The 15 th International Topical Meeting on Nuclear Reactor Thermal -Hydraulics, NURETH-15 HYDROGEN...

The 15th

International Topical Meeting on Nuclear Reactor Thermal - Hydraulics, NURETH-15 NURETH15-248 Pisa, Italy, May 12-17, 2013

HYDROGEN SAFETY – ECART AND MELCOR MODELING

G. Manzini, S. Morandi and F. Parozzi RSE, Power Generation System Dept. - Nuclear and Industrial Plants Safety team, via Rubattino,

54, 20134, Milano, Italy

[email protected], [email protected], [email protected]

ABSTRACT

Safety of hydrogen is inherent to both conventional and nuclear sectors and its

challenges are very different such as production, easy leaking, low-energy

ignition, wide range of fuel-air mixtures flammability limits, metals

embrittlement, explosion. About nuclear power plants, the Fukushima Dai-ichi

NPP accident of 2011 further highlighted the importance of these phenomena.

This work is focused on modeling of hydrogen – air fast deflagrations, as they are

some of the most significant phenomena in case of hydrogen leakages in both

conventional and nuclear power plants accident scenarios.

In particular, the paper describes the modeling of this phenomenon finalized to

improve ECART numerical tool (lumped parameters code dedicated to the

simulation of accidental scenarios), enabling it to analyze accident scenarios with

hydrogen explosions.

A first validation of improved ECART code is currently in progress by means of

comparison with results of other computational tools simulations like MELCOR

and experimental data coming from the FP5 SAFEKINEX research project.

1. INTRODUCTION

The accidents that involved the nuclear units of Fukushima Dai-ichi in 2011 put in evidence the

consequences of hydrogen explosions on the plant buildings (Fig. 1), as well as the significant

capability of spreading radioactive pollutants to the external environment [1].

Indeed, the phenomenon of hydrogen explosion is modeled in severe accident codes through

simplifying assumptions that can be managed with the lumped-parameter approach typically

used for the thermal-hydraulics of severe accident computer codes.

The model described in this work simulates explosions in conventional and nuclear accident

scenarios, considering only fast deflagrations and not detonations phenomena. It calculates the

main effects of air - hydrogen mixtures explosions in terms of pressures and temperatures.

The hydrogen – air explosion modeling is finalized to improve ECART numerical tool (lumped

parameters code dedicated to the simulation of accidental scenarios, [2]) in order to enable it to

the analysis of accident scenarios with hydrogen leakages in conventional or nuclear power

plants.

mailto:[email protected]



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Figure 1 – Consequences of hydrogen explosion

in one unit of Fukushima Dai-ichi (Photo from

Air Photo Service/AP Photo/La Presse) [1].

The ECART model is dedicated to air – hydrogen mixture fast deflagrations and it is internally

divided into two sub-models relating, respectively, to the outdoor (open spaces) and indoor

(confined spaces) explosions. In particular, the former uses the so-called TNT (TriNitroToluene)

model with appropriate coefficients dedicated to take into account the efficiency of the explosion

and the geometry of areas surrounding the explosion (obstacles acting as shields). With regard to

explosion in confined environment, a sub-model based on results of FP5 SAFEKINEX [3]

research project was created. In particular, after a substantial thermodynamic analysis, some

interpolating functions of the above results have been developed.

Therefore, both sub-models consist of a set of correlations that provide the values of temperature

and pressure required for a calculation in the first approximation of the effects of explosion.

Both approaches were implemented, first of all, inside stand-alone sub-models.

The outdoor explosion sub-model calculates pressure as a function of time and distance from

explosion and the indoor explosion sub-model calculates pressure and temperature as a function

of time.

By means of such sub-models, some numerical simulations of sample cases were performed and

the main results are summarized (explosions of clouds of air – hydrogen mixture with different

sizes and concentrations). Later, a validation of a WiP version of ECART, in which these models

were implemented, was carried out by comparisons with results of experimental sample cases

(see Chap. 2, 3).

