Characteristics of fuel rod behaviour during LOCA

Characteristics of fuel rod behaviour during LOCA
Paul Scherrer Institut Characteristics of fuel rod behaviour during LOCA G. Khvostov ESB RIA-LOCA Seminar, Januar, Böttstein Castle

Contents Introduction Thermo-Mechanical (T-M) behaviour.
The general flow-chart of events occurring in a fuel rod during the LOCA. Fuel rod-related requirements to be fulfilled for the corresponding DBA. Thermo-Mechanical (T-M) behaviour. LOCA specific boundary conditions vs. those during the normal operation. High-T cladding creep, plastic instability. Empiric relations of the cladding burst. Main effects of cladding ballooning and burst. Axial contraction strain. Effect of axial constraint. Effects of heat-up rate. Effects of base irradiation before the LOCA transient. Effects of pellet-cladding bonding. Effects of fuel fragmentation and relocation. Effects of axial gas flow. Effects of azimuthal temperature non-uniformity. Effects of O- and H- content. Cladding material properties degradation. High-T oxidation. Realistic distribution of O in cladding vs. the concept of ECR. Break-away oxidation. Secondary degradation via H- up-take from the stagnating steam. ESB RIA-LOCA Seminar, Januar, Böttstein Castle 2

Tensile hoop stresses in cladding: DP →  > 0 (~50-70 MPa)
The general flow-chart of events occurring in a fuel rod during the LOCA De-pressurization & Loss of coolant & decay heat-generation Tensile hoop stresses in cladding: DP →  > 0 (~50-70 MPa) Cladding heat-up Cladding oxidation H- uptake - Cladding creep-out - Plastic instability (ballooning) Burst Cladding quench at re-flood Cladding embitterment ~3-5 mins ESB RIA-LOCA Seminar, Januar, Böttstein Castle 4

Tensile hoop stresses in cladding: DP →  > 0 (~50-100 MPa)
The general flow-chart of events occurring in a fuel rod during the LOCA De-pressurization & Loss of coolant & decay heat-generation Tensile hoop stresses in cladding: DP →  > 0 (~ MPa) Cladding heat-up Cladding oxidation H- uptake - Cladding creep-out - Plastic instability (ballooning) Burst Cladding quench at re-flood Cladding embitterment (1)Thermo-Mechanical behaviour ~3-5 mins ESB RIA-LOCA Seminar, Januar, Böttstein Castle 5

Tensile hoop stresses in cladding: DP →  > 0 (~50 MPa)
The general flow-chart of events occurring in a fuel rod during the LOCA De-pressurization & Loss of coolant & decay heat-generation Tensile hoop stresses in cladding: DP →  > 0 (~50 MPa) Cladding heat-up Cladding oxidation H- uptake - Cladding creep-out - Plastic instability (ballooning) Burst Cladding quench at re-flood Cladding embitterment (2) Cladding Materials degradation ~3-5 mins ESB RIA-LOCA Seminar, Januar, Böttstein Castle 6

 Derivation is not straight forward
Fuel rod-related requirements to be fulfilled, which relates to T-M behaviour: Coolabillity Assembly flow blockage: (1) non-mechanistic approach Use of bundle tests to determine correlation between burst strain and rod co-planer average one Single-rod burst strains determination using special correlations/codes (NUREG-0630, Falcon, FRAPCON/FRAPTRAN, etc.): Empiric correlation average ≈ 0.5 burst  Derivation is not straight forward Geometric considerations (e.g. pitch-to-rod-diameter ratios); Bundle size (small or large); Assembly specific features .. Max. assembly blockage (through empiric correlations)  Scarcity of blockage data ESB RIA-LOCA Seminar, Januar, Böttstein Castle 8

Assembly flow blockage: (2) mechanistic approach
Fuel rod-related requirements to be fulfilled, which relates to T-M behaviour: Coolabillity Assembly flow blockage: (2) mechanistic approach The use 3-D T-H analysis for all the rods, including rods interaction and coupling to the detailed TH calculation (e.g. DRACCAR code) Calculation (left) VS data (right) for Phebus LOCA 215 R test using DRACCAR Analysis is not straight forward because of complex feed-back effects, effects of initial conditions, etc. Yet unavailable in STARS;  Development of coupled sub-channel code with Falcon is planned ESB RIA-LOCA Seminar, Januar, Böttstein Castle 9

