I think there are plenty to choose from in this brilliant dystopian novel. However, one of the biggest I would say comes in Chapter Nine, when Helen, clearly struggling with the grief of losing her husband, hallucinates that Randy is actually Mark, her dead husband and kisses him:

She pulled him back and whirled the chair so that he faced her. Her eyes were round. He could see beads of perspiration at the corners of her nose, and on her forehead. "You are Mark," she said. "Don't you believe me? Here, look!" She lifted the mirror from the desk and thrust it before his face.

Helen is obviously presented as going through the denial stage of her grief, where she feels that she can regain Mark through imposing her feelings on Randy. Even the way she kisses him seems to suggest that she feels she can "subdue and mould and change him" into being Mark, who of course, is lost forever. The stress of the nuclear holocaust combined with the loss of her own husband is causing Helen to experience a bitter internal conflict as she battles to cling on to sanity and what she knows to be real.