ERMSAR 2012, Cologne March 21 – 23, ON THE ROLE OF VOID ON STEAM EXPLOSION LOADS

ERMSAR 2012, Cologne March 21 – 23, Introduction : on steam explosion TROI-30 (KAERI)

ERMSAR 2012, Cologne March 21 – 23, On the role of void : motivations Since about 10 years, the focus has been put of the premixing stage in order to be able to provide a reasonable initial state: – Distribution of melt, liquid and vapor After important improvements of modeling and code capabilities and functionalities, focus is put back on explosion stage The models in explosion are based on conceptual pictures which need to be justified and improved – These concepts were built with assumptions of small local void because it was estimated that void would suppress the explosion – Recent reactor applications tend to converge on the existence of a quite important void during premixing (high temperature, quite small drops) => The impact of void on the explosion, as calculated by the codes, has to be investigated in details

ERMSAR 2012, Cologne March 21 – 23, Description of exercise performed in SARNET frame Two models under development in EU (not much more in the world !) – IDEMO : IKE Stuttgart (support from GRS) – MC3D-EXPLO : developed by IRSN with support from CEA and EdF. – Both with two very different assumptions on pressurization mechanisms Joint IRSN – IKE work in two steps – 1-D (analytical) configuration with no specific link to real situation – 2-D (analytical) configurations  One at experimental scale (based on TROI exp)  One at reactor scale (~PWR)

ERMSAR 2012, Cologne March 21 – 23, Models in IDEMO and MC3D IDEMO - The “micro-interaction“ approach – Fragments are in quasi-equilibrium with gas and part of coolant  m-fluid – Dilatation or vaporization of m-fluid  Pressurization – Model parameter : Rate en entrainment of coolant in m-fluid (fe) MC3D - The direct vaporization approach – Vapor film production around fragments  Pressurization micro-interaction: fragment Water +vapor interacting homogeneously with fragments Direct vaporization : Each fragment is surrounded by a vapor film. drop

ERMSAR 2012, Cologne March 21 – 23, Models in IDEMO and MC3D : role of void Flow maps – MC3D : separation in bubbly and droplet regions  B = 0.3  D = 0.7  Drop and fragments are homogenously ditributed  A cut-off at 70 % void is expected – IDEMO  One single pattern, independent of conditions  Was reconsidered after 1-D exercise results to introduce a cut-off of HT’s when void is larger than 70 % m-fluid = fragment + gas + entrained water

ERMSAR 2012, Cologne March 21 – 23, Models in IDEMO and MC3D : role of void Fragmentation – Evaluated differently but similar laws – Fragment size is a user input  Improvements underway for both codes Heat and mass transfers - Pressurization – IDEMO : micro-interaction model, with water entrainment rate proportional to the fragmentation rate independent on void:  E = f e.F f e =Cte ~ 7 as recommended value (volume of water in m-fluid = f e x volume of fragments)  All the void is put in the m-fluid – MC3D :  Direct vaporization through a heat balance at film interface  explicit cut-off of vaporization if fragment in droplet flow since the fragment is not in contact with water

ERMSAR 2012, Cologne March 21 – 23, Recent changes in MC3D Up to version 3.6 – Assumption of a very transient heat transfer with fixed, user input, transfer coefficient (50 kW/m²/K) The TREPAM experiments (CEA/IRSN, quenching of small filaments) showed: – no transient stage – HT is quasi-steady and reasonably evaluated with classical film boiling models (e.g. Epstein-Hauser), even for pressure slighly above critical pressure (240 bars) and high velocity (40 m/s) In version 3.7 : – no transient stage : HT with film boiling (Esptein-Hauser) – Fragmentation characteristics slightly reconsidered – Recent new model for drop fragmentation are not used in this exercise

ERMSAR 2012, Cologne March 21 – 23, Summarizing … Code main parameters for calculations: – Note : heat transfer 10 times higher in IDEMO Only cases with fe = 7 (IDEMO) and version 3.7 (MC3D) discussed here

ERMSAR 2012, Cologne March 21 – 23, Dimensional study 3 different melt fractions : 1, 3 and 5 % 10 different void fractions : 5, 10, 20, 30…90 %. characteristic UO 2 /ZrO 2 corium at 3000 K (no solidification effect)

ERMSAR 2012, Cologne March 21 – 23, Results for the 1-D configuration Despite the simplicity of the configuration, the exercise gave important results for the analysis of the codes and impact of the void The general patterns of pressurization history are different – Higher and sharper peaks in IDEMO => peak pressure less significant – Stabilization of pressure at the bottom in IDEMO whereas slow decrease in MC3D – Two characteristic pressure plateaus in IDEMO

ERMSAR 2012, Cologne March 21 – 23, Results for the 1-D configuration The pressure peaks are largely higher in IDEMO in most cases In both cases the reduction due to the void is limited The pressure can reach high values even at very high void with IDEMO – The behavior at high void has been since then revisited In contrast, the impulses are of the same order and nearly insensitive to the void

ERMSAR 2012, Cologne March 21 – 23, D configurations – Of course, situations idealized and not realistic – Should always exists some layer around mixture with liquid melt and low void Experimental scale Reactor scale Typical caseTROI experimentsPWR case Water level x radius 1 m x 35 cm3.6 m x 2.45 m Mixture radius14 cm70 cm Melt masses (1%, 3%, 5%) 5.5 kg, 15.7 kg, 26 kg 400 kg, 1200 kg, 2000 kg Physical time simulated (ms) 1030 Homogenous mixture of fragments + water + vapor Surrounded by pure water

ERMSAR 2012, Cologne March 21 – 23, D Experimental case – Similar results for both codes – Quite similar to 1D with unobvious impact of void  Still quite high pressures can be reached in premixture, Strong reduction at wall

ERMSAR 2012, Cologne March 21 – 23, – Void favors fragmentation by increasing coolant velocity – But escalation more difficult => impact depends on the scale – Water enters into the mixture at the shock front – Ex : flow for 5, 30, 50 and 70 % 2D Experimental case

ERMSAR 2012, Cologne March 21 – 23, – Similar patterns of explosion, the larger scale allows a stronger escalation  Some calculations reached the code limit (~3000 bars) for MC3D 2D Reactor case

ERMSAR 2012, Cologne March 21 – 23, Conclusions Interesting analytical exercise allowing a better understanding and easier comparison of the code behavior Peak pressures in general higher with IDEMO but impulses in the same range – Small visible impact of different modeling – Note that loads are comparable to experimental results Unobvious impact of void with limited effect due to complex interactions – Pressure in premixture zone can be very high even if strong venting effects (smaller pressure at the wall) – Need for verification of behavior at highly supercritical pressures Since no strong reduction is obtained with void, the behavior at high void should be carefully analyzed