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Transcript of TMI-2
The scale can be applied to any event associated with nuclear facilities, as well as the transport, storage and use of radioactive material and radiation sources. INES purpose:
facilitate communication and understanding between the technical community, the media and the public on the safety significance of events. Classification:
by considering three areas of impact: people and the environment, radiological barriers and control, and defence-in-depth. The scale is designed so that the severity of an event is about ten times greater for each increase in level on the scale. . . Events without safety significance are called “deviations” and are classified Below Scale / Level 0 2006: Fire in Nuclear Waste Volume Reduction Facilities of the Japanese Atomic Energy Agency (JAEA) . . Unexpected events during normal conditions may result into overexposure of a member of the public in excess of statutory annual limits. e.g. Low activity lost or stolen radioactive source, device or transport package .
. . • Exposure of a member of the public in excess of 10 mSv.
• Exposure of a worker in excess of the statutory annual limits . . . . . . ...... . . . . . . . ... • Exposure in excess of ten times the statutory annual limit for workers.
• Non-lethal deterministic health effect (e.g., burns) from radiation. • Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls.
• At least one death from radiation. Limited release of radioactive material likely to require implementation of some planned countermeasures •Several deaths from radiation Significant release of radioactive material likely to require implementation of planned countermeasures. Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures . . International Nuclear Event Scale Accidents All over the world more than 49 nuclear accident took place
since the very first starting of the technology The first incident occurred in 1952 in the Canadian Candu , when there was a power surge and partial loss of coolant in the NRX reactor which resulted in significant damage to the core.
The control-rods could not be lowered into the core, because of mechanical problems and human errors.
Three rods did not reach their destination, and were taken out again by accident. The fuel-rods were overheated, resulting in a meltdown.
The reactor and the reactor-building were seriously damaged by hydrogen-explosions. The seal of the reactor-vessel was blown up four feet.
level 5 It’s obvious that US is the country with the biggest share of accidents as the table shows about 17 accident within 47 year only TMI-2 Accident The Accident Causes and Impacts In 1979 a nuclear accident occurred in Three Mile Island nuclear power plant "Unit 2" as a consequence of human and equipment errors.
The accident hasn't significant impacts on the environment or the human health.
However, this accident helped a lot to improve the operational training and improving the safety provisions before discussing the accident we may need to introduce
The INES system
The Accidents Rate allover the world
The Component of the Reactors Similar to TMI AT 4 am a partial core melt down melt
down of the three mile island unit 2
reactor occurred ,
this accident occurred due to
technical and human errors Three Mile Island What is nuclear power plant?
It’s a facility which make use of the heat resulting from fission process to convert water into steam which in turn moves turbines to generate electricity ,
It consists of
Primary loop used to carry heat generated
Secondary loop used to cool the rector
Feed water pumps
Valves like PORV
Turbine The accident At 4 am the movement of water through secondary loop stopped because the maintenance crew had been removing resin-coated pellets from polisher number 7
What is resin- coated- pellets?
Type of filters used to absorb unwanted minerals from water
While transferring resin-coated-pellets the flow of water through the loop become blocked
This causes the steam turbine to stop and the temperature of the primary loop to increase rapidly First they didn’t realize the blocked flow of
secondary loop , the light of tripped steam
turbine alerted them.
Now the pressure and the temperature
where increasing rapidly in the reactor core. Where are plant operators?!!!!!! What they should do?
The pumps started up as they were supposed to, but the operators failed to notice that the valves to the pipes leading to auxiliary pumps were closed, this meant that they were unable to add water to the secondary loop!!!!!!!!!!!!!!!!!!
8-minutes passed before the operators noticed that the light indicators for the valves were closed, at that time the valves were manually opened, and the secondary loop was back to work on its normal state. They have to stop temperature and pressure increasing and brought them to a safe limit
In 9 seconds 69 control rods fill into the core, so we are expecting that no chain reactions were going to happen and temperature won’t increase again.
