“SDC” and “SDC discussions”
related to
“Practical Elimination [PE] of
Accident Situations”
GIF SDC Task Force Member
Yasushi OKANO
Contents
1. Descriptions in the SDC Phase 1 Report
2. Case study on LWR
3. Guideline of “PE of Accident Situation” for GIF SFR
a. Application to the design
b. Challenges to the containment &
Candidates of situations to be practically
eliminated
c. Design requirements
d. Principles for Demonstration
•
This presentation includes the discussions now under by GIF SDC Task Force
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 2
Descriptions in the SDC Phase 1 Report
General Idea for PE
• IAEA SSR 2/1 - Expression “Practical Elimination” is:
“The possibility of certain conditions occurring is considered to have
been practically eliminated if it is physically impossible for the
conditions to occur or if the conditions can be considered with a high
level of confidence to be extremely unlikely to arise”.
• SDC descriptions in Appendix C:
– “Practical elimination" of accident situations is a matter of judgment
– Each type of sequence must be assessed separately,
– Taking into account the uncertainties due to the limited knowledge
of some physical phenomena and the cost of implementation.
– This judgment cannot be solely based on probabilistic exclusion
criteria, but should be combined with careful deterministic
assessment of all potential mechanisms leading to potentially large
radioactive releases
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 3
Descriptions in the SDC Phase 1 Report
General manner for PE Identification
• The first design objective is
– to make such situations physically impossible.
• Mitigation of the consequences of some accident situation
– should be excluded by design where feasible,
– because the implementation of additional mitigation
devices, or the R&D necessary for demonstrating their
effectiveness, may be prohibitively expensive or
difficult to prove effective under DECs.
• However, for situations that are physically possible,
– Design process has to consider, within economic and
physical constraints, all situations independent of their
probability.
• Robust demonstration is necessary
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 4
Case study on LWR [EPR]
List of situations required as “practically eliminated” for PWR
Melting core in vessel
• Accident sequences involving containment by-pass
• Global hydrogen detonation and steam explosions
threatening the containment integrity
• Reactivity accident resulting from fast introduction of
cold or deborated water
• High pressure core melt situation
• Melting core in pool
• Fuel melting in the spent fuel pool (if the spent full pool
is not inside a containment building)
Ref. of this slide: “Safety for GEN 3 Reactors: EPR Case”, IRSN, Belgium, 21 Oct. 2010
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 5
Guideline of “PE of Accident
Situation” for GIF SFR
• Application to the design
Specific design measures for prevention of occurrences of
“significant” situations (for these situations, reasonable
design measures are impossible to cope with) shall be
provided
in order to rationalize that “the situations can be excluded
from the design consideration.”
• The situations are postulated as
– extreme initiating events,
– severe accident phenomena and
– situations arise from specific event sequences.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 6
Guideline of “PE of Accident
Situation” for GIF SFR
• Approach to identify PE situation candidates
– Analyze failure modes of the containment, and
– Challenging phenomena and their initiating events.
Challenges to the containment &
Candidates of situations to be practically eliminated
Challenges to the
containment
機械的破損
Mechanical
loads
熱的破損
Thermal loads
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
バイパス
Bypass
External
events
Slide 7
Challenges to the
containment
Candidates for
situations practically
eliminated
Direct heating
By dispersed fuel
Prompt criticality
/recriticality in RV
B
Large sodium
spray fire
Molten fuel-coolant
interaction in RV
Fuel
Core
Severe
damage assembl reactivity
in ATWS y failure insertion
A
Mechanical loads
c
Deflagration/detonation of
hydrogen
Large
sodium leak
in the CV
Fuel
Core
Severe
damage assembl reactivity
in ATWS y failure insertion
A
B
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
C
Molten fuelcoolant
interaction
Sodium-concrete
reaction
RV meltthrough
Debrisconcrete
interaction
Large
sodium leak
in the CV
RV meltthrough
D
D
RV meltthrough
D
Slide 8
Candidates for
situations practically
eliminated
A
Core damage in
ATWS
ULOF
Coherent
sodium boiling
ULOHS
Coherent
Molten fuel
Molten fuel
compaction sodium boiling compaction
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
B
Fuel assembly
failure
UTOP
Molten fuel
compaction
Molten fuel
compaction
Slide 9
Candidates for situations
practically eliminated
C
Severe reactivity
insertion
RV melt-through
Heat removal failure
in core damage
(IVR failure)
Large bubble
ingress into
the core
D
Core damage in
LOHRS
Significant
Core
failure of the
core support assemblies’
oscillation
structure
Failure of
Failure of dedicated
dedicated
structures* due to
structures* due to
thermal loads
mechanical loads
Loss of heat
sink(LOHS)
Loss of
reactor
level(LORL)
*; e.g., core catcher
Same as the
previous figure below “direct heating”
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 10
Challenges to the
containment
Thermal loads
Containment wall
Base floor concrete
heating by contacted
erosion
sodium pool
Heating of the
containment
Large sodium
pool fire
Sodium vaporization/
condensation
RV meltLarge
RV meltthrough sodium leak through
in the CV
D
D
Heating by
gaseous/volatile
fission products
Loss of
heat sink
(LOHS)
Core
damage
ATWS,LOHRS
Fuel assembly
fault
Coolant boiling
before core melt
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Sodium-concrete Debris-concrete
reaction
interaction
Large
sodium
leak in
the CV
RV meltthrough
D
RV meltthrough
RV meltthrough
D
D
Slide 11
