ERMSAR 2015, Marseille March 24 – 26, 2015

Modelling of Hydrogen Generation during Spent Fuel

Pool LOCA using Thermal Hydraulic and Severe

Accident Code SOCRAT/V5

A.Vasiliev

Nuclear Safety Institute of Russian Academy of Sciences (IBRAE),

B.Tulskaya 52, 115191 Moscow, Russia

ERMSAR 2015, Marseille March 24 – 26, 2015

1. Purpose

2. Hints to Model SFP LOCA

3. Main Results of Modelling by SOCRAT Code

4. Modelling of Zr Oxidation in H2O-O2-N2 Gas

Mixtures in Application to SFP LOCA

5. Conclusions

Content

of Presentation

ERMSAR 2015, Marseille March 24 – 26, 2015

Importance

In the course of postulated spent fuel pool loss of coolant accident (SFP

LOCA) at NPP, a huge amount of hydrogen can be generated due to

chemical reactions of zirconium and steel with steam-oxygen-nitrogen

atmosphere.

Highly explosive mixture of hydrogen and oxygen may threaten to

containment integrity.

Loss of SFP water may result in fuel assemblies heat-up, oxidation,

melting and relocation accompanied by radioactive nuclides release.

This is why SFP LOCA modelling in SA codes is necessary.

ERMSAR 2015, Marseille March 24 – 26, 2015

Difficulties in Modelling SFP LOCA Realistically

Thermo-hydraulic, thermo-mechanical, chemical and severe accident processes

under SFP LOCA conditions are extremely complicated and very far from

complete understanding.

It is not clear how to model specific phenomena in codes keeping in mind that the

phenomena are modelled in codes with many assumptions and limitations.

Scientific community has not come to consensus yet how far to severe accident

range would SFP LOCA develop.

ERMSAR 2015, Marseille March 24 – 26, 2015

Gas Flows in Spent Fuel Pool during Loss of Coolant Accident

(LOCA)

Break 10 mm

N2, O2

H2O (steam)

Some N2 and O2 would entry SFP!

Densities of N2 and O2 are larger than

density of H2O!

It is necessary to consider oxidation in

vapour-oxygen-nitrogen mixtures!

ERMSAR 2015, Marseille March 24 – 26, 2015

B. Jaeckel, J. Birchley, L. Fernandez-Moguel. Spent Fuel Pool

under Severe Accident Conditions. Proc. 22nd Int. Conf. on Nuclear

Engineering (ICONE22), July 7-11, 2014, Prague, Czech Republic.

ICONE22-30729.

M. Steinbrück, F. Oliveira da Silva, H.J. Seifert. High-Temperature

Oxidation of Zircaloy-4 in Steam-Nitrogen Mixtures. NuMat2014:

The Nuclear Materials Conference, Clearwater Beach, Florida,

USA, October27-30, 2014.

Zr oxidation in H2O-O2-N2

mixture is very far from

classical parabolic law!

SFP LOCA Investigation

stakeholders

EDF, France

PSI, Switzerland

IRSN, France

IBRAE, Russia

NRCKI, Russia

ERMSAR 2015, Marseille March 24 – 26, 2015

Hints to Model SFP LOCA in Realistic Way

1. To estimate the thermal behaviour adequately it is necessary to divide SFP into two or several channels.

2. When evaluating radiative heat transfer from SFP structures to external boundaries in upward, sideward and downward directions we should take into account the temperature profiles in system. The temperature at periphery is less than the temperature in the centre.

3. After pool dry-out, the convective gas velocity established in FAs is about 10-20 cm/s due to hydraulic resistances, so the convective cooling is limited.

4. It is necessary to adequately take into account the high chemical power of Zr oxidation in steam-oxygen-nitrogen mixtures.

Radial Temperature Profile in SFP after

Pool Dry-Out T

x

The temperature profiles are non-uniform and three-dimensional!

Representative Scenario of Hypothetical

SFP LOCA at Balakovo NPP Unit 1 with

VVER-1000 Nuclear Reactor Storage time, years Pool “a” Pool “b” Pool “c”

3 days

30 days

0.8

2.2

3.6

4.7

5.8

10.2

65 10

16

60

30

2

1

1

60 4

13

0

30

4

0

0

38 3

23

5

0

0

0

0

Power, MW 6.78 5.91 3.82

Total power, MW 16.51

Pool “a” Pool “b” Pool “c”

No break

Olny loss of water supply

(blackout)

SFP Pool Dry-Out

Geometry of

Radiative Heat

Exchange in SFP

R

H

d

D qrad,s

qrad,u

Water

red colour – high

heat load

green colour – low

heat load

Propagation of Burning Wave

32

)2/(

3T

D

dBrad

d

D

rad

Rt

2

R

3 30 0.8 2.2 3.6 4.7 5.8 10.2 FA storage times

t 50,000 s

T1500K

Linear Kinetics Results in Drastic Augmentation

of Oxidation rate and Heat Generation!