At the same time (PORV) opened to decrease pressure to a safe limit then it was supposed to close automatically but it didn’t. The valve stuck open and steam continued to rush out the valve, while the operators didn’t notice that.
To remove the temperature from primary loop auxiliary feed water pumps should have started pumping water to the secondary system Back to primary loop, as the pressure level dropped, the emergency core cooling system automatically turned on, at that point the operators made a serious error, seeing the gauges that the pressurizer contains high level of water; they manually overrode the emergency core cooling system, and reduced the flow of water from high pressure injection pumps.
Due to the opened valve and the lack of cooling water, a Loss of Coolant accident was happening, but the operators believed just the opposite.
2:18 minutes passed before the plant operators realized the relief valve was stuck open.
If the relief valve have been left open another 30 minutes, the core of the reactor might have been without coolant and completely meltdown. The crisis worsens Under normal conditions, the core of the reactor was covered by 1.8 meter of water, but with the relief valve stuck open this water slowly drained away. The temperature in the core rose faster, some parts of the reactor heated up, and the uranium fuel pellets begin to melt. Due to the accumulation of hydrogen in the reactor vessel, the operators have to release hydrogen to atmosphere to avoid the explosion; this causes the increase of radiation levels around the reactor. At the same time, chemical reaction was taking place between the water turning to steam and zirconium alloy of the fuel rods, zirconium took the oxygen from the steam molecules and formed zirconium oxide. This process freed hydrogen from the steam molecules, hydrogen gas is highly explosive. . TMI Cleanup 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
And about 100 tonnes of damaged uranium fuel had to be removed from the reactor vessel .
All without hazard to cleanup workers or the public.
Defueling the TMI-2 reactor vessel was the heart of the cleanup
The damaged fuel remained underwater throughout the defueling. 1980
Approximately 43,000 curies of krypton
were vented from the reactor building.
The first manned entry into the reactor building took place.
An Advisory Panel for the Decontamination of TMI-2, composed of citizens, scientists, and State and local officials, held its first meeting in Harrisburg, PA. 1984
The reactor vessel
head (top) was
Defueling began. 1986
The off-site shipment of
reactor core debris began. 1988
GPU submitted a request for
a proposal to amend the TMI-2
license to a "possession-only"
license and to allow the facility
to enter long-term monitoring storage. 1990
Defueling was completed.
GPU submitted its funding plan
for placing $229 million in escrow
for radiological decommissioning of the plant. 1991
The evaporation of accident-generated
NRC published a notice of opportunity
for a hearing on GPU's request for
a license amendment. 1992
NRC issued a safety evaluation
report and granted the license
The processing of 2.23 million gallons accident-generated water was completed.
NRC issued a possession-only license.
The Advisory Panel for Decontamination
of TMI-2 held its last meeting.
Post-Defueling Monitoring Storage began. Health Effects
The health effects of the 1979 Three Mile Island nuclear accident are widely, but not universally, agreed to be very low level.