Challenges to the
containment
Candidates for
situations practically
eliminated
By pass
Coolant boundary failure
due to thermal loads
Loss of
heat sink
primary/secondary boundary
failure due to mechanical loads
Large scale rupture Prompt criticality Molten fuel-coolant
of steam generator /recriticality in RV interaction in RV
tubes
Same as the
following figure bellow “Core damage in LOHRS”
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Same as the
previous figure
Same as the
previous figure
Slide 12
Candidate of Situation to be PE
A] Abnormal reactivity insertion lead to prompt criticality
A-1 Large bubble ingress into the core
A-2 Core configuration change by earthquake beyond the
reference of the design basis
A-3 Core displacement due to significant failure of the
core support structure
B] * Large scale sodium spray combustion inside containment
* Hydrogen accumulation and deflagration/detonation due to
sodium-concrete reaction inside the containment
C] Containment bypass due to large scale rupture of SG tubes
D] Loss of heat removal system (LOHRS)*
D-1] Loss of coolant level (LORL)
D-2] Loss of heat sink (LOHS)
*Design requirements of LOHRS for PE are in “DEC” slide.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 13
Design requirements (1/5)
• Situation A-1]
Abnormal reactivity insertion lead to prompt criticality
(Large bubble ingress into the core)
• Reason to choose
Spatial distribution of void reactivity coefficient can be
positive in the center of the core
• Design requirements
– Prevent gas entrainment from cover gas.
– Limit gas accumulation in the primary coolant system by
the geometric conditions of structures and components.
– Provide gas release paths where gas accumulation might
occur.
– Ensure the gas ingress or entrainment and transportation
into the core doesn’t cause prompt criticality.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 14
Design requirements (2/5)
• Situation A-2]
Abnormal reactivity insertion lead to prompt criticality
(Core configuration change by earthquake beyond the reference of
design basis)
• Reason to choose
Potential for reactivity insertion accident since the core is not in the
most critical configuration
• Design requirements
– Provide function to detect ground motion and shutdown reactor
automatically .
– In order to assure the control rod insertion, relative displacement
between the core and the control rod drive lines shall be limited
with sufficient margin against design basis earthquake.
– Reactivity insertion due to fuel assemblies’ motion and
deformation of the core support plate shall be limited to prevent
core damage.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 15
Design requirements (3/5)
• Situation A-3]
Abnormal reactivity insertion lead to prompt criticality (Core
displacement due to significant failure of the core support structure)
• Reason to choose
Potential for reactivity insertion accident since the core is not
in the most critical configuration
• Design requirements
– Core support structure shall ensure sufficient margin in its design
for design-basis load such as earthquake and total weight of fuel
assembly, also integrity shall be secured against the
environmental conditions such as neutron irradiation,
temperature, and sodium during its life time .
– Significant deformation or failure shall be detectable by ISI or
plant parameter monitoring. The core support function shall be
ensured without unstable fracture if the detectable deformation or
failure is postulated.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 16
Design requirements (4/5)
• Situation B]
– Large scale sodium spray combustion inside the containment
– Hydrogen accumulation and deflagration/detonation due to
sodium-concrete reaction inside the containment
• Reason to choose
–
–
If large amount of sodium is leaked inside the containment, containment
failure may occur due to temperature and pressure generated by severe
combustion.
In case of leaked sodium and concrete contact, hydrogen is generated
and may cause explosion.
• Design requirements
– For design basis measure, sodium leak countermeasures is
necessary for sodium leak detection and mitigation of combustion.
– Large scale sodium spray combustion due to sodium piping break
inside the containment is necessary to be prevented.
– Prevent leaked sodium and concrete contact, or provide
countermeasure against hydrogen accumulation and explosion.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 17
Design requirements (5/5)
• Situation C]
Containment bypass due to large scale rupture of SG tubes
• Reason to choose
Significant sodium-water reaction might cause failure of the
primary and the secondary coolant boundary in the
intermediate heat exchangers.
• Design requirements
– For design basis, countermeasures shall be provided for the leak
detection, pressure relief, isolation and treatment of reaction
products.
– The boundary between the primary and the secondary coolant
system in the intermediate heat exchangers and the secondary
coolant system boundary inside the containment shall maintain
their boundary function against pressure loads generated by
sodium-water reaction. Due consideration shall be taken in
determination of the water leak rate.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 18
Principles for setting up a demonstration of
Practical Elimination
• Demonstration is made on a case by case approach.
Deterministic basis, supplemented by probabilistic studies.
• General principles for deterministic demonstration:
– Look for complete list of Practical Elimination situations,
– Introduce provisions to mitigate the consequences of initiating event
• Emphasis should be placed on]
– Prevention of situations leading to “cliff edge effects”.
– Efficiency & reliability of mitigating provisions cover a wider domain
– Less sensitive to common mode failure
• Probabilistic studies]
– to ensure completeness and to establish expected frequency.
IAEA-GIF Workshop on Safety of SFR, Vienna, 10-11 June 2014
Slide 19