Regulatory view of hydrogen management at the Krško NPP
Tomi Živko
Slovenian Nuclear Safety Adminstration
Litostrojska 54
1000 Ljubljana, Slovenia
[email protected]
Srđan Špalj, Andreja Peršič
Nuklearna elektrarna Krško, Slovenian Nuclear Safety Administration
Vrbina 12, Litostrojska 54
8270 Krško , Slovenia, 1000 Ljubljana, Slovenia
[email protected], [email protected]
ABSTRACT
The Krško NPP and the Slovenian Nuclear Safety Administration (SNSA) have been
for a longer time paying attention to the problem of hydrogen in the containment. Findings of
the IAEA's RAMP mission concerning hydrogen were included in the first Periodic Safety
Review of the Krško NPP. It resulted in studies which showed that there was no danger due to
nonuniformity of hydrogen distribution in the case of accident. Hydrogen explosions
happened during the Fukushima accident and hydrogen problematics came into focus of
regulators and opearators. European Nuclear Safety Regulators Group requested from
national regulators to urgently consider additional protection of the containment from
hydrogen explosions and ovepressure. On the basis of SNSA regulatory decisions, the Krško
NPP decided to install passive autocatalytic recombiners (PARs) in 2013. The PAR system
will protect the containment in the case of design as well in the case of beyond design basis
accidents (BDBA). This will be the first case of licencing of BDBA systems at the Krško NPP
and therefore also a challenge for the regulator.
1
INTRODUCTION
The hydrogen in the containment can be produced in interaction between steam and the
zirconium cladding of the nuclear fuel. The reaction is exothermic, it starts at high
temperature and accelerates as temperature increases. Other sources of hydrogen during
design bases accidents are corrosion of metals and radiolysis of aqueous solution in the core
and sump. If the core is not successfully cooled, the molten core-concrete interaction (MCCI)
is expected to occur resulting in large quantities of carbon monoxide and hydrogen, which are
both flammable and non-condensable gases [1]. Hydrogen is not flammable below 4% of
volume concentration at 20˚ C and atmospheric pressure. The same limit for carbon monoxide
is at 12% volume concentration.
A lot of attention was paid to the problem of hydrogen generation in nuclear power
plants after the Three Mile Island (TMI-2) accident. During the accident, a hydrogen burn or
an explosion caused pressure to increase by 0.197 MPa in the containment building,
according to the Kemeny Commission [2]. However, it did not damage the containment of the
TMI-2 nuclear power plant. The Fukushima accident occured in March 2011. Hydrogen was
produced in big quantities at units of the Fukushima NPP that eventually led to hydrogen
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explosions blowing the roofs off of reactor buildings. These two accidents have caused the
worldwide response.
The objective of the paper is to present regulatory measures in Slovenia concerning
hydrogen control and the corresponding response at the Krško NPP which is the only NPP in
Slovenia. The rest of this paper is organized as follows. In the second chapter we
chronologically describe history of hydrogen management in Slovenia. The first subsection of
the third chapter presents regulatory view and work concerning the PAR modification.
Subsections 3.2 and 3.3 give relevant informations about capability of the PAR system to
handle DBA and BDBA. Technical description of PARs is presented in subsection 3.4. All
data from subsections 3.2, 3.3 and 3.4 are taken from the PAR application. A short summary
is given in the fourth chapter.
2
SURVEY OF HYDROGEN CONTROL AT THE KRŠKO NPP
The Krško NPP can successfully handle hydrogen problem in the case of a design basis
accident. Two redundant electric recombiners are provided in the containment. The capacity
of each electric recombiner is 47.2 litres of containment air per second. This is enough to keep
the hydrogen concentration at safe level in the case of design basis accident [3].
The RAMP (Review of Accident Management Programmes) mission in the year 2001
compared the Krško NPP Accident Management Programme (AMP) with international
standards and experience. The mission found out that the AMP for Krško NPP is generally in
accordance with the IAEA guidance documents as well as with international experience and
practice [4]. However, the question of a possible impact of non-uniform hydrogen distribution
in the containment was raised as well as the question related to adequacy of supposed
hydrogen source. The latter followed because a possibility of MCCI could not be excluded.
Installation of passive autocatalytic recombiners was also mentioned to be worth of
consideration [4].
These questions were soon addressed in the action plan which followed the Krško NPP
first Periodic Safety Review. The following actions were identified:
• analyses of possible non-uniform distribution of hydrogen within containment space,
• analyses of influence of potentially decreased corium coolability on the burnable gas
management and containment long term pressure management,
• analyses of effects of PAR system installation.
The first action resulted in a study in which relevant Krško NPP severe accident
scenarios were analysed [5]. The study found out that beside some local peaks of hydrogen
concentration in the reactor cavity, atmosphere in all other lower compartments had similar
hydrogen concentrations. The atmosphere in the containment dome was well mixed, too and
there was no significant stratification. The conclusion was that possible non-uniformities in
hydrogen distribution did not present a serious threat to the containment. It was later decided
to address the remaining two actions as a part of the Safety Upgrade Program (see the
following text).