High-T oxidation (at 1000-1200oC): Licensing approach (ECR)
Fuel rod-related requirements to be fulfilled, which relates to cladding materials degradation: Fuel integrity High-T oxidation (at  oC): Licensing approach (ECR) Equivalent Cladding Reacted is calculated using a simplistic parabolic-kinetics law, as applied to known temperature regimes in the samples. ECR is treated as integral measure of material degradation in response to the LOCA in question, in function of time and temperature. Threshold ECR value for cladding embrittlement is found from the results of the semi-integral quench tests. A failure map in terms of BJ-ECR from semi-integral quench tests under fully restrained conditions Results are test methodology dependant; ECR calculation does not reflect on everything Post-failure appearances of failed cladding ESB RIA-LOCA Seminar, Januar, Böttstein Castle 10

High-T oxidation (at 600-1400oC): Advanced analysis (DIFFOX)
Fuel rod-related requirements to be fulfilled, which relates to cladding materials degradation: Fuel integrity High-T oxidation (at  oC): Advanced analysis (DIFFOX) Profiles of O concentration along with reaction layer thickness. Two-sided oxidation. Schematic illustration for O distribution across cladding after oxidation at 1200 oC Effects of H. Transient effects ESB RIA-LOCA Seminar, Januar, Böttstein Castle 11

LOCA specific boundary conditions
Cladding temperature, hoop stress, stress bi-axiality LOCA causes high cladding temperature: Tc>>350 oC Target temperature and heat-up rate depend on both LHGR in this rod, and in the surrounding those (simulated by the heater in the Halden LOCA facility). Cladding temperature time evolution depending on LHGR in the rod and surrounding-heater Schematic for Halden LOCA facility Heater Fuel rod ESB RIA-LOCA Seminar, Januar, Böttstein Castle 13

LOCA specific boundary conditions
Cladding temperature, hoop stress, stress biaxiality Pressure differential yields tensile hoop stress in cladding, which is somewhat lower than stress due to PCMI at the End of Base Irradiation Experimental engineering hoop stress Calculated hoop stress due to PCMI at BI Measured characteristics of pressure during the LOCA in IFA-650.5 Stress biaxiality parameter changes into thin-shell gas-filled tube-specific value, ≈ 2. bLOCA = VS. bPCMI = 1.0 ESB RIA-LOCA Seminar, Januar, Böttstein Castle 14

High-T cladding creep, plastic instability
Uniform diametral expansion, ballooning, contraction In spite of relatively low tensile hoop stress, cladding heat-up eventually results in the on-set of high-T creep (T=650 oC) Criterion #1: Commonly accepted criterion of the ballooning on-set: dP = 0 (at this stage the diameteral expansion is still uniform ) Measured rod pressure in IFA test Criterion #2: Plastic instability (local ballooning)  Franklin stability relation: tcriterion 2 > tcriterion 1 ESB RIA-LOCA Seminar, Januar, Böttstein Castle 16

High-T cladding creep, plastic instability
Cladding burst Ballooning eventually results in cladding burst. The failure mode of the cladding (e.g. the appearance and size of the balloon) is strongly dependent on the burst temperature. Typical failure mode for Zircalloy cladding in a-, (a+b)- and b-phase For burst in a-phase (< 820°C), the burst is violent and the opening is square- shape. For bursts in the two-phase region ( °C), the burst opening is narrow and characterized by the V-shape splits at both ends. In the b-phase region (>960°C), pinhole ruptures occur and the cladding surface exhibits the characteristic "orange-peel" appearance. ESB RIA-LOCA Seminar, Januar, Böttstein Castle 17

Empiric relations of the cladding burst
Rupture temperature (according to NUREG-630) It is well established, that Tburst is decreasing function of pressure in the rod, essentially depending on heat-up rate. Chapman deduced the correlation for Tburst as function of engineering hoop stress at the moment of rupture, and heat-up rate in the range from 0 to 28 oC/s. ESB RIA-LOCA Seminar, Januar, Böttstein Castle 19

Modern results on burst temperature for HBU rods cladding of different types are consistent ESB RIA-LOCA Seminar, Januar, Böttstein Castle 20

Maximum strain (according to NUREG-630) NUREG-630 summarized the burst-test data that had been conducted before, and pointed to the characteristic dependency of max. burst on Tburst, also influenced by the heat-up rate. The low- and high-temperature peaks correlate with a- and b-phase of Zircalloy, respectively. a (hcp) a+b b (bcc) The intermediate drop of burst strain is consistent with the transition to the two-phase region . Super-plasticity of the cladding in the b-phase material is to do with the second peak. Peak circumferential strain as function of burst temperature for various heat-up rates ESB RIA-LOCA Seminar, Januar, Böttstein Castle 21

