The failure of an oxygen generator on the International Space Station will force the crew to switch to a backup oxygen generator system next week, NASA officials said Thursday. A balky Elektron unit, which generates oxygen from the electrolysis of waste water, failed permanently earlier this month after months of intermittent problems. The crew has been using oxygen from a Progress spacecraft docked to the station, but that source of oxygen will be exhausted by Monday. The crew will then turn to another backup system, "candles" that generate oxygen through an exothermic chemical reaction. Such candles were used on the Mir space station; one caused a fire on the station in 1997 that briefly threatened the crew. There are enough candles on the station to support the crew until a new Progress spacecraft is launched next month, carrying additional oxygen supplies. The crew can also draw upon oxygen stored in the Quest airlock module if needed.