WASHINGTON (AP) – Nuclear detectors that shouldn’t be triggered by cat litter and other harmless materials will be installed this fall at major seaports and border crossings, the government said Friday.

The rollout of the high-tech systems to detect radiological substances arriving in the United States will be at the Port of New York and New Jersey, Homeland Security Secretary Michael Chertoff said.

The department also is expanding a test program to prevent nuclear and dirty-bomb materials already in the U.S. from entering New York City, he said.

The estimated 670 detectors currently in place at ports and borders have long frustrated Homeland Security officials because of false positives triggered by medical supplies, cat litter, banana truckloads and other innocuous materials with low levels of naturally occurring radiation.

“We don’t want to send the red flag up every time someone moves a shipment in of perfectly respectable granite,” Chertoff told reporters in announcing $1.1 billion in contracts to three companies to help develop and deploy the systems. Officials said the new detectors are expected to reduce about 831,000 false positives each year to 15,000.

The department wants to have 80 of the new detectors in place by this fall, Chertoff said, and 1,400 for the nation’s 317 ports of entry by 2011. The new scanners cost an estimated $350,000 each, about double the price of each detector currently in place.

Congressional investigators in March questioned whether the cost of the scanner upgrades would be worth the results. The Governmental Accountability Office continues to analyze a Homeland Security cost-benefits plan justifying the expense, said GAO assistant director Jim Shafer.

While the new scanners may cut down on false positive rates, “these things are marginal gains” against “extremely high costs,” Shafer said in an interview Friday.

Penrose Albright, who led Homeland Security’s border nuclear detection program before leaving the agency a year ago, said the new scanners will “dramatically complicate the lives of people who want to smuggle materials.”

But Albright noted that the detectors still won’t be alerted to uranium or plutonium shielded by thick cases of lead. And installing them above speeding traffic on highways or bridges – as Homeland Security is considering in metropolitan New York – raises questions about how vehicles would then be stopped, he said.

Chertoff said the department was looking at ways to put the detectors in a variety of transportation byways – roads, rails and seaports – outside New York. Scanners already are in place at the Holland Tunnel and John F. Kennedy International Airport, said Vayl S. Oxford, director of the department’s domestic nuclear detection office.

The three companies awarded the contracts are Raytheon Co. and Thermo Electron Corp., both of Waltham, Mass., and Canberra Industries Inc., of Meriden, Conn.