Safe Operations
Three Mile Island Unit 1 has a proven track record for safe and
reliable operations. The plant has set four separate world records for continuous days of operations. This has been accomplished because safety is the number one priority at Three Mile Island Unit 1. If TMI can't operate safely, it won't operate at all. The safety of the plant, personnel and the community is paramount.
Keeping the plant modern ensures the highest standard of safe operations. Three Mile Island spends about $10 million a year on capital investments to improve and modernize equipment and enhance plant operations and safety. In the past 5 years, Exelon Nuclear has invested approximatley $500 million into plant equipment to ensure continued safe operations and to ensure essential electricity is supplied to the region. In fall 2009, TMI will complete its largest capital project to date, when new replacement steam generators are installed in the plant. The cost to replace TMI's steam generators is a $300 million investment. The steam generators are an essential system to the future reliability of Three Mile Island. To learn more about the steam generator replacement, click here.
Exelon Nuclear also invests in its people to ensure safe operations. Licensed plant operators receive over 200 hours of training each year, which equals one week of training for every five weeks of operations. To become a plant operator and receive an NRC license, 18 months of intense initial training is required. Trainees spend 12 weeks in the classroom, 25 weeks in the control room simulator practicing various scenarios that are needed to manipulate the plant, and 16 weeks training inside the plant. At the end of 18 months, trainees must pass final examinations including an NRC administered written examination, a simulator examination and an in-plant exanimation to receive their NRC license. In order to maintain their operating licenses, operators are required to take a written re-qualification NRC examination and operating tests. The NRC examination is administered every two years, while the operating tests are administered annually.
Additionally, the plant's systems and components are routinely tested and inspected to ensure continued safe and reliable operation and performance. All nuclear energy plants are subject to a regular and rigorous program of the Nuclear Regulatory Commission (NRC) oversight, inspection, preventive and corrective maintenance, equipment replacement, and extensive equipment testing. These programs ensure that nuclear plant equipment continues to meet safety standards, no matter how long the plant has been operating. Every two years, the plant shuts down for refueling and maintenance work. These outages allow personnel to perform major maintenance work that could not be performed with the plant online and to modify the plant if necessary to ensure continued safe, reliable operations.
All equipment at nuclear energy plants have intended safety functions and are designed to respond in the safest manner to any condition at the plant. Certain equipment is designed to automatically shut down the plant if the need arises due to a condition outside of normal operations.
Three Mile Island was built with redundant and multiple barriers to prevent radioactive material from escaping during the operation of the plant. The first barrier is the fuel itself: the solid ceramic uranium pellets. During the process of fission, the uranium atoms that make up the pellets split, which creates the heat essential to turn water into steam that turns the blades of the turbine-generator. Fission creates radioactive by-products that remain locked safely inside the ceramic pellets.
The pellets are sealed in metal fuel rods that are approximately 13 feet high and half an inch in diameter. Fuel rods are made of the metal alloy zirconium, which resists heat, radiation and corrosion. The rods are bundled together into fuel assemblies. The fuel assemblies make up the nuclear reactor core, which is about 12 feet in diameter. The reactor core is inside the reactor vessel, which has steel walls eight inches thick. The reactor vessel sits inside a shield wall, which is made of steel-reinforced concrete and is about five feet thick. All of this is inside the Reactor Building.
TMI Unit 1's Reactor Building is also steel-reinforced concrete, about four feet thick. The Reactor Building wall also includes a steel liner. This structure is designed to contain radiation that could escape from the reactor vessel in the unlikely event of a major accident that involves fuel damage.