A system for monitoring a state of a reactor core in a nuclear reactor may include an internal monitoring device located inside the reactor core, the internal monitoring device including one or more internal sensor arrays configured to take measurements of conditions of the reactor core at different vertical regions within the reactor core to generate internal measurement data; an external monitoring device located in the reactor structure outside the reactor core, the external monitoring device including one or more external sensor arrays configured to take measurements of conditions of the reactor core at positions outside the reactor core corresponding the plurality of different vertical regions within the reactor core to generate external measurement data, and a transmitter configured to wirelessly transmit the external measurement data; and a receiver station configured to determine a state of the reactor core based on the external and internal measurement data.