Analysis of loss of water in the Spent

Fuel Pools of Ignalina NPP after reactor

shutdown

Vileiniškis Virginijus, Kaliatka Algirdas Lithuanian Energy Institute

Technical Meeting on The Management of Spent Fuel at Shutdown Reactors, Including Those to be Shut Down Prematurely

11-13 June 2018, Vienna

Outline

• Introduction

• Spent Fuel Storage and Handling System

• RELAP/SCDAPSIM analysis of loss of water in SFP

after the shutdown of reactor

• ASTEC analysis of loss of water in SFP after the

shutdown of reactor

• Analysis of loss of water accident in SFP three

years after the shutdown of reactor

• Conclusions

2

Introduction (1)

• At the Ignalina NPP two RBMK-1500 Russian design

channel-type graphite-moderated boiling water

reactors are shut down for decommissioning (31

December of 2004 and 31 December of 2009).

• The reloaded from the RBMK-1500 reactor FA

remain in the pool for at least a year, after

which they may be removed to be cut in a

“hot” cell. • The two fuel bundles (each 3.5 m long placed

one above the other) are separated and

placed into the special shipping casks. 3

Introduction (2)

• The shipping casks with SFA are stored in pools until

loading into protective casks CASTOR or CONSTOR for

transportation to dry SF storage facility.

• After final shutdown, the total amount of SFA was about

22000 (about 2500 tons).

• The last SFA was unloaded from the reactor on 25

February 2018.

• The most challenging consequences for the SFP can occur

in the case of loss of heat removal.

4

Introduction (3)

• The heat removal from the SFAs in SFP can be lost due to

failure of heat removal system or in the case of

uncompensated leakage of water from SFPs.

• The analysis of loss of water was performed using

RELAP/SCDAPSIM and ASTEC computer codes.

• Two cases were analysed:

– Safety evaluation of SFP at Ignalina NPP just after the shutdown

of Unit 2 reactor (situation on January 01 of 2010).

– Situation after three years after the permanent shutdown of

Unit 2 reactor of Ignalina NPP.

5

Spent Fuel Storage and Handling System(1)

• Spent fuel storage and handling system is designed to

implement the following basic processes:

– Holding & storing SF extracted from the reactor (prior to cutting

process, cooling period is at least for 1 year);

– Cutting SFA into fuel bundles and placing them into 102 pcs.

shipping casks;

– Holding 102 pcs. shipping casks with cut FA assemblies in

storage pools;

– Transporting SF outside from the units (after cooling in pools for

at least 5 years) to the dry storage facility.

• Dry Spent Fuel Storage in the Casks during 50 years.

6

Spent Fuel Storage and Handling System(2)

• Spent Fuel Storage and Handling System of each reactor

unit consists of 12 pools:

– Two pools (Compartments 236/1, 236/2) for cooling uncut SF

extracted from the reactor;

– Five pools (Compartments 336, 337/1,2, 339/1,2) used for

storing SF in 102 pcs casks after cutting;

– One pool for collecting SFA prepared for cutting, cutting hanger

from SFA, transporting SFA to the hot cell and 102 pcs casks in

the hot cell and backward to pools (Compartment 234);

7

Spent Fuel Storage and Handling System(3)

– Two pools (Compartments 338/1, 338/2) to perform operations

to load the shipping casks with the FA into the transport casks

and store the 102 placed shipping casks;

– Transport corridor (Compartment 235) intended to transport

SFA and shipping casks loaded with spent fuel assemblies

between the pools;

– Transport corridor (Compartment 157) intended to transport

fresh fuel and reactor assemblies from the fresh fuel assembly

preparation bay to the reactor and return spent fuel and reactor

assemblies from the reactor to the storage pools.

8

Spent Fuel Storage and Handling System(4)

9

RELAP/SCDAPSIM analysis of loss of water

in SFP after the shutdown of reactor (1)

• For the analysis single

compartment 336 with

the highest decay heat

load (171.6 kW) selected

with 1428 SFA.

• Remaining

compartments of SFP are

modelled as single

compartment.

• Adiabatic condition on

outer surface of wall.

10

011

010

RELAP/SCDAPSIM analysis of loss of water

in SFP after the shutdown of reactor (2)

11

0

200

400

600

800

1000

1200

1400

1600

1800

0 250 500 750 1000 1250 1500 1750

Te

mp

era

ture

, oC

Time, h

Pool wall - inner

Pool wall - outer

Max. fuel

Steam

Temperatures of fuel and SFP structures

0,00000

0,00001

0,00010

0,00100

0 250 500 750 1000 1250 1500 1750

Tota

l H2

gen

era

tio

n r

ate

, kg/

s

Time, h

bgth-0

Hydrogen generation rate

0

1

10

100

1 000

10 000

100 000

0 250 500 750 1000 1250 1500 1750

Oxi

da

tio

n h

ea

t g

en

era

tio

n,

W

Time, h

bgthq-0

Generation of heat At the time moment 780 h exothermic steam – zirconium

oxidation reaction starts.

RELAP/SCDAPSIM analysis of loss of water

in SFP after the shutdown of reactor (3)

12

Pressure inside the fuel rods

At the time moment 822 h - rupture of fuel rods claddings and release of gasses to the pool.

