a discussion on the cause and effect of major nuclear accidents

1. Nuclear Power Plant Disaster
Chernobyl and Three Mile Island
Prepared by:
Mohd Zulkarnaen Bin Zasni CE087557
Hidayat Bin Achmad Hanafi CE087547

2. Causes
Chernobyl 3 Mile Island
The Chernobyl accident in 1986 was
the result of a flawed reactor
design that was operated with
inadequately trained personnel.
A series of operator actions,
including the disabling of automatic
shutdown mechanisms, preceded
the attempted test early on 26
April.
By the time that the operator
moved to shut down the reactor,
the reactor was in an extremely
unstable condition.
The interaction of very hot fuel
with the cooling water led to fuel
fragmentation along with rapid
steam production and an increase
in pressure.
In 1979 at Three Mile Island nuclear
power plant in USA a cooling
malfunction caused part of the core
to melt in the # 2 reactor. The TMI-2
reactor was destroyed.
The operators believed the relief
valve had shut because instruments
showed them that a "close" signal
was sent to the valve.
This in turn caused the reactor to
shut down automatically.
At this point a relief valve failed to
close, but instrumentation did not
reveal the fact, and so much of the
primary coolant drained away that
the residual decay heat in the reactor
core was not removed.

3. Chernobyl Impact
The accident caused the
largest uncontrolled
radioactive release into
the environment ever
recorded for any civilian
operation, and large
quantities of radioactive
substances were released
into the air for about 10
days.
Two radionuclides, the
short-lived iodine-131 and
the long-lived caesium-
137, were particularly
significant for the
radiation dose they
delivered to members of
the public.
It is estimated that all of
the xenon gas, about half
of the iodine and
caesium, and at least 5%
of the remaining
radioactive material in
the Chernobyl 4 reactor
core (which had 192
tonnes of fuel) was
released in the accident.

4. Chernobyl Impact
The casualties included fire
fighters who attended the
initial fires on the roof of the
turbine building. All these were put out in
a few hours, but
radiation doses on the
first day were estimated
to range up to 20,000
millisieverts (mSv),
causing 28 deaths – six
of which were firemen –
by the end of July 1986.
In the years following the
accident, a further 220,000
people were resettled into
less contaminated areas,
and the initial 30 km radius
exclusion zone (2800 km2)
was modified and
extended to cover 4300
square kilometres.

5. Chernobyl Today
Chernobyl unit 4 is now enclosed in a large concrete shelter which was erected quickly
(by October 1986) to allow continuing operation of the other reactors at the plant.
Some 200 tonnes of highly radioactive material remains deep within it, and this poses an
environmental hazard until it is better contained.
A New Safe Confinement structure is due to be completed in 2016, being built adjacent
and then moved into place on rails. It is to be a 20,000 tonne arch 108 metres high, 150
metres long and spanning 257 metres, to cover both unit 4 and the hastily-built 1986
structure.
Construction started in April 2012 and is expected to take four years. The hermetically
sealed building will allow engineers to remotely dismantle the 1986 structure that has
shielded the remains of the reactor from the weather since the weeks after the
accident.
This task represents the most important step in eliminating nuclear hazard at the site -
and the real start of decommissioning. The NSC will facilitate remote handling of these
dangerous materials, using as few personnel as possible.

6. Chernobyl Today
In the early 1990s, some US$400 million was spent on improvements to the remaining
reactors at Chernobyl, considerably enhancing their safety.
Energy shortages necessitated the continued operation of one of them (unit 3) until
December 2000. (Unit 2 was shut down after a turbine hall fire in 1991, and unit 1 at the
end of 1997.)
Almost 6000 people worked at the plant every day, and their radiation dose has been
within internationally accepted limits. A small team of scientists works within the wrecked
reactor building itself, inside the shelter.
Workers and their families now live in a new town, Slavutich, 30 km from the plant. This
was built following the evacuation of Pripyat, which was just 3 km away.
When it was announced in 1995 that the two operating reactors at Chernobyl would be
closed by 2000, a memorandum of understanding was signed by Ukraine and G7 nations to
progress this, but its implementation was conspicuously delayed.

