atw Vol. 63 (2018) | Issue 5 ı May
coolability. As the activities have to
produce a reasonable contribution to
the safety enhancement, the following
conditions were defined
• Effectiveness, it means a quantified
benefit to safety (prevention of
early FP releases and minimize FP
releases) plus confirmed physical
fruitfulness with sufficient margin
• Reasonable technical feasibility
• No negative impact to reactor operation
(including all procedures/
activities during outages)
• Simplicity – applicability under
SA condition (limited personal
capacity, limited accessibility …)
• At least partial independence of
functionality assurance in comparison
with existing emergency
systems
• Consistency of approaches with
other utilities operating VVER-1000
(or reactors of similar power) and
VVER-1000 designers
Taking into account above defined
conditions the activities were defined
for following six main topics which
have very well defined structure and
relations each to other
• Primary circuit depressurization
under SA conditions
• Corium cooling with water injection
into RPV (new acronym IVR-IN
is used)
• In-vessel retention with external
reactor vessel cooling strategy
(originally used IVR, but newly
IVR-EX acronym is used)
• Ex-vessel corium cooling strategy
(ExVC)
• Containment response to severe
accident and long term issues
• Severe accident initiated in the
spent fuel pool
Short description of the effort done up
to know is included in following
sub-chapters excluding the last topic,
of which activities are not yet initiated,
but some analyses were already
performed in past.
3.4.1 Primary circuit depressurization
under SA conditions
The activities related to this topic were
mostly focused on the conditions for
the IVR-EX strategy as the primary
pressure is strogly determining
success of this strategy. The analytical
investigation using the MELCOR 1.8.6
code was performed to identify if the
depressurization initiated after the
entry to SAMGs is sufficiently fast to
meet predefined criterion – to reach
the pressure reduction below 2 MPa-g
till the time of starting relocation of
corium into the lower plenum of RPV.
The analytical studies, for variant
opening of one or two safety valves or
additionally the system for a removal
of gases from the pressurizer, confirmed
that the safety valves if can be
kept open are sufficient, but the contribution
of the gas removal system is
negligible. The important condition is
that the valves must be kept open, the
modification of a control of safety
valves is in the final phase of its
preparation with an expectation to
initiate an appropriate technical
negotiation with the SONS.
3.4.2 Corium cooling with
injection into RPV
(IVR-IN strategy)
The extensive analytical investigation
was performed with the MELCOR
1.8.6 code to analyze possibility of a
termination of the core degradation
due to water injection into the RPV.
The analyses varied with the assumed
mass rate of the water injected (to
alternate among the repaired LPI and
HPI, and the new alternative system
called TB50, which has the lowest
mass rate of water injection and also
relatively lower maximum pressure
head). The second variations were in
timing of the water injection activation,
it means different configurations
of core degradation at the time of
water injection initiation. The last
variation was in the initiating event as
the most of analyses were performed
for the large break LOCA initiated
envent (plus loss of all active systems)
and as an alternative the scenario
initiated with postulated SBO was
chosen with the early depressurization
after the entry to SAMGs and also
loss of all active systems.
The conclusion from the analytical
work is that the results are of course
uncertain, but they show that the
initiation of water supply before starting
of the corium relocation into the
lower plenum should prevent the loss
of RPV integrity. More detailed, the
analysis predicted un-failed RPV wall
if the lower head is not dried out, but
such answer seems to be too optimistic
due to some modeling assumptions in
the MELCOR code which favoured
cooldown with even very small
remaining mass of water in the lower
head.
3.4.3 Strategy IVR-EX
The greatest effort was done for this
topic during last two years and the
activity will continue with the experimental
program on the RPV coolability.
The first issue related to the IVR-EX
strategy is the flooding of reactor
cavity and long term water supply
to the cavity. The design of the
VVER-1000/320 containment is absolutely
un-favorable for this IVR-Ex
strategy, because the recirculation
sump is located one floor below the
cavity and the water from the containment
is drained to this sump. So it
is absolutely impossible to close the
water circulation inside of the containment,
like it is in case of the
AP1000 or VVER-440/213 reactors.
The water for flooding of the cavity
must be injected from sources outside
of the containment. The feasibility
study was performed to investigate all
necessary modifications to be done
for a possibility to cool the RPV
from outside. The preliminary project
solution was prepared for new systems
on fast cavity flooding (based on
pressurized tanks located on roof of
the auxiliary building) with the
second part for feeding water from the
second pair of pressurized tanks. This
solution enabled to cover the first
6 hours by this passive system and
thus to have a way sufficient time for
an activation of the new active system
which is capable to feed water from
own tanks till 72 hours of the accident.
The injection of the water is
not the only condition, but the second
one is the protection of water releases
from the cavity. The injection of
water is assumed via. the venting
system channels and those must be
equipped with new valves to prevent
water overflow to the containment.
Generally it means to install 9 new
valves which must all of them be
surely closed before starting of injecting
water to the cavity. This is the
most critical part of the solution,
because of possible failure, which
would cause the loss of the IVR-EX
strategy. The second critical point is
the control of water level for the
second set of pressurized tanks. The
control is proposed to prevent loss of
this water due to overflow, but this
control can be tested and tuned up
only at the experimental facility, so its
credibility is not too high.
The second issue is the release of
steam produced during the cooling
of RPV by boiling. The design drawings
of the Temelín NPP showed that the gap
between the RPV itself and its
supporting ring should be sufficient
(including thermal expansion of RPV),
but the data from measurement after
the vessel installation were not available.
So at least part of this gap was
measured during the outage at the unit
2 in 2016 and it confirmed sufficient
gap. But the gaps among the boxes of
thermal and biological shielding seem
OPERATION AND NEW BUILD 303
Operation and New Build
Continuous Process of Safety Enhancement in Operation of Czech VVER Units ı J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik
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