Date: April 12, 1979
Article: TMI defects spelled out
Author: Lisa A. Pawelski
Special to The Dickinsonian
For the past two weeks, the publics understanding of the Three Mile Island reactor malfunction has been clouded by an explosion of media misinformation. The sensationalist character of some media coverage has only aggravated the confusion surrounding events at the nuclear plant. Only a full-scale federal investigation will enable us to understand what has actually happened on the Island since March 28. Meanwhile, we can only state the few scientific details that are apparent.
In a fission-type reactor, the water which bathes the nuclear fuel elements is heated by the energy-producing reactions which occur in the fuel core. This so-called “primary water” normally transfers its heat to a separate secondary water loop. It is the secondary water which drives the steam turbines which produce electricity.
Early on the morning of March 28, the secondary water at Three Mile Island stopped flowing. The primary water could no longer transfer its heat effectively to the secondary system; as a result, the reactor core and its surrounding primary water began to heat up. At least one back up cooling system failed. “Emergency core cooling” was automatically brought into effect, but it is alleged that a worker at the reactor turned the emergency system off after it had been automatically activated. An overflow of hot water and steam was released from the cement-enclosed reactor vessel to the floor of the reactor building.
There are several possible radioactive species which might appear in the overflowed primary water. Fission reactions involve the splitting of a heavy atom into two or more light atoms. The process releases neutrons, which can bombard the primary water and transform it into “tritiated” water. This means that some of the hydrogen atoms in the water molecules collect extra neutrons. Tritiated water is a low-energy radioactive emitter.
Secondly, trace amounts of metal ions can slough off the insides of the water pipes. Neutron bombardment of these ions produces some “activated ions” which can be radioactive.
Thirdly, if any of the fuel elements are damaged, actual fission products might be released into the primary water system, and hence, into the atmosphere.
Radiation which could result from the about three species – tritiated water, activated ions, and fission products – is being monitored near the plant. Gases such as xenon and krypton have been found; however, both of these species are short-lived and have minimal biological impact. Iodine, which readily accumulates in man’s thyroid gland, has been observed in milk from several Harrisburg-area dairies, but the amount of radioactive iodine present does not present a health hazard.
The precaution of advising pregnant women and small children to leave the immediate vicinity of the reactor was offered because rapidly dividing cells – such as those present in fetuses and growing children – are more susceptible to radiation damage.
Cleanup procedures at TMI have been complicated by a bubble of gas which appeared in the reactor vessel on Saturday, March 31. If that bubble had expanded such that fuel elements were not covered with water, the temperature of the core would have risen appreciably. In addition, electrolysis of water – the splitting of water molecules into their elemental components – had produced concentrations of potentially explosive gases within the reactor building. However, neither of these problems now exists.
Decontamination will require temporary storage of the primary water which leaked to the floor of the reactor building. The fuel assembly of the reactor may have to be replaced, but it appears that the reactor vessel itself had not been damaged.