Fluid dynamic effects in the fuel element top nozzle area during refilling and reflooding

Directorate-General For Research And Innovation

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Industrial research and development

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• Sciences et techniques

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 13 Mo

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Commission of the European Communities
nuclear science
and technology
FLUIDDYNAMIC EFFECTS
IN THE FUEL ELEMENT TOP NOZZLE AREA
DURING REFILLING AND REFLOODING
Report
EUR 10165 EN
Blow-up from microfiche original Commission of the European Communities
nuclear science
and technology
FLUIDDYNAMIC EFFECTS
IN THE FUEL ELEMENT TOP NOZZLE AREA
DURING REFILLING AND REFLOODING
A. HAWIGHORST, H. KRONING, D. MEWES
R. SPATZ, F. MAYINGER
UNIVERSITÄT HANNOVER
Gallinstrasse, 46
D-3000R
Contract No. SR-001 -80-D
FINAL REPORT
The work reported in this document was carried out as a jointly funded project of
the Commission of the European Communities and the Bundesminister für
Forschung und Technologie
Directorate-General Science, Research and Development
1985 EUR 10165 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Information Market and Innovation
Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting on behalf
of then is responsible for the use which might be made of the following
information
ECSC—EEG—EAEC Brussels - Luxembourg, 1985 III
LAY OUT
This report is presented in one volume, divided into 3 distinct parts,
which are as foLLows :
Part 1 : Investigations of gas/Liquid counter-current flow in
vertical channels
Part 2 :s of steam/watert flow in
fuel element geometries
Part 3 : Investigations on drop size determination IV
0. Introduction to the Final Report
Safety aspects are very important for power supply with nuclear power
plants. High-capacity nuclear power plants are usually equipped with
water-cooled nuclear reactors. For an environmentally acceptable and
safe operation of these plants, which are mostly equipped with
pressurized water or boiling water reactors, it is an important
prerequisite that an accident caused by a loss of coolant - due to a
leakage in the primary system - can be controlled.
In order to avoid that radioactive products are released in an uncon­
trolled way, it is necessary to provide a quick and reliable emergency
cooling of the reactor core, producing decay heat. This emergency core
cooling must make sure that the fuel rods, whose temperature rise con­
siderably due to subsequent disintegration of radioactive isotopes, are
rapidly rewetted by means of emergency core cooling water and are thus
cooled down to safe temperatures.
The rewetting process is extremely dependent upon the fluiddynamic
processes in the fuel assemblies of the reactor, as well as in the areas
located above, i.e. the fuel element top nozzle and the upper plenum of
the reactor pressure vessel. The feeding of water into the reactor core
and the subsequent cooling of the fuel rods can be simply affected by
discharge hindrances in the fuel assembly top nozzle and in the upper
plenum. This is the case, if a steam cushion has developed in this area,
the discharge of which is affected negatively by two-phase mixtures in
the pipe work going to the leak. Furthermore, it is possible that a
liquid or foam layer is present between tie-plate and grid-plate, which
can also act as obstacle against the discharge of the steam which was
produced in the fuel assembly. The cooling and rewetting process can be
improved considerably by means of a downward-flowing liquid. A liquid or
foam layer between tie-plate and grid-plate can develop due to the fact
that water droplets, which are carried along by the discharging steam
during the cooling and evaporization processes in the lower areas, are
separated and accumulated there.
The evaluation and prediction of the emergency core cooling behaviour of
water-cooled nuclear reactors takes place with the use of extensive computer programmes, which are based on experimental data and which must
be verified in model experiments. These computer codes, however, are
still needing a clear analysis of the flow processes in the area of the
fuel element top nozzle, among others. It is only with this knowledge
that the emergency core cooling behaviour can be predicted in a more
reliable and safer way than before.
It was the objective of this research project to contribute to a better
physical understanding of the fluiddynamic processes in the area of the
fuel element top nozzle and so to improve emergency core cooling
calculations. The results are to be used mainly for the multinational
research project 2D/3D. Therefore, experimental and theoretical invest­
igations about the entrainment and countercurrent behaviour of gas/liquid
flows have been implemented within this project. Fluiddynamic processes
in the fuel element top nozzle area were simulated during the reflooding
and refilling phase.
Based on special internals as single and multiple-hole orifices, basic
phenomena of fluiddynamics were studied first with air-water. Subse­
quently, investigations of the system steam/water were conducted in
another, newly developed test facility. The reactor geometry was appro­
ximated step by step, until a complete reactor fuel assembly top nozzle
was constituted. The system pressure was 4.8 bars (abs), in accordance
with the conditions in the reactor pressure vessel at the end of the
blowdown phase. The water was initially fed in at saturation temp­
erature, then, as a second step, fed in at subcooled condition relative
to the steam temperature, in order to be able to study condensation
effects as well.
The report at hand is divided into three parts, which contain a detailed
description of the research executed from March 1st, 1980 until September
30th, 1984.
Part I covers investigations on gas/liquid countercurrent flows in the
fluid system air/water.
Part II contains countercurrent flow studies in the system steam/water,
including the investigation of condensation effects. VI
Part III gives a detailed description of the research on droplet size
determination.
The research project was supported at 50% each by the Federal Minister of
Research and Technology and the Commission of the European Communities. VII
PART 1
INVESTIGATIONS OF GAS/LIQUID COUNTER
CURRENT FLOW IN VERTICAL CHANNELS