ERMSAR 2012, Cologne March 21 – 23, 2012

High temperature reaction between UO2 and sea salt deposit

M. Takano1, T. Nishi1, M. Kurata1, M. Amaya2, F. Nagase2

1 Nuclear Science and Engineering Directorate2 Nuclear Safety Research Center

Japan Atomic Energy Agency

Session “Fukushima and perspectives for SARNET”, paper 7.2

ERMSAR 2012, Cologne March 21 – 23, 2012 2

Contents of talk

1. Purpose of this study

2. Experimental

3. Decomposition and volatilization of salt

4. Reaction in sea salt + UO2 mixture

5. Reaction at salt / UO2 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–UO2 system was investigated as

the preliminary examination to obtain fundamental data.

ERMSAR 2012, Cologne March 21 – 23, 2012

2. Experimental (1)

4

Preparation of reactants· Sea salt deposit prepared from seawater, dried at 230 °C

NaCl/MgCl2·aH2O/MgSO4·bH2O/CaSO4·cH2O/KCl = 87.8/5.8/2.9/1.8/1.7 (mol%)

(Detected as NaCl, KMgCl3·6H2O, MgSO4·H2O, CaSO4 by XRD)

· NaCl as simplified reference

a=6, b=1, c=0

· UO2 powder and sintered pellet (~94%TD)

· Salt only

· Pelletized mixture of salt+UO2

· UO2 pellet in salt bed

Schematic of heating system

Reaction systems

CharacterizationXRD, ICP/AES, SEM/EDX

in Ar or moist Ar+5%H2

at 815~1198 °C

ERMSAR 2012, Cologne March 21 – 23, 2012

2. Experimental (2)

5

Experimental matrix

Specimen

Sea salt (pelletized)

NaCl (pelletized)

Sea salt + UO2 mixture(pelletized)

NaCl + UO2 mixture(pelletized)

Sea salt + sintered UO2 pellet

Gas flow

Ar

Ar

ArMoist Ar+5%H2

ArMoist Ar+5%H2

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/UO2 = 70.2/29.8 (wt%), NaCl/UO2 = 68.3/31.7 (wt%) = 90/10 (mol%)

pO2 in Ar = 5–10x10-6 atm, pO2 in moist Ar+5%H2 = ~10-15 atm at 1002 °C

ERMSAR 2012, Cologne March 21 – 23, 2012 6

3. Decomposition and volatilization of salt

Weight reduction of salts in heated specimens

pH~2 in downstream trap water (HCl, H2SO4)

Impact on the structural materials ?

·NaCl: Volatilization above M.P. (~800 °C), evolution of HCl

·MgCl2·aH2O: Decomposition to MgOHCl then to MgO, evolution of HCl (<800 °C)

·MgSO4·bH2O: Decomposition to MgO, evolution of SOx

·CaSO4·cH2O: Decomposition to CaO (?), evolution of SOx

·KCl: Eutectic with NaCl and volatilization?

Weight reduction, analyses by XRD & ICP

CaMg3(SO4)4

(800~1000 °C)

Evolution of corrosive gases

ERMSAR 2012, Cologne March 21 – 23, 2012 7

4. Reaction in sea salt + UO2 mixture (1)

XRD profiles of products in Ar flow

XRD analysis of products

·NaCl+UO2 system

No new phases

No change in UO2 phase peak positions

·Sea salt + UO2 system

UO2, MgO, NaCl (disappeared at 1198°C)

No calcium compounds

Considerable peak shift of UO2 phase(smaller lattice parameter)

Suggests formation of (Ca,U)O2+x solid solution.

ERMSAR 2012, Cologne March 21 – 23, 2012 8

4. Reaction in sea salt + UO2 mixture (2)

Lattice parameter of UO2+x phase heated with salt

Lattice parameter of UO2 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 UO2 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, 2012 9

4. Reaction in sea salt + UO2 mixture (3)

SEI O Mg

U Ca

SEM and elemental mapping images for sea salt +UO2 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)O2+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 UO2 grains

UO2: 2~3 µm grains, round shape

ERMSAR 2012, Cologne March 21 – 23, 2012 10

5. Reaction at salt / UO2 pellet interface (1)

SEM images of UO2 pellet surface heated with sea salt for 3h.

UO2 pellet surface

Distinct UO2 grain boundaries

“Terraced” structure of exposed UO2 grain surface

Point analysis at UO2 grain surface

Ca/(Ca+U)

Mg/(Mg+U)

913 °C

0.02

≤0.01

1002 °C

0.02

0.03~0.04

Ca dissolution into UO2 bulk seems to proceed atlower temp. than Mg

Ca and Mg were faintly detected on the surfacenot as precipitation.

Estimated atomic fraction at UO2 grain surface

Effect of Ca and Mg dissolution, or corrosive gas ?

ERMSAR 2012, Cologne March 21 – 23, 2012 11

5. Reaction at salt / UO2 pellet interface (2)

Cross section of salt/UO2 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 UO2 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 UO2 bulk

Salt

UO2

S ClO

Mg NaCaU

20µmSEI

ERMSAR 2012, Cologne March 21 – 23, 2012 12

6. Summary

NaCl, main component of sea salt, volatilizes without reaction with UO2, however, it can be a

remarkable source of corrosive gas (HCl).

MgCl2·aH2O and MgSO4·bH2O decompose to MgO with evolution of corrosive gasses (HCl, SOx).

A part of Mg can dissolve into UO2 phase.

CaSO4·cH2O decomposes probably to CaO. Ca dissolves into UO2 phase to form solid solution,

from XRD and SEM/EDX analyses.

Mg and Ca intrusion or diffusion into dense UO2 bulk was not recognized under the experimental

conditions. Heat treatment for longer time with open crucible system will be examined.

High temperature reaction between UO2 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.