Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

homogeneous heat-releasing debris bed is considered

in [14], with the critical conditions for

steady-state cooling characterized by the Dryout

Heat Flux (DHF). DHF is determined for top-fed

and bottom-fed debris beds from several twophase

models differing by the treatment of porous

and interphase drag. Aleatory uncertainties due

to randomness of the debris bed formation scenario

and respective physical parameters (particle

diameter, porosity) are quantified. It is shown that

the «model-to-model» differences are noticeable

on the cumulative distribution functions, however,

the lower coolability boundary, corresponding to

5% of cumulative distribution function of DHF, is

predicted by all models in quite narrow range. The

effects of the ranges and distributions of the input

parameters on the DHF and CDF are investigated.

We show that qualitative behavior of the CDF

and conclusions about the relative importance of

uncertainties in different input parameters do not

change if different probability distributions are

used. However, the lowest DHF values corresponding

to 5% CDF can be increased by about 25–30%

due to the elimination of, or making less probable,

the most prone to dryout combinations of parameters.

Narrowing of the uncertainty ranges of the

input parameters decreases the total uncertainty

of the DHF prediction, while the lower DHF boundary

(at 5% CDF) can either increase or decrease.

The latter depends on the variation of the mean

value which the changes in the ranges and distribution

function can entail.

The concept of «load versus capacity» is employed

in [15] to quantify the probability of failure (local

dryout). Possible choices of «load» and «capacity»

in terms of heat fluxes, thermal power or melt

mass as well as the results of Monte Carlo simulations

of distribution functions for the local heat

flux and the dryout heat flux at the debris bed

top point (defined as the extensions of one-dimensional

counterparts) are discussed in [15]. A surrogate

model for the dryout heat flux is developed by

the generalization of two-dimensional simulation

results. Dryout probabilities are obtained under the

conservative assumptions (neglecting the coolability

improvement due to side ingress of water

into a non-flat debris bed), and from the surrogate

model (see Table 2).

3.5 Progress in POMECO Experiment

The objective of the POMECO experiments is to

provide data for validation of the codes which can

assess the coolability of a debris bed formed in

fuel coolant interactions (FCI) during a postulated

Slope angle α Dryout probability P, [%]

M = 200 t, Q = 3 GW

M = 256 t, Q = 3.9 GW

2D model Conserv. 2D model Conserv.

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