Three Mile Island Generator Flaws

Monday, January 30, 2012 @ 03:01 PM gHale


Just hearing the phrase Three Mile Island conjures up all types of scary thoughts going back to the 1979 nuclear accident. Those thought all come back when after just one operating cycle, inspectors at Three Mile Island detected unexpected flaws in the facility’s new steam generators.

The good news is there is no indication radiation released.

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Officials said the flaws are well below regulatory thresholds, but the Nuclear Regulatory Commission (NRC) held a meeting to get more information.

Two 70-foot, 510-ton “once-through” generators sit on either side of the nuclear core. The company installed them in 2009. Each cost more than $140 million.

Each generator has more than 15,000 high-chromium nickel alloy tubes, through which flows hot radioactive water from the core. The radioactive water in the tubes is under high pressure to keep it from boiling. It heats nonradioactive water outside the tubes, turning it to steam that powers the plant’s turbines.

The tubes are one of the primary barriers between the radioactive and nonradioactive sides of the facility. If one were to break, radioactive water from the reactor core could pass into the steam mechanism and escape as steam into the atmosphere.

When TMI shut down in October to change fuel, inspectors discovered some of the tubes had unexpected wear marks — from rubbing against each other.

An eighth-inch of space separates each tube. They should not be touching.

The generators had been in operation only 22 months and they should give the plant another 25 years of life.

An official from AREVA, Inc., the French company that manufactured the generators, told a meeting of the NRC, “We did not expect a tube touching another tube.”

The reason the tubes are wearing against each other and in the pattern they are, he said, “is not immediately obvious to us right now.”

A detailed study is underway to determine the cause.

TMI ran multiple tests in November, and ultimately found tube-to-tube wearing in 257 of the 31,194 tubes in the two generators. In most cases, it had abraded less than 10 percent of the tube wall, but in several cases close to 20 percent of the tube wall was gone. The tubes are less than four hundredths of an inch thick.

All tubes with more than 15 percent of the wall gone — seven total — were plugged before the plant restarted at the end of November.

“In no case was the depth of wear even close to the regulatory limit,” said TMI spokesman Ralph DeSantis.

Although officials did not expect tube-to-tube wear, wearing at the location of plates that support the tubes is well known. He said the allowed limit of wear is 40 percent through the tube wall.

The new flaw “is not considered a safety issue,” said DeSantis. “We feel confident we have a good program in place to manage it.

DeSantis said every single tube will undergo inspection again at the next refuelling.

“We’re not going to compromise safety on any of this,” he said.

Only one other facility uses the AREVA generators: Arkansas Nuclear One in Russellville, AK, which has been running them since 2005.

Once officials told Arkansas about the findings at TMI, they reviewed their previous inspection results and discovered their generators also were susceptible to tube-to-tube wear.

Arkansas officials offered multiple explanations to the NRC on Thursday why their inspections had failed to detect the flaws.

They also said their review of the data indicated more tubes show wear over time, but that the wearing appears to cease after it abrades about one-quarter of the tube wall.



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