Analytical support to experiment QUENCH-17 and first post-test calculations with ATHLET-CD C. Bals, T. Hollands, H. Austregesilo Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Germany
Content Short description of experiment QUENCH-17 Pre-test calculations Objectives and analytical model used for ATHLET-CD Parameter variations, results and conclusions First post-test calculations Analytical model used for ATHLET-CD Results Conclusions 2
Short description of experiment QUENCH-17 (1) Performed by KIT at the QUENCH facility on 30-31 January 2013 Test Purpose: Examine the formation of a debris bed inside a completely oxidized bundle region without melt formation and investigate the coolability during reflood Test Bundle: - 9 unheated inner rods with Zry-4 cladding, - 12 heated outer rods with Hf cladding, - 4 Hf corner rods (with thermocouples), - Hf shroud tube Test bundle /J. Stuckert: Status of the preparation of the QUENCH-Debris experiment, 18 th IQWS, Nov. 2012, KIT/ Debris bed formation: - inner rods filled with pre-fragmented zirconia pellets, - conditions of pre-oxidation to produce complete oxidation of Zry-4 claddings within Δh ~ 0.5 m, - collapse of oxidized region before / at start of water cooling 3
Short description of experiment QUENCH-17 (2) Test conduct /J. Stuckert: Conduct and first results of the QUENCH-17 experiment, KIT, Febr. 2013/ Oxidation: Stepwise increase of electrical power to 10 kw heats the bundle to a max. temperature of ~ 1700 K; after 10000 s stepwise increase to 12 kw to reach a max. temperature of 1800 K; after 64000s power increase to 14 kw (argon and steam flow both 2 g/s) end of oxidation phase at 72000 s (devices show complete oxidation at 650 mm elevation) Debris bed formation: Collapse of surviving Zry-4 claddings by application of an axial mechanical force material relocation and formation of heterogeneous debris bed at the two middle grid spacers was confirmed by response of fluid thermocouples Debris bed cooling: Initiation of reflood at 77773 s with 10 g/s water flow, power reduction to 4 kw (steam flow stopped, argon injection from the top) 4
Short description of experiment QUENCH-17 (3) Debris collected between two upper grid spacers (KIT) consisting of pre-segmented pellets and larger cladding tube fragments Debris bed cooling: - short temperature increase of ~ 150 K during delay period between stopping the steam flow and starting the water injection; - temporary temperature reduction at start of refilling due to rapid steam generation; - second temperature increase of ~ 120 K during refilling of the lower volume; - after 78200 s reduction of debris temp. together with first rod quenching at bottom of bundle; - steady progress of quench front up to the debris region; then constant behaviour of evaporation rate indicates slower quenching inside the debris bed; - stagnation of water level at 78720 s at bundle elevation of ~ 850 mm due to shroud rupture; - after ~ 70000 s increase of hydrogen release (rod inner side oxidation, shroud outer oxidation); 5
Pre-test calculations: Objectives Determine boundary conditions for the test; 6 SARNET partners involved (IRSN, IBRAE, NRI, PSI, RUB, GRS) Definition of conditions (mainly power, bundle temperatures, flow rates of steam and argon) for a controlled pre-oxidation of the Zry claddings, where no formation of melt occurs to allow debris bed formation consisting of solid fragments; therefore, temperature escalation has to be avoided; total oxidation of Zry claddings should be reached over a height of h 0.5 m; within a time as short as possible (to avoid shift operation of experimenters) 6
Pre-test calculations: Analytical model used for ATHLET-CD (1) Input data: derived from standard dataset used for previous QUENCH tests (QUENCH-16) Nodalization: - Central BUNDLE (90%) with 3 rod types: ROD1 ( 1 unheated inner rod) ROD2 ( 8 unheated rods, inner ring) ROD3 (12 heated rods, Hf cladding), - outer bundle region BYPASS (10%), - cross connection BUNDLE-BYPASS, - Inlet INPIPE, outlet OFFPIPE - 5 grids (at unchanged positions), - Structure Shroud (Hf) with Insulation and Cooling Jacket inner wall, - Cooling Jacket (JACKETTUBE) with structure OUTERWALL - Fills (STFILL, ARFILL, JACINAR) - No corner rods simulated - Shroud outer wall not considered regarding oxidation 7
Pre-test calculations: Analytical model used for ATHLET-CD (2) Code version: modified ATHLET-CD version with material properties for Hf / HfO 2 and Hf oxidation kinetics (as given by KIT) on the basis of version 2.2c (Hf / HfO 2 data are adopted for ROD3 and SHROUD) Hf oxidation kinetics: K=K 0 e E/(RT) with K 0 =0.76 kg/m 2 s 1/2, E=7.8423 10 4 J/mol results in lower rates compared to Zry; exothermal heat Δh Hf = 630 kj/mol = 3.53 10 6 J/kg Hf ( vs. Zr: Δh Zr = 1.21 10 7 J/kg Zr ) Initial and boundary conditions: (from QUENCH-16 as recommended by KIT) Pressure p= 2.1 bar, Initial gas temperature (steam, argon): T 0, gas = 600 K 8