7. Chernobyl Lesson
While no-one in the West was under any
illusion about the safety of early Soviet
reactor designs, some lessons learned
have also been applicable to Western
plants.
Certainly the safety of all Soviet-designed
reactors has improved vastly. This is due
largely to the development of a culture of
safety encouraged by increased
collaboration between East and West, and
substantial investment in improving the
reactors.
Originally the nuclear chain reaction and
power output could increase if cooling
water were lost or turned to steam, in
contrast to most Western designs. It was
this effect which led to the uncontrolled
power surge that led to the destruction of
Chernobyl 4.
All of the RBMK reactors have now been
modified by changes in the control rods,
adding neutron absorbers and consequently
increasing the fuel enrichment from 1.8 to
2.4% U-235, making them very much more
stable at low power.
Automatic shut-down mechanisms now
operate faster, and other safety mechanisms
have been improved. Automated inspection
equipment has also been installed. A
repetition of the 1986 Chernobyl accident is
now virtually impossible, according to a
German nuclear safety agency report.

8. Chernobyl Lesson
Since 1989, over 1000 nuclear engineers
from the former Soviet Union have visited
Western nuclear power plants and there
have been many reciprocal visits.
Over 50 twinning arrangements between
East and West nuclear plants have been put
in place. Most of this has been under the
auspices of the World Association of
Nuclear Operators (WANO), a body formed
in 1989 which links 130 operators of nuclear
power plants in more than 30 countries.
Many other international programmes were
initiated following Chernobyl.
The International Atomic Energy Agency
(IAEA) safety review projects for each
particular type of Soviet reactor are
noteworthy, bringing together operators
and Western engineers to focus on safety
improvements.
These initiatives are backed by funding
arrangements. The Nuclear Safety
Assistance Coordination Centre database
lists Western aid totalling almost US$1
billion for more than 700 safety-related
projects in former Eastern Block countries.
The Convention on Nuclear Safety adopted
in Vienna in June 1994 is another outcome.

9. Chernobyl In Memories

10. Chernobyl Reactor

11. Chernobyl Reactor

12. 3 Mile Island Impact
Some radioactive gas was
released a couple of days after
the accident, but not enough
to cause any dose above
background levels to local
residents.
There were no injuries or
adverse health effects from
the Three Mile
Island accident.
The Three Mile
Island accident caused
concerns about the possibility
of radiation-induced health
effects, principally cancer, in
the area surrounding the
plant.
Because of those concerns,
the Pennsylvania
Department of Health for 18
years maintained a registry
of more than 30,000 people
who lived within five miles
of Three Mile Island at the
time of the accident. The
state's registry was
discontinued in mid 1997,
without any evidence of
unusual health trends in the
area.
The cleanup of the
damaged nuclear reactor
system at TMI-2 took
nearly 12 years and cost
approximately US$973
million. The cleanup was
uniquely challenging
technically and
radiologically.
Plant surfaces had to be
decontaminated. Water
used and stored during the
cleanup had to be
processed.

13. 3 Mile Island Impact
100 tonnes of damaged
uranium fuel had to be
removed from the reactor
vessel -- all without hazard to
cleanup workers or the
public.
A cleanup plan was
developed and carried out
safely and successfully by a
team of more than 1000
skilled workers. It began in
August 1979, with the first
shipments of accident-generated
low-level
radiological waste to
Richland, Washington.
In the cleanup's closing
phases, in 1991, final
measurements were taken
of the fuel remaining in
inaccessible parts of the
reactor vessel.
Approximately one percent
of the fuel and debris
remains in the vessel.
Also in 1991, the last
remaining water was
pumped from the TMI-2
reactor. The cleanup ended
in December 1993, when
Unit 2 received a license
from the NRC to enter Post
Defueling Monitored
Storage (PDMS).
In October 1985, after
nearly six years of
preparations, workers
standing on a platform atop
the reactor and
manipulating long-handled
tools began lifting the fuel
into canisters that hung
beneath the platform.
In all, 342 fuel canisters
were shipped safely for
long-term storage at the
Idaho National Laboratory,
a program that was
completed in April 1990.

