  The paper describes an ongoing effort in developing a declarative system for
supporting operators in the Nuclear Power Plant (NPP) control room. The focus
is on two modules: diagnosis and explanation of events that happened in NPPs.
We describe an Answer Set Programming (ASP) representation of an NPP, which
consists of declarations of state variables, components, their connections, and
rules encoding the plant behavior. We then show how the ASP program can be used
to explain the series of events that occurred in the Three Mile Island, Unit 2
(TMI-2) NPP accident, the most severe accident in the USA nuclear power plant
operating history. We also describe an explanation module aimed at addressing
answers to questions such as ``why an event occurs?'' or ``what should be
done?'' given the collected data.
  This paper is *under consideration* for acceptance in TPLP Journal.