Pre-test calculations: Simulation of the pre-oxidation period Investigation of the following parameters: Inlet flow rate of steam and argon: 3 5 8 g/s; Power ramp dep. on inlet flow rate to get a max. temperature of ~ 1750 K; External electrical resistance: 3.5 4.5 5.5 mω/rod; Options for steam oxidation model: IOXMOD=15 (Cathcart-Prater/Courtright), =19 (Leistikov-Prater/Courtright); oxide layer thickness for transition from parabolic to linear rate: 100 200 300 µm - no transition; Variation of shroud insulation thickness to get flat temperature profile; Position of grid 3 (shift from 550 mm to 450 mm elevation) 9
Pre-test calculations of pre-oxidation period: Parameter variation optimal profile of power ramp for short oxidation of cladding between 650 and 1150 mm elevation without escalation of temperatures: - steep first increase to get quick oxidation of hottest position (950 mm), - second more flat increase where max. value of power is reached after complete oxidation of hottest position, - third period of constant power to keep temperatures at ~ 1750 K; Selected power ramp for different inlet flow rates as boundary condition for pre-test calculations 10
Pre-test calculations of pre-oxidation period: Results (1) 650mm 1150 mm Results of reference calculation: Flow rate 8 g/s, max. power 29 kw Complete oxidation of Zry claddings between 650 and 1150 mm elevation was reached after 35000 s 11
Pre-test calculations of pre-oxidation period: Results (2) Results for low flow rate: Flow rate 3 g/s, max. power 14 kw, oxidation kinetics: no transition from parabolic to linear rate Complete oxidation of Zry claddings between 650 and 1150 mm elevation was reached after t = 65000 s (vs. experiment with flow rate 2 g/s: t = 72000 s) higher flow rates resulted in a shorter test time for the aimed amount of oxidation 12
Pre-test calculations of pre-oxidation period: Results (3) Hydrogen generation: Flow rate 3 g/s, max. power 14 kw ~ 80 g hydrogen were produced within the time period needed for the total oxidation of Zry-claddings between 650 and 1150 mm elevation similar to the test result up to 67000 s; the measured increase of hydrogen generation rate in the later time period of the test (after power increase to 14.3 kw) was not simulated by ATHLET-CD (constant power); Comparison of measured hydrogen generation with pre-test calculation (flow rate 3 g/s) 13
Pre-test calculations of pre-oxidation period: Conclusions Max. temperature T max > 1800 K results in an escalation due to quick oxidation where melting could not be avoided; Max. temperature T max < 1700 K showed very slow oxidation at levels below 750 mm and above 950 mm bundle height; therefore, max. temperature should be kept between 1750 and 1800 K Higher flow rates resulted in a shorter test time for the aimed amount of oxidation In simulations of pre-oxidation period with low flow rates (3 g/s), similar as used in the test (2 g/s), the predicted time for complete oxidation of Zry-4 claddings between 650 and 1150 mm bundle elevation was 65000 s (18 h), compared to 72000 s (20 h) as indicated in the test; The amount of hydrogen generation up to the time when the simulation (flow rate 3 g/s) reached the aimed amount of oxidation (t = 65000 s) was predicted similar to the experiment; the later increase of hydrogen production in the test was not calculated by the code (no consideration of inner oxidation, shroud leak); 14
First post-test calculation: Analytical model used for ATHLET-CD (1) Nodalization: Application of the MEWA model for simulation of debris bed cooling, bundle was modelled in more detail in radial direction: - BUNDLE1 with ROD1 (inner unheated rod) - BUNDLE2 with ROD2 (inner unheated ring) - BUNDLE3 with ROD3 (outer heated ring), - outer bundle region BYPASS, - cross connections CROSSFL1/2/3, - 4 grids in each bundle region, - active fill BOTH2OINJ (water cooling), additional fill TOPARSHR (simulation of argon flow through shroud crack) - others as previous 15
Post-test calculation: Analytical model used for ATHLET-CD (2) Code version: modified ATHLET-CD version with material properties for Hf / HfO2 and Hf oxidation kinetics on the basis of version 2.2c (as used for pre-test calculations); Initial and boundary conditions: (as received by KIT) Inlet flow rate for steam and argon: 2 g/s Debris bed simulation with MEWA (IKE): - calculates quenching of particle beds; - MEWA was started with a time dependent criterion; activated at t = 77500 s (experiment indicates debris relocation at top of grid spacer 2) resp. t = 77800 s (quench initiation); - size of debris bed was defined by input data: horizontally over region of ROD1 and ROD2, vertically from 0.392 m to 0.77m corresponding to first estimations considering test results (3 thermo-fluid volumes); - after activation of MEWA differential equations of ECORE model are switched off in the integration module FEBE; - boundary conditions for the remaining ATHLET-CD equations are defined by special coupling routines of MEWA; - particle diameter 3.5 mm (height of pellet segments) resp. 5 mm (consideration of larger parts of cladding in collected debris); 16
Post-test calculation: First main results (1) Hydrogen generation: Simulation with boundary conditions from the test results in good agreement of hydrogen generation during heat up and pre-oxidation period until ~ 67000 s; the increase of hydrogen generation rate after the power increase and before start of cool down (67000 77800 s) was still underestimated; Comparison of hydrogen generation of post-test calculation 17