About that, also simulations by means of MELCOR [4] numerical tool (Lumped parameters code

selected for US-DOE Safety Analysis Application) have been performed about the same cases

with the aim to compare the results with those of ECART simulations.

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It must be remarked that ECART tool contained already numerical modules dedicated to thermo-

fluid-dynamics and combustion, then the work consisted of an integration of those modules in

order to obtain better results, and not of a simple implementation of the correlations previously

mentioned. A validation of the complete / improved version of ECART code is currently in

progress by means of comparison with results of other computational tools simulations,

references and experimental data.

(This work has been financed by the Research Fund for the Italian Electrical System under the

Contract Agreement between RSE S.p.A. and the Ministry of Economic Development - General

Directorate for Nuclear Energy, Renewable Energy and Energy Efficiency in compliance with

the Decree of March 8, 2006).

2. NUMERICAL MODELING

2.1 Generalities

The approach was focused on the developing of some correlations able to calculate the pressure

and temperature of the fluids in the scenarios of interest as a function of time. This, in particular,

with the purpose of improving the potentialities of calculation of ECART simulation tool, which

development is carried out by the authors of this work [2].

ECART is focused on the consequences simulation of accidents in risk installations. It was

originally created to calculate the concentration of airborne radiotoxic substances inside nuclear

power plants in the case of a severe accident. As it is not related to a specific design, nuclear or

not, it can simulate the airborne transport of dangerous substances throughout a generic system

of rooms, pipes or plant components, together with the removal and the air-entrainment

mechanisms which may occur in the presence of structures, liquid sumps or water sprays.

The problem of complete analysis of complex systems or industrial installations is still quite

difficult using CFD codes, then, the possibility offered by the lumped-parameter fast-running

approach of ECART appears interesting.

A free version of the code and the related documentation can be downloaded from http://fus-

se.frascati.enea.it/ECART.htm.

With the aim to have reference data to compare the results of ECART and evaluate its

calculation potential about hydrogen explosion scenarios, also the code MELCOR [4] was used.

MELCOR is a fully integrated computer code that models the progression of severe accidents in

light water reactor nuclear power plants (it is developed at Sandia National Laboratories for the

U.S. Nuclear Regulatory Commission). A broad spectrum of severe accident phenomena in both

boiling and pressurized water reactors is treated in MELCOR in a unified framework. The main

use of MELCOR is the assessment of NPP severe accident source terms.

The code is composed of an executive driver and a number of modules, or packages, that

together model the major systems of a reactor plant and their coupled interactions.

http://fus-se.frascati.enea.it/ECART.htm

http://fus-se.frascati.enea.it/ECART.htm

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So far, hydrogen explosions in confined and open spaces were simulated by ECART (WiP

version) and only hydrogen explosions in confined space scenarios were simulated by MELCOR

(it requires some other hypotheses and modeling in order to be used for open space simulations,

as the work about that is in progress).

2.2 ECART Modeling

2.2.1 Confined spaces

The pressure model is simply based on experimental results fittings (FP5 SAFEKINEX project

[3]) and, with regard to gas temperature, on perfect gases thermodynamics.

The starting point of the model is the cubic law, defining a basic coefficient KG:

31V

dtdp

KMax

G ⋅

= (1)

where:

KG: explosion index [MPa⋅m⋅s-1

],

Maxdt

dp

:maximum rate of pressure rise [MPa⋅s

-1],

V : volume of the container/ box [m3].

Essentially by the Thermodynamics of an (almost) perfect gas mixture we have that:

i

f

i

f

p

p

T

T= (2)

( ) ( )( )v

HHif cn

qnTT

⋅∆⋅

=− 22

(3)

iv

HH

i

f

i

f

Tc

qx

p

p

T

T

⋅

∆⋅+== 221 (4)

where:

pi, pf: initial and final values of pressure in the container [Pa];

Ti, Tf: initial and final values of temperature in the container [K];

xH2: hydrogen initial mole fraction [molH2/mol];

∆qH2: hydrogen / oxygen reaction heat released [J/mol];

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cv: molar heat capacity of gaseous mixture (mean value) [J/(mol⋅K)].

