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2003 Segmented Shutdown at Callaway (11/2010)

In October 2003, operators at the Callaway nuclear plant in Missouri allowed the reactor to passively shut down from the buildup of Xenon-135. For 106 minutes the operators performed ancillary actions while relying on an informal estimation that Xenon-135 levels were high enough to prevent the reactor from restarting.

When the control rods were finally inserted, personnel outside of the control room were under the impression the control rods were being used to shut down the reactor. No one outside of the control room was aware that the reactor had actually shut down 106 minutes earlier until the incident was accidently uncovered 40 months later during a review of past reactor shutdowns.

When the incident was brought to the attention of plant management, all levels of management refused to investigate it. When the incident was brought to the attention of the Nuclear Regulatory Commission (NRC), only a nominal investigation occurred which resulted in several minor findings. The Institute for Nuclear Power Operations (INPO) likewise refused to investigate it.

During this event, safety was compromised because the reactor was not actively controlled during its shutdown and it could have inadvertently become critical again without further operator action. Xenon buildup caused the reactor to become subcritical. The failure to ensure shutdown conditions via complete insertion of control rods for nearly two hours left the reactor vulnerable to inadvertently re-attaining criticality through either xenon burnout or moderator temperature reduction.

The issue brief describes the chronology of actions (and inactions) during this event and the many lessons that should be learned from it.

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