atw Vol. 63 (2018) | Issue 2 ı February
ENVIRONMENT AND SAFETY 86
second and third area comprises the
operating containment compartments
and the containment dome. The
fourth area includes all the compartments
outside the missile protection
cylinder, the periphery of the containment.
The volume of the RB annulus is
subdivided into four areas with a total
volume of 50,000 m 3 . The first area is
the annular gap, located above elevation
21.5 m, which has a total volume
of 14,900 m 3 . This area, in turn, is
divided into six axial levels along the
height of the gap. It is connected to
the lower part of the annular gap
( second area) below elevation 21.5 m
and has a free volume of 4,300 m 3 . In
this area vertical fire protection walls
with metal sheets are located, which
do not allow atmospheric flow in
azimuthal direction. The third area
comprises several separate annulus
rooms located on building floors at
elevation 6 m to 21.5 m. The annulus
rooms at elevation 6 m to 9 m are
separated from the annular gap by
ventilation systems. The connections
between these separate rooms are
provided with fire protection doors
and fire protection flaps, which automatically
close, if the room temperature
exceed ~70° C. The fourth area
represents all annulus rooms below
elevation 6 m with a total volume of
23,100 m 3 . Those rooms have only a
negligible atmosphere exchange with
the annular gap above.
Moreover, the model consists of
all relevant plant systems used during
accidents (e.g. the RB annulus exhaust
air system) or operational systems
foreseen as SAM measures in the
SAMG handbook (e.g. the annulus
air supply/suction system and the
annulus air recirculation systems).
The filtered containment venting
system and the hydrogen recombination
system with about 65 PARs
installed inside the containment are
introduced in the input deck as well,
using the modelling capabilities of the
engineered safety features, integrated
in the COCOSYS code.
The COCOSYS model also includes
the containment design leakage of
0.25 vol.-%/d into the RB annulus.
For the base case analyses the design
leakage is assumed to be at the most
unfavorable place in the area of the
cable penetrations at elevation 12 m
(Figure 1 right side), e.g. the leakage
is located opposite to the single
suction point of the RB annulus
exhaust air system, operated in case
of an accident. In addition, leakages
are defined from the environment
through the auxiliary building main
gate into the lower annulus rooms
(leakages represented by red arrows
in Figure 1).
1.2 Selected representative
Severe Accident Scenarios
Two representative and different
severe accident scenarios – the base
cases – have been selected for the
analyses. Some characteristics of the
scenarios are summarized here, the
timing of main events is provided in
Table 1:
• MBL – a medium break LOCA with
a failure of the emergency core
cooling system after the emergency
water supply tank inventory is
empty; core degradation starts
delayed; sequence results in a
maximum water inventory in the
containment sump and a late
filtered containment venting.
• ND* – a transient with a failure of
steam generator feedwater supply;
failure of injection of active
emergency core cooling systems;
primary circuit depressurization
procedure to avoid reactor pressure
vessel failure at high-pressure;
core degradation starts early;
sequence results in a minimum
water inventory in the containment
sump and an earlier containment
venting.
The two representative base cases
were already used in earlier analyses
[2], [3] with respect to the reassessment
of other mitigative SAM measures.
In both cases, no melt relocation
from the reactor cavity into the containment
sump after melt penetration
of the biological shield was assumed,
just water ingress into the cavity and
therefore extended steam production.
As melt relocation into the sump with
cooling of the relocated melt amount
seems to be a realistic scenario leading
to reduced production of combustible
gases, two additional variant calculations
were done with melt relocation
into the containment sump. Furthermore,
a series of COCOSYS variant
calculations were carried out in order
to investigate the influence of the
following specific aspects:
• Operation/failure of the RB annulus
exhaust air system installed for
accident conditions.
• Variation of the size of containment
leakages into the reactor
building annulus: design leakage
(base case) and a 10 times larger
leakage.
• Variation of the containment
leakage location in the area of
containment cable penetrations.
Moreover, the efficiency of different
SAM measures for mitigation of the
consequences in the RB annulus,
documented in the SAMG handbook
of the reference plant, was analysed.
These measures are as follows:
• Use of RB annulus air supply/
suction system – provision of a
controlled ventilation to reduce
the hydrogen concentration in the
annulus.
• Use of RB annulus air recirculation
system – mixing of the annulus
atmosphere and elimination of gas
stratification.
• Use of emergency air filtration
system – extraction of air from the
RB annulus through a filtration
system to reduce the release of
radionuclides into the environment.
The following SAM measure was
additionally investigated as a possible
alternative method for hydrogen
reduction in the annulus. It is related
to a optional recommendation of the
RSK [1].
• Implementation of a small number
of PARs in the RB annulus upper
part to prevent combustible gas
mixtures.
2 Results – Quantification
of the effectiveness of
selected AM measures
Selected results are presented in the
following only for one base case
scenario (MBL) with the operation of
RB annulus exhaust air system used in
case of accidents and for some variant
Scenario
Start of steam/water
leak flow into
containment
Start of
core melting
RPV failure and
melt release
into cavity
Water ingression into
cavity and possible
melt release into sump
Start of filtered
containmentventing
ND* 1.4 hr 3.5 hr 6.5 hr 17.1 hr 66.5 hr
MBL 0.0 hr 5.8 hr 8.9 hr 13.5 hr 82.2 hr
| | Tab. 1.
Timing of characteristic events of severe accident progression of base case scenarios.
Environment and Safety
Investigation of Conditions Inside the Reactor Building Annulus of a PWR Plant of KONVOI Type in Case of Severe Accidents with Increased Containment Leakages ı Ivan Bakalov and Martin Sonnenkalb
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