Radioactive contamination at the Hanford Site’s double-shell Tank AZ-101 does not appear to be from a leak within the tank, the Department of Energy said Wednesday.
A leak was considered possible when a robotic crawler pulled out of the annulus, or space between the shells of the tank, on May 18 was found to be contaminated with radioactive waste. However, analysis of the radioactive isotopes contaminating the robot did not match the current contents of the tank, according to preliminary results from the DOE investigation. In the check of the filter on an annulus air monitor that activated an alarm during the incident, its contamination was found to match that on the crawler.
Washington River Protection Solutions (WRPS), the Hanford tank farms contractor, is continuing to check the annulus, using a video camera lowered down risers. In addition, historical and operational records for the tank are being studied to learn more about the possible source of the contamination.
The space between the shells might be contaminated during transfers of waste in and out of the tank or by precipitation leaking into the annulus and carrying contamination with it. The crawler might have stirred up some previous contamination, causing it to also contaminate the air filter, DOE said.
The crawler was being used in the annulus of Tank AZ-101 as one step in an ultrasonic inspection of tank integrity. The tank, which has held waste since 1976, contains 800,000 gallons of waste. When the crawler was extracted, the unit had contamination at three times the planned limit. One worker also had low levels of contamination on a leg of his protective clothing, according to WRPS.
The contaminated section of clothing was cut off, per Hanford protocol, and workers immediately exited the area. Follow-up checks found no other contamination of the crew at a change trailer used for removing protective clothing, and all workers were cleared for normal duty. All were wearing supplied air respirators as protection against chemical vapors associated with tank waste.
