CONTAINMENT BEHAVIOUR IN SEVERE ACCIDENTS
LWR SEVERE ACCIDENT SOURCE TERM:
T. Kress, D. Powers, R. Lee and L.
PART 2: FISSION
PRODUCT RELEASE, TRANSPORT AND BEHAVIOR IN THE CONTAINMENT
U. S. Nuclear Regulatory Commission
Fission product releases to the environment, or source terms, arise as a result
of a highly diverse group of phenomena involved in any particular severe
accident sequence. For light water reactors (LWRs), these phenomena include
fission product release, transport and behavior in core and primary system and
in the containment. They include core heatup, fuel element degradation and
melting, pressure vessel attack and failure, possibly high pressure melt
ejection, interaction of core debris with concrete, retention of fission
products within the reactor coolant system, effects of hydrogen burns or
detonations, retention of fission products by suppression pools or ice beds,
late revolatilization of fission products from surfaces, and, clearly the
effect of containment integrity or containment bypass and time and location of
containment failure, if it occurs. Because of the multiplicity of accident
sequences that can occur for a given plant as well as the diversity of the, as
yet, imperfectly understood severe accident phenomena, it is not surprising
that probabilistic risk assessments such as, for example, those documented in
NUREG-1150 have indicated large uncertainties in source terms which represent a
significant contribution to the uncertainty in the absolute value of risk
because of the difficulty and expense involved in performing prototypic
experiments, substantial reliance has been placed on the development and
validation of detailed mechanistic computer codes for analyzing severe accident
phenomena and the source terms associated with them. This paper discusses the
extensive research and other efforts that have taken place over the last decade
to address the technical issues which bare on being able to describe
quantitatively the source term(s) and its characteristics. It also summarizes
our present state of knowledge and points out areas where additional research
will add further to our understanding. Finally, this paper discusses the NRC's
efforts in revising the licensing source term (TID-14844) and the applications
of this revision, especially for siting and design of future power plants.
INSIGHTS INTO SEVERE ACCIDENT PHENOMENA FROM LEVEL 2
Vattenfall Energisystem AB, P.O. Box 528, S-16216 Stockholm,
In 1992-94 a PSA Level 2 Study was performed for the Ringhals 2 NPP - a
three-loop W-PWR on the west coast of Sweden.
This paper gives an overview of the study with emphasis on phenomena and their
importance for processes resulting in activity releases.
The following phenomena were included as possible causes of activity releases
from the containment:
The largest contribution to rapid overpressurization of the containment is
derived from hydrogen deflagration.
- Hydrogen Deflagration/Detonation
- Direct Containment Heating
- In-vessel ans Ex-vessel Steam Explosions
- Global Vessel Failure
- Thermal Attack of Pentrations
- Basemat Failure due to Core Concrete Interaction
The Level 2 Study also includes a sensitivity analysis where the impact on the
results from uncertainties in the phenomena are estimated. This is shortly
described in this paper together with the effect of human errors.
Finally the results of the study are summarized and discussed.
PREDICTION OF THERMOHYDRAULICS OF MODEL CONTAINMENTS
CEC, Ispra, Italy
THERMALHYDRAULICS TESTING OF THE PHEBUS-FP CONTAINMENT
I. Shepherd, E. Hontanon, S. Gaillot, L.
Herranz, E. Bonanni, K. Akakane
Safety Technology Institute Commission of the European Communities
Joint Research Centre 21020 Ispra (Va), Italy fax +39 332 785053, phone +39 332
CEA Cadarache, France, CIAMAT, Madrid, Spain, NUPEC,
In the Phebus experiments fission products and structural materials from a
realistic source will enter the containment vessel in the form of aerosol
particles. One of the objectives of the test series is then to see whether
steam condensation onto these aerosols will influence the rate at which they
are removed from the atmosphere. The relative humidity is the most important
parameter in determining this condensation rate so each Phebus-FP test has a
different target humidity that should be reached.
