The scheme that officials now seem to favor for bringing the crippled nuclear reactor at the Three Mile Island power plant to a "cold shutdown" is exactly the same method Henry Ford used to cool the engine in the old Model T.

The builders of the Three Mile Island plant, Babcock & Wilcox Co., have proposed cooling the reactor by using what is called a "thermal convection" or "natural circulation" system.

They would hook up a cooler - like the large air conditioning coolers that sit atop office buildings - to what in the normal operation of a nuclear power plant is the steam generator. Very cold water would then be pumped through a separate "secondary" system of pipes to chill the steam generator.

When the heated reactor coolant water - confined to another set of pipes - flowed into the chilled steam generator, it would become colder and thus heavier. This chilled reactor coolant water would sink, and push the already cool water ahead of it back into the reactor to resume the process.

"You get thermal pumping this way," explained Dr. Robert M. Bernero, assistant director of materials safety for the Nuclear Regulatory Commission. "You don't need an electri pump. Henry For cooled his Model T that way. The Model T Ford didn't have a water pump."

While NRC officials have not given final approval to this proposal, they say it appears to have a number of major advantages over other alternatives.

The residual heat removal system, which is normally used to bring a nuclar reactor to cold shutdown, requires pumping contaminated water out of the contaminated structure and into the auxiliary building, increasing the risk of new radiation leakage.

Dr. Harold Denton, NRC operations chief, noted that the scheme proposd by Babcock & Wilcox "does not require taking contaminated water outside the containment vessel."

Since the reactor coolant water under the Babcock & Wilcox scheme would in effect pump itself, this would allow plant officials to shut down the 9,000-horsepower pump currently being used to circulate the water. This would have the advantage of minimizing the risk that the pump might fail - allowing temperatures inside the reactor to rise again. It would also lessen the reliance by officials on instrumentation located inside the containment structure. The second instrument in two days broke down today as a result of excessive exposure to high levels of radiation.

Shutting down the big pump would also in itself accelerate the prcess of cooling the reactor. "That big pump is generating more heat now than the core is," Bernero said.

Denton said that if the plan is approved it will take about five days to put it in operation. In the meantime, officials would continue using the present cooling system to maintain the core stable at 281 degrees Farenheit and 1,100 pounds per square inch pressure.

Denton said B&W officials feel that over a 10-day period their scheme would bring the temperature down to "between 150 and 200 degrees, and the pressure would be down to about 100 punds per square inch."

"It sounds very good to me," Bernero said. "I think you can cool down to a very cool level - to cold shutdown - this way. It takes a careful look, but I know this plant reasonably well, and I think it sounds good."