CANDU Safety #12: Large Loss of Coolant Accident - Canteach

CANDU Safety #12:
Large Loss of Coolant Accident
F. J. Doria
Atomic Energy of Canada Limited
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 1

Overview
λ Event sequence for a large break loss-of loss of-coolant coolant accident
(LOCA)
λ Acceptance criteria used to assess the results of the analysis
λ Normal operating conditions
λ Fuel and pressure tube behaviour during the transient
λ Fission product release behaviour
λ Containment behaviour
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 2

Large LOCA with ECC Available
λ Event Sequence
– A large break occurs in a large diameter pipe in PHT,
discharging coolant into containment
– PHT depressurizes causing coolant voiding and an increase in
reactivity
– Reactor power increases until the reactor is shutdown on a
neutronic trip (i.e., high power, high-rate high rate power) or process trip
(i.e., low PHT pressure, low pressurizer level, high containment
pressure etc.)
– The PHT flow decreases fastest in the core pass downstream of
the break; and coolant stagnation occurs
– Onset of fuel dryout results in an increase in fuel temperature
– Once the PHT pressure is reduced to the ECC activation
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 setpoint, ECC is activated; the two loops are isolated from each
3

PHT Pumps
Postulated
Break Locations
1) Pump Suction
2) Inlet Header
3) Outlet Header
Primary Heat Transport System
Steam Generators
Feeders
Reactor/Fuel Channels
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 4

Break in Primary Heat Transport System
λ Critical pass is the
pass downstream of
the break location
λ Key of analysis is to
stagnate the flow in
the channel
the channel STEAM GENERATOR
PRESSURIZER
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ROH
pressure gradient across
one core pass reduced
BREAK IN REACTOR
INLET HEADER
RIH
REACTOR CORE
RIH
ROH
PUMP
OUTLET
FEEDER
PIPES
INLET
FEEDER
PIPES
PHTS.DWG

Analysis Acceptance Criteria
λ Dose limits are not exceeded
λ Two independent shutdown systems will arrest the reactivity
and power excursion, and will maintain the reactor in a
shutdown state
λ Fuel channel integrity is not compromised
λ The structural integrity of the containment must be maintained
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 6

Large Break LOCA Safety Analysis
λ Involves determining:
– Fuel normal operating conditions
– Fuel and fuel channel temperatures during LOCA transient
– Fission product release during transient
– Containment analysis
– Dose analysis
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 7

Fuel conditions prior to the onset of the accident
– Normal operating conditions is modelled by the ELESTRES
computer code
– Main Input Requirements
λ Fuel element (pellet and sheath) dimensions &
properties
λ Power/burnup history
λ Coolant temperature and pressure
– Important Output Parameters
λ Fission product distribution (gap, grain boundary and
grain bound)
λ Internal gas pressure
λ Fuel temperatures
λ Pellet strain
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 8

Transient Temperatures
– Thermalhydraulic behaviour is calculated by the CATHENA
circuit model
λ Reactor Inlet Header Break (RIH)
– 20%, 25%, 30%, 35%, 40% & 100%
λ Pump Suction Pipe Break (PS)
– 40%, 45%, 50%, 55%, 60%, 70% & 100%
λ Reactor Outlet Header Break (ROH)
– 80%, 90%, 95% & 100%
– A critical break size is identified for each break location and
a detailed CATHENA single-channel single channel analysis is performed:
λ 35% RIH , 55% PS & 100% ROH
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Circuit Model of Primary Heat Transport System
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 10

Depressurization in Inlet Headers (35% RIH
Break)
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ECC Flows
λ No ECC flow into the intact loop,
since
– intact loop pressure > broken loop pressure
(intact loop was isolated from the broken
loop)
– Intact loop pressure > ECC supply pressure
λ LOCA signal & loop isolation
initiated at 9 s after accident
λ Loop isolation complete by 29 s
λ ECC flows into the broken loop, 3
stages
– High-pressure High pressure injection initiation (38 s)
– Medium-pressure Medium pressure injection (293 s)
– Low-pressure Low pressure injection (678 s)
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 12

Break Survey for RIH breaks (Flows)
CATHENA Circuit Model
λ large breaks ==>
sustained reverse flow
λ small breaks ==>
forward flow
λ medium breaks ==>
stagnation
λ Critical break is 35%
RIH
Flow (kg/s)
200
100
0
-100
-200
-300
-400
-500
Coolant Flow at Center of Downstream Core Pass (RIH)
0 10 20 30 40 50
Time (s)
EARLY FLOW STAGNATION
20% RIH 25% RIH 30% RIH
35% RIH 40% RIH 100% RIH
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 13

Break Survey for RIH breaks (Sheath
Temperature)
CATHENA Circuit Model
λ Temperature transient
corresponds to flow
conditions
λ Flow stagnation ==>
temperatures increase
λ Break in flow
stagnation ==>
temperatures drop
λ Critical break is 35%
RIH
Temperature ( o C)
RIH Break Survey- Outside Sheath Temperature at Bundle 7
(center channel) of Critical Core Pass
1200
1000
800
600
400
200
0
0 20 40 60 80 100
Time (s)
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 14
20% RIH
25% RIH
30% RIH
35% RIH
40% RIH
100% RIH

