The FBI has been sent a chunk of a bathroom wall from the world's biggest nuclear power plant so it can investigate the most bizarre of what has been a rash of bomb threats against nuclear plants in the United States.

Officials of the Tennessee Valley Authority were so concerned about a written warning on a men's room wall at the Browns Ferry, Ala., plant last Jan. 11 that they cut out the handwriting and sent that part of the vinyl wall to the FBI for fingerprint analysis. As of yesterday, the FBI said it had not seen the handwriting on the wall.

"Thousands of people used that bathroom," said Lew Barnett, a spokesman at the FBI office in Birmingham, Ala. "If it had been a real threat, probably some action would have already been taken."

In the two months since an incident at Wilmington, N.C., where extortion was attempted against a nuclear facility there on Jan. 29, there have been 15 bomb threats against nuclear facilities in the United States, in addition to the Brown's Ferry incident. No fewer than 11 were made against the Diablo Canyon nuclear plant at San Luis Obispo, Calif., which is run by the Pacific Gas & Electric Co.

The most recent of the 11 telephoned threats against Diablo Canyon came Feb. 21, when an anonymous woman called the General Hospital in San Luis Obispo and said: "You better get a bunch of beds ready. A bomb is going to go off shortly at Diablo."

The strangest of the threats came at the Browns Ferry plant on the TVA near Athens, Ala. on Jan. 11, when a security guard discovered the following message on a bathroom wall at the world's biggest nuclear plant:

"Viet Nam will be miniscule compared to the catastrophe that will soon befall BFNP [Browns Ferry Nuclear Plant]. Don't worry about a layoff; worry about a lay under. Plastic explosives will demolish the plant if placed in strategic locations. Viet Nam taught me a lot of things besides how to kill gooks. Soon."

The incident was reported immediately to the FBI and the Nuclear Regulatory Commission. The NRC put the report into its public document file at its Birmingham, Ala. field office, which is how it became public yesterday.

A TVA spokesman said that TVA "did not take it lightly" even though the handwritten message "didn't say a bomb would go off.

"Anybody can write a message" the spokesman said. "That doesn't mean anybody can get a bomb into the plant."

Despite the lack of a bomb, the TVA cut the offending piece of the bathroom wall out and sent it to the FBI for fingerprint analysis. Since it is a government plant, all employes persumably have their fingerprints on file with the FBI, which by yesterday said it had not yet received the wall sent by the TVA.

The NRC's Morris Howard, director of the division of safeguards inspection, said all the bomb threats made in the last three months had been referred to the FBI for investigation. He said no bombs were found at any of the facilities threatened.