Caple AUF900 Manual de usuario descargar pdf (Pagina 249)

247

ENSI Erfahrungs- und Forschungsbericht 2011

head dominates over thermal expansion in the flow

limiter area. In Figure 2b the change in distances

between the points with respect to time. The max-

imum distance between the edge of the flow lim-

iter and the IGT casing is given at 0.25 mm. Thus,

clamping is defined as

Di (t) -0.5 mm, for any

pair i at any time t > 0 but before the nozzle welds

fail. Analysis of results presented in Figure 2b sug-

gests that at given melt pool depth of 1.9 m and a

location of the IGT close to the bottom center, the

IGT is not clamped in its housing during the entire

time until global vessel wall failure takes place. This

result suggests that in the considered case of 1.9 m

melt pool, the dominant vessel failure mode is IGT

failure and it happens at least 1 hour before the

global vessel failure. Other melt pool depths and/or

other locations of the IGT might result otherwise,

but are not covered here and will be the subject of

further inquiry. In another piece of work the effect

of low fluid Prandtl number (Pr) on heat transfer

from a melt pool to the vessel wall has been exam-

ined [3]. The integral effect of the local heat trans-

fer enhancement due to corium melt low Prandtl

number is found to be significant, and the vessel

wall temperature is considerably increased. How-

ever, we also found that this effect occurs later

than the rapid acceleration of vessel creep in con-

sidered case of 0.7 m deep melt pool. Thus global

vessel failure is not sensitive to the local heat trans-

fer enhancement caused by fluid low Pr.

2. Progress in DEFOR Activity

A conservative-mechanistic approach to predict

the debris agglomeration is proposed, based on

simulations of the VAPEX FCI code and validated

by the DEFOR-A experimental data (see Figure 3).

It is assumed that mass fraction of agglomerated

debris is proportional to the mass fraction of com-

pletely liquid droplets and thin-crust particles («liq-

uid» or «glue» particles).

Figure 1:

(a) Mesh of the IGT

section with 11553

tetrahedral elements

and 18435 nodes,

and (b) geometry with

the location of the flow

limiter.

Figure 2:

(a) Displacement of points

along the flow limiter area

and four pairs of points for

which the distances bet-

ween points are measured

with respect to time;

(b) The change in distances

(t) = D

(t) – D

) for

each pair i and time t

where D

) is the distance

between the points in pair

i at time t = 0. The vertical

lines correspond to the

estimated time of nozzle

welds creep acceleration,

nozzle welds melting, and

global vessel wall creep

acceleration.

(a) (b)

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