The core of any nuclear reactor presents a particularly harsh environment for sensors and instrumentation. The reactor core also imposes challenging constraints on signal transmission from inside the reactor core to outside of the reactor vessel. In this letter, an acoustic measurement infrastructure installed at the Advanced Test Reactor (ATR), located at Idaho National Laboratory, is presented. The measurement infrastructure consists of ATR in-pile structural components, coolant, acoustic receivers, primary coolant pumps (PCPs), a data acquisition system, and signal-processing algorithms. Intrinsic and cyclic acoustic signals generated by the operation of the PCPs are collected and processed. The characteristics of the intrinsic signal can indicate the process state of the ATR (such as reactor startup, reactor criticality, reactor attaining maximum power, and reactor shutdown) during operation (i.e., real-time measurement).