Three Mile Island Minor Airborne Radioactivity Incident Explanation

On Saturday, November 21, 2009, at about 4:00 pm installed radiation monitors inside the containment building at Three Mile Island Unit One showed higher than normal readings. The normal actions for such indications were taken. People in the area were directed to leave the building while the source of the radioactivity was located. Once they left the area, they would have been surveyed to determine if any radioactive material had gotten on their clothing or skin. All of the workers would have been wearing dosimeters – devices that measure how much radiation they receive.

Aside: Even trained nuclear workers sometimes get a little bit confused about the distinction between radioactive material and radiation. I had a trainer in my early days who had a rather earthy mnemonic – he taught me that radioactive material was like crap while radiation was like the smell that comes from crap. He said that helped him remember that getting radioactive material on his clothes and skin was a bad thing that would require washing to get rid of it – and that he would never want to ingest radioactive material. He told me that it also helped him to remember that you could stand a bit of radiation now and again, but that you would not want to get too close to a source and you would definitely want to limit your time of exposure to the “smell” coming from radioactive material. End Aside.

Since TMI Unit 1 is currently in the middle of a repair period that includes replacement of its steam generators, people at the scene suspected that the source of the airborne radioactivity causing the indication was suspended particles released during pipe grinding and cutting. Since all piping in a primary coolant system is welded, it is always necessary to cut lines during a steam generator replacement and those lines often contain radioactive materials.

Before conducting any cutting, radiation health specialists would have surveyed the pipes to ensure there were no surprisingly high readings indicating a concentration of activated corrosion products, but it would never be a surprise to find that the inside of the piping had higher than background levels of contamination. The pipes in the primary coolant system carry very hot water through a system of metal pipes and through a nuclear reactor. Inevitably, there will be some amount of corrosion inside the pipes and some of that corrosion would have been exposed to neutrons in the core and become activated. Nukes have a rather cute name for the corrosion found inside primary pipes – we call it CRUD. (Which happens to be an acronym for Chalk River Unidentified Deposits.)

People working in the maintenance area would generally be suited up in anti-contamination clothing, wearing gloves and rubber overshoes, hoods and face/eye protection. There was most likely an area outside the work boundaries where people would not be suited up. The people who were not in anti-c’s would have been the primary people of concern when the radiation monitor indicated that there was some airborne contamination.

All of the work was taking place inside the reactor containment building. According to the NRC Event posting, no contamination was found outside of the reactor building. The highest dose received by any worker was 40 mrem. To put that in context, the locally assigned annual limit for an occupational nuclear worker is 2,000 mrem. The legal annual limit is 5,000 mrem. It is not unusual for a nuclear worker to receive more than 40 mrem during routine maintenance work involving primary system piping. Heck, I got more than that during several reactor compartment inspection tours when I was not even doing any system work and I was careful to avoid hot spots.

All in all, it was a pretty boring event. The plant owners took action to notify the appropriate stakeholders because they knew that there would be media interest.

Not surprisingly, there were some media outlets that engaged in a bit of sensationalism and used scary words like spills and contamination. There were even a few that tried to use the incident to stimulate memories of the events at TMI-2 in 1979.

After all, there is usually not much to fill news time on a Saturday evening other than college football results.

NRC Press Release on the event dated November 23, 2009

Additional background information (Posted November 24, 2009 0156)

I was curious about how TMI Unit 1 has been performing in recent years. Here is the plant’s capacity factor data over the last three full years: (available from nei.org at the following shortened URL – http://bit.ly/5LShHy)

Capacity factor (2008) – 106.7
Average Capacity factor (2006-2008) – 102.7

(Because of the way that maximum power ratings are determined and the way that steam plants can perform if the heat sink is at a lower temperature than assumed, it is possible to operate at greater than 100% of “rated” power over a significant portion of the year. However, achieving an AVERAGE Cf of more than 100% over a three year period is very impressive and puts TMI Unit 1 in second place out of 104 operating nuclear plants in the US by that measure.)

Update: (Posted November 25, 2009 at 0309)

Fact checked summary report available at Reuters UPDATE 1-NRC monitors Pa. Three Mile Isl after contamination