ERMSAR 2012, Cologne March 21 – 23, 2012 High temperature reaction between UO 2 and sea salt deposit M. Takano 1, T. Nishi 1, M. Kurata 1, M. Amaya 2, F. Nagase 2 1 Nuclear Science and Engineering Directorate 2 Nuclear Safety Research Center Japan Atomic Energy Agency Session “Fukushima and perspectives for SARNET”, paper 7.2

ERMSAR 2012, Cologne March 21 – 23, Contents of talk 1. Purpose of this study 2. Experimental 3. Decomposition and volatilization of salt 4. Reaction in sea salt + UO 2 mixture 5. Reaction at salt / UO 2 pellet interface 6. Summary

ERMSAR 2012, Cologne March 21 – 23, 2012 Salt deposit Seawater feed by fire-pump Solidified fuel melt 1. Purpose of this study 3 Image of salt deposit on the damaged fuel Research works for retrieving the fuel debris in Fukushima Daiichi NPP (1F) have been started — chemical and physical form, mechanical and thermal properties, etc. Peculiar event in the 1F severe accident — seawater was pumped into cores for urgent cooling. Salt deposit may have been formed and reacted with debris — especially on the high temp. surface of crust and corium exposed above water level. High temp. reactivity between salt and U-Zr-O system should be investigated to specify the influence of salt components. In this study, salt–UO 2 system was investigated as the preliminary examination to obtain fundamental data.

ERMSAR 2012, Cologne March 21 – 23, Experimental (1) 4 Preparation of reactants · Sea salt deposit prepared from seawater, dried at 230 °C NaCl/MgCl 2 ·aH 2 O/MgSO 4 ·bH 2 O/CaSO 4 ·cH 2 O/KCl = 87.8/5.8/2.9/1.8/1.7 (mol%) (Detected as NaCl, KMgCl 3 ·6H 2 O, MgSO 4 ·H 2 O, CaSO 4 by XRD) · NaCl as simplified reference a=6, b=1, c=0 · UO 2 powder and sintered pellet (~94%TD) · Salt only · Pelletized mixture of salt+UO 2 · UO 2 pellet in salt bed Schematic of heating system Reaction systems Characterization XRD, ICP/AES, SEM/EDX in Ar or moist Ar+5%H 2 at 815~1198 °C

ERMSAR 2012, Cologne March 21 – 23, Experimental (2) 5 Experimental matrix Specimen Sea salt (pelletized) NaCl (pelletized) Sea salt + UO 2 mixture (pelletized) NaCl + UO 2 mixture (pelletized) Sea salt + sintered UO 2 pellet Gas flow Ar Moist Ar+5%H 2 Ar Moist Ar+5%H 2 Ar Temperature (°C) 815, 913, 1002, 1198 (10 min) 815, 1002 (10 min) 815, 1002 (30 min), 1198 (15 min) 1002 (30 min) 815, 1002 (30 min) 1002 (30 min) 913, 1002 (3 hr) Sea salt/UO 2 = 70.2/29.8 (wt%), NaCl/UO 2 = 68.3/31.7 (wt%) = 90/10 (mol%) pO 2 in Ar = 5–10x10 -6 atm, pO 2 in moist Ar+5%H 2 = ~ atm at 1002 °C

ERMSAR 2012, Cologne March 21 – 23, Decomposition and volatilization of salt Weight reduction of salts in heated specimens pH~2 in downstream trap water (HCl, H 2 SO 4 ) Impact on the structural materials ? ·NaCl: Volatilization above M.P. (~800 °C), evolution of HCl ·MgCl 2 ·aH 2 O: Decomposition to MgOHCl then to MgO, evolution of HCl (<800 °C) ·MgSO 4 ·bH 2 O: Decomposition to MgO, evolution of SOx ·CaSO 4 ·cH 2 O: Decomposition to CaO (?), evolution of SOx ·KCl: Eutectic with NaCl and volatilization? Weight reduction, analyses by XRD & ICP CaMg 3 (SO 4 ) 4 (800~1000 °C) Evolution of corrosive gases

ERMSAR 2012, Cologne March 21 – 23, Reaction in sea salt + UO 2 mixture (1) XRD profiles of products in Ar flow XRD analysis of products ·NaCl+UO 2 system No new phases No change in UO 2 phase peak positions ·Sea salt + UO 2 system UO 2, MgO, NaCl (disappeared at 1198°C) No calcium compounds Considerable peak shift of UO 2 phase (smaller lattice parameter) Suggests formation of (Ca,U)O 2+x solid solution.

