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ENSI Erfahrungs- und Forschungsbericht 2011

Project Goals

The safety impacts of air ingress on nuclear fuel

elements at high temperature have been studied

for many years, in accident situations involving

failure of the reactor pressure vessel (RPV) lower

head, shutdown conditions with the upper head

removed [1] and with, or in spent fuel ponds after

accidental loss of coolant [2]. The presence of air

can lead to accelerated oxidation of the Zircaloy

cladding compared with that in steam, owing to

the faster kinetics, while the 85 % higher heat of

reaction drives this process further. Air ingress is

typically associated with poor heat transfer; the

combined effect of these factors can give rise to an

increased rate of core degradation. Furthermore,

the exposure of uranium dioxide to air at high tem-

peratures can lead to increased release of some fis-

sion products [3]. The situation is kept under con-

tinual review, with experimental and modelling

studies performed, notably within the European

Union Framework SARNET project [4], and the In-

ternational Source Term Programme (ISTP) [5], in

which PSI takes part.

The MELCOR code is the major tool in use in Swit-

zerland for analysis of severe accidents in light

water reactors, from initiating events through to

potential release of radionuclide fission products

to the environment. Version 1.8.6 [6] is now es-

tablished as the current production version while

MELCOR 2.1 is still undergoing assessment. Use

of MELCOR is backed-up by SCDAP-based codes

[7], [8], for more detailed treatment of thermal

hydraulics and core degradation. The air ingress

model is being implemented in both MELCOR and

SCDAP/Sim.

The present three-year project running from 2009

to 2012 comprises two complementary activities

being pursued in tandem. The first of these is a

continuation of the previous PSI-ENSI collabora-

tion [9]. The model is being implemented in MEL-

COR to enable simulation of integral experiments

and plant or spent fuel transients. In the second

activity PSI is participating in the OECD Sandia Fuel

Project (SFP) [10], which will provide a prototypic

dataset under large scale fuel pond loss of cool-

ant conditions for validation of MELCOR code and

air oxidation models. The intended result is an im-

proved tool for plant and fuel pond simulation to

support PSA investigations and source term stud-

ies.

Work Carried Out

and Results Obtained

This section is divided into two parts. The first

presents a status of modelling activities during

the first year of the present PSI-ENSI collabora-

tion [11], which concentrate mostly on its imple-

mentation in MELCOR and SCDAP/Sim, the lat-

ter to enable validation against PARAMETER-SF4,

an air ingress experiment. The second part pres-

ents an overview of the SFP project, the experi-

ments planned therein and the sought-for results.

An indication is given of further work that could

be carried out, including a possible generalisa-

tion of the model to advanced cladding materi-

als such as Zirlo

and M5

, that feature in cur-

rent new reactors.

Part 1: Air oxidation modelling

Review of state of knowledge

Experimental Activities

The last formal reviews of activities in the experi-

mental area was presented at ERMSAR2008 [12],

and TOPSAFE2008 [13]. The status of studies con-

tinuing since then has been regularly reported in

the present series of annual progress statements.

During the last year further separate effects exper-

iments have been performed at KIT [14], concen-

trating on nitriding of homogenised alpha-Zr(O).

The latest air ingress bundle transient experiment,

QUENCH-16, was performed in July 2011[15]. It

comprised a comparatively minor degree of pre-

Figure 1: Sample results of QUENCH-16, indicating the

main phases of the experiment, the thermal response

during air ingress and the oxidation excursion during

reflood.

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