11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 1 Evaluation of the Oxygen-Induced Zircaloy Embrittlement in ICARE/CATHARE Ladislav Belovsky ALIAS CZ s.r.o., Czech republic Presented at the 11 th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 2 Motivation for Development of a New Model (ZROB) ICARE/CATHARE: … applicable also for LOCA and beyond DBA analyses Acceptance criteria for ECCS for LWRs ( 10CFR50.46 ): Evaluation of post-quench cladding embrittlement (17% ECR by B-J since 1973). The 17% ECR criterion currently under revision by USNRC: –High burnup (hydrogen, pre-oxide). –New Zr-based alloys Exp. research indicates that embrittlement is a combined function of : –Oxygen content & distribution metal (beta phase) –Hydrogen content (& distribution ?) in metal Modeling of Zircaloy embrittlement in ICARE/CATHARE in two steps: –1. step: Oxygen-induced embrittlement (O-diffusion in beta phase) –2. step: Impact of hydrogen onto embrittlement (O-solubility & diffusion, hydrides, …)

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 3 Modeling Features & Assumptions ZROB receives beta layer boundaries from oxidation module (ZROX or UZRO) ZROB calculates 1D oxygen diffusion in beta layer >970 °C ( oxygen-free ZR in ZROX ): – Oxidizing surface :- Beta layer (ZR) always covered with O-stabilized alpha layer (ZRO). - Boundary concentration at ZR/ZRO: Zircaloy-Oxygen phase diagram – Non-oxidizing surface : Zero oxygen flux. –Uniform meshing, Cylindrical coordinates, Implicit finite-difference method, Gauss elimination. – Initial condition : Constant concentration profile (as-received material ). ZROB deduces from the oxygen concentration profile in the beta layer : 1.Thickness of beta layer with less than specified O-concentration ( …, 0.6, 0.7, … wt% O ). 2.Fractional saturation of beta layer. 3.  embrittled Zircaloy components after quenching (Chung-Kassner 1 and/or Pawel 2 criterion).

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 4 Diffusion Equation for Oxygen in ZR Layer Oxygen mass balance in i th segment: Oxygen fluxes at segment boundaries: J i = D i ·ΔC/ΔR CNCN C N-1 C i+1 CiCi C i-1 C2C2 C1C1 RNRN R N-1 R i+1 RiRi R i-1 R2R2 R1R1 rNrN r N-1 r i+1 riri r i-1 r2r2 r1r1 r N+1 i-th segment (regular) inner segment outer segment r C thickness of ZR layer J i+1 JiJi J N+1 J1J1 Example of two-sided oxidation

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 5 Oxygen diffusion coefficient in ZR layer > 970 °C (  -Zr) : D = 2.63·10 -6 exp(-28200/(1.987·T)) J. Nucl. Mat. 68 (1977) < 820 °C (  -Zr) : D = 1.32·10 -4 exp(-48200/(1.987·T)) J. Nucl. Mat. 67 (1977) °C

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 6 Oxygen solubility S O in ZR layer at ZR/ZRO interface >1007 °C : S O = exp(5.02 – 8220 / T[K]) [wt%] As-received Zircaloy (Chung-Kassner 3 ) °C : S O = 5.246·10 -3 ·(T[K]-1233) < 970 °C : S O = 0 T [  C ] Oxygen concentration [ at% ]  -Zr ZrO2  -Zr inner clad surface outer surface Phase diagram Zry-O SOSO ZRO ZR

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 7 Input & Output Data Input : – MACR xxxxUser name of the oxidizing Zircaloy macro-component (eg. CLAD1). – CINI Initial O-concentration in as-received Zircaloy: 0.1 wt% ( ) – COXX User defined critical O-concentration: 0.55 wt% ( ) – DTMX Max. length of internal sub-time step within global Δt: 0.5 s ( ) – NMAX Max. number of concentration points in ZR: 15 ( ) Output : – Fractional saturation of beta layer FBS = C AV / S O –C AV : Average concentration of oxygen in ZR layer –S O : Boundary concentration of oxygen in ZR (oxygen solubility) – Thickness THICXX within ZR with max. COXX [wt%] oxygen (another six variables THIC04 to THIC09 are automatically calculated for 0.4 to 0.9 wt%) –If embrittlement criterion fulfilled < 400 K (Chung-Kassner 1 or Pawel 2 ), component state  DISLOCAT.

