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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