Post-test calculation: First main results (2) 650mm 1150 mm Oxide layer thickness: ROD2 (Zry) ROD3 (Hf) Complete oxidation of Zry claddings at 650 mm elevation was reached after t = 56000 s (vs. experiment at t=72000 s); Complete oxidation of Zry claddings at 1150 mm (Δh 0.5m) after 78000 s agreement with time when quenching starts in the test Max. oxide layer thickness for Hf claddings low compared to that of Zry Oxide layer thickness: outer inner layer ROD2 noticeable internal oxide layer thicknesses at upper bundle elevations as shown from first evaluations of the test are not calculated by the code as activation temperature (T=1600 K) was not reached above 1150 mm; highest values of internal oxide layer in the simulation are ~ 100 µm at the 850/950 mm level; 18
Post-test calculation: First main results (3) Oxide layer thickness of grids: GRID3 and GRID4 are totally oxidized after ~ 50000 s; agreement with first results from the experiment, where grid spacer 4 was completely oxidized after the test; 19
Post-test calculation: First main results (4) Rod temperatures (lower upper bundle region): Good agreement of temperatures at lower elevations (350 750 mm), where the debris bed is built after the pre-oxidation period; Temperatures above 1000 mm are overestimated in the simulation (but underestimated inner oxide layer! Criterion for activation of inner oxidation has to be extended); Max. temperatures (850/950 mm) are underestimated by ~ 100 K during the pre-oxidation period in comparison to corrected test data (but good agreement of hydrogen generation!); 20
Post-test calculation: First main results (5) Graphic demonstration of temperatures in ECORE region (left) and MEWA (right) at t = 77800 s (initiation of reflood) 21
Post-test calculation: First main results (6) Rod temperatures during quenching: without (left) with (right) MEWA simulation Cooling within debris region (350 650 mm) is calculated very similar in both simulations in satisfactory agreement with measured data; Cool down of positions above the simulated debris bed (750 950 mm) occurs too early for both simulations, but total quenching occurs at the right time at ~ 78800 s if the debris bed model MEWA is used; 22
Post-test calculation: First main results (7) Rod temperatures during quenching: additional MEWA test calculations with parameter variations Increased particle diameter (left) The increase of particle diameter from 3.5 to 5.0 mm (collected debris contains larger parts of broken cladding) accelerates the cool down at 850 / 950 mm positions due to increased permeability; temperatures within the debris bed remain nearly unchanged; Simulation of the shroud leak (right) The consideration of the shroud leak (h = 850 mm, opening time 78720 s), where 90 % of the injected water was lost through the leak, results in significantly delayed quenching for elevations 850 / 950 mm; 23
Post-test calculation: First main results (8) Comparison of calculated grid temperatures with test data: Pre-oxidation period (left) Good agreement of calculated temp. for GRID2 (350 mm) with measured test data at the same level; wide band of test data measured at the top of grid spacer 2 (400 mm), where higher measured temp. coincide with calculated rod temperatures at levels 350 / 450 mm; Quenching (right) Cool down of GRID2 generally agrees well with test data beside a slightly too fast rewetting; the simulation calculates the rewetting of grids slightly faster compared to rods at the same level, whereas this is not the case for the test. 24
Post-test calculation: Conclusions First post-test calculations with an extended nodalization for the application of the debris bed model MEWA shows generally good agreement of temperatures and oxidation behaviour which can be derived from the measured hydrogen mass. The measured increase of oxidation rate in the last phase of pre-oxidation, which is probably mostly attributed to considerable inner oxidation at upper positions is not calculated by the code in the right extend; this leads to an underestimation of hydrogen generation of ~ 20 % at the end of the test The application of the MEWA model for the cool down phase shows that the cooling of the debris bed could be simulated with satisfactory results for the temperature decrease (particle diameter 3.5 mm). The parameter variation with higher particle diameter of 5 mm for the debris, as it seems reasonable considering the configuration of the collected debris after the test, shows a too early quenching. The consideration of the shroud leak did not show the expected concordance with test data regarding the temperature decrease at upper levels. In spite of remaining instability problems for the application of MEWA in the late phase of refilling this first post-test calculation of test QUENCH-17 contributed to the verification of MEWA within ATHLET-CD.. 25
Acknowledgement The development and validation of ATHLET-CD are sponsored by the German Federal Ministry of Economics and Technology BMWi. Thanks for your attention. 26