Understanding the effects of reflooding in a reactor core beyond LOCA conditions

Transcription

1 Understanding the effects of reflooding in a reactor core beyond LOCA conditions F. Fichot 1, O. Coindreau 1, G. Repetto 1, M. Steinbrück 2, W. Hering 2, M. Buck 3, M. Bürger IRSN, Cadarache (FR) 2 - KIT, Karlsruhe (GE) 3 - IKE, Stuttgart (GE) ERMSAR 2010, May , Bologna

2 Motivation Reflood is a prime accident management measure to terminate a nuclear accident However, the success of reflood cannot be proved in all situations Water progression in the core may be too slow or not sufficient to cool down all materials, in particular after collapse and even melting of rods. Reflood may cause temperature excursion connected with increased hydrogen and FP release and additional melting of materials TMI2, LOFT, PBF, CORA.. but only qualitative hydrogen measurements Coolability of a degraded core is a matter of high priority (SARNET-SARP, OECD-GAMA) ERMSAR 2010, May , Bologna 2

3 Reflood Water injection steam production Cooling effect Chemical heat production Q c h (T Zr - T c ) Q R R H (K/t) 0.5 Influenced by Zr + 2 H 2 O ZrO H kj T Flooding rate Cooling medium Pressure Hydraulic diameter Core state: rods or debris Oxidation kinetics Temperature (Arrhenius type) Degree of pre-oxidation Surface available for oxidation Material composition Steam availability ERMSAR 2010, May , Bologna 3

4 QUENCH facility Bundle with fuel rod simulators of ~2,5 m length Electrically heated: ~1 m; max 70 kw Fuel simulator: ZrO 2 pellets Quenching (from bottom) with water or saturated steam Off-gas analysis by mass spectrometer (H 2, steam ) Extensive instrumentation for T, p, flow rates, level, etc. Removable corner rods during test ERMSAR 2010, May , Bologna 4

5 Hydrogen release, g QUENCH test High temperatures above 2200 K Eutectic melt formation H 2 before reflood H 2 during reflood Temp. at initiation of reflood Low flooding rate Temperature, K Steam starvation QUENCH bundle tests can be split into those: With successful flooding leading to immediate cooldown and low hydrogen release With temporary temperature escalation and strong hydrogen production ERMSAR 2010, May , Bologna 5

6 Conclusion of 10 years QUENCH program Main factors affecting cooling and chemical heat production during reflood have been identified. The heat balance at initiation of reflood is crucial for further development of reflood process. Melt formation, relocation, and oxidation as well as steam starvation before reflood are the most important factors for enhanced oxidation after reflood. Reflood rate must be > 1 g/(s rod) for successful cooldown. Experimental results must not be directly applied to plant analyses, but they help to improve the SA code systems. ERMSAR 2010, May , Bologna 6

7 Impact of reflooding on the damaged rods What happens when water is injected? The collapse of rods is likely to occur if Zr claddings are sufficiently oxidized ERMSAR 2010, May , Bologna 7

8 TMI-2 Observations [1] 100 wt % 1000 µm, largest amount of material 4000 µm 90 wt % 1000 µm, largest amount of material 4000 µm d= wt % 1000 µm, largest amount of material in [ µm] ε 0.63 d= wt % 1000 µm, largest amount of material in [ µm] and [ µm] ε 0.47 d=5.0 Particles characterized by diameters ranging in a wide interval i.e. from 0.1 to 10 mm but most of the particles (in weight) have a size greater than 1 mm. [1] D.W. Akers et al., TMI-2 core debris grab samples, examination and analysis, 1986 ERMSAR 2010, May , Bologna 8

9 TMI-2 and bundle tests Observations The upper debris bed in TMI-2 is similar to the debris bed of fuel fragments in the upper bundle in LOFT LP-FP-2 and PBF SFD 1-4 (despite quite large in scale and scenario) In TMI-2, the particle size distribution is not homogeneous neither in radial nor in the axial direction but 4 zones with distinct characteristics have been identified the progress of quench front in such debris bed is likely to be multi-dimensional Fragment size distribution seems to be primarily determined by fragmentation pattern before accidental transient Embrittlement of fuel claddings is the second key parameter as thermal or mechanical shock will lead to their collapse ERMSAR 2010, May , Bologna 9

10 Fuel Fragmentation Fuel fragmentation under normal conditions At low BU: the initial number of cracks depends on the power As BU : the number of cracks and the max number of radial fragments is 10 [1] -15 [2] Fuel fragmentation due to fission gas bubbles from «Les combustibles nucléaires», CEA [1] L.A. Walton and D.L. Husser, Fuel pellet fracture and relocation, 1983 [2] J. Bonnin, La fragmentation du combustible, 1995 Fission gas distributed between intergranular gas, intragranular gas and pores. During thermal transient, intergranular bubbles can grow leading to unstable crack propagation. The proportion of gaz release strongly increases after 30 GWd/t U Few data on this mechanism ERMSAR 2010, May , Bologna 10

11 Criteria for fuel rod collapse Embrittlement criteria for Zr fuel cladding in SA conditions [1] : Single tube specimens filled with ZrO 2 pellets, heated by induction and then cooled down Parameters investigated: extend of cladding pre-oxidation, T at onset of quenching The mechanical behaviour of the cladding mainly depends on the initial oxide scale thickness. Cracking occurs if ZrO 2 thickness > 150 µm [1] Hofmann P. et al., Quench behavior of Zircaloy fuel cladding tubes. Small-scale experiments and modeling of the quench phenomena, FzKA 6208, 1999 ERMSAR 2010, May , Bologna 11

