Caple AUF900 Manual de usuario descargar pdf (Pagina 253)

251

ENSI Erfahrungs- und Forschungsbericht 2011

ter of the bed prevents the side coolant from flow-

ing into the center of the bed. For the bed with a

heap-like shape (cone) as shown in Figure 5b, the

dryout power density is 535 kW/m

. Surprisingly,

instead of increase in dryout power density, it de-

creases by 41 %, compared with the value of the

top-flooding bed (910 kW/m

). This is because the

heap-like bed has the height of around three times

the top-flooding bed, given the same mass of the

debris. The special geometry results in a high void

zone near the tip of the cone, where dryout oc-

curs first. Notably, due to the high rate of steam

flow through the tip, the dryout zone and its tem-

perature do not vary rapidly with increasing power

load. In other words, the steam flow plays a role

in coolability. An axially stratified debris bed with

a fine-particle layer settle atop a larger-size particle

layer is considered (see Figure 5c). The coolability

of such bed is determined by the top layer in top-

flooding scheme, since the capillary force across

the interface of the two layers prevents coolant

from reaching the lower layer. Such a barrier, how-

ever, can be alleviated if coolant is supplied from

the bottom through the annular gap as shown in

Figure 5c where it is assumed that a 200-mm-thick

layer with 0.5-mm-diameter particles sits atop a

474-mm-thick layer with 1-mm-diameter parti-

cles. The porosity is 0.45 for both layers. The dry-

out power density is calculated to be 356 kW/m

which is 63 % higher than the values (218 kW/m

)

of the top-flooding bed packed with the 0.5-mm-

diameter particles. The dryout positions for the

three beds can be seen in Figure 6. The co-current

two-phase flow in the multi-dimensional beds rais-

es the vulnerable location of dryout upward.

Extensive sensitivity-uncertainty analysis has been

performed for 1D debris bed coolability [12]. Cali-

brated «classical» model has been proposed based

on the optimization with regards to available ex-

perimental data. Comprehensive sensitivity analy-

sis suggests that up to 70 % of the uncertainty in

the debris bed coolability is due to the uncertainty

in the particle size distribution.

4. Progress in SERA Activity

The SERA study was focused on physical mecha-

nisms and material effects on triggerability and en-

ergetics of single droplet steam explosion.

The MISTEE experiment [13] using WO

-CaO mix-

ture as corium simulant provided data on the pro-

cesses in which the eutectic and non-eutectic com-

positions affect steam explosion energetics of a sin-

gle droplet. At lower superheat (100 °C) a notice-

able differences in preconditioning and conversion

ratio between the eutectic and non-eutectic melts

were observed. Such observations can be rational-

Figure 7:

Debris size distribution of

eutectic and non-eutectic

-CaO melt droplets,

with a 100 °C superheat,

which underwent a steam

explosion.

Figure 8:

Morphology of the

fragmented debris.

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