2. What occurred during the accident in 1979? Describe in detail the events that led to the eventual radiation leak. Please use scientific terminology to describe the event.

Answer 1

The accident you are referring to is the Three Mile Island nuclear accident, which occurred on March 28, 1979, near Harrisburg, Pennsylvania, USA. It is classified as a partial meltdown of the reactor core, which resulted in a significant but contained release of radioactive gases.

Sequence of Events Leading to the Accident

1. Initial Conditions:

   • Reactor Design: Three Mile Island’s Unit 2 (TMI-2) was a pressurized water reactor (PWR) designed to generate electricity through the fission of uranium fuel. The reactor core is surrounded by a robust containment structure designed to prevent the release of radioactive material.

2. The Incident Begins:

   • Mechanical Failure: The accident was triggered by a combination of mechanical failures and human error. Early in the morning, a series of mechanical failures led to a loss of coolant flow to the reactor core. A malfunctioning non-nuclear auxiliary feedwater system prevented the adequate replenishment of coolant in the primary system.

3. Automatic Safety Features:

   • Reactor Trip: The reactor automatically shut down (or "tripped") due to the loss of coolant. However, the primary coolant system became compromised, and safety valves opened as designed, but failed to close. This led to the primary coolant escaping into the containment building.

4. Operator Response:

   • Misinterpretation of Data: Control room operators encountered various alarms and readings that were difficult to interpret amidst increasing pressure, temperature, and water level in the reactor vessel. Misdiagnosis of the situation led to improper decisions, including the failure to manually close the safety valves.

5. Core Heat-Up:

   • Fission Product Generation: As coolant levels continued to drop and the reactor core was exposed to air, fuel rods began to overheat. At approximately 2:00 PM, the temperature reached a critical point where cladding on the fuel rods began to degrade. The melting of the fuel rods released fission products, including cesium-137 and iodine-131, which significantly increased the risk of radioactive release.

6. Release of Radioactive Gases:

   • Containment Breach: Over the course of several hours, a partial meltdown occurred. The degradation of the core materials and subsequent chemical reactions resulted in the formation of hydrogen gas due to the reaction of zirconium from the fuel cladding with steam. This gas accumulation posed a significant explosion risk within the containment.

7. Containment Measures:

   • Controlled Release: To prevent catastrophic failure of the containment structure, operators vented a small amount of contaminated gas to the atmosphere, which resulted in the release of low levels of radioactive iodine and other isotopes. This decision was controversial and raised public concern.

Aftermath and Consequences

The incident at Three Mile Island was significant both in terms of its immediate consequences and long-term implications for nuclear power regulation:

• Public Fear: The release of radioactive material provoked widespread public fear over the safety of nuclear power.
• Regulatory Changes: In response, the U.S. Nuclear Regulatory Commission (NRC) implemented stricter regulations and improved operator training protocols aimed at preventing similar accidents in the future.
• Economic Impact: The accident also had lasting economic effects on the nuclear industry and energy policy in the United States, with many new reactor proposals being halted or abandoned.

The Three Mile Island incident served as a pivotal moment in the history of nuclear power, leading to changes in safety culture, risk assessment, and emergency response protocols in the industry worldwide.