ERMSAR 2012, Cologne March 21 – 23, Reaction in sea salt + UO 2 mixture (2) Lattice parameter of UO 2+x phase heated with salt Lattice parameter of UO 2 phase Mg: T. Fujino and K. Naito (1970) Ca: T. Yamashita and T. Fujino (1985) Most part of Ca and probably a part of Mg dissolved into UO 2 phase to form solid solution. Much smaller than fcc uranium oxide phase range (1002 and 1198 °C) From L.P., Ca/(Ca+U) ~ 0.3 or Mg/(Mg+U) ~ 0.18 Initial mixture composition, Ca/(Ca+U) ~ 0.15 Mg/(Mg+U) ~ 0.45 Effect of Ca is insufficient to explain the L.P. variation.

ERMSAR 2012, Cologne March 21 – 23, Reaction in sea salt + UO 2 mixture (3) SEM and elemental mapping images for sea salt +UO 2 mixture heated in Ar flow Observation by SEM/EDX Na: Solidified NaCl (disappeared at 1198 °C) Mg: Fine precipitation of MgO Ca: Distribution overlapped with U Supports the formation of (U,Ca)O 2+x For product at 1198 °C, Ca/(Ca+U): 0.06~0.07 (< estimation from L.P.) Mg/(U+Mg): varies widely (picking MgO) Na, K: 0~0.4 wt% Cl, S: not detected 1198 °C Point analysis on UO 2 grains UO 2 : 2~3 µm grains, round shape

ERMSAR 2012, Cologne March 21 – 23, Reaction at salt / UO 2 pellet interface (1) SEM images of UO 2 pellet surface heated with sea salt for 3h. UO 2 pellet surface Distinct UO 2 grain boundaries “Terraced” structure of exposed UO 2 grain surface Point analysis at UO 2 grain surface Ca/(Ca+U) Mg/(Mg+U) 913 °C 0.02 ≤ °C ~0.04 Ca dissolution into UO 2 bulk seems to proceed at lower temp. than Mg Ca and Mg were faintly detected on the surface not as precipitation. Estimated atomic fraction at UO 2 grain surface Effect of Ca and Mg dissolution, or corrosive gas ?

ERMSAR 2012, Cologne March 21 – 23, Reaction at salt / UO 2 pellet interface (2) Cross section of salt/UO 2 interface Elemental mapping images for the cross section of specimen heated at 1002°C for 3h. Dense salt deposit remained because of semi-closed crucible system — contact of Ca and Mg with UO 2 pellet was insufficient for reaction. Ca and Mg were not detected at 5 µm from interface — intrusion or diffusion into the bulk did not proceed under the experimental conditions. No microstructural change in UO 2 bulk Salt UO 2 S Cl O Mg Na Ca U 20µm SEI

ERMSAR 2012, Cologne March 21 – 23, Summary NaCl, main component of sea salt, volatilizes without reaction with UO 2, however, it can be a remarkable source of corrosive gas (HCl). MgCl 2 ·aH 2 O and MgSO 4 ·bH 2 O decompose to MgO with evolution of corrosive gasses (HCl, SO x ). A part of Mg can dissolve into UO 2 phase. CaSO 4 ·cH 2 O decomposes probably to CaO. Ca dissolves into UO 2 phase to form solid solution, from XRD and SEM/EDX analyses. Mg and Ca intrusion or diffusion into dense UO 2 bulk was not recognized under the experimental conditions. Heat treatment for longer time with open crucible system will be examined. High temperature reaction between UO 2 and sea salt deposit was examined in the temperature range 815 –1198 °C. Experiments on the sea salt + U-Zr-O system will be performed soon. — influence on the phase relationship? Thank you for your attention.