Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Figure 7: Comparison of phase II thermal response up to

first ignition: Data, MELCOR, PSI model.

The phase II analyses are reported by Jaeckel [24].

The flame front slowly propagated downward,

reaching the bottom of the bundle after about a

further 6 hours (figure 8). A problem in the downward

propagation of the zirconium fire is that a

flat power profile along the heated fuel pins is

assumed. The slight temperature-dependence of

the heater wire resistance causes the power profile

to be peaked at regions with higher temperatures.

Due to limitations in MELCOR, qualitative assessment

of the effect was made using a constant

tilted power profile which resulted in closer agreement

with the data.

The radial fire propagation into the peripheral fuel

elements was also simulated with MELCOR (figure

9). There was insufficient oxygen to consume all

the metallic cladding during this downward propagation,

and like in the phase I experiment burning

continued for about 4 days at a low air flow rate.

As can be seen, the fire spread outwards from the

centre at the same time as propagating downward.

The first ignition at the radial locations occurred at

progressively lower elevations during the the fire

spreading.

Figure 9: Radial fire propagation into the peripheral fuel

elements. The colours indicate the radial node in the

MELCOR model.

In phase II the gas composition at the fuel bundle

outlet was measured during the entire experiment.

A very important finding was the strong nitrogen

consumption during the downward propagation

of the fire, followed by release of nitrogen during

the upward propagation as the zirconium nitride

was reoxidised. The nitriding reaction of zirconium

in the oxygen-starved region may deliver as much

heat as the oxidation by oxygen. The reoxidation

is also exothermic.

The final benchmark report of the SFP phase II

experiment will be released in February 2013. The

present analysis forms the basis of PSI contribution

to the benchmark. Work continues on this topic.

Figure 8: Calculated and measured downward propagation

of flame front in fuel bundle in phase II experiment inclusive

calculations with the PSI air oxidation model.

ENSI Erfahrungs- und Forschungsbericht 2012 229

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