0 4000 8000 12000

Время, с

0

2

4

6

8

10

Мощность

реакции

окислен

ия

, Вт

/твэл

T = 1273Kair parabolic

steam parabolic

steam+nitrogen linear

Total power generation of one FA can be increased by 1 kW even at temperature 1000С !

Solo Fuel Assembly (FA) Hanging in Air

The single FA can be effectively cooled by

convection in air!

Except 3 day, 30 days and possibly 0.8 years

storage time.

Storage

time,

days

FA

power,

W

3 days 94.2

30 days 31.8

0.8 5.75

2.2 3.29

3.6 1.52

4.7 1.4

5.8 1.2

10.2 0.9

Ensemble of FAs in SFP

The behaviour of FAs Ensemble is completely different

compared to single assembly!

3 30 0.8 2.2 3.6 4.7 5.8 10.2

SFP Nodalization Scheme for SOCRAT

water inlet

1

2

3

4

17

18

Water volume

coolant outlet at pressure p(t)

SF Pool

rigid wall

Boundary condition

switch

Dome

Concrete

Air volume

FA_4.5y FA_7.5y FA_6.0y FA_9.0y FA_3.0d FA_1.5y

water flow outlet

BASKETS

rigid wall

CH

1

CH

4

CH

44

CH

0

FA_3.0y

Boundary condition switch

CH

2

BBASKETS

3d - 3y

CH

11

CH

111

CH

3

CH

33 CH

5

CH

7 CH

55

CH

6

CH

66

CH

77

CH

8 CH

88

Concrete

CH

a CH

b

Water Mass Level

Pool “a” Pool “b” Pool “c”

Hydrogen Production

Temperature, Nuclear and Chemical Power

Pool a

Chemical heat generation is very high!

Radial radiative heat fluxes to concrete walls

Masses of Materials in SFP

0 100000 200000 300000 400000

Time, s

0

40000

80000

120000

Mass

, k

g

Materials in pool aUO2

Zr

ZrO2

ss

FeO NiO Cr2O3

H2 Production: Total and due SS Oxidation

0 100000 200000 300000 400000

Time, s

0

400

800

1200

1600H

yd

rogen

pro

du

ctio

n, k

g

pool atotal H2

H2 due to ss oxidation

Characteristic Weight Gain Dynamics

during Cladding Oxidation in Air at 1000C

Oxygen and nitrogen contents in mixture are 50% both (O 50 N 50)

Experiment

M.Steinbrueck et.al.

Analytical model

A.Vasiliev

Even N2 concentrations 1%, 5% or 10% result to oxidation enhancement

from 10% to 300%!

Characteristic Weight Gain Dynamics

during Cladding Oxidation in Air at 1000C for different N2 Content

Oxygen and nitrogen contents in mixture are 50% both (O 50 N 50)

Kdiff –

enhancement

factor of oxygen

diffusion effective

coefficient in

ZrO2+ZrN layer

H2 Generation for ilim=3 and ilim=4

iIim=3 – classical parabola

ilim=4 – enhanced oxygen diffusion coefficient in ZrO2 + ZrN layer

Kdiff is equal 3

Summary

The thermo-hydraulic and severe accident code SOCRAT/V5 was used for estimation of

hydrogen release during spent fuel pool accident with LOCA.

The representative scenario of hypothetical SFP LOCA at Balakovo NPP Unit 1 with

VVER-1000 nuclear reactor was considered.

The choice of cladding oxidation model strongly influences the hydrogen generation rate

– a key parameter for evaluation of hydrogen recombinating system effectiveness in NPP.

It was shown that the mass of hydrogen released during the SFP loss-of-coolant accident

can be about 3000 kg (even not taking into account the hydrogen released during molten

core-concrete interaction).

The maximum calculated hydrogen release rate was obtained in the case of enhanced

oxidation in steam-oxygen-nitrogen mixtures compared to the classical parabolic

oxidation in pure steam.