The Phebus containment vessel is designed so that the temperatures of all the
surfaces and the sump can be regulated: they can either be held constant or
changed in a pre-determined manner. The experimenters then have to choose
values of the surface temperatures that produce the desired humidity when steam
and hydrogen are entering the vessel at a specified rate and afterwards when
the aerosols are settling.
The first approach in determining the best surface temperature for the vessel
was to run computer codes such as JERICHO and CONTAIN that are used for
calculating the consequences of an accident in a full-sized reactor. The
results of these calculations demonstrated a sensitivity to heat and mass
transfer correlations. The codes have not been validated in geometries similar
to the Phebus vessel so a priori there were few grounds for preferring one
correlation to another.
In this paper we summarize blind pre-test calculations performed with the
codes JERICHO, MELCOR, CONTAIN, CONTEMPT and CONT, show how the results
compared with the thermalhydraulic experiments and go on to analyze the reasons
for the discrepancies in the code results.
Armed with the knowledge of these tests another benchmark exercise was
organized with boundary conditions corresponding to measured values from the
experiments. Codes using the well-mixed hypothesis as well as multi-compartment
and three dimensional codes were used. Comparing the results of the
calculations with measurements contributed to further understanding of the
thermalhydraulic behaviour of the Phebus-FP vessel and a better understanding
of the uncertainties of the codes.
EXPERIMENTAL INVESTIGATION AND ANALYSIS OF HYDROGEN
S. B. Dorofeev, A. A. Efimenko, A. S. Kochurko,
V. P. Sidorov
Russian Research Centre "Kurchatov Institute", Moscow, 123182,
A review of hydrogen combustion research at Kurchatov Institute is presented.
Results on the spontaneous detonation scaling methodology and on the loads from
different combustion and explosion modes are summarised.
Criterion for spontaneous detonation onset possibility and its application to
severe accident in a nuclear power plant is discussed. Theoretical and
experimental results on spontaneous detonation onset conditions are summarised.
Three series of large scale turbulent jet initiation experiments have been
carried out in KOPER facility (50 m3 and 150 m3). Series
of jet initiation experiments in initially confined H2 - air
mixtures have been carried out in KOPER facility (20 - 46 m3).
Turbulent deflagration/DDT experiments were carried out in large scale confined
volume of 480 m3 in RUT facility. Transition to detonation was
observed at min. of 12.5% H2. Results showed, that the
characteristic volume size should be used for conservative estimates in
accident analysis. Series of experiments on detonation transition from one
mixture to another of lower sensitivity has been carried in DRIVER facility.
The experiments were aimed on the estimation of the minimum size of a
detonation kernel. The received results are in a good agreement with the 7
3D computer codes 3ET and b02 have been developed for description of loads from
detonations. Series of large scale H2 detonation experiments have
been carried out in RUT facility (16 - 25 %H2, two initiator
locations). Experimental results form a database on detonation loads on
reactor-relevant scale and complex 3D geometry. B02 code was evaluated against
experimental data. Good agreement of main loading parameters was observed for
fully developed detonations. Effect of DDT location on loads has been studied
experimentally (UTR facility) and numerically in 1D geometry. It was found,
those peak overpressures depend strongly on DDT location, and can be
significantly higher than that from detonation. Impulses depend mainly on the
mixture volume. Data of large scale detonation, deflagration and DDT
experiments (RUT facility) confirm these observations. Experimental data on
turbulent flame propogation and on resulting loads (including DDT events) were
received in large-scale RUT experiments.