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 8 Results: Numerical Against Analytical Solution Non-moving boundary diffusion problem in a slab, thickness l = 0.7 mm –Outer surface: oxidizing, boundary concentration from Chung-Kassner 3 correlation –Inner surface: zero oxygen flux –Initial O-conc.: 0.1 wt% (1000 wt ppm) –Constant temperature 1000 ° C, 1400 ° C Analytical solution (Carslaw & Jaeger 4 ) : –Oxygen concentration C(x, t) after t seconds at distance x from the surface: Numerical solution by ZROB : –Clad diameter 9 m (  slab) –Default input data Comparison : Good agreement (see next figures)

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 9 Results: Numerical Against Analytical Solution Cont’d Temperature 1000 °C Distance from the outer cladding surface [micron] Oxygen concentration [kg/m 3 ] Rel. difference (A - ZROB) / A [%] Analytical ZROB Rel. difference [%] 60 s 400 s 1140 s 60 s 400 s 1140 s

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 10 Results: Numerical Against Analytical Solution Cont’d Temperature 1400 °C Distance from the outer cladding surface [micron] Oxygen concentration [kg/m 3 ] Rel. difference (A - ZROB) / A [%] Analytical ZROB Rel. difference [%] 40 s 480 s 40 s

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 11 Results: Sensitivity to Meshing and Time step Temperature 1400 °C Number of concentration points NMAX [-] Max. relative difference [%] 40 s 480 s Internal sub-time step DTMX [ s ] Max. relative difference [%] 40 s 480 s

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 12 Results: Isothermal Oxidation (As-received Zircaloy) Temperature 1200 °C

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 13 Results: Isothermal Oxidation (As-received Zry) Cont’d Temperature 1300 °C

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 14 Results: Transient Oxidation (As-received Zry) Linear heat-up and cool-down between 800 and 1300 °C at 1 °C/s

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 15 Absorbed hydrogen C H increases the oxygen solubility S O in beta phase. CEA 5, 6 experimental data available for 1200 °C. Saturation of this effect at  600 wppm H. Billone ( ANL, 2005 ) 7 : Fit to CEA data (additive term to Chung-Kassner 3 correlation): S O = exp(5.02 – 8220 / T) + 0.6· (1 - exp[-0.006· C H ]) [wt%] T[K], C H [ wppm]. The increased solubility limit accelerates the filling of beta phase with oxygen. Oxygen Solubility in Hydrided Zircaloy °C

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 16 Beta layer poor in oxygen (~ < 0.6 wt% ) disappears faster in hydrided Zircaloy. FBS = C AV / C B, FBSA = (C AV – C INI ) / (C B - C INI ), where C INI … initial oxygen conc. in as received Zry. Results: Isothermal Oxidation (Hydrided Zircaloy)

11th International QUENCH Workshop, October 25-27, 2005, Forschungszentrum Karlsruhe, Germany 17 Conclusions Zry embrittlement module ZROB available since mid 2005 (ICARE2-V3mod1.4). –Applied embrittlement criteria: Chung-Kassner (1980) & Pawel (1974) … to be revised. Effect of hydrogen is under testing: –Increased oxygen solubility due to H:ready for implementation into ZROB –Increased oxygen diffusion coefficient : –Impact of hydrides onto embrittlement : References [ 1] H. M. Chung, T. F. Kassner: NUREG/CR-1344 (1980). [ 2] R. E. Pawel: Oxygen diffusion in beta Zircaloy during steam oxidation. J. Nucl. Mat. 50 (1974). [ 3] H. M. Chung, T. F. Kassner: Pseudobinary Zircaloy-Oxygen Phase Diagram. J. Nucl. Mat. 84 (1979) [ 4] H. S. Carslaw, J. C. Jaeger: Conduction of Heat in Solids. Oxford, Clarendon Press, 2 nd edition (1959), p.100. [ 5] L. Portier et al.: 14 th Int. Symp. Zirconium Nucl. Ind., June 13-17, 2004, Stockholm, to be published by ASTM. [ 6] J-C. Brachet et al.: NRC Nucl. Safety Research Conf., Oct , 2004, Washington. [ 7] M. C. Billone: LOCA Embrittlement Criterion. Argonne National Laboratory (April 2005). … to be experimentally investigated