Main effects of cladding ballooning and burst
Axial contraction strain. Effect of axial constraint The condition of cladding material volume conservation during ballooning implies either cladding contraction, or reduction of the wall thickness equivalent to the reached diametral expansion. In the a-phase, at low temperature, ‘stretching’ of the cladding wall is unlikely, and contraction is necessary for the development of the balloon. High-speed-movie frames showing axial cladding contraction and ballooning An axially constrained cladding shows up significant reduction of the biaxiality parameter after the balloon. Hoop strain is considerably low in constrained tubes, but difference is generally believed to be more pronounced for burst in a-phase ESB RIA-LOCA Seminar, Januar, Böttstein Castle 23

Effects of heat-up rate The effect of the increase in heat-up in low-T region is credited with the (1) strain-rate hardening (reduction of stress relaxation due to the shorter time available), and (2) with the effect on ab transformation , and (3) the effect of the expansion of emergent oxide on stress biaxiality. H < 10 oC/s H > 25 oC/s Peak circumferential strain as function of burst temperature for the lower (left) and higher (right) heat-up rates ESB RIA-LOCA Seminar, Januar, Böttstein Castle 24

Effects of heat-up rate The effect of the increase in heat-up in low-T region is credited with the (1) strain-rate hardening (reduction of stress relaxation due to the shorter time available), and (2) with the effect on ab transformation, and (3) the effect of the expansion of emergent oxide on stress biaxiality. H < 10 oC/s H > 25 oC/s Peak circumferential strain as function of burst temperature for the lower (left) and higher (right) heat-up rates ESB RIA-LOCA Seminar, Januar, Böttstein Castle 25

Effects of heat-up rate The effect of the increase in heat-up in low-T region is credited with the (1) strain-rate hardening (reduction of stress relaxation due to the shorter time available), and (2) with the effect on ab transformation, and (3) the effect of the expansion of emergent oxide on stress biaxiality. H < 10 oC/s H > 25 oC/s Peak circumferential strain as function of burst temperature for the lower (left) and higher (right) heat-up rates ESB RIA-LOCA Seminar, Januar, Böttstein Castle 26

Effects of heat-up rate In the high-T range, the effect on the burst parameters is due to the oxidation during the ballooning. The thinning of the specific ‘b-phase grooves’ is the predominant mechanism for the high-T ballooning. The less heat-up rate, the more time for the oxygen diffusion, which strengthens the cladding. H < 10 oC/s H > 25 oC/s A groove in the cladding wall after high-T burst ESB RIA-LOCA Seminar, Januar, Böttstein Castle 27

Effects of base irradiation before the LOCA transient FGR and relative reduction of the rod free volume are essentially dependant on LHGR history and rod design. These processes inevitably results in increase of the rod pressure, which would evidently affect the characteristics of burst in the hypothesized LOCA. Time evolution of internal gas pressure in the high- (left) and low- (right) power PWR fuel rods (calculation for the two ‘idealized cases of IAEA programme FUMEX’) ESB RIA-LOCA Seminar, Januar, Böttstein Castle 28

Effects of pellet-cladding bonding In case of HBU fuel, long-term mechanical contact may result in result in bonding. The bonding may have a suppressing effect on the ballooning, just like a mandrel in the burst tests that causes axial constraint. Zry-2 Liner Mechanical effect of pellet-cladding bonding: Suppressed axial contraction results in no ballooning. Pellet Bonding and closed gap typical of KKL high-burnup fuel ESB RIA-LOCA Seminar, Januar, Böttstein Castle 29

Effects of pellet-cladding bonding If the bonding layer is broken, a great amount of fission gases can be released a-thermally from the HBS pores, and from the locally closed gap. Such ‘burst’ FGR is able to modify inner rod pressure, and thus affect the course of the ballooning and burst parameters. Zry-2 Liner Pellet Bonding and closed gap typical of KKL high-burnup fuel Radial distribution of the calculated and EPMA-concentration of Xe across the pellet RIM ESB RIA-LOCA Seminar, Januar, Böttstein Castle 30

Effects of fuel fragmentation and relocation The recent experimental studies by HRP (left) and Studsvik (right) have shown considerable susceptibility to fragmentation and relocation. This observation may have considerable effect on the LOCA consequences analysis relating to core- coolability and withdrawability Just after Two days later Defueled fuel rod after a burst test at Stunsvik (top), and fuel fragments collected under the rod (bottom) just after, and two days after the test Gamma-scan of Halden LOCA test in IFA showing significant ejection of the fuel from the rod ESB RIA-LOCA Seminar, Januar, Böttstein Castle 31

Effects of fuel fragmentation and relocation (FRELAX model) Moreover, axial fuel relocation was shown by the modeling to be able to affect cladding temperature, due to the ‘hot-spot’ effect from the fuel relocated into the ballooned region. The modeled thermal effect of axial fuel relocation in IFA test ESB RIA-LOCA Seminar, Januar, Böttstein Castle 32