So, with the same composition (xH2, cv) and initial temperature (Ti), the mixture will have the

same final temperature (Tf) and therefore the same ratio between the temperatures (Tf / Ti) and the pressures (pf / pi) corresponding to initial and final states.

The values of that ratio can be calculated in closed-form with the following expression (based on fittings of exp. results [3], numerical scientific notation):

pf/pi = -7.7505437E-08(xH2%)5 + 1,1675447E-05(xH2%)

4 - 4.9263486E-04(xH2%)

3 - 3.8631742E-

03(xH2%)2 + 6.3120965E-01(xH2%) - 1.9295087E+00 (5)

(Ti= 293.15 K)

While the values of KG can be expressed in closed-form as follows (based on fittings of exp.

results [3], numerical scientific notation):

pi= 0.1 MPa ⇒ KG= -9.2915799E-07⋅(xH2%)5 + 2.6212963E-04⋅(xH2%)

4 - 2.5070655E-

02⋅(xH2%)3 + 9.4432432E-01⋅(xH2%)

2 - 1.1349571E+01⋅(xH2%) + 3.7939734E+01 (6)

pi= 0.5 MPa ⇒ KG = -5.6597687E-07⋅(xH2%)6 + 1.1719357E-04⋅(xH2%)

5 - 8.7227693E-

03⋅(xH2%)4 + 2.7172619E-01⋅(xH2%)

3 - 3.0197646E+00⋅(xH2%)

2 + 9.2994156E+00⋅ (xH2%) +

3.6854281E+00 (7)

pi= 1 MPa ⇒ KG = -1.5529513E-06⋅(xH2%)6 + 3.3006398E-04⋅(xH2%)

5 - 2.5879701E-

02⋅ (xH2%)4 + 9.0512526E-01⋅(xH2%)

3 - 1.3661800E+01⋅(xH2%)

2 + 8.5859490E+01⋅(xH2%) -

1.8144386E+02 (8)

pi= 3 MPa ⇒ KG = -3.0172288E-06⋅(xH2%)6 + 6.2894474E-04⋅(xH2%)

5 - 4.8045699E-

02⋅(xH2%)4 + 1.6224736E+00⋅(xH2%)

3 - 2.3479106E+01⋅(xH2%)

2 + 1.4803566E+02⋅(xH2%) -

3.2804872E+02 (9)

(Ti= 293.15 K)

where xH2% is the hydrogen percent initial mole fraction.

By this way, we can obtain the maximum values of pressure (Eq. 5, pf) and of its derivative (Eq. 6 - 9, 1, dp/dtMax) and, by the perfect gas model (Eq. 4), the corresponding values of temperature

(Tf) and of its derivative (dT/dtMax), in order to obtain first approximation trends of p and T over time.

2.2.2 Open spaces

In the case of explosion in open spaces (unconfined vapor cloud explosion), the equivalent TNT

(TriNitroToluene) model is applied [5], [6], although the model, at least tendentially, overstates the pressure values close to the explosion and underestimates them at high distances.

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Essentially, such a method computes the pressure wave by applying known graphics and

correlations to the TNT equivalent mass that would produce the same effects as the actual amount of hydrogen involved in the explosion.

The TNT mass, mTNT [kg], which releases the same amount of heat as the hydrogen mass, is computed as:

186.4.1232.0 2

22H

TNT

HHTNT m

q

qmm ⋅≅

∆

∆⋅⋅=

η (10)

where:

mTNT: TNT equivalent mass [kg];

η: explosion efficiency (0 ≤ η ≤ 1).this factor is due to presence of obstacles acting as shields in the area surrounding the exploding mixture (actual values ranging from 0.1 to 0.2);

mH2: hydrogen mass [kg];

∆qH2 : hydrogen / oxygen reaction heat released [MJ/kg];

∆qTNT : TNT reaction heat released [MJ/kg];

The scaled distance z’ [m/(kg/bar)1/3

] is defined as:

31

0 )/('

pm

zz

TNT

= (11)

where z[m] is the distance from the explosion origin and p0 [bar] is the initial pressure.

