THE PLUTONIUM FACTOR by Michael Bagley (Allison & Busby: $13.95). This is a hard-to-put-down thriller, and on an important subject besides. A small group of former military men and a psychotic scientist bent on extortion hijack an airplane and its cargo of plutonium fuel and assemble an atomic bomb that they hide within a third of a mile of the prime minister's office in London. British security services, completely in the dark in the beginning, are charged with finding and defusing the bomb in time to avert catastrophe. A first novel, this is not only well written but seems technically realistic. It deals with one of the pre-eminent nightmares that reasonable people are having these days--that the availability of nuclear fuel combined with widening knowledge about the manufacture of nuclear weapons makes such events possible. The scene at the prime minister's Cabinet meeting when an expert details precisely what effect a relatively small, Hiroshima-size atomic explosion could have on central London and its residents is chilling. The author has felt obligated to include the usual sex scenes, but they do not detract too much from the flow.