Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

(a)

(b)

Figure 1:

Non-clamping of the

IGT located closest

to the bottom center

(a) With CRGT and

top cooling, and (b)

Without CRGT and top

cooling.

(a)

(b)

Figure 2:

Clamping of the IGT located

farthest from the

bottom center (a) With

CRGT and top cooling,

and (b) Without CRGT

and top cooling.

considered as the dominant vessel failure mode. At

the expected time of IGT failure, the debris bed is

still solid. In the continuation study, an IGT located

farthest from the bottom center is considered as

well as the influence of CRGT and top cooling (as

SAM measure). In Fig. 1, the non-clamping of the

IGT located closest to the bottom center is shown

for both cases with and without CRGT and top

cooling. All the clamping pairs are greater than the

clamping threshold which also takes into account

the expansion of the IGT housing. The uncertainty

range of the global vessel failure time is obtained

by considering the influence of the melt properties.

Clamping of the IGT is possible, as shown in

Fig. 2, for the IGT located farthest from the bottom

center for both cooling and non-cooling cases.

However, it happens in the time window where

IGT can still fail due to weld failure. Further study

is necessary to identify the time of IGT failure due

to weld failure. Also for future work, we need to

identify locations of non-clamped IGTs as this will

determine the region where melt can be available

for ejection.

Lastly, it should be mentioned that a design of a

new facility to perform experiments for debris remelting

and melt interactions with vessel structures

is being developed. The key physical phenomena

to be addressed in the experiment are (i) remelting

of debris which consist of components with

significantly different melting temperatures (simulants

of oxidic and metallic corium debris) at different

conditions (heat-up rate, debris composition,

size distribution, porosity, etc.), (ii) melt interaction

with IGT mock-up, (iii) possible melt leak through

the IGT, and (iv) flow, ablation and freezing of the

melt containing solid particles through an open

IGT/CRGT nozzles in case of IGT/CRGT ejection.

3. Progress in DECO Activity

Melt fragmentation, quenching and long term

coolability of porous debris bed in a deep pool

of water under reactor vessel is employed as a

severe accident (SA) mitigation strategy in several

designs of light water reactors (LWR). Properties

of the debris bed such as particle size distribution,

porosity, fraction of agglomerated debris and

geometrical configuration of the debris bed are

the factors which define if the decay heat can be

removed from the debris bed by natural circulation.

DECO goal is to develop deterministic (data and

mechanistic models) and probabilistic frameworks

for assessment of the risk associated with formation

of non-coolable debris bed.

3.1 Progress in DEFOR-A Experiments

A series of confirmatory DEFOR-A tests has been

carried out with a binary mixture of heavy oxides

ZrO2-WO3 as a corium melt simulant material.

ENSI Erfahrungs- und Forschungsbericht 2012 235

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