Influence of Pre-Oxidation on Mechanical Properties of Zr1Nb Alloy

Transkript

1 Influence of Pre-Oxidation on Mechanical Properties of Zr1Nb Alloy Olga Bláhová New Technologies - Research Centre in Westbohemian Region University of West Bohemia, Plzen 1

2 Zirconium Alloys The use of Zr alloys expanded in 1950 s Application in nuclear power generation: nuclear fuel cladding Favourable mechanical properties and corrosion and radiation resistance Further development: alloys for fuel with a long burnout period The present alloy: % Nb, 3 ppm H, 20 ppm N, 100 ppm C, 840 ppm O 2

3 Focus of Research Nuclear plants security enhancement in case of LOCA type accident a project of the Ministry of Industry and Trade, CZ UJP Praha a.s.: corrosion testing, metallography, hydrogen content measurement, pressure tests, safety criteria University of West Bohemia in Pilsen: nanoindentation measurement, X-ray diffraction, SEM, EDAX Aim of the study: - Clarification of mechanism of oxidation processes (hydriding, long fuel cycle, oxide dissolution) - Formulation of oxide dissolution model - Construction of pseudo-binary phase diagrams - Proposed modification of oxidation and safety criteria 3

4 LOCA Accident Loss Of Coolant Accident Failure of primary circuit piping Loss of coolant and pressure Temperature increase in the core (1,000 C 1,200 C) Rapid material oxidation in water - steam mixture Emergency water cooling quenching of material Negative impact on mechanical properties requirement for safe handling of material Oxidation criteria: ECR 17% equivalent cladding reacted - Zircaloy-4 K-criterion (by UJP) -Zr1Nb 4

5 LOCA Simulation Pre-oxidation simulation of long-term operation HTO high temperature oxidation Quenching water + ice Parameters: temperature, time, heating and cooling rates Migration of oxygen and hydrogen atoms in material: Oxidation Hydriding of material - embrittlement, degradation of mechanical properties Occurrence of stresses 5

6 Experimental Specimens and Methods Specimens of cladding tubes, length 30 mm, 9 mm, wall thickness 0.6 mm Corrosion testing various environments and conditions Corrosion kinetics Mechanical testing (compression, microhardness) Metallography, X-ray diffraction, H 2 and O 2 levels, EDAX, WDX (UJV Řež) 6

7 Microstructure of Material upon HTO External oxide ZrO 2 α-zr(o) phase: α-zr phase stabilized by oxygen greater brittleness & hardness α-zr (prior phase β-zr) higher elongation and toughness, lower hardness provides required mechanical properties (toughness) 7

8 Nanoindentation Measurement NanoIndenter XP (MTS) Parameters NI XP: Load: 10 μn to 10 N Loading unit resolution: μn ( 5.1 μg). Max. indentation depth: 500 μm Depth measurement resolution: 0.01 nm Indenters: Berkovich, Vickers, ball, cube corner tip Methods: indentation test cyclic indentation CSM method scratch test profilometry 8

9 Nanoindentation Measurement h = f(f) function: indentation curve Indentation hardness: F E r = H IT = max A Indentation modulus of elasticity: S 2.. π A p p 2 s EIT = ( ν i ) E r 1 ( ν ) (ISO/DIS Metallic materials Instrumented indentation test for hardness and materials parameters. ISO Central Secretariat, Geneva 2002) 9 E i

10 Nanoindentation Measurement Berkovich indenter 8 mn load Indent spacing 5 µm Determination of α-zr hardness 10

11 Experimental Specimens Pre-oxidation - simulation of long-term operation - simulation of the reactor environment - steam at 425 C, the pressure of 10.7 MPa, till 365 days oxid thickness: 0, 2, 10, 30 µm HTO high temperature oxidation - different temperature: C - different time: min Different cooling rates - quenching water + ice - furnace 11

12 Exposure Time vs. Hardness C 1200 C-p 1150 C 1150 C-p 1100 C 1100 C-p 1050 C 1000 C 950 C HIT [GPa] HIT [GPa] t [min] t [min]

13 Exposure Time vs. Modulus EIT [GPa] C 1200 C-p 1150 C 1150 C-p 1100 C 1100 C-p 1050 C 1000 C 950 C C 1200 C-p 1150 C 1150 C-p 1100 C 1100 C-p 1050 C 1000 C 950 C 100 EIT /H IT t [min] t [min]

14 Hardness vs. Ductility Brittle samples: H IT > 4 GPa ductility [%] C 1200 C-p 1150 C 1150 C-p 1100 C 1100 C-p 1050 C 1000 C 950 C HIT [GPa] 14

15 Conclusions Use of nanoindentation measurement: - Evaluation of changes upon high-temperature oxidation, exposure time and temperature - Additional input in Corrosion and Oxidation databases - Evaluation of influence pre-oxidation on properties Correlations with results of other analytical methods: - Formulation of models of oxidation, hydriding and oxygen dissolution - Specification of safety criteria with greater accuracy - Construction of pseudo-binary phase diagrams The study was conducted under project no. 2A - 1TP1/037 of the Ministry of Industry and Trade of the Czech Republic 15