Nuclear Power Plant Disaster

Chernobyl and Three Mile Island

Prepared by:

Mohd Zulkarnaen Bin Zasni CE087557

Hidayat Bin Achmad Hanafi CE087547

Causes

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 Mile IslandChernobyl

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.

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.

Chernobyl TodayChernobyl 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.

Chernobyl TodayIn 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.

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.

Chernobyl LessonSince 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.

Chernobyl In Memories

Chernobyl Reactor

Chernobyl Reactor

3 Mile Island ImpactSome 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.

3 Mile Island Impact100 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 percentof 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.

3 Mile Island TodayFrom 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.

3 Mile Island TodaySince 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.

3 Mile Island LessonTraining 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.)

3 Mile Island LessonApplying 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.

3 Mile Island Flashback

3 Mile Island Reactor

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_Island_accident