Severe Accident Scenarios:

Indian Perspective

FR13, 4-7 Mar 2013, Paris

P.Chellapandi Indira Gandhi Centre for Atomic Research

Kalpakkam, India

Approach Towards CDA for Future FBRs

• Target reliability for shutdown system to be

considerably enhanced w.r.t PFBR: to be achieved

by introducing third level shutdown systems

On probabilistic basis

• Target reliability of DHR system to be enhanced w.r.t

PFBR: Innovative features to be introduced

• Combined analysis of reactor physics, energy

release and mechanical consequences, validated

with experiments

Assessment of mechanical consequences with

pessimistic assum on energy release

Two initiating events, that could lead to a CDA: (1) Loss of

flow without reactor shutdown leading to ULOFA

(Unprotected Loss of Flow Accident) and (2) Uncontrolled

withdrawal of control rods introducing reactivity ramp leading

to UTOPA: Unprotected Transient Overpower Accident .

In the UTOPA scenario, the in-pin motion of molten fuel

caused by fission gas pressure, called 'fuel squirting' or 'fuel

extrusion', introduces a lot of negative reactivity in the core

and hence stabilize the reactor at a higher power level

without causing any core melting / boiling / disruption.

Hence, UTOPA is not energetic

ULOFA is analyzed in detail, since it leads to a CDA that

could pose major threat to the structural integrity of primary

containment.

Severe Accident Scenario of PFBR

3 phases: Pre-disassembly, transition and Disassembly

Flow reduction immediately leads to coolant temperature rise

in core that gives positive reactivity, which however, is

dominated by the negative reactivity due to the radial thermal

expansion of core and the net reactivity is negative. This

results in decrease in power.

Power to flow ratio however increases subsequently, resulting

in high coolant temperature rise and voiding in the upper part

of the highly rated fuel channel.

Core voiding spreads radially outward and axially downward

increasing positive reactivity contribution. When the voiding

spreads into the central part of the core, the net reactivity

becomes positive, initiating a series of events: power

excursion, clad dry out, rapid temperature rise in the fuel and

clad and ultimately melting of fuel and clad.

ULOFA Scenario

The molten materials would be swept out of the core by the

shearing force of the coolant vapor (dominant force in the case of

fresh fuel) as well as by the accumulated fission gas pressure

(dominant in the case of irradiated fuel).

If there are sufficient negative reactivity introduced from Doppler

and fuel displacement, the core could become subcritical and the

accident terminates.

Else, with the high rate of positive reactivity addition, core attains

a super prompt critical condition and sub subsequently into

disruptive condition.

Consequence is large thermal energy release & vaporization of

significant portions of fuel and structural materials of the core.

Under an idealized condition, a mixture of molten materials at the

bottom with vapor phase at the top could be conceived at the end

of disassembly phase

ULOFA Scenario .. Contd….

Mechanical energy release depends upon the reactivity addition

rate in the disassembly phase, which in turn depends upon the

assumptions made on the sodium void propagation, fuel

displacement / slumping characteristics, reactivity feedback

mechanisms, cross section data, nature of temperature

distributions assumed for the disrupted core and cross section

data employed in the analysis

One of the important parameters influencing the coolant void

generation/propagation is flow halving time. With lower value, the

coolant voids could generate below the core top and spread

rapidly to the core centre, resulting in high positive reactivity rate

in the disassembly phase. With higher value, the coolant boiling

starts at the upper portion of active core, which introduces

negative reactivity due to the high neutron leakage.

Work Potential - contd..

Mechanical energy release when vapor phase expands

from its initial pressure to one atmosphere

Scenario Reactivity

addition rate

Energy

Release

Low flow halving time of 2 s, coherent core

lumping, absence of feedbacks, flat temperature

distribution across the core at the end of

disassembly phase and use of conservative cross

section data (CV2M cross section set),

200 $/s 1000 MJ

Conservative slumping model: active core zone

divided into three. The molten fuel from middle

one third occupies the core lower portion and fuel

from top one third occupies the middle portion.

65 $/s 100 MJ

A flat temperature distribution of the core at the

end of disassembly phase 50 $/s 268 MJ

Longer flow halving time of 8 s, incoherent core,

presence of all feedbacks, realistic temperature

distribution across the core and use of realistic

cross section data (ABBN cross section set),

10.5 $/s < 1 MJ

Work Potential – contd..

