Chapter 34: Nuclear Energy: Safety – Accidents – Three Mile Island – Edited by Dr. Mir F. Ali
The very first dreadful nuclear accident at the Three Mile Island Unit 2 (TMI-2) Nuclear Power Plant, Pennsylvania, USA, was occurred at 4 in the morning on March 28, 1979. It lasted only five days as the crisis was officially declared to be over on April 2, 2012 but it provoked massive fear, stress, and confusion. Rumors were dispersed piercingly about an uncontrolled release of radiation from the plant and on March 30th, the governor of Pennsylvania ordered the evacuation of children and pregnant women living within 5 miles of the plant. In spite of the fact that there were no deaths or injuries to plant workers or members of the nearby community, it was considered to be the most serious accident in US commercial nuclear power plant operating history and it amplified the long global squabble over the safety of nuclear energy.
Here is a summarized version of the activities based on the World Nuclear Association that took place on March 28, 1979:
1. 4:00 am on that day:
- The reactor was operating at 97 percent and it involved a relatively minor malfunction in the secondary cooling circuit (TMI-2) which caused the temperature in the primary coolant to rise. This in turn caused the reactor to shut down automatically which only took about one second;
- Within seconds of the shutdown, the pilot-operated relief valve (PORV) on the reactor cooling system opened, as it was supposed to. About 10 seconds later it should have closed. But it remained open, leaking vital reactor coolant water to the reactor coolant drain tank;
- The operators believed the relief valve had shut because instruments showed them that a “close” signal was sent to the valve. However, they did not have an instrument indicating the valve’s actual position;
- Responding to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief valve, cooling water surged into the pressuriser, raising the water level in it;
- Based on their training, operators responded by reducing the flow of replacement water;
- It caused steam to form in the reactor primary cooling system, pumping a mixture of steam and water which caused the reactor cooling pumps to vibrate;
- Because the severe vibrations could have damaged the pumps and made them unusable, operators shut down the pumps; and
- This ended forced cooling of the reactor core. However, as reactor coolant water boiled away, the reactor’s fuel core was uncovered and became even hotter. The fuel rods were damaged and released radioactive material into the cooling water.
2. At 6:22 am on that day:
- Operators closed a block valve between the relief valve and the pressuriser;
- This action stopped the loss of coolant water through the relief valve. However, superheated steam and gases blocked the flow of water through the core cooling system
- Throughout the morning, operators attempted to force more water into the reactor system to condense steam bubbles that they believed were blocking the flow of cooling water; and
- During the afternoon, operators attempted to decrease the pressure in the reactor system to allow a low pressure cooling system to be used and emergency water supplies to be put into the system.
3. At 7:50 pm on that day:
- Operators restored forced cooling of the reactor core when they were able to restart one reactor coolant pump. They had condensed steam so that the pump could run without severe vibrations.
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