Recent results of combined hydrogen injection/ignition experiments are
presented. The experiments are aimed on the investigation of possible
consequences of deliberate ignition at dynamic conditions. Experiments include
the large-scale tests on the effects of igniter location, ignition time,
injection rate (0.1 - 1 kg/s) and injection point on the combustion mode. The
possibility of initiation of local detonations due to ignition at dynamic
conditions was observed in the tests. The experiments showed a good local
mixing and large-scale hydrogen concentration nonuniformities. Multiple
explosions at continuous injection were observed. The possibility of local
detonations resulting from deliberate ignition should be taken into account in
* Research sponsored by the US NRC and KfK,
CRITERIA FOR TRANSITION FROM DEFLAGRATION TO DETONATION IN H2-AIR-STEAM MIXTURES
C. C. Chan, W. A. Dewit, G. W. Koroll
AECL Research Whiteshell Laboratories Pinawa, Manitoba ROE 1LO
During a loss of cooling accident, hydrogen (or deuterium) can be formed due to
metal-steam reaction. This hydrogen can leak into the containment building to
form a combustible mixture. The pressure loading on the containment resulting
from a hydrogen burn depends on whether the burn is a deflagration or a
detonation. Direct initiation of detonation is very unlikely because it
requires a high energy source such as solid explosive that is not present
inside a reactor. However, a detonation is still possible by way of a
Deflagration to Detonation Transition (DDT). For insensitive
H2-air-steam mixtures, flame acceleration is the most probable
mechanism for transition to detonation. This paper describes some recent
results of a study on DTT resulting from flame acceleration and based on these
results, establishes the criteria for DDT for these insensitive mixtures.
The propagation of a freely expanding flame is intrinsically unstable. Due to a
feedback mechanism between the combustion induced flow and the combustion
itself, a flame can accelerate very rapidly if obstructions are placed along
its path. If appropriate conditions (in terms of the composition of the
mixture, the flame speed and the configuration of the obstruction) are present,
a DDT can occur. Presently, these conditions for H2-air-steam
mixtures have not been determined. This paper describes a systematic study of
flame propagation in a duct filled with obstacles to identify the transition
limit (in terms of mixture composition), the transition distances and the
critical flame speed leading to a DDT. Experiments were performed in a 28 cm
diameter, 6-m-long combustion duct. Baffle type obstacles, having a blockage
ratio (blocked area to cross-sectional area ratio) of 0.4 were mounted along
the duct to induce turbulence in the unburned gas. Piezoelectric pressure
transducers were mounted along the wall of the duct to monitor the location of
a DDT as well as flame speed just prior to the transition process. Results
showed that the range of mixture (in terms of the H2 concentration)
for which DDT has been observed reduces as the steam concentration increases.
DDT was not observed in any mixture containing more than 30% of steam. Results
also showed that DDT did not occur if a flame had not accelerated to a speed
corresponding to a flame Mach number greater than 1.5. The transition limits
and the critical flame speed are necessary conditions for DDT; both of these
criteria have to be satisfied before a DDT can occur.
CATALYTIC HYDROGEN MITIGATION IN POST-ACCIDENT
G. W. Koroll1, W. A. Dewit1,
W. R. C. Graham2
1Containment Analysis Branch AECL Research - Whiteshell Laboratories
Pinawa, Manitoba ROE 1LO
2Chemical Engineering Branch AECL Research - Chalk River
Laboratories Chalk River, Ontario KOJ 1JO
Catalytic recombination is a means of passive hydrogen removal from a
post-accident containment atmosphere. It is attracting the attention of the
utilities, designers and regulators as simple backfittable means of improving
the margins of safety for hydrogen. The current generation of catalytic
recombiners has demonstrated high capacity, resistance to fouling and generally
good safety orientation under most fore seeable conditions. This paper
describes progress in catalytic recombiner development, with emphasis on the
activities at AECL.
The essential feature of the AECL recombiner is a novel catalyst material which
is unmatched in wetproofing and thermodynamic range of operation. The catalyst
is the product of the 20 years of engineering of high-quality catalysts to
separate hydrogen isotopes in heavy water manufacture, a continuing key
technology area in AECL. The catalyst is a nuclear product manufactured
exclusively by AECL. It is successfully used to control hydrogen in other
nuclear applications such as in the transport and storage of wet radioactive
materials, and in fuel reprocessing operations. The active catalyst material is
platinum, matrixed in an inorganic wetproofing support and bonded to a
stainless steel substrate by a proprietary process. The formulation has
extraordinary stability and robustness. Problems such as spalling and
evaporation of wetproofing at high temperatures are eliminated. For this
application, the catalyst is manufactured in thin, tough sheets with low mass
for fast heat dissipation and low bulk for compact size per unit capacity.
