A panel of independent scientists yesterday challenged a key Navy finding that the USS Iowa turret explosion was deliberate, saying that the alleged "foreign material" found in the turret after the blast could be explained as the routine presence of seawater and gun-cleaning fluids, and not debris from an incendiary device, as the Navy contends.

The scientists from Sandia National Laboratories, appearing before the Senate Armed Services Committee, also used videotapes and diagrams to explain the experiment they designed to demonstrate how the Iowa's sailors could have improperly operated the hydraulic ram in the Iowa's No. 2 turret, possibly setting off an explosion by accidentally pushing five bags of gunpowder with too much speed and pressure into the loading end of the 16-inch gun.

The Sandia experiment was duplicated by the Navy Thursday in a test at the Naval Surface Weapons Center at Dahlgren, Va. A series of gunpowder bags was attached to 860 pounds of weights and dropped onto a steel plate to simulate the pressures of an "overram" of a 16-inch gun barrel.

The experiment ignited the gunpowder, and the Navy reopened its Iowa probe and suspended all firings of 16-inch guns in the service's four battleships.

The Sandia ram simulation and its challenge of "foreign materials" further undermine the Navy's official investigation, which concluded last September that the Iowa blast resulted from a "wrongful intentional act" and was "most probably committed" by Gunner's Mate Second Class Clayton M. Hartwig, 24, who died along with 46 other crewmen after a blast ripped through the Iowa's turret on the morning of April 19, 1989. The explosion occurred as the World War II-vintage battlewagon was conducting firing exercises 330 miles off the coast of Puerto Rico.

The Navy yesterday reiterated that it has reopened its investigation and "will continue to test additional theories dealing with the possible causes of the explosion."

Defense Secretary Richard B. Cheney said he is reserving final judgment on the Navy's handling of the case.

Richard L. Schwoebel, a Sandia physicist who supervised the new study in New Mexico, told senators that the lab's extensive experiments "suggest another way, a very simple scenario, in which the explosion could have occurred" by accident.

He said two factors would have to be present for an accidental explosion: first, the "overram" that created severe pressures on the powder bags and, second, a certain configuration of the gunpowder pellets that fill a liner on the top of each bag to even out its weight.

Schwoebel acknowledged that the experiment "represented the maximum . . . energy the rammer system on Iowa could deliver" to gunpowder bags, but he defended this approach as necessary to establish that accidental ignition of the bags in the loading process was possible even though such an event has never happened in tens of thousands of successful 16-inch gun firings over the decades.

In a terse statement, the Navy acknowledged that on the 18th test of the Sandia-designed experiment, an "unexplained ignition" of the powder occurred. The test results stunned senior Navy officials and prompted Navy Secretary H. Lawrence Garrett III to reopen the Iowa investigation and suspend indefinitely all live firings by the four U.S. battleships on active duty.

Navy and Sandia officials say that a full-scale direct test with a 16-inch gun system is necessary to prove that accident theory.

The test developments and Sandia's new examination of the evidence, while still inconclusive as to the cause of the Iowa explosion, nonetheless "cast grave doubts on the Navy's findings," according to Sen. Sam Nunn (D-Ga.), the committee chairman.

Rear Adm. Richard L. Milligan, who supervised the Navy investigation into the Iowa disaster, asserted last year that there could be no accidental explanation for the blast, and further asserted that the presence of unique foreign chemicals in the blast-damaged turret proved that a homemade incendiary device had been placed in the gun.

On that basis, Milligan focused on Hartwig, who supervised the gun's loading that morning and who was characterized in the investigative reports as an unstable and suicidal loner who was despondent about the breakup of his relationship with another seaman, Kendall Truitt.

Kathleen Kubicina, Hartwig's sister who took up the cause of his innocence over the past year, said after yesterday's hearing, "I would just like the Navy to admit that they made a mistake" and to acknowledge that "we don't know what happened."

In separate testimony, Frank C. Conahan, assistant comptroller general of the General Accounting Office, also told the committee that he agreed with the former commanding officer of the Iowa, Capt. Fred P. Moosally, who alleged that the quality, experience and training of sailors in the battleship fleet is inferior to the rest of the Navy.

The Navy challenged Moosally's testimony, but Nunn told Conahan yesterday, "Much of what Moosally had to say is borne out by your investigation."

The Sandia study focused on the Navy's claim that Hartwig had probably built a homemade bomb using a plastic jug, steel wool, brake fluid and an "igniter" chemical, such as calcium hypochlorite.

The Navy asserted that because it found traces of these chemicals in the breech of the 16-inch gun, these foreign materials were evidence of an "igniter device."

But Sandia found that the same calcium and chlorine residues "are readily detectable throughout the entire region of both Turret 1 and Turret 2 of the USS Iowa, Turret 2 of the USS New Jersey and Turret 2 of the USS Wisconsin."

"The presence of these elements is consistent with the maritime environment and the cleaning operations carried out in the turrets," the Sandia report said, adding that some of the chemicals also were present in the "gun sock" used to clean the guns.

The Sandia officials said they could not explain some of the steel wool "iron fibers" described in the Navy's investigation, but when Sandia asked to examine them, the Navy could not find them, Schwoebel said, adding that in the last several days they have been found and will be examined in followup testing. He pointed out that steel wool had been used in the turret to clean up after the explosion.