"Powering Down," Feature Article, April 2003

powering down

After a half-century of safety testing for the nuclear industry, a key heat-transfer lab is losing its home.

by Harry Hutchinson, Executive Editor

Nuclear reactors can generate heat in all kinds of ways. And that's the problem. They're efficient, and the electricity they produce is cheap, and very profitable. On the other hand, they breed a sort of waste whose hazards have no historical precedent.

So, of course, there are those who are for nuclear power and those against. Arguments on both sides can range from the economic to the political, moral, and ecological.

But put all arguments aside, and one thing remains: More than 100 nuclear reactors in the United States produce about 20 percent of the country's electricity. So no one can afford to turn them off any time soon.

Companies like Framatome ANP and Westinghouse, which design and service nuclear plants, continually tweak hardware, including fuel assemblies, to make reactors more efficient. And before they put their new ideas into service, the companies submit them to high-power tests.

Columbia University's Heat Transfer Research Facility has been the only place to go for key safety testing. Since the days of the Atoms for Peace program during the Eisenhower years, the lab has tested generations of nuclear reactor fuel assemblies.

The NRC Stamp of Approval

According to the lab, its clients over the years have included all the designers of pressurized water reactors in the United States and others from much of the world. There is no lab quite like it in the U.S. and few anywhere. It is the only one the Nuclear Regulatory Commission recognizes as qualified to perform such qualification programs.

But this summer, everyone will have to turn somewhere else. Columbia University has said it will close the lab at the end of June.

The lab's previous director has already left, and the winding down of operations is being overseen by an acting director, Carlos Fighetti, a chemical engineer who has worked at the test facility since 1968. "What we're doing is not the mission of the university anymore," he said.

When the lab opened in 1950, there were no national laboratories, and the research center worked for the Atomic Energy Commission.

Four engineers and two technicians may put in an extra 25 hours in a week to monitor a test of a fuel assembly. Power will be inched up to find the point of critical heat flux at a given pressure and flow.

As Fighetti sees it, one of the lab's liabilities is its size. It takes up several floors of very valuable real estate.

Many at the lab point out that it has paid for itself. According to Victor Carrano, senior engineer at the site, the laboratory can bring in as much as $2 million for perhaps six to eight tests in a good year.

But as Fighetti pointed out, "The measure of research is dollars per square foot." Large testing labs like the Heat Transfer Research Facility, in Columbia's Prentis Building on West 125th Street, and many others like it, are "dying elephants," Fighetti said.

Testing the Grid

The tests are primarily concerned with one small, but significant feature of a reactor core.

A core contains as many as 3,000 fuel assemblies, bundles of long, slender rods containing enriched uranium. Controlled fission among the bundles heats water to begin the series of heat-transfer cycles that send steam to the turbines that will drive generators.

An area of the assembly that is often a target of redesign is the spacer grid. The grids, placed every foot and a half or so along each bundle, make sure the rods keep their proper distance from each other. The spacer grids also influence the flow of water around the rods. Even small changes in the design of the spacer can make a difference in the efficiency of a reactor.

In a pressurized water reactor, the reactor coolant system stream reaches about 590°F, or more than 310°C—far past the boiling point in atmospheric conditions. To avoid boiling, other than for circulation of minor bubbles, the reactor coolant system is pressurized to more than 2,000 pounds per square inch, about 150 times the pressure of a standard atmosphere.

The superheated water flows in a closed loop to heat another stream of water. When it has transferred much of its heat, the reactor coolant heads back toward the reactor vessel.

The more efficiently the water flows among the rods, the more efficient the plant will be in converting the heat of fission eventually to electricity.

As senior advising engineer for thermohydraulics at Framatome ANP in Lynchburg, Va., Dave Farnsworth has been a long-time customer of the lab. Framatome sends fuel assemblies there to be tested for safety and reliability before they ever receive pellets of fissionable fuel.

The lab subjects test bundles to massive doses of direct current under simulated reactor conditions. Then, its staff watches and records what happens.

Spacer grids are designed to give the coolant a swirling motion around the rods and promote the transfer of heat.

The one thing no one wants is the formation of a small layer of steam on the surface of a rod, Farnsworth said. That is the point of critical heat flux, which will stop, or at least sharply cut down, the transfer of heat to the coolant. The condition can cause the temperature of a rod to spike by 1,000°F, he said. The rod is made of a heat-resistant alloy, but an almost instantaneous heat rise of that magnitude can burn the metal and release radioactive material into the reactor coolant system.

It is this point that the lab is particularly looking for. Safe margin of operation for a plant will be 20 to 30 percent below that maximum power level.

Wired to stand the heat, a test bundle is in place for a test at Columbia's heat-transfer lab. The cables will carry so much current that testing can be done only during off-peak hours.

In a reactor, a single fuel assembly may consist of 225 rods arranged in a grid 15-by-15 square. For heat-transfer testing, a representative bundle of five by five will serve. The bundles are heavily wired to the direct current and then sealed in what look like sewer pipes. They are actually small-scale closed systems where test water will circulate under pressure, approximating the reactor coolant system in a power plant.