Following the Fukushima accident, the question of hydrogen handling became one of
urgent topics for nuclear industry and regulators. After the adoption of the stress tests
specifications by European Nuclear Safety Regulators Group (ENSREG) and the European
Commission, the SNSA immediately issued a decision for the Krško NPP to perform the
extraordinary safety review [6]. The program of the review was completely in line with the
adopted stress tests specifications. The Krško NPP's report was reviewed by the SNSA, open
issues were clarified and the Slovenian national report was adopted, which was very much
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based on the report of the Krško NPP. The 15 European Union countries with nuclear reactors
as well as Ukraine and Switzerland performed the stress tests and were subjected to the peer
review. Among the results of stress tests there was also a compilation of recommendations
addressed to national regulators which was published in July 2012 [7]. The question of
hydrogen handling for protection of containment integrity was recognized as very important
in the compilation. Correspondingly, the national regulators were required to urgently
consider this topic.
Regulatory actions in Slovenia were not limited to following of international initiatives.
Besides the decision for the plant to perform the extraordinary safety review, the SNSA also
issued a decision in September 2011 requiring from the plant to reassess the Severe Accident
Management strategy, the existing design measures and procedures and to implement safety
upgrade for prevention of severe accidents and mitigation of its consequences. This
evaluation was finished in January 2012. It resulted in the Safety Upgrade Program (SUP) in
which was presented the program of implementation of measures corresponding to Design
Extension Conditions (DEC).
Plant states corresponding to DEC were derived from the following combinations of
events, specific for the plant design and site location [8]:
• Combination of seismic event (Peak Ground Acceleration up to 0.6 g), Loss of
Coolant Accident (LOCA) and Station Blackout (SBO),
• Combination of seismic event (PGA up to 0.6 g) and external flooding,
• Combination of seismic event (PGA up to 0.6 g), loss of Ultimate Heat Sink (UHS)
and SBO,
• Combination of large commercial aircraft accident and fire.
These combinations of events were obtained on the basis of deterministic and
probabilistic assessments, engineering judgement, international recommendations and the
Krško NPP specific analyses [9]. The SUP was reviewed and approved by the SNSA in
February 2012 [10]. The SUP included various modifications and equipment procurement.
Part of the SUP was also installation of the PAR and Passive Containment Filter Vent (PCFV)
systems in order to improve protection of the containment integrity. The SUP foresees the
installation of the first two DEC systems (PAR and the PCFV) during 2013 outage.
3
IMPROVEMENT OF HYDROGEN CONTROL AT THE KRŠKO NPP
3.1
Regulatory view
In May 2013, the Krško NPP submitted an application to SNSA to approve the
installation of passive autocatalytic recombiners and documents which are affected by the
modification: Updated Safety Analysis Report (USAR), Technical Specifications (TS) and
Design Extension Conditions Technical Specifications (DEC TS). The goal of the PAR
system is protection and prevention of the containment damages due to overpressure from
hydrogen and carbon monoxide explosions in the case of Design Basis Accidents (DBA) and
BDBA. The modification foresees removal of the two existing electric recombiners and
installation of two safety related PARs and additional twenty non-safety related PARs for the
case of BDBA. All units will be a model NIS PAR 44 KKH [11].
Non-safety related PARs are DEC equipment. Slovenian regulations currently do not
have precise formulation of DEC and how DEC should be described in USAR. It was decided
to add a new chapter to USAR which would cover DEC. Currently, the DEC chapter has two
sections: General description of DEC systems and site characteristics, Design inputs for DEC
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systems, structures and components. The latter section has following subsections: Seismic
design, Extreme Weather Conditions Design, Releases of radioactive materials design,
Separation and single failure criteria, DEC instrumentation design requirements,
Environmental qualification design. This organisation of the DEC chapter supports systems
PAR and PCFV. The regulatory view is that different organisation of the chapter and more
details will be required during course of time, when new DEC systems will be added.
Limiting conditions for operation (LCO) for 2 safety related PARs are given in TS,
while Limiting conditions for operation for all DEC systems (DEC-LCO) are given in DEC
TS. There is an important difference between LCO and DEC-LCO. Namely, if the DEC-LCO
is not fulfilled and the required action is also not fulfilled, then the shutdown conditions for
plant are not required, as would be case if LCO and required action would not be fulfilled.
Instead of achieving shutdown conditions, operator must report the SNSA and ask an
approval for continuation of operation. The reason for the difference is that DEC systems are
used for accidents that are very unlikely to occur.
Currently (August 2013), the PAR application is under review of the SNSA.
3.2
Hydrogen treatment in the case of DBA
As already mentioned, two existing electric hydrogen recombiners will be replaced with
two safety related PARs. Their goal will be to keep hydrogen volume concentration in the
containment below acceptable conservative limit of 4% in the case of DBA. The number of
needed PAR units was determined as follows. The worst design basis case of LOCA leads to
production of about 250 kg of hydrogen in 100 days [12]. One PAR unit with a conservatively
assumed hydrogen depletion rate 0.095 kg/hr keeps total the hydrogen mass in the
containment (green line in Fig. 1) well below limiting concentration. Additional safety related
PAR unit will be installed for the reason of redundancy.