Function and performance of the AECL recombiner were demonstrated in the 6.6
m3 and 10.7 m3 Containment Test Facility
(CTF) vessels at AECL Whiteshell Laboratories (WL) using a 1/10-scale test
model and a full-scale prototype recombiner. The AECL recombiner removes
hydrogen (and carbon monoxide) at a high capacity and is resistant to
foreseeable poisons and fouling agents in containment, during normal operation,
or in an accident.
The AECL recombiner is self-starting in the range of temperatures 20 to
150oC at a low H2 concentrations (<1.0% V) and starts at
low temperatures in a condensing atmosphere. Humid performance at low
temperatures is considered a vital test of wetproofing effectiveness and is
critically important in containments where pressure suppression systems are
The nominal capacity for hydrogen recombination is 5.0
0.2 kg/h per
m2 of inlet area to the recombiner in 5.0% H2 in air at
25oC. The capacity is increased by approximately 30% per metre of
chimney length. The capacity is insensitive to the presence of the diluents
such as steam, CO2, or N2 as long as the minimum
stoichiometric amount of oxygen is available to recombiner the hydrogen. The
capacity increases directly in proportion to the initial pressure in the range
1-2 atm. The capacity increases in proportion to the concentration of the
limiting reactant. Finally, the capacity for hydrogen removal increases in
proportion to the inlet area to the recombiner housing. This simple scaling
parameter was verified in identical tests with the 1/10-scale test model and
full-scale prototype recombiners.
EXPERIMENTS ON VESSEL HOLE ABLATION DURING SEVERE
B. Raj Sehgal, J. Andersson, N. Dinh, T.
Nuclear Power Safety, Royal Institute of Technology
100 44 STOCKHOLM Sweden
A program of simulant material experiments is planned to investigate the
physical processes that occur during the progression of a postulated melt-down
accident in a light water reactor (LWR). The experiments will employ glass type
melt materials at ~1000 K to 1700 K to represent the corium
(UO2-ZrO2) melt. These simulant materials have the
advantage that they are oxidic and their composition can be changed to widely
vary the physical properties, e.g., viscosity, thermal conductivity etc.
Additionally, because of their high temperature, they will model the radiation
heat transfer and film boiling phenomena, which play a role in the corium melt
interactions with structures and water. These materials can be electrically
heated, form crusts and display slurry type viscous flow behaviour on cooling.
Another advantage is that their cost is generally less than U.S.$ 1.00/kg.
Specific experiments proposed in the initial program are pertinent to (a) the
interaction of corium melt and the lower head of the pressure vessel, (b) the
interaction of corium melt with in-vessel and ex-vessel water pools. We expect
that experiments on in-vessel ex-vessel melt and debris bed coolability will
also become a part of this program of experiments on melt-structure water
The first set of experiments performed investigate the ablation process in a
LWR vessel, as the melt discharges from the vessel to the containment through
the failure location, be it a penetration, or the initial opening during
through the creep-rupture process. The simulant material chosen is a mixture of
Pbo and B2O3 which melts at ~1000 K and has very low
viscosity at 1000 K and above. The vessel material chosen is lead, which melts
at 600 K. The objective of these experiments is to determine if a crust formed
retards the heat transfer to the vessel wall or is swept out by the flowing
melt and is ineffective. The extent of the final size of the hole is affected
significantly by the presence or absence of the crust.
Scaling of the hole ablation process has been performed. It appears that the
experiments performed with a ? 10 to 100 l of melt will cover the values of the
parameters for the prototypic accident conditions. A one dimensional code for
melt flow and vessel interaction, called HAMISA (hole ablation in severe
accidents) has been completed, and a two dimensional code is being prepared.
The process is basically two dimensional, as has been demonstrated in the
experiment with a thick plate, having an initial hole of 10 mm.
PASSIVE CONTAINMENT COOLING AFTER A CORE MELT ACCIDENT
F. J. Erbacher
KfK, Karlsruhe, Germany
VALIDATION OF RASPLAV/SPREAD CODE AGAINST CORINE
A. Popkov1, V. Chudanov1, P.
Vabushchevich1, V. Strizhov1, J. C.