Single Channel Model for 35% RIH Critical Break
Boundary Conditions from Circuit Analysis Applied
λ Header boundary
conditions (pressure,
enthalpy, void)
λ 6 inlet feeder
segments
λ Inlet end-fitting end fitting
model
λ 12 channel segments
(for 12 bundles)
λ Outlet end-fitting end fitting
model
λ 7 outlet feeder
segments
Inlet header Outlet header
Inlet Feeder
Outlet Feeder
Inlet End-fitting Fuel Channel Outlet End-fitting
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 15

Temperature ( o C)
Single Channel Analysis of 35% RIH Critical
Break
Channel 06 (7.3 MW); Outer Elements; Sheath Temperatures
1400
1200
1000
800
600
400
200
0
Critical Pass (Broken Loop)
0 50 100 150 200
Time (s)
Bundle 5 Bundle 6 Bundle 7 Bundle 8
Non-Critical Pass (Broken Loop)
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 16
Temperature ( o C)
1400
1200
1000
800
600
400
200
0
0 50 100 150 200
Time (s)
Bundle 5 Bundle 6 Bundle 7 Bundle 8

Detailed Fuel Element Analysis
λ Objective of analysis is to predict the potential for sheath
failure during the LOCA
λ Boundary conditions from single-channel single channel CATHENA analysis
is provided to ELOCA code
– coolant temperature,
– coolant pressure,
– sheath-to sheath to-coolant coolant heat transfer coefficient,
– ELESTRES normal operating conditions, and
– power pulse
λ For excessive straining: the increase in sheath temperature
and high internal gas pressure in conjunction with low coolant
pressure may result in sheath failure
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 17

Fuel Element Failure Mechanisms
λ The ELOCA code is used to generate a sheath failure map
(threshold map) in conjunction with failure criteria, for
• no
example
no excessive example
straining- straining 5% strain less than 1000 oC • no-oxide no oxide cracking- cracking 2% strain greater than 1000 oC • no beryllium-braze beryllium braze penetration
• no oxygen embrittlement
• no fuel melting
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 18

Detailed Fuel Analysis by ELOCA code
λ Sheath temperatures for a 30% RIH LOCA scenario
λ Outer elements of bundle position 6
λ High-powered High powered Channel O6 (7.3 MW)
EARLY FLOW STAGNATION
BREAK IN FLOW STAGNATION
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 19

Pressure Tube Behaviour
λ Pressure tube deformation is expected for LOCA breaks
resulting in high pressure tube temperatures at high channel
pressures
λ Objective of analysis is to determine pressure tube
temperature at time of contact with its surrounding calandria
tube
λ Moderator temperatures are sufficiently low enough to prevent
film boiling (dryout) on the outer surface of the calandria tube
after the pressure tube-calandria tube calandria tube contact
λ Localized hotspots on the pressure tube due to bearing-
pad/pressure tube contact results in localized deformation of
tube
λ 24-May-01 No channel failures CANDU during Safety - a #12 large - Large LOCA.ppt break Rev. 0 LOCA
20

Temperature ( o C)
1000
900
800
700
600
500
400
300
200
100
0
Pressure Tube Temperatures
Critical breaks: Pressure Tube Temperature
at Bundle 7 of 7.3 MW Channel
55% Pump
Suction 100% Reactor
35% Reactor Outlet Header
Inlet Header
0 10 20 30 40 50 60
Time (s)
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 21

Barriers to Fission Product Release
1) Uranium-Dioxide Fuel Matrix & 2) Sheath
3) Primary Heat Transport System, 4) Containment & 5) Boundary
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 22

Fission Product Release
λ The gap inventory plays a key role in the fission products released released
during the transient (i.e., sheath failure ==> retention & transport transport
of
fission products in the gap ==> released to heat transport system) system)
λ Secondary release phenomena may include diffusion, fuel
oxidation, Zircaloy-UO
Zircaloy UO2 reaction and fuel cracking
λ LOCA Methodology
– Entire gap inventory is assumed to be released upon sheath failure failure
– 1% of fission products residing on grains and grain boundary is assumed to
be released: to account for the possibility of secondary release mechanisms
λ Fission Product Retention in Heat Transport System
– large surface area in primary heat transport system (for example, example,
endfitting,
feeders) for fission product deposition
– The retention of fission products in the system are currently not not
credited in
the analysis; however, plan to credit in the future
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 23

Example of Fission Product Release Transient
I-131 Release (TBq)
2500
2000
1500
1000
500
0
Transient I-131 Release for 50%
Pump Suction Break
0 20 40 60 80
Time (s)
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 24

Pressure (kPa(g))
Containment Behaviour
λ Pressure increases rapidly due to large amount of heat
rejected to the containment atmosphere
λ Peak Pressure is below design pressure of 124 kPa (g)
λ Dousing system limits peak pressure
Containment Pressure Transient for
100% ROH LOCA
100
80
60
40
20
0
0 100 200 300 400 500
Time (s)
24-May-01 CANDU Safety - #12 - Large LOCA.ppt Rev. 0 25