The variables ps [Pa] and iTNT [Pa·s/(kg/bar)1/3

] are fitted by the following equations:

1 ≤z’≤ 10

)01,2(6 '1013.1 −⋅⋅=∆= zpp

MaxS (12)

)91,0('203 −⋅= ziTNT

(13)

10 < z’≤ 200

)16,1(5 '1083.1 −⋅⋅=∆= zppMaxS

(14)

)06,1('335 −⋅= ziTNT

(15)

Whence the impulse i of the hydrogen mass can be calculated as:

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31

0

⋅=

pm

ii TNTTNT (16)

So, the values of ps and i (and so the rate of pressure increase) can be computed as functions of

the distance from the explosion point.

2.3 MELCOR Modeling

The code models the combustion of gases in control volumes without modeling the reaction kinetics and the flame front propagation. The deflagration (only deflagrations are modeled. no

detonations) and diffusion flame models are derived from HECTR 1.5 code [4]. A simple diffusion flame model allows for the burning of hydrogen-rich mixtures upon entry to volume

containing oxygen.

The combustion ignition and the combustion rate are simulated as functions of different variables

(mole fractions etc.). The rate is modeled varying with time.

3. MAIN RESULTS

3.1 Confined spaces

In this section, comparisons between ECART, MELCOR and some experimental results of

SAFEKINEX project [3] are summarized for the following scenarios:

1. Vessel with V= 0.0028 m3 (stationary volume) containing a mixture of H2 and air

(normal), Figs. 2 ÷5:

a. Initial pressure p0= 0.1, 0.5, 1, 3 MPa;

b. Initial hydrogen mole fraction of mixture xH2 %= 5 ÷ 70 %.

2. Vessel with V= 0.006 m3 (stationary volume) containing a mixture of H2 and air

(normal), Fig. 6÷9:

a. Initial pressure p0= 0.1, 0.5, 1, 3 MPa;

b. Initial hydrogen mole fraction of mixture xH2 %= 5 ÷ 70 %.

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pMax (p0= 0.1 MPa, V= 0.0028 m3)

0

0,2

0,4

0,6

0,8

1

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 0.1 MPa, V= 0.0028 m3)

0

100

200

300

400

500

600

0 20 40 60 80XH2 [mol/mol]

(dp

/dt)

Max

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR

Figure 2 – Maximum values of pressure - pMax and of pressure increase rate - (dp/dt)Max as

function of xH2% (initial hydrogen mole fraction of mixture). Vessel with V= 0.0028 m3.p0= 0.1

MPa.

pMax (p0= 0.5 MPa, V= 0.0028 m3)

0

1

2

3

4

5

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]

pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 0.5 MPa, V= 0.0028 m3)

0

500

1000

1500

2000

2500

0 20 40 60 80XH2 [mol/mol]

(dp

/dt)

Max

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR


function of xH2 % (initial hydrogen mole fraction of mixture). Vessel with V= 0.0028 m3, p0= 0.5

MPa.

pMax (p0= 1 MPa, V= 0.0028 m3)

0

2

4

6

8

10

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]

pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 1 MPa, V= 0.0028 m3)

0

1000

2000

3000

4000

5000

0 20 40 60 80XH2 [mol/mol]

(dp

/dt)

Max

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR

Figure 4 – Maximum values of pressure - pMax and of pressure increase rate - (dp/dt)Max as function of xH2 % (initial hydrogen mole fraction of mixture). Vessel with V= 0.0028 m

3, p0= 1

MPa.

