WASHINGTON/DETROIT (Reuters) – The top U.S. auto safety regulator on Tuesday imposed a relatively modest $70-million fine on air-bag supplier Takata Corp (7312.T) and ordered it to stop making inflators that use ammonium nitrate. The National Highway Traffic Safety Administration cited the chemical as a factor in explosive air-bag ruptures that caused seven deaths and nearly 100 injuries in the United States. NHTSA said millions of cars equipped with potentially defective Takata air bags may not be repaired until the end of 2019 because not enough replacement parts are available.