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MELTDOWN AT 3 MILE
ISLAND
AKSHIT ARORA(201)
YASH BHAYANI(202)
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BACKGROUND INFORMATION
Most significant in the history of US commercial
nuclear power generation history
Partial core meltdown of Unit 2 of Three Mile
Island Nuclear Generating Station at DauphinCounty, PA
4 am on March 28, 1979
Operated by the General Public Utilities and its
regional subsidiary, the Metropolitan EdisonCompany
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Aerial View of Three Mile Island the two reactors to the
left are the cooling towers of Unit 2 where the accident
occurred
BACKGROUND INFORMATION
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A Typical Nuclear Reactor with its components
BACKGROUND INFORMATION
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Events leading to the accident occurred a few seconds
after 4 am on March 28, 1979
ACCOUNT OF THE
ACCIDENT
Elapsed Time Event
00:00:00 Pumps feeding water to the secondary loop are shut down
This was the first of two independent system failures that led to the near
meltdown of the Three Mile Island Nuclear Reactor.
00:00:01 Alarm sounds within the TMI control room
This alarm is disregarded by the operators.
00:00:02 Water pressure and temperature in the reactor core rises
The failure of the secondary loop pump had stopped the transfer of heat
from the Primary Loop to the Secondary Loop. The rise in temperature andpressure is considered to be part of the normal plant operations, and hence
was ignored.
00:00:03 Pressure Operated Relief valve (PORV) automatically opens
When the pressure of steam in the reactor core rises above safe limits, the
pressure relief valve is designed to automatically open, releasing the excesssteam to a containment tank.
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00:00:04 Backup pumps for the secondary loop water system automatically turn on
Four seconds into the accident, the secondary loop water pumps are
automatically turned on. This is indicated to the operators by the presence
of lights on the control panel. The operators are not aware that the pumps
have been disconnected and are not functioning.
00:00:09 Boron and Silver control rods are lowered into the reactor. PORV light goes
out, indicating valve is closed.
Lowering of the control rods into the reactor core slows down the rate of
the reaction. The effect of which is also a reduction in the heat produced by
the reactor. When the PORV light went out, the operators incorrectly
assumed that the valve was closed. In reality the valve was not only open
but was also releasing steam and water from the core. This was now a LOCA
(Loss of Coolant Accident)00:02:00 Emergency Injection Water (EIW) is automatically activated.
The EIW is a safety device that causes water to flow into the reactor core. It
is designed to ensure that when there is a LOCA, the water in the core
remains at a safe level. In the past the EIW system has turned itself on
when there has been no leak so the operators are not unduly concerned bythis.
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00:04:30 Operators observed that the water level in the Primary System is rising
while the pressure is decreasing.
When they observe that the water level in the core was rising, the
operators shut off the EIW system.
00:04:30 Water level in the core still appeared to be rising.
In actuality the water level in the core was dropping, and turning off the
EIW increased the amount of steam being produced by the reactor core.
The combination of steam and water was still being released through the
PORV.
00:08:00 Operator noticed that the valves for the secondary loop backup pumps
were off.
8 minutes into the accident, the closed valve was noticed by an operator.
Once he turned the valves back on the Secondary Water loop is functioning
correctly.
00:45:00 Water level in primary loop continued to drop.
At this point in the accident the operators still do not suspect a LOCA. The
instrument checking the radiation has not registered an alarm, and the
gauges in the control room are wrongly indicating that the water level is up.
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01:20:00 Primary loop pumps start to shake violently.
Steam produced by the lack of cooling water in the core passes through the
primary loop pumps and causes them to shake. Assuming they are not
functioning correctly the operators turn off two of the four pumps.
01:20:00 Remaining two pumps in the primary loop turned off.
The automatic shut down of the two remaining pumps in the primary loop
caused the water within the nuclear core to stop circulating. This in turn
caused the heated core to convert more water into steam, further reducing
the transfer of heat away from the core.
02:15:00 Water level dropped below the top of the core.
Once the top of the core is exposed the steam is converted to super heated
steam. This reacts with the control rods and produces hydrogen and other
radioactive gases.