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pMax (p0= 3 MPa, V= 0.0028 m3)

0

5

10

15

20

25

30

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 3 MPa, V= 0.0028 m3)

0

2000

4000

6000

8000

10000

12000

0 20 40 60 80XH2 [mol/mol]

(dp

/dt)

Max

[M

Pa/s

]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR


3, p0= 3

MPa.

pMax (p0= 0.1 MPa, V= 0.006 m3)

0

0,2

0,4

0,6

0,8

1

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]

pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 0.1 MPa, V= 0.006 m3)

0

50

100

150

200

250

300

350

400

0 20 40 60 80XH2 [mol/mol]

(dp

/dt)

Max

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR


3, p0= 0.1

MPa.

pMax (p0= 0.5 MPa, V= 0.006 m3)

0

1

2

3

4

5

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]

pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 0.5 MPa, V= 0.006 m3)

0

500

1000

1500

2000

0 20 40 60 80XH2 [mol/mol]

(dp

/dt)

Max

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR


function of xH2 % (initial hydrogen mole fraction of mixture). Vessel with V= 0.006 m3, p0= 0.5

MPa.

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pMax (p0= 1 MPa, V= 0.006 m3)

0

2

4

6

8

10

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 1 MPa, V= 0.006 m3)

0

500

1000

1500

2000

2500

3000

3500

0 20 40 60 80XH2 [mol/mol]

(dp/d

t)M

ax

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR


function of xH2 % (initial hydrogen mole fraction of mixture). Vessel with V= 0.006 m3, p0= 1

MPa.

pMax (p0= 3 MPa, V= 0.006 m3)

0

5

10

15

20

25

30

0 20 40 60 80

XH2 [mol/mol]

pM

ax [

MP

a]

pMax EXP

pMax ECART

pMax MELCOR

(dp/dt)Max (p0= 3 MPa, V= 0.006 m3)

0

2000

4000

6000

8000

10000

0 20 40 60 80XH2 [mol/mol]

(dp/d

t)M

ax

[M

Pa/

s]

(dp/dt)Max EXP

(dp/dt)Max ECART

(dp/dt)Max MELCOR


3, p0= 3

MPa.

3.2 Open spaces

As regards the simulation of hydrogen deflagration in open spaces with ECART, two scenarios were analysed:

A. H2 – Air (normal) cloud with initial volume of V= 0.0028 m3 located outdoors (Fig. 10):

− Initial pressure p0= 0.1 MPa;

− Initial hydrogen mole fractions of mixture / cloud xH2 %= 5, 10, 20, 30, 40, 50, 60, 70 %.

B. H2 – Air (normal) cloud with initial volume of V= 0.006 m3located outdoors (Fig. 11):

− Initial pressure p0= 0.1 MPa;

− Initial hydrogen mole fractions of mixture / cloud xH2 %= 5, 10, 20, 30, 40, 50, 60, 70 %.

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pMax (T0= 293,15 K, V0= 0,0028 m3 - p0= 0,1 MPa)

0,100

0,105

0,110

0,115

0,120

0 2 4 6 8 10 12 14 16 18 20

l [m]

pM

ax [

MP

a]pMax ([H2]%=5)

pMax ([H2]%=10)

pMax ([H2]%=20)

pMax ([H2]%=30)

pMax ([H2]%=40)

pMax ([H2]%=50)

pMax ([H2]%=60)

pMax ([H2]%=70)

Figure 10 – Maximum values of pressure - pMax as function of distance from initial cloud (mixture of H2 and air) for various xH2 % (initial hydrogen mole fraction of mixture / cloud).

Initial volume of cloud V= 0.0028 m3, p0= 0.1 MPa.

pMax (T0= 293,15 K, V0= 0,006 m3 - p0= 0,1 MPa)

0,100

0,105

0,110

0,115

0,120

0 2 4 6 8 10 12 14 16 18 20

l [m]

pM

ax [

MP

a]

pMax ([H2]%=5)

pMax ([H2]%=10)

pMax ([H2]%=20)

pMax ([H2]%=30)

pMax ([H2]%=40)

pMax ([H2]%=50)

pMax ([H2]%=60)

pMax ([H2]%=70)

Figure 11 – Maximum values of pressure - pMax as function of distance from initial cloud (mixture of H2 and air) for various xH2 % (initial hydrogen mole fraction of mixture / cloud).

