The Three Mile Island 2 nuclear reactor should be stably shut down in exactly one week, the Nuclear Regulatory Commission announced yesterday.

Victor Stello, director of the NRC's division of operating reactors, said the team of scientists watching the reactor plan to start relying completely on cooling by natural circulation May 2, more than a month after the reactor was damaged.

Ever since the near-disaster at the Middletown, Pa., plant on March 28, engineers have been working to cool the system's intricate loops of steam and water pipes enough so that they are all "solid," or all water, with no steam anywhere.

In an ordinary reactor, that situation would be called "cold shutdown." But, Stello said, "This is not a normal reactor. Ti's a damaged reactor, and 'cold shutdown' is not appropriate in describing it."

Damage to the radioactive fuel core has created pockets where cooling water cannot reach very well, so temperatures will not be uniform throughout the system, Stello explained. Instruments yesterday showed that while the coolant measured 175 degrees Fahrenheit, some sensors inside read 271 degrees. Pumps are still being used to drive the collant water around the reactor, and the heat is still being drawn off to produce steam.

The goal is to have both existing steam generators "solid" water by April 30, Stello said, in preparation for shutting the pump off May 2.

"There is no danger whatever of any [fuel] reheating," Stello said. "If it starts to heat up you just restart the pumps, that's all."

Critics have worried that damaged, loose fuel pellets may have fallen to the bottom of the reactor during the March 28 incident where they could heat dangerously without being registered on thermocouples, or temperature sensors, that are normally located near the top of the fuel assembly.

Stello said radiation-monitoring instruments indicate that the "profile" of the internal core remains normal, showing no significant loose fuel anywhere inside the reactor. "If the [fuel] pellets did fall out they're still approximately in the right position," he said.

Scientists at the Three Mile Island site are putting in further modifications for the "long-term cooling condition" that the plant will be in for the foreseeable future, Stello continued. These include a new set of charcoal filters and a separate, new filter system to catch radioactive iodine that has been leaking sporadically from the plant's auxiliary building. Iodine emissions yesterday were below any minimum detectable level, Stello said.

The changes also include construction, now under way, of two closed loops of piping to take heat from the two steam generating units. The steam generators are now cooled by a main condenser, and the nw units will allow the system's continued operation without the condenser, if necessary.