Three Mile Island NRC Report

By Patrick Mondout

What follows is part of the Nuclear Regulatory Commission's (NRC) report regarding the accident at Three Mile Island.

Introduction

This section will provide a brief description of the accident at Three Mile Island Unit 2 that occurred on March 28, 1979. Also, the core damage and release of radioactive material will be discussed.

		Babcock & Wilcox Design
		NRC image

Background Design

Three Mile Island is a Babcock & Wilcox design pressurized water reactor. The reactor coolant system consists of the reactor vessel, two steam generators, four reactor coolant pumps, and the pressurizer. The above figure shows the layout of the reactor coolant system inside the reactor building (containment).

		Pressurizer Relief Valve
		NRC image

Loss of Coolant Pressure

Following a reactor trip, the pressurizer relief valve stuck open (shown above), causing a gradual loss of coolant pressure. Upon sensing the loss of coolant pressure, safety equipment automatically started, but was stopped in order to prevent the pressurizer from overfilling with water. Also, the amount of water being removed from the coolant system to the purification system was increased because of the high level in the pressurizer. The resultant low pressure and high temperature caused the coolant to boil. The low coolant pressure required the turning off of the reactor coolant pumps. With little or no cooling available to remove decay heat, the reactor fuel rods started to crack and break down due to the high temperatures. Some radioactive fission products (mostly gases such as xenon and krypton) were released into the coolant.

Core Damage

Later, the pressurizer relief valve was isolated, and reactor coolant pumps were restarted. The starting of the reactor coolant pumps caused cold water to be pumped onto the now very hot and very brittle fuel rods, causing severe core damage.

		Major Release Point
		NRC image

Release of Fission Products

Fission products (mostly gases) escaped from the damaged reactor core into the reactor coolant system. Due to the stuck open pressurizer relief valve, the pressurizer relief tank pressure increased to the point of rupturing the rupture disk. The coolant was now being released into the containment building atmosphere. The coolant entering the containment was being collected in the sump. Upon reaching a high sump level, the sump pumps automatically started and pumped the water to the auxiliary building. The fission product gases in the coolant were picked up by the auxiliary building ventilation and blown out the plant stack.

Containment Contamination

The rupture of the pressurizer relief tank severely contaminated the containment. Also, due to the high concentration of fission products in the water from the containment sump and the water in the purification system, the auxiliary building had to be evacuated.

Major Release Point

The major release point was the waste gas system. The fission product gases were being stripped out in the volume control tank and vented to the waste gas system. A leak in the waste gas system allowed the gases to be blown out the ventilation stack via the auxiliary building ventilation system.

		Void Area
		NRC image

Core Damage

Television cameras, lowered into the reactor core, indicated a large void area where the top half of the core should be. Also, a bed of rubble (fuel pellets, fuel rod debris, etc.) was found on the top of the lower half of the core. The exact condition of the lower portion of the core was not determined for several more years.

		Fuel Melt
		NRC image

Fuel Melt

Disassembly of the reactor core revealed a significant amount of fuel melt. A portion of the molten core had flowed laterally after melting through the stainless steel core support assembly. That material then flowed down to the lower portion of the reactor vessel, where it cooled and became a solid material again. As shown in the diagram, some of the lower portion of the fuel assemblies remained intact.

		Vessel Cutaway
		NRC image

Material Removal

By the end of May, 1989, most of the internals of the vessel had been removed. The diagram above shows the cutaway of the vessel as of May, 1989. The material removed from the vessel has been placed in canisters and shipped from the site.

Radiological Consequences

The principal cause of exposure for individuals in the pathway of the release was noble gases (primarily xenon). Below is a summary of the radiological consequences of the Three Mile Island accident.

Maximum Projected Offsite Dose:
Less than 100 millirems

Average Dose to Population:
Approximately 1.4 millrems/person

Projected Additional Cancers:
0 to 1

Bibliography

Mike Gray, Ira Rosen, The Warning: Accident at Three Mile Island, W.W. Norton & Company, 2003.
Wilborn Hampton, Meltdown: A Race Against Nuclear Disaster at Three Mile Island: A Reporter's Story, Candlewick 2001.
Bonnie A. Osif, Anthony J. Baratta, Thomas W. Conkling, TMI 25 Years Later: The Three Mile Island Nuclear Power Plant Accident and Its Impact, Pennsylvania State University Press, 2004.
J. Samuel. Walker, Three Mile Island : A Nuclear Crisis in Historical Perspective, University of California Press, 2004.
M.S. Wood, Suzanne M. Shultz, Three Mile Island: A Selectively Annotated Bibliography, Greenwood Press, 1988.