Defense in depth safety measure to ensure Nuclear Power Plant Safety

1. Safety Measure of Nuclear Power Plant
Defense in Depth (DiD)
1st Review Training Course (RTC)
On
“Reactor Engineering”
Presented by:
Nadia Aziz
Executive Trainee (Scientific)
Nuclear Power Plant Company Bangladesh Limited (NPCBL)

2. Int. Nuclear Safety Advisory Group (INSAG), IAEA
The International Nuclear Safety Advisory Group (INSAG)
of the IAEA has settled Defense in Depth for Nuclear Safety
in its report Defense in Depth in Nuclear Safety (INSAG-10,
1996)
2
INSAG-10 describes purposes, method, implementation,
perspectives of DiD and INSAG-12 describes that DiD is one
of safety fundamentals that underlie the safety of NPPs

3. What is Defense in Depth?
 Defense in Depth (DiD) is a safety philosophy
that guides the design, construction, inspection,
operation, and regulation of all nuclear facilities.
Objectives :
 To protect the health and safety of the public and
plant workers.
 To protect the environment and ensuring the
operational readiness of the facility.
3

4. How is Defense in Depth Achieved?
 This approach recognizes that imperfections,
failures, and unanticipated events will occur
and must be accommodated in the design,
operation and regulation of NPPs.
 DiD is implemented through a lot of measures
such as multiple barriers, redundant and
diverse safety systems, high quality
Maintenance/ operation, physical security and
emergency preparedness.
4

5. Barriers
For water reactors the barriers confining
fission products are typically:
• Fuel matrix
• Fuel cladding
• Primary coolant boundary
system
• Primary containment steel-
lined
• Secondary containment
Defence in depth applies to the protection
of their integrity
against internal and external events.
Example of Multiple
Barriers. This does not
mean levels of DiD are
equal to multiple barriers.
5

6. Barriers
6

7. On 28 March1979, one of the two reactors at Three Mile Island in
USA experienced a mechanical or electrical failure in water pumps
that helped cool its core. Employees were not aware of the
malfunction and control room instruments did not show the leak of
cooling water had left too little around the reactor. The nuclear fuel
overheated and roughly half the reactor's core melted. The
incident was rated a five on the seven-point International Nuclear
Event Scale: Accident with wider consequences.
Three Mile Island (TMI-1) NPP Accident
7

8. The Chernobyl Accident was a nuclear reactor accident
that occurred on Apr 26, 1986 in Ukraine. A total of about
30 people, including operators and firemen, died as a
result of direct exposure to radiation. One positive void
coefficient possessed by the nuclear reactor.
Chernobyl NPP Accident
8

9. Following a major earthquake, a 15-metre tsunami disabled the
power supply and cooling of three Fukushima Daiichi reactors,
causing a nuclear accident on 11 March 2011. All three cores largely
melted in the first three days. The accident was rated 7 on the INES
scale, due to high radioactive releases over days 4 to 6, eventually a
total of some 940 PBq (I-131).
Fukushima Daiichi NPP Accident
9

10. *Level-4 and Level-5 are added to “Defense in Depth” after Chernobyl Accident in
1986.
10

11. Level-2: Prevention of
Progressing to
Accident from Incident
Level-1: Prevention of Incident
Occurring
Level-3: Prevention of
Radioactive Material
being released into
Environment
<Design Flow of Defense in Depth>
8)Decay Heat Removal System
Defense in Depth
7) Reactor Protection System
Cooldown
2) Fail-Safe Design
3) Interlock Design
1) Self Regulating Characteristics
4) Sufficient Design Margin
5) High Quality Educational Training
6) Thorough Managing Q.A.
Concrete Measures on “Defense in Depth”
Shutdown
9)Engineered Safety System
(1) (ECCS)
10)Engineered Safety System
(2) (Containment System)
Confining
11

