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Stephen B. Morton
FILE - In this Nov., 20, 2013, file photo, radioactive waste, sealed in large stainless steel canisters, are stored under a five-feet of concrete in a storage building at the Savannah River Site near Aiken, S.C. South Carolina says Nevada's demand for the U.S. government to remove weapons-grade plutonium that was secretly trucked between the two states last year contradicts its claim that moving the radioactive material is dangerous. (AP Photo/Stephen B. Morton, File)

RENO, Nev. — Nevada's demand for the U.S. government to remove weapons-grade plutonium that was secretly trucked to a site north of Las Vegas last year contradicts its claim that moving radioactive material is dangerous, according to lawyers for South Carolina, where the shipment originated.

South Carolina and the U.S. Energy Department filed their first responses this week to Nevada's legal challenge in the 9th U.S. Circuit Court of Appeals. Nevada appealed after a federal judge in Reno refused to temporarily ban any more shipments to the state and added a request to remove the plutonium already there.

The Trump administration has promised no more plutonium will be transported to Nevada from the Savannah River Site in South Carolina or any other nuclear facility. But government attorneys say the courts have no legal power to undo the shipment that's already been made.

South Carolina says moving the plutonium from the Nevada National Security Site would "repeat the risks of effects that Nevada claimed in district court would cause it irreparable harm."

"The public's interest does not favor unnecessary shipments of defense plutonium over the nation's highways," South Carolina Attorney General Alan Wilson wrote in a brief filed Monday.

He argued that the plutonium is stored safely at the Nevada National Security Site and should stay there until the federal government is ready to move it to its final home in New Mexico.

The filing did not reference a March 13 warning from an independent government advisory board about "significant" earthquake dangers at the site's facility where officials say the plutonium is stored 60 miles (97 kilometers) from Las Vegas.

"A seismically induced high explosive violent reaction could result in unmitigated dose consequences" to the public, the Defense Nuclear Facilities Board told Energy Secretary Rick Perry in a letter last month.

The board said the facility is operating with "unknown risk," which a 2007 hazards study revealed.

The shipment of one-half metric ton (1,102 pounds) of plutonium — which was kept secret until January — was part of the government's effort to comply with an earlier court order to remove at least a metric ton (2,204 pounds) of the material from the Savannah River Site by Jan. 1, 2020.

The Energy Department unsuccessfully appealed the December 2016 order, arguing that removing the plutonium by the deadline was "simply impossible" if it also complied with environmental laws.

Nevada's lawyers said that months later, "the impossible happened" and the Department of Energy completed an analysis declaring the Nevada site a prime candidate for indefinitely housing the material.

"DOE's previous capacity, safety, security and surveillance concerns magically disappeared," Nevada said in its appeal.

The government's new filing says Nevada took those comments out of context.

It says references to potential radiation exposure and difficulty meeting the 2020 deadline were tied to an initial plan to dilute the plutonium into waste — a process known as "down blending" — for disposal.

"The weapons-useable plutonium at issue here is not being 'down blended' or turned into waste. ... The plutonium here is being repurposed for weapons production," government lawyers wrote.

Nevada's concerns that the shipping containers may experience corrosion and are unsafe for storage come from an earlier federal declaration, which discussed handling plutonium in a different form. The government disclosed in its filing that the plutonium shipped to Nevada was in solid metal form and lacks the same concerns.

State Attorney General Aaron Ford criticized federal officials for releasing key information "only when it is convenient and beneficial for them to do so."

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"The Department of Energy's recent disclosure demonstrates why Nevada's lawsuit was necessary," Ford said in a statement. "Unfortunately, Nevada only learns this critical information through litigation."

Democratic Gov. Steve Sisolak vowed to "do what it takes to get this unwanted weapons-grade plutonium out of" Nevada.

U.S. Judge Miranda Du in Reno denied a motion seeking to block any shipments pending the appeal. She said the matter was moot because the plutonium already had been shipped.

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Associated Press writer Ken Ritter in Las Vegas contributed to this report.