Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

The melt has high melting temperature (1231 °C)

which reduces the gap between the test and plant

accident conditions. It also provides better visualization

of the melt-coolant interaction details

because, and it has one element of the prototypic

corium component ZrO 2 . Dependencies of the

agglomerated debris fraction on the pool depth

were measured in four tests (A10, A11, A12, A14)

with about 20 kg of melt at about 60–200 °C

superheat. Similar results and tendency of increasing

fraction of agglomerated debris in case of

higher melt superheat were confirmed in comparison

to those obtained in the previous DEFOR-A

tests with another melt simulant (Bi 2 O 3 -WO 3 ) [6].

Results of the tests generally confirm that data produced

in the DEFOR-A experiment are not sensitive

to variations of melt material, at least for binary

oxidic type of melt.

The effect of jet free fall height on the particle size

distribution was also assessed in the new series of

tests. A range between 0.7 m (A11), and 0.0 m

(A12, A14 with release of melt under water) was

investigated. Remarkably, we found no big difference

between the particle size distributions and

morphologies obtained in case of melt release above

water level (jet free fall height more than 0.2 m).

In case of melt release under water the size of

the debris increases and morphology of the debris

changes from spheroid to flap-like particle. This

suggests that there is a change in the hydrodynamic

breakup regime. In general obtained in the

new series of the tests size distributions are similar

to those observed in the previous DEFOR-A test

with Bi 2 O 3 -WO 3 melt and agree with the data

from FARO experiments with prototypic corium

melt mixtures, larger jet diameters and jet free fall

heights. These findings are quite encouraging with

respect to possibility of using DEFOR-A data for

validation of particle formation and agglomeration

models and further work in this direc-tion is

reported in [7], [8].

3.2 Progress in DEFOR-MISTEE Tests

This work is motivated by the insights from Debris

Bed Formation tests (DEFOR-S and DEFOR-A) carried

out with WO 3 -Bi 2 O 3 as corium simulant material.

Analysis of DEFOR debris revealed strong influence

of water subcooling on particle morphology

(round shape or sharp edges), which was apparently

created in different fragmentation modes

(hydrodynamic breakup and solid fracture) [9].

The changes in particle morphology from mostly

round shape to mostly sharp edges (fractured) at

relatively small changes (~10–20 K) of water subcooling

were explained [10] by the effect of transition

from film to nucleate boiling on the particle

thermal stress. Experimental observations [9] and

predictions [10] also suggested that smaller particles

(below 1 mm) have higher chances to avoid

fracturing.

A series of confirmatory DEFOR-S type experiments

was carried out in a small scale MISTEE (micro

interactions in steam explosion energetic) facility.

DEFOR-MISTEE tests [11] were performed by

quenching small scale jets of different binary oxide

melt simulants (WO 3 -Bi 2 O 3 and WO 3 -ZrO 2 eutectic

compositions) in water. The analysis of debris

generally confirmed a transition in particle size

distribution between low and high water subcooling.

The transition occurs at ~50 K water subcooling

for WO 3 -Bi 2 O 3 and ~60 to 70 K for WO 3 -ZrO 2 .

The average particle size increases with decrease

in subcooling. WO 3 -Bi 2 O 3 material had a tendency

to produce more round shaped particles with low

water subcooling i.e. less than ~40 K while the presence

of round shaped particles was limited to ~0.3

mm at higher water subcooling i.e. above ~40 K.

Mass fraction of fractured particles increases along

with subcooling in all the tests.

Particle sizes and fraction of round shape WO 3 -

ZrO 2 particles were consistently larger than those

of WO 3 -Bi 2 O 3 particles, suggesting that there are

considerable differences in thermo-mechanical

properties important for the fragmentation modes.

In general results of the small scale tests carried

out in MISTEE facility is in good agreement with

previous experimental data obtained in a larger

scale facility (DEFOR) with WO 3 -Bi 2 O 3 [9] and with

results of simulations [10]. Further experiments

and analysis are necessary to develop quantitative

particle size distribution and morphology maps.

3.3 Progress in PDS Activity

Boiling and two-phase flow inside the bed serves

as a source of mechanical energy which can reduce

the height of the debris bed by so called «selfleveling»

phenomenon. However, to be effec-tive

in providing a coolable geometrical configuration,

self-leveling time scale has to be smaller than the

time scale for drying out and onset of re-melting

of the bed. The goal of this work is to assess characteristic

time scale of particulate debris spreading.

The PDS activity covers experimental and analytical

studies concerning the self-leveling phenomenon.

The experimental studies provides valuable data in

terms of empirical closure dependence of the par-

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ENSI Erfahrungs- und Forschungsbericht 2012

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