A test can take 20 to 25 hours during the course of a week and will capture 80 to 100 data points. A data point records the system's reaction to a specific combination of variables, chiefly flow, temperature, pressure, and power.

According to Carrano, the tests begin at a power level considerably below the expected critical heat flux point. "We take power up gradually until the thermocouples begin to react sharply, then back off," he said.

The process water gets so hot that it cannot be released directly into the city sewer system.

When you reach the basement of the building, you can hear the current of water running behind the cellar wall. It is an artesian well that provides water that will cool down the test water system.

The lab currently keeps five motor-generator sets: four from General Electric, each with a 2,100-horsepower motor driving a 1,500-kilowatt generator, and an Allis Chalmers setup with one 5,000-hp motor driving two generators that together produce a total of 3,500 kW.

The lab has two 13,000-volt feeders coming in to power the generator sets. There is another 13,000-volt line for the rest of the building's needs.

The lab has done some testing for boiling water reactors in the past, and it was for one of those several years ago that it set its record for power applied in a test, 12.5 megawatts.

Easing the Current Drain

Because it puts such a drain on the electricity grid, the heat-transfer lab can run the generator sets only during off-peak demand hours, at night and on weekends. According to Carrano, a typical test will be set up and perhaps yield a few data points one night during a week, sometime between 10 p.m. and 8 a.m., and the rest of the work will be done on the following Saturday and Sunday by a team of four engineers and two technicians.

"We joke about a second shift coming in," Carrano said. There is no overtime pay for the engineers.

Although the actual data capture can take less than a week, preparations begin nine months to a year in advance for a new bundle. For instance, there is usually a six-month wait for the specialized tubing, Carrano said.

The rods are packed with precision-machined ceramic cylinders, which provide the mechanical backup that allows the rods to withstand the high pressures. Then, up to seven thermocouples are installed in the ceramics in precise locations. The ceramics are thermal conductors and electrical insulators for the thermocouples. The rods stretch about a half-inch as they heat up during testing.

Complicating the process is the geometry of the test rod. The interior diameter increases toward the center of each assembled rod to simulate the higher neutron activity at the center of a fuel assembly.

According to Carrano, "We receive the tubes in halves from the supplier. We then send them to the ceramic guy who has to fit each ceramic by hand in the half tube. This is because of the variable wall thickness. Each successive ceramic (they are about 1 1/2 inches long) must have a slightly different taper, so it fits snugly against the changing inner wall. The tubes are sent back to us with all the ceramics and our people then put the rods together. The customer tells us where to locate the thermocouples and we install them with the ceramics and weld the tube halves together, as well as attaching the top and bottom electrodes, which are nickel-plated copper."

Although they may be used more than once, the rods will wear out from stress. Thermocouples also fail eventually and when they do, the entire rod has to be replaced.

"There are basically two types of tests—uniform wall thickness and non-uniform" Carrano said. "It is the non-uniform that is the challenging one and the type for which we have the NRC seal of approval."

According to Farnsworth at Framatome, the test data from the lab has been vital to the nuclear industry. "For almost all reactors in the U.S.," he said, "safety analysis developed from research done at Columbia."

One of four General Electric motor-generator sets that power the lab's tests. Each set consists of a 2,100-horsepower motor driving a 1,500-kilowatt dc generator.

Now that the lab in New York is closing, Framatome, which has its world headquarters in Paris, is working on a research site of its own. According to Farnsworth, the company "is developing expertise to test PWR spacer grids" at a facility in Karlstein, Germany.

For the safety data to be applicable in the United States, the new Karlstein site will have to be cleared by the Nuclear Regulatory Commission. "My job will be to prepare the report to the NRC asking for approval of the new lab," Farnsworth said.

The lab is full of vintage equipment with no place to go. According to Carrano, there have been inquiries from various places—companies, universities, and national labs—about collecting the hardware, but so far there have been no takers.

"I'm sure the university would be willing to let someone just take it," Carrano said. Columbia has to clear the space to make room for whatever it has in mind for the Prentis Building. "It would be a shame to see it cut up," he said. "These are my babies."

Beyond confirming that it has decided to shut the lab, the university isn't commenting on its plans.

The motor-generator sets also can still be used for high-power testing. Engineers at the lab argue that the power of the dc generator is better suited to their kind of testing than is the output of a rectifier, which has become the more common lab tool for applications requiring direct current.

According to Carrano, for instance, "The rectifier output has ac components in it, which are not representative of nuclear heating."

William Begell, the ASME Fellow who heads Begell House, the technical book publisher in New York, was director of the lab in the late 1950s. He said, "The lab has had a tremendous amount of experience in doing research." Then he added: "Maybe someone could restore it."

During a tour of the facility, one could see how Begell values his old workplace. There was a pile of used test
assemblies stacked against a wall. He called it "a history of the nuclear power industry."