While Channel 4 is setting hearts palpitating Sunday night with NBC's fictionalization of a nuclear threat by terrorists, Metromedia's Channel 5 will be offering a well-researched look at government preparedness for this possibility.

"The Ultimate Terror," a report by Jackson Bain and Jan Richards, takes an hour, starting at 9 p.m., to detail the history of nuclear weapons, the actual nuclear extortion threats the FBI has received since 1975 and the personnel and equipment developed to stop the incidents.

All the participants in the documentary agree the threat is more than real, it's imminent. The FBI, the scientist and the commander of the government's Nuclear Emergency Search Team, commonly called NEST, all say "sooner or later" a terrorist group or a psychotic working alone will build a nuclear device. What the terrorist wants is attention, says scientist Ted Taylor, and a nuclear threat is "an instant display."

Scary questions are abundant, as well as some haunting reminders from the windswept desert in New Mexico where the first nuclear weapon was tested 38 years ago. This hour leaves the frightening re-creations to Channel 4. What you get here, reminiscent of the 1950s government lectures and films on air raid preparedness, are assurances that the government can take care of matters.

Quite expectedly, the ultimate scenario is the most interesting segment. And, naturally Bain, the narrator and coproducer, goes to New York City for a test of reaction capability.A simulated terrorist's note is received at the mayor's office. All officials swing into action. Emergency centers are alerted. The note's validity is ascertained, fingerprints are enlarged, and the SWAT teams descend.

Equipment in helicopters picks up neutrons and gamma rays radiating from the bomb. Ground crews close in. The public and the media aren't notified. Says O.R. Revell of the FBI: "Panic of any type is much more dangerous than any plausible device." Besides: "Chances of getting away are nil."

This is the first time, according to the station, that the FBI and the Department of Energy have discussed NEST and its contingency plans on television. That makes the hour worthwhile to those interested in the nuclear power discussions and in government accountability.