ERMSAR 2012, Cologne March 21 – 23, 2012 1

ON THE ROLE OF VOID ON STEAM EXPLOSION LOADS

ERMSAR 2012, Cologne March 21 – 23, 2012 2

Introduction : on steam explosion

TROI-30 (KAERI)

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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

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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)

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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:fragmentWater +vapor interacting homogeneously with fragments

Direct vaporization :Each fragment is surrounded by a vapor film.

drop

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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 %

droppletdropletbubblybubblyglobal fFfFf

m-fluid = fragment + gas + entrained water

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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 = fe.F fe =Cte ~ 7 as recommended value

(volume of water in m-fluid = fe 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

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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

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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

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1-Dimensional study

3 different melt fractions : 1, 3 and 5 %

10 different void fractions : 5, 10, 20, 30…90 %.

characteristic UO2/ZrO2 corium at 3000 K (no solidification effect)

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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

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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

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2D 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 case TROI experiments PWR case

Water level x radius

1 m x 35 cm 3.6 m x 2.45 m

Mixture radius 14 cm 70 cm

Melt masses (1%, 3%, 5%)

5.5 kg , 15.7 kg, 26 kg

400 kg, 1200 kg, 2000 kg

Physical time simulated (ms)

10 30

Homogenous mixture of fragments + water + vaporSurrounded by pure water

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2D 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

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– 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

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– Similar patterns of explosion, the larger scale allows a stronger escalation

Some calculations reached the code limit (~3000 bars) for MC3D

2D Reactor case

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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