VGB POWERTECH 7 (2021) - International Journal for Generation and Storage of Electricity and Heat

VGB PowerTech 7 l 2021
Study on the integrity of containment against hydrogen threats
Review of PSA results
Selection of initiating events among
PSA results for severe accident analysis
Severe accident analysis
(using the MAAP5 code)
3D analysis of hydrogen behavior and
combustion in the containment
(using the GASFLOW-MPI code)
3D analysis of structural response for
the integrity of containment
(using the ABAQUS code)
under severe accident conditions The
Westinghouse-type 650 MWe PWR was
used as the reference PWR for the evaluation.
. A detailed assessment of the hydrogen
threat in the containment was conducted
using a 3D analysis. F i g u r e 1
shows the assessment procedure in this
study. In section 2, in order to review the
integrity of the containment against hydrogen
threats, accident scenarios were first
chosen for the analysis of severe accidents.
Subsequently, a severe accident analysis
was performed to determine the mass and
energy discharges to the containment during
severe accidents. With the obtained
mass and energy data, the static pressure
in the containment was calculated in the
event of hydrogen combustion in section 3.
Then, the responses of the containment
structure were evaluated on the basis of
the combustion static pressure of hydrogen.
Finally, the integrity of the containment
against hydrogen threats under severe
accident conditions was evaluated in
section 4 and conclusion.
2. Selection of accident
sequences and analysis
Analysis for
in-containment
against hydrogen
threats
Analysis for the structure of
containment against
hydrogen threats
Fig. 1. Overall procedure of the comprehensive analysis on the integrity of containment against
hydrogen threats.
Tab. 1. Level 1 PSA sequences of the reference
plant.
Sequences Percent [%]
LOCCW 70.99
LOOP 6.06
MLOCA 2.08
SLOCA 1.22
Other transients 19.65
Tab. 2. Key characteristics of the reference
plant (WH-650MWe) [5].
Parameter
Value
Reactor Power (MWth) 1,876
Mass of zircaloy (lb) 25,240
RCS operating pressure (psig) 2,235
Hot leg/ Cold leg temperature ( o F) 616.6/549.4
Containment design pressure (psig) 44.8
Containment free volume (ft 3 ) 1.44x10 6
Hydrogen control system
PARs
the atmosphere in the containment with
the progress of the severe accident, were
evaluated. Accident sequences were chosen
based on the PDS (Plant Damage Status)
[5]. In addition, the characteristics of
the accidents and the location where hydrogen
is discharged to the containment
were considered. As a result, the LOCCW
(Loss Of Component Cooling Water) scenario,
and LOOP (Loss Of Offsite Power)
scenario were selected as the accident cases
for the analyses, and the accidents with
the same characteristics of transient accidents
and discharge points were excluded.
Additionally, the SLOCA (Small Break Loss
Of Coolant Accident) and MLOCA (Medium
break Loss Of Coolant Accident), which
are ranked high in terms of the PDS, were
selected. Ta b l e 1 shows the selected scenarios
are dominant sequences in PSA results.
The mass and energy discharge quantities
for the selected accidents were calculated
using the MAAP5 severe accident analysis
code [7]. The MAAP5 is an integrated system
analysis code used to evaluate severe
accidents of NPPs. As a result of these analyses,
the gas composition and thermal-hydraulic
results, including the hydrogen
concentration in the containment, were
obtained. Only PARs were considered for
hydrogen control. The NUKEM correlation,
which is the most conservative among
the hydrogen removal rate correlations,
was applied to calculate the PAR removal
rate.
The MAAP5 nodalization was composed of
the containment comprised 28 nodes,
55 flow paths, and a total of 237 heat sinks,
as shown in F i g u r e 2 . Key characteristics
of the reference plant for severe accident
analysis were shown in Ta b l e 2 . Ta -
b l e 3 summarizes the results of the severe
accident analysis obtained using the
MAAP5, including the timing of the occurrence
of major events such as core uncover,
core melt progression, corium relocation
into the lower head, and reactor vessel failure.
In addition, it is possible to check the
maximum average hydrogen concentration
of the containment in each accident scenario.
From the accident analysis results,
the highest average hydrogen concentration,
6.43 vol. %, is detected in the case of
MLOCA. This value confirms that hydrogen
is sufficiently controlled below the
regulatory standard, which stipulates that
the hydrogen concentration must be managed
at an average value of 10 vol. % or
less. F i g u r e 3 . shows the typical steam
Accident sequences were selected for the
analysis of severe accidents. To choose the
accident sequences, accident scenarios
based on the results of a PSA (Probabilistic
Safety Assessment) of the reference plant
were referred. A 3D analysis of the containment
under severe accident conditions was
performed to evaluate its structural integrity
against hydrogen threat. First, the
thermal-hydraulic conditions, such as the
pressure, temperature, and composition of
Tab. 3. Severe accident progression and maximum hydrogen fraction in containment.
Cases LOCCW LOOP MLOCA SLOCA
Core Uncover [h] 3.5 2.25 0.04 4.77
Max. Core Temperature
exceeds 2500K [h]
Corium relocation into Lower
Head [h]
6.74 3.37 7.69 5.67
8.87 5.75 9.28 7.10
Reactor Vessel Failure [h] 12.88 6.03 10.67 8.38
Max. Hydrogen fraction [-] 0.05067 0.04522 0.06434 0.06333
71