Regulators believe some workers at the Palisades Nuclear Plant got higher doses of radiation during a special project than the company says.
According to the Nuclear Regulatory Commission, workers got a radiation dose of up to 2,800 millirem (mrem) during the month long project.
That's more than half the dose, or 5,000 mrem, the federal government permits over an entire year. But Entergy, the company that owns Palisades, believes the highest dose was only 1,800 mrem.
“The problem is not how much dose they received but the fact it was not properly measured,” NRC spokeswoman Viktoria Mitlyng said.
The special project at the plant in Covert, Michigan started February 6th and lasted through March 8th last Spring. 185 workers helped replace control rod drive mechanismsthat slide in and out of the reactor vessel that contains the nuclear fuel when it is operating. The rods have had several problems in the past, causing water leaks.
A lot of the work involved machining and welding atop the reactor vessel. The potential radiation dose to workers was relatively high compared to many other repair jobs because of the location near the vessel and the length of time it took to complete the job, according to Billy Dickson, Branch Chief for Health Physics and Incident Response at NRC.
“Being able to properly project dose (for a worker on a specific project) is vitally important for them to meet their requirements of having a dose to a worker that’s as low as possible in any circumstances at the plant,” Mitlyng said.
“When they do jobs in high radiation areas, it is important that the doses be measured correctly,” she said.
Workers on the project did not place dosimeters, which measure radiation doses, properly on their bodies. Sometimes there weren’t enough dosimeters on each worker; six instead of seven. Other times the dosimeters weren’t placed in the correct places on the body.
Entergy offered tungsten vests to workers to shield some of the radiation away from vital organs. The vests were an optional “new tool” available specifically for this project. Those who wanted to use them could, according to Palisades spokeswoman Lindsay Rose.
At times, workers wore dosimeters under the vests. But the vests didn’t protect certain parts of the torso, particularly the workers’ sides and gonads, the report said. So the actual radiation dose may be unknown.
“No quantitative data was available for inspectors review since (Palisades) did not maintain records of which workers wore shield vests or when the shield vests were worn,” an NRC report says. Interviews and photos of the workers leads the NRC to believe most workers wore the shields most of the time.
“Although (Palisades) has calculated the potential unmonitored (radiation) dose, the calculations have substantial uncertainties, which are not resolvable,” the report says.
The real problem is not how the vests were worn, but how they fit into the final dose calculation, Dickson said. He’s confident 2,800 mrem was the highest dose received.
“We are fairly confident in those numbers. We believe those numbers are as accurate as we can get them,” Dickson said.
Palisades officials will have the last chance to make their case for their 1,800 mrem radiation dose at a meeting on Tuesday. That number includes the calculation of a “virtual” dosimeter, using data from other dosimeters the workers wore. But the NRC has already “questioned the validity” of this method, finding it does not meet federal requirements.
“The NRC had said in their finding that our procedures didn’t fully reflect NRC guidance. We certainly agree with that and we’re making changes to our fleet procedures to make sure that they do so that we can avoid this sort of situation in the future,” Palisades spokeswoman Lindsay Rose said.
“We did the very best that we could during the course of our work to prevent as much radiation exposure as possible to our workers and to make sure that it would be assessed correctly,” she added. “As it turns out the NRC doesn’t feel that we did that correctly. We took every action that we had at our disposal to ensure that it would be done correctly."
In addition to the shields, workers placed shield packaging around the vessel head when possible, and collected information about radiation levels in real time during the work, Rose said.
Entergy has a company rule that only allows its workers to get a dose of up to 2,000 mrem per year, Rose said. For comparison, a person getting a full body CT scan receives a dose of about 1,000 mrem of radiation.