02:15:00 Hydrogen gas is released through PORV.
Since the Pilot Operated Relief Valve is still in the open position it allows the
hydrogen gas produced to be released along with the steam.
02:20:00 Operator from next shift arrived and closes PORV backup valve.
02:20:30 Operators received first indication that the radiation levels were up.
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02:45:00 Radiation alarm sounds and a site emergency is declared.
At this point half the core is uncovered and the radiation level of the water
in the primary loop is 350 times its normal level.
03:00:00 Due to higher radiation levels a General Emergency is declared.
There is still confusion as to whether the core is uncovered or not. There
are some that feel the temperature readings may be erroneous.
07:30:00 Operators pump water into the primary loop and open the PORV backup
valve to lower the pressure
09:00:00 Hydrogen within the containment structure explodes
The explosion is recorded by the instruments in the control room. It is
dismissed as just being a spike caused by an electrical malfunction. The
sound of the explosion heard is thought by some to be a ventilator damper.
15:00:00 Primary loop pumps are turned on
By now a large portion of the core has melted and there is still hydrogen
present in the primary loop. Water from the primary loop pumps is
circulated and the core temperature is finally brought under control.
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The reactor had come very close to a meltdown, and
was damaged beyond repair. Cleanup took more than adecade with costs of around $ 970 million.
Metropolitan Edison, the plant's owner, had assured
that everything was under control. But, there were
conflicting statements about radiation releases. Schools
were closed and residents were urged to stay indoors.Farmers were told to keep animals under cover and on
stored feed.
The State Governor advised the evacuation of pregnant
women and pre-school age children within a five-mileradius of the Three Mile Island facility. Within days,
140,000 people had left the area.
ACCOUNT OF THE
ACCIDENT
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Some of the factors which contributed the build-up of the
accident were: A water purifier which was under maintenance and
a leaking valve that caused the pumps in the external
circuit to close down.
Emergency pumps were also blocked off during
maintenance work Heat started increasing within the reactor core, and
this let open the safety valve to emit the heat; but the
valve malfunctioned letting off the coolant beyond
the intended levels. Design flaw, which indicated by way of a light on a
control panel that the PORV was closed though it
was actually open.
ANALYSIS OF
ACCIDENT
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The accident was worsen by wrong decisions made
because the operators were overwhelmed withinformation, much of it irrelevant, misleading or
incorrect.
Decision making under stress; over 100 alarms went
off in the control room during the first few minutes of
the accident. This added to the confusion withoutproviding any useful information to the operators.
The inadequate training of the employees at the
facility. The training was the responsibility of
Metropolitan Edison and Babcock and Wilcox.
ANALYSIS OF
ACCIDENT
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RADIOACTIVE MATERIAL RELEASE
Around 480 petabecquerels (13 million curies) ofradioactive noble gases(primarily xenon) werereleased by the event.
However these gases were considered relatively
harmless, and only 481 to 629 GBq (13 to 17 curies)of thyroid cancer-causing iodine-131 were released.
Total releases according to these figures were arelatively small proportion of the estimated37 EBq (10 billion curies) in the reactor.
It was later found that about half the core hadmelted, and the cladding around 90% of the fuel rodshad failed with five feet of the core gone, and around20 tons of uranium flowing to the bottom head of thepressure vessel, forming a mass of corium.
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AFTERMATH
advised the evacuation "of pregnant women and
pre-school age children ... within a five-mile
radius of the Three Mile Island facility." Within
days, 140,000 people left the area.
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CLEANUP
Three Mile Island Unit 2 was too badly damaged
and contaminated to resume operations; the
reactor was gradually deactivated .
Cleanup started in August 1979 and officiallyended in December 1993, having cost around
US$975 million. Initially, efforts focused on the
cleanup and decontamination of the site,
especially the defueling of the damaged reactor.
Starting in 1985 almost 100 tons of radioactive
fuel were removed from the site, the defueling
process was completed in 1990, and the damaged
fuel was removed and disposed of in 1993