Effects of axial gas flow (FRELAX model) The model-variables and local fuel-rod characteristics of interest are assumed to be localized in the three elements: Gas plenum accommodate the largest part of the initial filling-gas. Junction-element, which is acting as the resistance to gas flow – driven by the pressure differential – between the plenum and balloon. model Balloon-area involves all the variable part of the total free-volume of the rod. ESB RIA-LOCA Seminar, Januar, Böttstein Castle 33

Effects of axial gas flow (FRELAX model) The resistance of the junction is treated as a variable linked with the axial profile of the cladding circumferential strain, (z,t), in consideration of the mode of fuel-cladding bonding : The Poiseuille’s law was adopted in the model to describe the axial gas transport through the junction. To determine the local effective hydraulic diameter as function of axial co-ordinate, the parallel connection of the initial and emergent paths for the gas to flow is assumed. The condition of the initial fuel-pellet bonding is involved in the calculation of the variable part of the effective hydraulic diameter. ESB RIA-LOCA Seminar, Januar, Böttstein Castle 34

Effects of axial gas flow The delayed axial gas redistribution in the fuel rod during the ballooning was shown by modeling to be eventually able to considerably affect gas pressure in the rod, particularly if the distance between the plenum and the ballooned region: specifically to cause the delay of burst moment by up to 1min. The predicted time-evolution of gas pressure for the hypothesized LOCA scenario in PWR at BOC without (left) and with (right) fuel-cladding bonding ESB RIA-LOCA Seminar, Januar, Böttstein Castle 35

Effects of azimuthal temperature non-uniformity The azimuthal non-uniformity of temperature was shown to have great effect on the burst strain, e.g. in the REBECA experiments at KfK. The FALCON code uses time dependent Damage Index (DI) as measure of cladding failure. Critical DI (CDI), corresponding to failure is expected to be 1.0 for single rod tests, but to drop to 0.5 in the rod bundle, due to the azimuthal non-uniformity. Accounting for this effect has allowed to interpret the data for cladding circumferential deformation in the Halden LOCA test IFA , using KKL high-burnup fuel sample. Peak hoop strain versus azimuthal temperature difference in the rupture section as determined from the post-test Calculated axial profile of cladding hoop strain for different predicted DI versus measured one after Halden LOCA test in IFA ESB RIA-LOCA Seminar, Januar, Böttstein Castle 36

Effects of O- and H- content. Apart from cladding embrittlement, concentration and distribution of O an H in cladding have considerable influence on the T-M behaviour during the LOCA, due to: effect of O- and H- concentrations on phase transition temperatures, BWR (Zry-2) Typical Hydrides- distribution in BWR and PWR HBU rod cladding at RT PWR (Zry-4) Hav=600 ppm Hmax= ppm H=300 ppm as well as due to the above-mentioned effect of oxidation on the stress biaxiality (a-phase) and cladding strengthening during the heat-up ESB RIA-LOCA Seminar, Januar, Böttstein Castle 37

Cladding material properties degradation
High-T oxidation. Concept of ECR. Simplified calculation for oxygen absorption, as controlled by the diffusion in a single equivalent-reaction zone. The bases of the parabolic kinetics model for cladding oxidation Rate constants for various ‘Models’ based on the parabolic kinetics approach ESB RIA-LOCA Seminar, Januar, Böttstein Castle 39

High-T oxidation. Concept of ECR. Once equivalent thickness of cladding reacted is calculated, the relative ECR can be determined. To ensure the post-LOCA ductility, this calculated value is not to exceed an appropriate safety limit, which is to be determined based on the special quench tests. An example of the tests for the determination of the ECR safety limit ESB RIA-LOCA Seminar, Januar, Böttstein Castle 40

Break-away oxidation. Secondary degradation via H- up-take (1) Rapid oxidation and embrittlement was found possible if cladding temperature exceeds 1204 oC (2200 F), even though the calculated ECR could be below the safety limit, which has become the bases one one more oxidation-related safety limit. (2) Break-away oxidation, leading to embrittlement, occurs in Zr-based cladding at long-term exposure to temperature between 650 and 1100 oC, where the oxide turns into specific non-protective form. Some materials, particularly Russian E-110, have shown particular susceptibility to this. Breakaway oxidation was found to be sensitive to manufacturing conditions: especially the surface finish, and quality of the initial material. An example of brake away oxidation on the sample surface (3) Enhanced hydrogen absorption on ID:. Two peaks of H- concentration are observed at the ID, apart from, but in the vicinity of rupture (presumably due to H- stagnation there) ESB RIA-LOCA Seminar, Januar, Böttstein Castle 41

Thank you for attention
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Characteristics of fuel rod behaviour during LOCA

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