12. 12

13. 13

14. Doppler effect
 238U is a principal neutron
absorber in the resonance
energy region in reactor.
The absorption increases
as the temperature rises.
 Because U-238 becomes
easy to absorb a neutron
at higher temperature, and
a neutron made of nuclear
fission is likely taken by U-
238, fission reactions of U-
235 could decrease.
Incident neutron energy (eV)
Cross
section
(barn)
Fig. Resonance of radiative
capture cross-section at 6.7 eV
It's temperature dependence
(1) Doppler Broadening (Capture Cross Section of 238U )
14

15. Cross
Section
(Barn)
Resonance
Absorption Region
Absorption
Cross-section
Area
Image of Doppler
Effect
Reonance Area
When Fue
Usual Resonance Area
under Normal Operation
Neutron
Energy
Absorption
Cross-section
Area
Image of Doppler Effect
Resonance Area
When Fuel Temp.
Rises
Energy (eV)

16. Self Regulating Characteristics
16

17. Density (void) effect
 Thermal expansion of heated moderator
(water) decreases it's density. A neutron
and water become hard to collide because
the distance between molecules increases,
and the speed of neutron becomes hard to
decline. As a result, the neutrons more
often leak out of the reactor.
(2) Void Effect in BWR
17

18. Source：T. Nakagawa, et. Al. “Curves and Tables of Neutron Cross Section in JENDL-3.3”, JEARI-Data/Code 2002-020, Part-Ⅱ
Nuclear
Fission
Cross
Section
(barn):
x10
-24
(cm
2
)
Neutron Energy (eV)
Hardening of Neutron Spectrum (Decreasing Slowdown Effect)
Decreasing Neutron Fission
Cross Section by Hardening
of Neutron Spectrum
Slowdown
Hardening of
Neutron Spectrum
n
n
n
238U
235U
239Pu
Thermal Neutron (0.025eV)
Keep energy by
through Void
n
Lost energy after Collision
with Water Molecule
Void
18

19. Self Regulating Characteristics
19

20. 20

21. Reliability of the safety related equipment
21

22. 22

23. Confinement
23

24. Confinement
24

25. Nuclear Disaster Preparedness
25

26. Nuclear Disaster Preparedness
26

27. How DiD was Broken
at Fukushima
Accident?
*Evacuation of residents near NPP
*Radiological contamination in
environment
Prevention
of
abnormal
operation
and
failures
Control of
abnormal
operation
and
detection of
failures
1st level 2nd level 3rd level 4th level 5th level
Control of
accident
within the
design basis
Control of
severe
plant
conditions
Mitigation of
radiological
consequence
s
*Loss of off-
site power
after
earthquake
*Attacked by
Tsunami
*SBO by
flooding
*No cooling of
fuel and CV
(containment)
*No immediate CV
venting
*No immediate
alternative cooling
*Fuel melting and
hydrogen leakage
from CV followed
by hydrogen burn
27

28. Lessons Learned from
Fukushima Accident on DiD
 Fukushima accident suggests
 DiD on prevention of
severe accident  DiD
relating to mitigation of
severe accident.
 Not only reinforcement of
level 4, 5 but also levels
1, 2, 3.
 DiD layers worked well just after severe earthquake.
 However, low frequency but high consequence events
(extreme tsunami) did occur and could breach
essentially all layers of DiD.
 Protection against external hazards must be enhanced
according to the DiD concept, which itself is
believed to be valid even after the accident.
SA Prevention
SA Mitigation
28

29. Iodine Tablet (DiD 5th level)
 Role of KI taking during radiological
emergency preparedness
 Evacuation is the most effective but
rather expensive. Taking KI can be a
reasonable, and inexpensive supplement to
in-place sheltering and evacuation.
 However, side-effect has to be considered.
 What is iodine tablet?
 Iodine tablet is a salt of potassium
iodide, KI.
 Benefit of taking KI during a
radiological accident
 When KI is ingested, it is taken up
by the thyroid gland. it will
effectively saturate the thyroid
gland in such a way that inhaled
radioactive iodine will not be
accumulated more in the thyroid gland.
29

30. 30
Thank you
for your attention