Average local radiation exposure was equivalent to a chest X-ray
Maximum local exposure equivalent to less than a year's background radiation. Several studies were conducted by the Pennsylvania Department of Health. A 1981 study reported that if the accident had had any effect on infant death rates, there would have been a significant increase in the six months after the accident. Instead, the infant death rate was lower than normal. A 1982 study found that the incidence of congenital hypothyroidism within a 10-mile radius of the plant was well within a normal range in the year after the accident. A 1982 study found no measurable impact on infant mortality within a 10-mile radius of the plant, compared with infant mortality rates for all Pennsylvania for 1977-1979. A 1985 study found no significant difference in cancer mortality within a 20-mile radius of the plant during the five years preceding the accident and the five years following it. Environmental Effect Major reasons that Lead to The operators Confusion: Unprofessionalism of the operators Training may have been adequate for the operation of a plant under normal circumstances; insufficient attention was paid to possible serious accidents
The depth of understanding, even of senior reactor operators, left them unprepared to deal with something as confusing as the circumstances in which they found themselves
The specific operating procedures, which were applicable' to this accident, are at least very confusing and could be read in such a way as to lead the operators to take the incorrect actions they did The lack of attention to the human factor in nuclear safety
The lessons from previous accidents did not result in new, clear instructions being passed on to the operators, and didn’t reach those individuals and agencies that most need to know about them The Misconnection between Workers in the Reactor The maintenance crew had been removing resin-coated pellets from the polishers without the previous knowledge of the current shift operator. Operators Defocusing The operators failed to notice that the valves on the pipes leading to the auxiliary pump were closed; one indicator was obscured by a hanging caution tag, the other may have been obscured by an operator’s body as he leaned over the panel. The Inadequate Design of The Control Room The control panel is huge, with hundreds of alarms, during the first few minutes of the accident, more than 100 alarms went off, and there was no system for suppressing the unimportant signals so that operators could concentrate on the significant alarms. Indeed; The major accident at Three Mile Island would have been a minor incident. But, wherever we looked, we found problems with the human beings who operate the plant, with the management that runs the key organization, and with the agency that is charged with assuring the safety of nuclear power plants I. Within the Accident II. TMI-2 Cleanup : The only radioactive substances to escape into the environment were some
17 curies of radio-iodine
13 million curies of inert gases such as argon and krypton. TMI Meltdown : Radioactive gas release : Radioactive material reached the cooling water
when the operators shut down the pumps as a result of uncovering the fuel rods which became hotter and were damaged
even though the Three Mile Island meltdown caused no significant environmental degradation or increased injury to any person
all of the harmful emission products dissolved in the water and condensed out on the inside of containment surfaces. Even if containment had been severely breached, little radioactivity would have escaped Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29 and 30,
operators used a system of pipes and compressors to move the gas to waste gas decay tanks.
The compressors leaked, and some radioactive gas was released to the environment.
These went through high-efficiency particulate air (HEPA) filters and charcoal filters which removed most of the radionuclides, except for the noble gases,
the estimated total of which was about 370 PBq
With short half-life and being biologically inert, these did not pose a health hazard. The initial concern of the scientists and technicians was to vent molten pieces of the core from bursting the reactor pressure-vessel and destroying the containment.
By November 1979, the base of the containment was flooded to a depth of 6 feet, and highly radioactive water was still leaking in at a rate of 1,000 gallons a day, threatening vital equipment. A month later they began to release the krypton through a tall chimney - some 2,000 people opted to leave the area while the operation was in progress - and on 23 July two engineers clad in protective clothing and using special respirators briefly entered the containment building. TMI-2 cleanup operations produced over 10.6 megalitres of accident-generated water that was processed, stored and ultimately evaporated safely Lessons & Changes on Nuclear Industry after TMI In complex systems any part of the system
might be interacting with other parts in an
unanticipated ways, In this case where several
indicators gave the operators different
information the operators assumed
the indicators were wrong, because
they didn’t expect this complex scenario. Identifying human performance
as critical; improved operator training
Use human factors principles to design
the control panels.
Improved instruction to avoid
confusing signals Better indicators for status of valves
Upgrading and strengthening of
plant design and equipment
Improve design of control room
Shift of plant priorities from continuing
power production to safety. Immediate NRC notification of all events
Improved NRC licensing requirements
Expansion of NRC inspector program
Expansion of performance and safety inspections
Separate enforcement office in NRC Established Institute of Nuclear Power
Operations (INPO), the nuclear powers
Expansion of NRC’s international activities to
Installation of additional safety and radiation equipment
Identification and sharing of all safety
related problems According to International Atomic Energy
Agency (IAEA) statistics, the accident
marked a turning point in the global
development of nuclear energy. We must never forget TMI; it taught us many lessons and helped us develop a more realistic sense of what actually happens in a nuclear accident. Thank You!