On the morning of March 22, 1955, James A. Dennis of Alexandria crouched in the chill Nevada desert, 3,500 yards from where an atomic bomb exploded with a blinding flash.

Dennis, who was an Army major at the time, said yesterday he and the 300 soldiers he commanded soon were covered with sooty, radioactive fallout. The next day Dennis was back in the desert at another atomic bomb test explosion and in 1962, he was directed to dismantle and dispose of a highly radioactive nuclear reactor that accidentally exploded the year before, killing three military personnel.

"Now I'm dying of blood and bone marrow cancer," said Dennis, who served in the Army 21 years, retiring as a colonel.

Dennis was one of several military veterans who appealed yesterday to a House Judiciary Subcommittee for legislation to allow them to sue the government for injuries they said they are suffering as a result of being exposed to atomic bomb testing in the 1950s.

Their suits are effectively blocked by the so-called Warner amendment to the 1985 Defense Appropriations Act and a doctrine that prohibits military personnel from suing the government for injuries sustained on active duty.

The amendment, little-noticed when offered last year by Sen. John W. Warner (R-Va.), has been under virulent attack by veterans who say they have been made ill by forced exposure to radioactive fallout.

The Warner provision protects federal contractors from claims for injuries arising out of atomic weapons testing programs. It states that the federal government should be substituted for the private contractor as the defendant in those cases.

But a Supreme Court ruling, known as the Feres Doctrine, states that the Federal Tort Claim Act does not extend to injuries to servicemen that arise out of the course of service.

"Accordingly the Warner amendment and the Feres Doctrine operate as a complete bar to suits by atomic veterans injured by the negligence of federal contractors during those tests," said Alvin M. Guttman, counsel to the National Association of Atomic Veterans. Congress, he said, has "denied the atomic veterans rights to pursue any claims in the U.S. courts whatsoever."

Warner said yesterday in an interview that he offered the amendment at the request of the Reagan administration. "No one brought to my attention this special category of tragic victim," he said.

Rep. Frederick C. Boucher (D-Va.) has introduced a bill that would give such veterans the right to sue the United States. The bill "neither eases their burden or proof nor does it address other legal obstacles which must be overcome in such litigation," said Boucher. "It does say to those who may have been injured in the service of their country that Congress made a mistake last year, a mistake that we are going to correct."

Warner met yesterday with Dennis and other atomic veterans and said afterward that he was "distressed by their rendition of their personal tragedies." However, he did not agree yesterday to endorse the Boucher bill or work to repeal his provision, as the veterans requested.

There are 250,000 servicemen who participated in 235 tests carried out between 1946 and 1962. More than 99 percent of the claims have been denied, according to Guttman