0

200000

400000

600000

800000

1000000

1200000

0 250 500 750 1000 1250 1500 1750

Pre

ssu

re i

n f

ue

l ro

d,

Pa

Time, h

pgas-1

Cladding rupture

0

2

4

6

8

10

12

0 250 500 750 1000 1250 1500 1750

He

at

tra

nsf

er

co

eff

icie

nt,

W/m

2K

Time, h

hthtc-302200100

Start of steam -

zirconium reaction

Heat transfer coefficient to the pool walls

RELAP/SCDAPSIM analysis of loss of water

in SFP after the shutdown of reactor (4)

13

Heat transfer in the 336 compartment

Sum of heat dissipated in the

walls and removed by air in the

room above SFP less than decay

heat generated in SFA

Temperature of spent fuel

increases 0

20

40

60

80

100

120

140

160

180

200

0 250 500 750 1000 1250 1500 1750

He

at

tra

nsf

er,

kW

Time, h

Through the cover of pool

To the wall of pool

TotalStart of steam -

zirconium reaction

Residual decay heat of SFAs

ASTEC analysis of loss of water in SFP after

the shutdown of reactor (1)

• For the analysis single

compartment 336 with

the highest decay heat

load (171.6 kW)

selected with 1428 SFA.

• Adiabatic condition on

outer surface of wall.

• SFP cover plate not

modelled.

14

ASTEC analysis of loss of water in SFP after

the shutdown of reactor (2)

15

• Processes modelled:

– Radial conduction between fuel and cladding;

– Axial, radial, azimuthal conduction inside each modelled element;

– Convective heat exchange;

– Heat exchanges between corium layers and with wall;

– Claddings oxidation;

– UO2 and ZRO2 dissolution by liquid Zr;

– Oxidation of U-Zr-O magma;

– Radial movement of materials;

– Decanting in lower part of pool;

– 2D movement of magma;

– Corium slumping, fragmentation;

– Fission products and structural materials transport;

– …

ASTEC analysis of loss of water in SFP after

the shutdown of reactor (3)

16

Water level in SFP 336 compartment

3.0

5.0

7.0

9.0

11.0

13.0

15.0

0 2 4 6 8 10

Level, m

Time, h

Start of Fuel Assemblies uncovering • t= 100 s – initiation of water leakage in the

SFP;

• t= 2.2 h –fuel uncovering and heat up in air

starts;

• t= 7.5 h –all SFAs are fully uncovered;

• t= 10.3 h – water level decreases down to the

bottom of SFP;

• t= 416.88 h - first cladding creep rupture;

• t= 416.88 h - start of FPs release from fuel

pellets (200 s later from first cladding

rupture);

• t= 877.26 h - first material slump of SFP floor;

• t= 964.34 h – calculation terminated after

melt through one layer of concrete wall.

0

500

1000

1500

2000

0 200 400 600 800 1000

Te

mp

era

ture

, K

Time, h

1000 oC

1500 oC

Maximal fuel temperature Fuel temperature increases until reaches melting

temperature of concrete wall (~1527 °C)

ASTEC analysis of loss of water in SFP after

the shutdown of reactor (4)

17

Hydrogen generation

Fraction of released fission products

Only highly volatile fission products

released:

• Near 100 % were released Ag, Br,

Cs, Cu, I, Kr, Rb and Xe.

• About 29 % - Se and Te.

• About 52 % - Sb.

0

20

40

60

80

100

120

140

0 200 400 600 800 1000

Ma

ss

, k

g

Time, h

Analysis of loss of water in SFP three years

after the shutdown of reactor (1)

18

• ASTEC V2.0R2 single pool model with 4

different fuel rod groups.

• The total decay heat in the SFP of Unit 2

three years after shutdown - 810 kW.

• Heat transfer through the concrete wall.

6.8

m

6.8

m

6.8

m

6.8

m

Water level

0.1

0.0

-1.623

4.377

6.9

11.177

15.277

16

.9m

26.277

11

m

+24.6 m

+7.7 m

ROD 1

ROD 4

ROD 2 ROD 3

POOL LOWER

TO ATMOSPHERE

PO

OL

WA

LL

0.5 m

Groups

Fuel rod

groups in

the model

SFA

decay

heat, kW

Amount of

SFAs in group

Group power,

kW

SFAs in compartment

“236/2” ROD1 0.153 166 ~25.4

SFAs in “236/2” compartment

ROD2 0.122 1182 ~144.20

SFAs in “236/1” and “234” compartment

ROD3 0.122 892 ~108.82

SFAs in shipping casks ROD4 0.9387 5661 ~531.40

Total: 7901 ~810

Analysis of loss of water in SFP three years

after the shutdown of reactor (2)

19

Water level in SFP Fuel temperatures

-2

0

2

4

6

8

10

12

14

16

18

20

0 30 60 90 120 150

Time, h

Wat

er l

evel

fro

m p

ool

bo

tto

m,

m.

top of fuel assemblies in heat structure "12221" (ROD4)

top of fuel assemblies in heat structures "12111", "12211", "12121" (ROD1-3)

start of uncovering of fuel assemblies

0

100

200

300

400

500

0 50 100 150 200 250 300 350 400 450 500 550

Time, h

Tem

per

ature

, oC

ROD1 ROD2

ROD3 ROD4

Fuel temperature increases, until reaches about 400 °C

Decay heat from the spent nuclear fuel is removed by air and by conduction through

walls of SFP building.

Conclusions

• For the analysis of the loss of water in the SFPs models were

developed using the RELAP/SCDAPSIM and ASTEC codes.

• Calculation, performed for the compartment with the highest decay

heat load covered by the steel sheets, shows that the temperature

of fuel is continuously increasing. But the heat up process is very

slow after total loss of water in the pool. Thus, the operators have

enough time to find the alternative means for the cooling of SFP.

• The preliminary ASTEC analysis for the situation three years after

the permanent shutdown of Unit 2 showed that SFAs can be cooled

by the air circulation.

20

Virginijus Vileiniškis

Virginijus.Vileiniskis@lei.lt

21