[en] In an accident situation, because any decisions that the operators make will depend on how instrumentation readings are ultimately interpreted, the issue of instrument uncertainty is of paramount importance. This uncertainty exists because instrument readings may not be available in the desired form - i.e., only indirect readings for a parameter of interest may exist, with uncertainty on which physical models may be used to deduce its value from these indirect indications -, or because readings may be coming from instruments whose accuracy and reliability in the face of the severe conditions produced by the accident are far from what may be expected under normal operating conditions. In following the EOPs, the operators must rely on instrumentation whose readings may not reflect the real situation. The Accident Management Advisor System (AMAS) is a decision aid intended to supplement plant Emergency Operating Procedures (EOPs) by accounting for instrumentation uncertainty, and by alerting the operators if they are on the wrong procedures, or otherwise performing an action that is not optimal in terms of preventing core damage. In AMAS, the availability and reliability of certain important instrument readings is treated in probabilistic, rather than deterministic terms. This issue is discussed in greater detail later in the paper, since it relates to one of the key characteristics of the AMAS decision aid. (author)