Yankee robot gets stuck while exploring underground pipe
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By Susan Smallheer STAFF WRITER - Published: March 10, 2010
VERNON — A small robot that was exploring the underground pipe tunnel believed to be the source of the radioactive tritium leak at the Vermont Yankee nuclear reactor has fallen victim to one of the contributing factors of the leak – radioactive "mud" from deteriorating concrete and radioactive water.
The robot, which its Entergy Nuclear handlers have dubbed Rover, is about the size of a remote-controlled toy car, said Larry Smith, spokesman for Entergy Nuclear.
The robot became mired in the sludge at the bottom of the tunnel, which was also partially responsible for the leak. The sludge plugged a drain in the tunnel, which was supposed to drain off the radioactive liquid to be treated later.
Instead, in a scenario supported by both Entergy Nuclear and the Department of Health, both steam trap drain lines associated with the advanced off-gas system developed leaks, and the drain, which was designed to trap and collect any leakage, was plugged. At least one of the leaks was estimated at 100 gallons a day, the health department said.
Those steam trap drain lines carry not just tritium, but radioisotopes cobalt-60, manganese-54 and zinc-65, Smith said.
Meanwhile, the Department of Health said other pipes could be corroded, singling out pipes buried in soil under the advanced off-gas building.
Smith said "conceptual plans" already exist about replacing the pipes during Vermont Yankee's regular refueling and maintenance outage, slated to start April 24 and last until May 19.
"We will deal with piping in our upcoming outage," said Smith, who said the reactor would refuel on schedule, despite the vote by the Vermont Senate two weeks ago, declining to approve another 20 years of operation for the embattled plant.
"We will refuel and we will start up," said Smith, rejecting rumors that the 38-year-old reactor might not refuel because of the uncertainty of its future.
Since the pipe tunnel's drain was unplugged and the leaks identified, the levels of radioactive tritium have dropped substantially in what was the most contaminated well.
Smith said Rover was one of several similar remote-controlled robots Entergy Nuclear engineers are using to track down the source of the leak or leaks, which spread tritium to the banks of the Connecticut River, contaminating several underground monitoring wells.
Smith said engineers were also using a long, telescopic pole with a camera on its tip.
While the tunnel does contain radioactivity, searchers are using the robots because there isn't enough space in the tunnel for a human, and Smith noted there were 36 pipes in the tunnel.
Both of the leaks that have been identified as probable sources of the tritium and other radionuclides found in soil near the leaks are in the hydrogen recombiner steam trap drain lines. The first drain line was sealed off Feb. 14, and the other was identified Friday, a hole about the size of a dime or quarter.
So far, the tritium has not shown up in any drinking water wells either on the site or off Entergy Nuclear property, and the Connecticut River so far has tested below detectable levels for tritium, according to the Department of Health.
The well, GZ-10, which once tested positive for 2.5 million picocuries per liter, tested Monday at 1.025 million picocuries per liter, although Entergy reported late Tuesday that it had dropped to 525,000 picocuries per liter. That well is in the excavation pit.
Another well, GZ-7, which is closer to the Connecticut River, is testing now at 1.121 million picocuries per liter, and another well, GZ-15, is testing at 593,000 picocuries per liter.
In all, seven groundwater monitoring wells have tested positive for tritium, which while it occurs naturally, is also a byproduct of the nuclear fission process.
"This is more evidence that tritiated groundwater has been moving away from the source … and down toward the Connecticut River," the Department of Health stated.
susan.smallheer@rutlandherald.com
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