Lutche2, J. M. Veteau
GEYSER/TONUS : A CODE FOR SEVERE ACCIDENTS CONTAINMENT
L. V. Benet, C. Caroli, P. Cornet, N.
Coulon, M. Durin*, J. P. Magnaud, M. Petit
91191 Gif Sur Yvette Cedex
In many countries, the safety requirements for future light water reactors
include accounting for severe accidents in the design process.
As far as the containment is concerned, mitigation features allowing to limit
pressure and temperature inside the building are to be assessed. There is also
a need to accurately to estimate local hydrogen concentrations in order to
evaluate the hydrogen risk.
For this purpose, codes have been developed in the past twenty years around the
world. they generally include models for all the physical phenomena involved
during severe accidents. Most of them are based on a multi-compartment approach
for the spatial description. This approach have well known limitations when
local concentrations must be evaluated in large volumes.
On the other hand, general purpose multi-dimensional thermal hydraulics
computer codes are able to predict complex situations such as stratification
occurrence, but they often lack some models which are necessary to be included
in the analysis of the containment under a severe accident. The ability to
treat long transients is also a concern with this type of codes.
For these reasons, CEA/DMT has undertaken the development of the GEYSER/TONUS
code. This code will allow the coupling of parts of the containment described
in a lumped parameter manner, together with meshed parts. Physical model of
classical lumped parameter codes, such as condensation, will be adapted for the
spatially described zones. The objective is to be able to treat complete
scenarios and optimized numerical methods are developed for the transient
problem. The code is built in the CASTEM 2000/TRIO EF system which allows,
thanks to its modular conception, to construct sophisticated applications.
In this paper, the GEYSER/TONUS code is described, and some applications are
* Presently at Commissariat à l'Energie Atomique,
IPSN/DPEI, CE Fontenay, BP
AD-HOC FILTRATION FOR SEVERE ACCIDENTS
S. R. Kinnersly
Filtration is a means of reducing the release of fission products to the
atmosphere during a severe accident if the containment function is bypassed. A
number of Western LWRs have pre-installed filters to remove fission products if
deliberate venting is needed to prevent containment failure. For UK gas cooled
reactors, ad-hoc filtrations a potentially valuable measure for reducing
releases if there is a leak during a severe accident and for extending the
range of severe accident management options available. An ad-hoc filter would
be rapidly assembled during a severe accident. The principles involved in
ad-hoc filtration may be more generally applicable to other reactor designs,
particularly those without a strong secondary containment or which are
susceptible to containment bypass.
The design of an ad-hoc filter requires consideration of :
- availability of materials;
- ease of construction and operation;
- stability of materials under heat and irradiation;
- filter efficiency (aerosols and vapours);
- filtration capacity;
- decay heat removal;
The focus here will be on generic aspects of heat and mass transfer in ad-hoc
filter design. Key filter parameters will be identified and discussed in terms
of projected loadings on the filter. The advantages and disadvantages of
alternative materials and designs will be considered.
Acknowledgement: This paper is based on work funded by the UK
Health and Safety Executive and by Nuclear Electric plc.
IODINE BEHAVIOUR IN CONTAINMENT
W. C. H. Kupferschmidt, J. C. Wren, J. M.
Research Chemistry Branch
CANADA ROE 1LO
Radioiodine is recognized as one of the most hazardous fission products that
can be released from fuel during a reactor accident. This is due to the
combination of its large inventory in fuel, short half-life and biological
Furthermore, iodine has volatile chemical forms, which increases the
probability of its release to the environment. However, iodine chemistry is
quite complex, and the behaviour of iodine under accident conditions can be
influenced by numerous variables. For example, the extent to which CsI, the
most likely form of iodine to enter containment, would undergo reaction to form
volatile I2 or organic iodides, is dependent on the dose rate,
solution pH and impurities present within containment. Surfaces also play an
important role in influencing iodine volatility, both as a sink and as a source
of impurities that can increase iodine volatility.
This paper highlights recent findings in iodine research and summarizes the
current state of understanding of iodine chemistry relevant to reactor safety.
This paper also puts forward recommendations for minimizing iodine release from