Soldiers exposed to radiation in early atomic tests may have received doses far higher than originally estimated, a Defense Nuclear Agency radiation expert conceded in San Francisco federal court. Navy Cmdr. R. Thomas Bell also admitted that the agency withheld some documents requested under the Freedom of Information Act from lawyers for the only three veterans who challenged Veterans Administration-established radiation exposure levels. Bell's testimony came during questioning in the landmark lawsuit by the National Assn. of Radiation Survivors against the VA. The association has challenged the constitutionality of an 1862 law that all but bars veterans from hiring lawyers in disputes with the VA over compensation. Some radiation-exposed vets contend that hiring lawyers is the first obstacle they must hurdle in their effort to claim VA payments for cancers they say were caused by the radiation exposure.