The Day the Sun Rose Twice — Donald Thomas

When Karl Rainer Andor came to Berlin for the last time it was sacrifice, not victory, that was uppermost in his mind. He intended to use the plutonium bomb he had elaborately planted to effect the reunification of Germany, but he didn't expect to survive. The 'allied' powers are concerned as much with scoring off each other as with finding the bomb - or with seducing or frightening Andor into telling them where it is. And eventually they are faced with the impossible task of evacuating the historic capital of Germany.