14. 3 Mile Island Today
From its restart in 1985, Three Mile Island Unit 1 has operated at very high levels of safety
and reliability. Application of the lessons of the TMI-2 accident has been a key factor in the
plant's outstanding performance.
At the time of the TMI-2 accident, TMI-1 was shut down for refueling. It was kept shut down
during lengthy proceedings by the Nuclear Regulatory Commission. During the shutdown, the
plant was modified and training and operating procedures were revamped in light of the
lessons of TMI-2.
In 1988 a 1.3% (11 MWe) uprate was licensed. For 1989, TMI-1's capability factor was 100.03
percent and the best of 357 nuclear power plants worldwide, according to Nucleonics Week.
In 1990-91, TMI-1 operated 479 consecutive days, the longest operating run at that point in
the history of US commercial nuclear power. It was named by the NRC as one of the four
safest plants in the country during this period.
In October 1998, TMI workers completed two full years without a lost workday injury.

15. 3 Mile Island Today
Since its restart, TMI-1 has earned consistently high ratings in the NRC's program, Systematic
Assessment of Licensee Performance (SALP).
In 2009, the TMI-1 operating licence was renewed, extending it life by 20 years to 2034.
Immediately following this, both steam generators were replaced as TMI's "largest capital
project to date“.
The plant had previously been operated by Metropolitan Edison Company (Met-Ed), one of
GPU's regional utility operating companies. In 1996, General Public Utilities shortened its
name to GPU Inc.
In 2000, PECO merged with Unicom Corporation to form Exelon Corporation, which acquired
British Energy's share of AmerGen in 2003. Today, AmerGen LLC is a fully owned subsidiary of
Exelon Generation and owns TMI Unit 1, Oyster Creek Nuclear Generating Station, and
Clinton Power Station. These three units, in addition to Exelon's other nuclear units, are
operated by Exelon Nuclear Inc., an Exelon subsidiary.

16. 3 Mile Island Lesson
Training reforms are among the most
significant outcomes of the TMI-2 accident.
Training became centred on protecting a
plant's cooling capacity, whatever the
triggering problem might be.
At TMI-2, the operators turned to a book of
procedures to pick those that seemed to fit
the event. Now operators are taken through
a set of "yes-no" questions to ensure, first,
that the reactor's fuel core remains covered.
Training has gone well beyond button-pushing.
Communications and teamwork,
emphasizing effective interaction among
crew members, are now part of TMI's
training curriculum.
Close to half of the operators' training is in a
full-scale electronic simulator of the TMI
control room. The $18 million simulator
permits operators to learn and be tested on
all kinds of accident scenarios.
Disciplines in training, operations and event
reporting that grew from the lessons of the
TMI-2 accident have made the nuclear
power industry demonstrably safer and
more reliable. Those trends have been both
promoted and tracked by the Institute for
Nuclear Power Operations (INPO).
On the reliability front, the median
capability factor for nuclear plants - the
percentage of maximum energy that a plant
is capable of generating - increased from
62.7 percent in 1980 to almost 90 percent in
2000. (The goal for the year 2000 was 87
percent.)

17. 3 Mile Island Lesson
Applying the accident's lessons produced
important, continuing improvement in the
performance of all nuclear power plants.
The accident fostered better understanding of
fuel melting, including improbability of a
"China Syndrome" meltdown breaching the
reactor vessel and the containment structure.
Public confidence in nuclear energy,
particularly in USA, declined sharply following
the Three Mile Island accident. It was a major
cause of the decline in nuclear construction
through the 1980s and 1990s.
The safety provisions include a series of
physical barriers between the radioactive
reactor core and the environment, the
provision of multiple safety systems, each
with backup and designed to accommodate
human error.
Safety systems account for about one
quarter of the capital cost of such
reactors. As well as the physical aspects of
safety, there are institutional aspects which
are no less important.
The barriers in a typical plant are: the fuel is
in the form of solid ceramic (UO2) pellets,
and radioactive fission products remain
largely bound inside these pellets as the fuel
is burned. The pellets are packed inside
sealed zirconium alloy tubes to form fuel
rods.
All this, in turn, is enclosed inside a robust
reinforced concrete containment structure
with walls at least one metre thick. This
amounts to three significant barriers around
the fuel, which itself is stable up to very
high temperatures.

18. 3 Mile Island Flashback

19. 3 Mile Island Reactor

20. Reference
1. http://www.world-nuclear.
org/info/chernobyl/inf07.html
2. http://totallycoolpix.com/2011/01/chernobyl
-25-years-later/
3. http://www.world-nuclear.
org/info/inf36.html
4. http://en.wikipedia.org/wiki/Three_Mile_Isla
nd_accident