Figure 1: Volume percent of hydrogen in containment in the case of the DBA LOCA [12].
Red line depicts the case without PARs, green line depicts the case with one PAR operating.
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3.3
Hydrogen treatment in the case of BDBA
The dimensioning sequence for PARs and PCFV was SBO. It was conservatively
assumed that severe accident management equipment was not credited and that DEC
equipment was not available for first 24 hours. Eventualy, the core would be melted and
relocated into the containment.
During the in-vessel phase of a severe accident, hydrogen is produced in the core by
oxidation of zirconium cladding by the steam at high temperature. When a vessel failure
occurs, the molten core relocates to the cavity that has already been flooded. Water from the
cavity quenches the core. As a result, water steam is produced and pressure in the
containment increases. The MCCI occurs following cavity dryout. Large quantities of
hydrogen and carbon monoxide are produced during MCCI resulting in the pressure increase.
Pressure in the containment is controlled by the PCFV which is supposed to open (6 bar) and
close (4 bar) at preset values [13]. The containment pressure transient for the dimensioning
sequence is shown in Fig. 2.
Carbon monoxide has similar behaviour with respect to burning as hydrogen. Therefore,
if carbon monoxide mass is correspondingly weighted, the so called equivalent carbon
monoxide mass is obtained. Hydrogen mass and equivalent carbon monoxide mass are added
up in order to obtain the so called equivalent hydrogen mass. It was found out that the
combustion of 408 kg of equivalent hydrogen mass could produce peak pressure of 6 bars,
which in turn could lead to 5 % probability of containment failure [14]. Pressures smaller than
6 bars and correspondingly, quantities of equivalent hydrogen mass smaller than 400 kg are
not considered as a serious challenge to the containment. Four hundreds kg of hydrogen need
3200 kg of oxygen for combustion. Therefore, the hydrogen and carbon monoxide problem is
effectively solved if the quantity of oxygen in containment is smaller than 3200 kg.
The initial quantity of oxygen in the containment is about 9600 kg as can be seen from
numbers on the right side ordinate axis of Fig. 3. It means that we have to eliminate 6400 kg
of oxygen to reach the state which is safe with respect to hydrogen combustion. Big quantities
of oxygen are depleted from the containment before the MCCI occurs, during first vent
opening (see Fig. 2 and Fig. 3). The rest of the oxygen can be used for oxidation of hydrogen
by PARs. Fig. 3 presents oxygen mass, hydrogen mass, equivalent carbon monoxide mass
and equivalent mass of hydrogen as function of time for the case of 18 PARs [13]. The
analysis shows that 22 PARs will keep mass of oxygen below 3200 kg at all times when
equivalent mass of hydrogen exceeds 400 kg. In this way, the needed number of PARs was
found to be 22. The locations of the PARs are chosen to be stiff in order not to change the
dynamic response of the elements of the platforms to which the PARs are mounted.
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Figure 2: Time evolution of containment pressure during transient [13].
Figure 3: The right ordinate axis gives oxygen mass (blue), equivalent hydrogen mass
(black), hydrogen mass (red) and equivalent CO mass (green) during transient [13].
3.4
Technical properties of PARs
A PAR NIS 44KKH is a passive device that contains no active moving parts. Its
housing consists of a stainless sheet metal box, open at the bottom and near the top. Forty-
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four catalytic cartridges are inserted into the housing. Cartridges are fabricated from
perforated stainless steel plates which hold the catalyst pellets. Recombination is
accomplished by the attraction of oxygen and hydrogen molecules to the surface of catalyst.
The PAR is designed for a self-starting and self-sustaining reaction. The hydrogen depletion
rate depends on the total pressure, gas temperature and hydrogen concentration. Mass of one
PAR is about 100 kg. PARs will be demonstrated operable during outage by visual
examination of each PAR enclosure what will ensure that there is no obstruction or blockage
of the inlets or outlets. One cartridge will be taken from 2 safety and from 4 non safety related
PARs. A surveillance bench test will be performed on these 6 cartridges. A schematic of PAR
NIS 44KKH is shown in Fig. 4.
Figure 4: Schematic of PAR NIS 44KKH [11].
4
CONCLUSIONS
The question of hydrogen management came again to the focus after the Fukushima
accident. The Krško NPP acted promptly in order to protect the containment integrity in the
case of severe accident. During the 2013 outage, two systems, PAR and PCFV will be
installed in order to protect the containment from explosions of flammable gases and
overpressure. Both systems fulfil requirements which follow from DEC. DEC represent
important step in improvement of nuclear safety. However, DEC is a new concept and is not
yet precisely defined in the existing Slovenian nuclear regulation. Some requirements
concerning severe accidents have already been incorporated into the regulation. Regulatory
work concerning DEC has to be continued.
1103.8
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