Initial volume of cloud V= 0.006 m3, p0= 0.1 MPa.

4. CONCLUSIONS

The fast running approach adopted for the lumped-parameter calculation method allowed a fast

and conservative evaluation of pressure and temperature values both in the case of open and confined space for the experimental situations here considered. The “correlations” model focused

on ECART improvement is still at the implementation stage, as well as the code version, that is presently used for the testing and validation process, involving sub-models and thermo-

hydraulics and chemical numerical modules of ECART.

About MELCOR, only simulations of confined space transients were carried out with default

models. Others simulations, related to transients in open space cases, are object of computational tests. These simulations need different scenario modeling, as the code is based on defined control

volumes, and this requires development stages longer than previous ones.

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5. ANNEX - HYDROGEN COMBUSTION AND EXPLOSION

The hydrogen oxidation process can be modeled as a one step reaction as:

H2 + ½ O2 → H2O + ∆q (17)

Where ∆q is the heat generated by the reaction which can be assumed equal to LHV (Lower Heating Value, 119.96 MJ/kg) because, in case of explosions in accidental scenarios, reactants and products are in gaseous phase (LHV is defined as the amount of heat released by combusting

a specified quantity - initial T= 25°C - and returning the temperature of the combustion products to 150°C, which assumes the vaporization heat of water in the reaction products is not recovered

– US DoE).

About others fuels, for example methane (CH4 - LHV: 47.13 MJ/kg) and propane (C3H8 - LHV:

46.28 MJ/kg) have values which are significantly lower than the hydrogen one.

The flammability limits of air - hydrogen mixtures are 4 - 75 vol% (values function of pressure

and temperature of the mixture) [7]. Thus the flammability range is extremely large.

About the rapidity of hydrogen combustion, it is known as such value is often high, generating

fast deflagrations or, sometimes, even detonations.

6. REFERENCES

[1] F. Parozzi, Fukushima, “Anatomia di un incidente”, Le Scienze (Italian edition of

Scientific American), n. 514, June 2011,

[2] F. Parozzi, S. Paci, “Development and validation of the ECART code for the safety analysis of nuclear installations”, 14th International Conference on Nuclear Engineering

ICONE14, Paper ICONE14-89275, Miami, Florida USA, July 17-20, 2006,

[3] FP5 EU-Project, “SAFEKINEX - SAFe and Efficient hydrocarbon oxidation processes by KINetics and Explosion eXpertise”, Programme on Energy, Environment and

Sustainable Development, Contract Number EVG1-CT-2002-00072, 2003 – 2006,

[4] SANDIA Labs, “MELCOR Computer Code Manuals”, Vol 1: Primer and Users’ Guide. Version 2.1, Draft version, September 2011,

[5] F.D. Alonso, E.G. Ferradas, J.F. Sanchez Perez, A.M. Aznar, J.R. Gimeno and J.M. Alonso, “Characteristic overpressure–impulse–distance curves for the detonation of

explosives, pyrotechnics or unstable substances”, Journal of Loss Prevention in the Process Industries 19, 724 – 728, 2006;

[6] A. Beccantini, A. Malczynski and E. Studer, “Comparison of TNT-equivalence approach,

TNO multi-energy approach and a CFD approach in investigating hemispheric hydrogen-air vapor cloud explosions”, Proceedings of the 5th International Seminar on Fire and

Explosion Hazards, Edinburgh, UK, 23-27, April 2007,

[7] V. Schroeder, K. Holtappels, “Explosion characteristics of hydrogen-air and hydrogen-oxygen mixtures at elevated pressures”, ICHS International Conference on Hydrogen

Safety, Congress Palace, Pisa, Italy, September 8-10, 2005.

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