atw 2018-02

atw Vol. 63 (2018) | Issue 2 ı February
Investigation of Conditions Inside the
Reactor Building Annulus of a PWR
Plant of KONVOI Type in Case of Severe
Accidents with Increased Containment
Leakages
Ivan Bakalov and Martin Sonnenkalb
1 Introduction and analysis method The severe accident at Fukushima Daiichi NPP resulted in
severe core damage and significant releases of hydrogen and radioactive materials from primary containment boundary
into or through the reactor buildings of three out of the six reactors (units 1 to 3). Based on analyses of the accident
progression it was realized that accidentally increased leaks from the inertized containment contributed to the
radionuclide and hydrogen release into the reactor building, thus leading to hydrogen explosions, severely damaging
the reactor building constructions.
The Fukushima Daiichi accident triggered
worldwide stress tests and
re-assessments of the NPP plant
safety. In Germany the process
resulted in an improvement and
extension of the existing severe accident
management (SAM) concept
by both additional preventive and
mitigative measures. The main improvements
in the mitigative domain
is a new concept of severe accident
management guidelines (SAMG) with
strategies and procedures intended to
be used by the plant crisis team for
mitigation of the consequences of
severe accidents. The SAMG concept
follows relevant recommendations
of the German Reactor Safety Commission
RSK [1].
Analyses of the hydrogen as well
as aerosol and noble gas behaviour
in case of increased containment
leakages into the reactor building
annulus of a German PWR KONVOI
reference plant under severe accident
conditions have been performed using
the GRS lumped parameter code
COCOSYS. The investigation carriedout
focusses on the assessment of the
efficiency of newly developed SAM
measures as described in the new
SAMG handbook or some measures
proposed in addition for a PWR
reference plant of KONVOI type. The
assessed strategies are related to the
mitigation of challenging conditions
inside the reactor building (RB)
annulus due to design based and
increased containment leakages
during severe accidents.
The analyses are based on previous
GRS investigations of the hydrogen
mitigation concept with passive autocatalytic
recombiners (PAR) inside
the PWR KONVOI containment [2] as
well as the reassessment of the effectiveness
of the filtered containment
venting concept of PWR KONVOI [3].
The main findings contribute to
further improvement of the planned
mitigative SAM measures in case of
enhanced containment leakages into
the reactor building annulus under
severe accident conditions.
1.1 COCOSYS plant model
The COCOSYS nodalisation scheme
of the PWR KONVOI plant with focus
on the RB annulus is presented in
Figure 1. The nodalisation of the
containment and the RB annulus is
developed in such a way that thermal
and gas stratification processes
expected under accident conditions,
local and global convection flows
between the compartments, and longterm
convection processes inside
the containment could be simulated
appropriately. Therefore, a refined
subdivision of the containment compartments
and RB annulus rooms and
free space was chosen. The model
considers all relevant gaseous and
liquid flows through different compartment
connections such as free
openings, fire protection doors, burst
membranes, drainages, etc. For the
purpose of heat and mass transfer
modelling inside the containment
and the RB annulus heat structures
representing the walls, floors, ceilings
and metal internals are introduced
into the model. With all these features
the model adequately represents all
relevant design specific features of the
PWR KONVOI reference plant – both
inside the containment as well as the
RB annulus.
The containment has a total free
volume of 70,000 m 3 . It is subdivided
into four areas which can have
different convection flow regimes
depending on the initial event of a
sequence and the break/discharge
location. The first area represents the
containment compartments, in which
the reactor pressure vessel and the
steam generators are located. The
| | Fig. 1.
COCOSYS nodalisation scheme of the RB annulus and location of containment penetrations through the containment steel shell.
85
ENVIRONMENT AND SAFETY
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