12 Planned QUENCH-Debris tests Investigation of formation and coolability of debris and melt in the core Use of Hf for structure component able to survive the test Melting points: Zry-4 ~ 2070 K, Hf ~ 2500 K ERMSAR 2010, May , Bologna 12

13 Bottom vs. Top flooding in boil-off experiments For bottom MEWA calculations on DEBRIS polydispersed beds show that interfacial friction is necessary to explain both top (counter-current ) and bottom (co-current ) flow. For top DHF measured in DEBRIS experiments with polydispersed beds (mixture of 6 mm,3 mm and 2 mm steel spheres with 50:30:20 mass parts) for both top and bottom flooding cases. Unified description of friction is of high importance in view of real, multidimensional (2D/3D) configurations where co- and countercurrent flow situations occur simultaneously ERMSAR 2010, May , Bologna

14 Quenching experiments: Tutu et al, BNL,1984 Bottom flooding with fixed rates dp = 3.2 mm, Initial Tp = 775 K, different fixed water inflow rates at bottom High inflow velocities: peak behavior, thick quenching front progression determined by heat transfer Low inflow velocities: plateau behavior, thin quenching front progression determined by friction Driven by lateral column and typical reactor cases: low inflow velocities simplified modelling possible Status of modelling: quite good agreement, especially concerning transition in behavior ERMSAR 2010, May , Bologna 14

15 Quenching experiments: DEBRIS, IKE Top flooding dp = 6 mm, initial Tp = 787 C Streak-like downwards propagation of quench front at side walls, due to temperature inhomogeneities and wall effect (higher porosity at wall), subsequent faster upwards filling over whole cross-section Status of modelling: quite good agreement ERMSAR 2010, May , Bologna 15

16 Quenching experiments: DEBRIS, IKE Bottom flooding dp = 6 mm, different initial sphere temperatures, water inflow at bottom driven by lateral column of same height as bed Good agreement between experiment and calculation at lower temperature of 360 C, strong decrease of quench front velocity above 400 C could not be reproduced so far Clarification still required New DEBRIS experiments, PEARL, PRELUDE, etc ERMSAR 2010, May , Bologna 16

17 PEARL PRELUDE (1/3) The objective of the PEARL program is to produce data for the validation of numerical codes to predict the consequences of the reflooding of a severely damaged reactor core in a debris bed configuration (Ø 500 mm, h= 500mm) A 2 step experimental approach has been adopted with a preliminary PRELUDE program, launched to test the performance of the induction heating system on steel particles and the instrumentation in a two phase flow, at atmospheric pressure, for a better design of PEARL Preliminary reflooding tests were carried out, involving a debris bed of 4 mm particles inside a Ø110 mm external diameter, and 100 mm height test section. Parameters investigated were: 1. Inlet water velocity at 1,3,6,8 mm/s (4 to 30 m 3 /h/m 2 ), in the range foreseen in the PEARL test matrix, 2. Power at 300W/kg (maintained or not during the reflooding phase), 3. Initial temperature before reflooding at 420K, 500K, 600K and 1000K This campaign ended with a heating sequence of a larger debris bed (Ø180 mm) up to 1000K at about 200 W/kg before the water injection. PRELUDE ERMSAR 2010, May , Bologna

18 PEARL PRELUDE (2/3) Example of PRELUDE Reflooding experiment (inlet water: 36 g/s i.e 5 m 3 /h/m 2 ) Using a high frequency generator (up to 400 khz) for the inductive furnace, the power distribution during the test was rather homogeneous inside the debris bed with particles of Ø 4 mm (as for particles of Ø 2 mm) Thermocouples inside the debris bed offer a fine illustration of the different phases of reflooding C Reflooding duration Front propagation Ø180mm mm height mm 55 mm 100 mm 155 mm 195 mm (s) Power distribution (W/kg) at a mean specific power of 200W/kg During the main part of the reflooding phase, about 60% of the water injected was converted into steam ERMSAR 2010, May , Bologna

19 PEARL PRELUDE (3/3) These tests (at 1 bar) will continue in 2010 to qualify the measurement of pressure inside the debris bed. These results will be extended with reflooding experiments in a homogenous debris bed with smaller particles (Ø2 to Ø1 mm diameter). Work on PEARL facility design has been completed in to start the construction at the end of The qualification tests are foreseen during the first half of 2011 to run experiments at high pressure, ranging from 1 to 10 bar according to the test matrix. In addition to the system pressure, the parameters investigated will be the inlet water velocity, the specific power and the water temperature (under saturation ranging from 50 K to 0) ERMSAR 2010, May , Bologna

20 Conclusions Although several phenomena are currently understood thanks to experimental programs dedicated to the reflooding of rods (QUENCH programme) or debris beds, further experiments are needed to investigate the processes leading to debris bed formation and the progression of the quench front in a large debris bed. The QUENCH-Debris experiments in KIT, the PEARL experiments in IRSN and the DEBRIS experiment in IKE should provide useful data in order to make progress in the modelling. More modelling and assessment should be done before codes can be considered as reliable to calculate the reflooding phase (result of recent OECD benchmark on TMI-2). In parallel to experiments, code improvements and validation will be done by several SARNET2 participants. ERMSAR 2010, May , Bologna 20