Synthesis of above results motivates to investigate the mechanical

consequences of a CDA over a wide range of work potentials

corresponding to the reactivity addition rates ranging from 25 $/s to 200 $/s.

Work Potential – contd..

1

10

100

1000

10000

25 50 75 100 125 150 175 200

Reactivity - $/S

Me

ch

an

ica

l E

ne

rgy

- M

J

Important Consequences of a CDA in SFR

a b c Deformations of

vessels Sodium ejection to

RCB Post Accident Heat

Removal

• Preliminary Deformations of Components (0 - 50 ms)

• Upward Motion of Sodium Slug (50 – 100 ms)

• Sodium Slug Impact on top shield and development of

Transient Forces on Reactor Vault (100-150 ms).

• Sodium Release to RCB during quasi-static state (150 –

900 ms)

• Post Accident Heat Removal Condition depending upon

coolability of core bubble (> 900 ms)

Mechanical Energy Release Scenario

0

50

100

150

200

250

300

350

0 25 50 75 100 125 150 175 200 225 250

Time - ms

En

erg

y r

ele

ase -

MJ

Strain energy absorbed by the vessel

100

MJ

1000

MJ

500

MJ

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700 800 900 1000

Work potential - MJE

ne

rgy

re

lea

se

- %

Energy balance in the vessel

Total energy

absorbed

upper portion

bottom portion

Sequence of Mechanical Loadings and

Energy Absorptions

Local deformations, get saturated at high mechanical energy release and

the vessel absorbs the energy uniformly, enhancing its energy absorbing

potential.

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10 12 14 16 18 20

Developed length from bottom - m

Dis

pla

ce

me

nt

- m

Radial displacement in the vessel

1000 MJ

100 MJ

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500 600 700 800 900 1000

Work potential - MJ

Str

ain

- %

Peak strain at top

Averaged strain

Peak to averaged strain

Membrane strains in the vessel

Main Vessel Deformations

Deformation becomes more uniform compared to lower work

potential cases. This is a favorable feature that the energy absorbing

potential is not linear and the vessel can absorb higher energy

without undergoing rupture locally upon application of higher energy

by the core bubble.

0

1

2

3

4

5

6

7

8

0 100 200 300 400 500 600 700 800 900 1000

Work potential - MJ

Imp

ac

t p

re

ss

ure

- M

Pa

Peak impact pressure on top shield Sodium slug impact scenario

-30

-20

-10

0

10

20

30

0 50 100 150 200 250

Time - ms

Imp

ac

t v

elo

cit

y -

m/s

1000 MJ

500 MJ

200 MJ

100 MJ

Slug Impact Loadings and Their Effects

In view of short duration of impact loadings and high mass inertia of top

shield structures, it has high potential to absorb higher impact loads and

hence, the integrity of top shield would not be of concern and do not

decide the acceptable work potential.

1000 MJ 600 MJ 800 MJ 900 MJ

100 MJ 200 MJ 400 MJ 500 MJ

Sodium Slug Impact w.r.t Sodium Release to RCB

Sodium slug decelerates to get separated from the top shield. Hence

the quasi-static pressure in the cover gas is tending towards

saturation with higher work potential

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500 600 700 800 900 1000

Work potential - MJ

Pre

ss

ure

- k

Pa

Quasi-static cover gas pressure

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800 900 1000

Work potential - MJP

re

ss

ure

- k

Pa

Pressure rise in RCB

Pressure rise in Reactor Containment Building

Since quasi-static pressure saturates, sodium release to RCB saturates

and hence containment loadings would attain saturation at higher work

potentials.

11 Tests on 1/13th scale mockups to

demonstrate the structural integrity of

DHX and to simulate sodium leak

Integrity of Main Vessel, Top Shield and DHXs

Main vessel capacity = 1200 MJ

DHX capacity = 500 MJ

Maximum sodium leak = 275 kg for 100 MJ

• Parametric study on mechanical energy release

values in the range 100 – 1000 MJ indicates that

primary containment has high potential to

withstand the transient forces generated by

energy release even more than 1000 MJ.

• Sodium ejection into the RCB through top shield

penetrations under sodium slug impact

phenomenon is limited with higher energy

• The deformations of decay heat exchangers

immersed in the sodium pool could limit the

acceptable work potential.

• For PFBR, this value is found to be 500 MJ from

simulated experimental study.

Conclusions

Thank You