There was a "blowdown" in College Park last week, when a nuclear reactor valve was opened to reduce pressure within the system. But there was no need to panic.

It was all part of a controlled -- and harmless -- experiment at the University of Maryland, whose department of chemical and nuclear engineering has just finished building a 15-foot-high model of a Three Mile Island nuclear reactor. The model will help campus researchers in their studies of how hot gases and liquids perform in a real reactor.

It is a stainless steel replica of the accident-stricken Unit 2 reactor at Three Mile Island near Harrisburg, Pa. The Susquehanna River plant was the site in 1979 of the nation's most serious nuclear power plant accident.

The College Park model, however, runs on electricity, not nuclear fission.

Nuclear and mechanical engineering professors say they will be able to run a series of tests and experiments on the university model, which is one-fifth the size of the real reactor, that would never be possible at a working nuclear plant.

"Our whole purpose is to understand physics better in order to provide scientific data that can be used to predict behavior of the reactor," said nuclear engineering professor Yih-Yun Hsu.

He and several graduate students watched as the "blowdown" caused pressure and water temperature to drop.

"If, say, there is a leak, what will happen next? We can calculate that, and the operator will have a better knowledge of guidance of what should be done," Hsu went on. "If you can anticipate conditions, the operator can handle an emergency much much better than the one on Three Mile Island."

Results of the university experiments, which will get under way this summer, will be published in professional journals.

Mechanical engineering professor Dirse W. Sallet, one of four faculty members directing the project, said that the model reactor is the only one of its kind on a university campus.

Construction of the model began a year ago with the help of four graduate students and 10 undergraduates.

Unlike the real reactor, fueled by uranium that heats water to 550 degrees Fahrenheit at a pressure of 2,250 pounds per square inch, the university model uses a 200-kilowatt heating element to raise the temperature of the water within the system to 400 degrees Fahrenheit at 300 pounds per square inch.

The pipes leading to and from the system's electrically heated reactor vessel contain small holes, where instruments can be inserted to gauge temperature, pressure and rate of water flow in the pipes, all critical elements in the experiments.

As the hot water moves through the vessel, it is pressurized so it does not boil. The water then passes through adjacent pipes to a steam generator that makes the steam driving the turbine.

Excess steam is emitted via a ventilation system in the parking lot outside the building..

There are 14 porthole-like viewing holes in the pipes, which allow researchers to see why and when a series of vent valves open and shut in response to pressure within the reactor during operation.

The project is funded by a three-year, $610,000 grant from the Nuclear Regulatory Commission and a $50,000 grant from the university. The Baltimore Gas and Electric Co. trained students to make the special welds needed for the stainless steel