One day next April, a message will be flashed to the Pentagon's National Military Command Center that an Air Force C141 transport carrying six nuclear weapons is in trouble over Nevada and the pilot is attempting to land in a rugged but uninhabited part of the state.

Succeeding messages will disclose that the plane has crashed, exploded and burned with apparent radioactive contamination being spread in a windswept dust cloud.

Thus will begin a unique and complex week-long training exercise expected to involve over 500 people, including the president, and designed to test how the government would handle an actual nuclear weapons accident.

It has been more than 10 years since the last real event - the Jan. 22, 1968 crash-landing on the ice southwest of Thule, Greenland, of an Air force B-52 bomber with four H-bombs aboard.

The B-52 burned on impact and the TNT used to detonate the nuclear fisson material in the weapons exploded, spreading plutonium over a wide area of the crash site. Cleanup at that time was limited since the burning plane melted the ice and much of it sank 800 feet to the bottom of North Star Bay.

According to Vice Adm. Robert R. Monroe, director of the Defense Nuclear Agency which is managing next year's exercise, "There is little experience remaining in the Defense Department [from the Thule period] to respond to such an occurrence."

Furthermore, new radiation detection techniques and reorganization of civilian and military units designed to respond to such an accident "increase the urgency for conducting the exercise," Monroe said during recent congressional hearings.

Though little publicized, training exercises for handling bizarre but possible nuclear incidents have been carried on in the past by one or another government agency.

Last year, for example, the Department of Energy ran NEST 77, an exercise that presumed terrorist had hidden a homemade atomic bomb in an urban area and sent a message to the president demanding $325 million or they would explode their device.

In that situation, DOE - which builds nuclear weapons - actually created a device and hid it in a building at the 900-square-mile Idaho National Engineering Laboratory.

The object of the exercise was to see how long it took DOE's Nuclear Emergency Search Teams (NEST) to find the device, determine its capabilities and deal with disposing of it.

Using helicopters fitted with radiation-sensing devices and trucks with similar equipment, the NEST team located the bomb in less than four hours.

As next year's simulated aircraft "accident" is presently planned, three of six nuclear weapons aboard are to be destroyed by fire, with resultant detonation of their high explosive triggers.

Radioactive material will be presumed to have spread throughout the area. To make the exercise more realistic, short-lived radioactive isotopes in measurable but not dangerous amounts will be placed around.

The other three bombs will be described as damaged.

The scenario also includes the presence of winds that carry some of the radioactive debris aloft and far beyond the crash area.

The crew of the C141 will suffer injuries and radiation contamination in the simulation and some will die.

To meet the disaster, the Pentagon will employ its Joint Nuclear Accident Coordinating Center. NEST teams will also take part.

Each of the three services will deploy units that will have responsibility for securing one of the burned-out weapons and one of the damaged ones.

Specially trained medical teams will handle the so-called injured and contaminated crew members.

Teams will come in to clean up the contaminated land.

One of the more difficult problems will be dealing with the public, both on a local and national level.

According to Defense officials, previous exercises have included use of reporters who are also military reservists. They were put on active duty during the exercise to play the role of reporters giving officials on the scene of what would be going on in the event of a real accident.

No determination has yet been made as to how that aspect will be dealt with next April.

In last year's NEST exercise, DOE officials found they needed secure radio frequencies for the exchange of information between search teams and command center personnel. Citizens band receivers, in civilian cars and trucks picked up their messages.

That exercise also ended up requiring evacuation of a large section of the so-called urban area downwind from where the device had been found, - in the event that an explosion occurred while it was being moved.

To handle that movement, the exercise officals had to work out public evacuation notices.

Next year's accident will be of a largernational scale and even President Carter may be called on to play a part, officials said. If it actually happens, the president's activities would come as part of dealing with the accident's national impact.

Between 1958 and 1968, according to Pentagon figures, there were 13 accidents involving nuclear weapons carried aboard Air Force planes.

The 1979 exercise, called NUACS, will help officials determine the "effectiveness of nuclear response organizations," Monroe said in a statement submitted to a Senate Appropriations subcommittee after closed hearings last spring. It would also "identify weaknesses in coordination and resolve problem areas," he added.