed along the experimental tube and in time. Comparison of model with test

ERMSAR-07 –FZK June07- Karlsruhe Modelling of VTT single effect tests: UJV Réz (ASTEC v1.3) Results CsI A relatively good agreement between the test and the calculation was found for all tests with CsI, see test 7 CsOH Vaporisation is slightly slower in the final phase of the test, after temperature rise beyond 700C. However the CsOH modelling tended to be too fast in this final phase. Values found for CsOH partial vapour pressure varied by 100x – a probable source of inaccuracy for the modellers

ERMSAR-07 –FZK June07- Karlsruhe Fortum engineering & VTT – Severe Accident Modelling (APROS SA) The circuit of the PHEBUS FP facility in FPT1 was calculated. The revaporisation model is based on the work by VTT and the revaporating mass flux J [kg/s] is calculated from: where: p S = saturation vapour pressure [Pa] M = molar mass of vapour[kg/mol] Q = volumetric flow rate of gas [m 3 /s] T = temperature [K] R= gas constant = [J K-1 mol-1] D = binary diffusion coefficient [m 2 /s] S = area of revaporisation surface[m 2 ] d h = hydraulic diameter of tube [m] Sh= Sherwood number. Size class Small (0.25 µm) Medium (0.84 µm) Large (3.4 µm) Iodine37 %39 %24 % Caesium6 %21 %73 % Tellurium44 %37 %19 % The following input size distribution (as mass fractions) was used for the modeled FP groups for modelling of FPT1.

ERMSAR-07 –FZK June07- Karlsruhe APROS SA Modelling of Phebus FPT1 Vertical steam generator U-tube diagram and nodalisation. (Hot leg has 3 nodes and the cold leg has 2 nodes)

ERMSAR-07 –FZK June07- Karlsruhe Distribution of iodine, caesium and tellurium in FPT1 and in APROS SA calculations + revaporisation + sedimentation options Calculation caseABCD RevaporisationYesNoYesNo SedimentationYes No All: deposited material was totally removed from the hot leg surfaces when the revaporisation model was turned on (A & C) – close to exptl. results Cs: when no revaporisation model, sedimentation active or not makes a significant difference (B & D). Cold leg deposits also over-estimated (OUT is the fraction passing through the system)

ERMSAR-07 –FZK June07- Karlsruhe 1)The REVAP circle have shown revaporisation can have a considerable source term impact as frequently these surface deposits are in equilibrium with the vapour. Thus changes in temperature, flow rate or atmosphere can result in a rapid revaporisation of Cs. Experimental 2)CsOH and Phebus FPT1 deposits (VTT & ITU testing) show Cs revaporises in steam atmospheres similarly to CsOH & explains the caesium mobility observed in the Phébus FPT1. 3) Cs revaporisation was also observed by IRSN after the FPT2 shutdown (it is a late phase risk). 4) Revaporisation can occur under different atmospheres and substrates. Cs oxide/hydroxide deposits on steel can revaporise under reducing, steam and inert atmospheres. Ceramic substrates show a slower and reduced release - probably with slower diffusion up through the pores. 5) AECL & FPT4 data also show that other major fission products such as Ru, Te or Mo revaporise as does Cs under various atmospheres. Modelling 6) Modelling by UJV has been able to reproduce the results for CsOH (single & mixed) deposits so that the Cs model appears to be correct. However some of the Cs vapour pressure data may not be very accurate. Cs 2 MoO 4 could also be a potential transport species. 9) The inclusion of revaporisation model by FORTUM in a full severe accident (SA) code has confirmed the Phebus FPT1 results. Cs fractional release beyond the steam generator is increased with all deposits in the hot leg (& upstream) being revaporised. Conclusions

ERMSAR-07 –FZK June07- Karlsruhe More complex scenarios will be difficult to resolve within this programme. Both integral & single effect testing will still be necessary. Particularly interactions with resuspension, thermophoresis & diffusiophoresis mechanisms (especially with heavy deposits) need to be understood to properly estimate the source term impact. Outlook