A system for monitoring a state of a reactor core included in a nuclear reactor, the system comprising: a nuclear reactor comprising a reactor core inside a reactor structure; an internal monitoring device located inside the reactor core, the internal monitoring device including one or more internal sensor arrays each configured to take measurements of conditions of the reactor core at each of a plurality of different vertical regions within the reactor core to generate internal measurement data, the internal monitoring device being configured to provide the internal measurement data to one or more data signal lines of the nuclear reactor; in which the one or more internal sensor arrays includes a conductivity sensor array including a plurality of conductivity sensors (220A-C), each of the plurality of conductivity sensors extending to an outer surface of a first case, each of the plurality of conductivity sensors corresponding to one of the plurality of different vertical regions within the reactor core, and in which the first case includes a plurality of protrusions extending outwards above each of the plurality of conductivity sensors, an external monitoring device located in the reactor structure outside the reactor core, the external monitoring device including one or more external sensor arrays each configured to take measurements of conditions of the reactor core at positions outside the reactor core corresponding to each of 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 outside the nuclear reactor; and a receiver station configured to receive the external measurement data and the internal measurement data, and to determine a state of the reactor core based on the received external and internal measurement data. a nuclear reactor comprising a reactor core inside a reactor structure; an internal monitoring device located inside the reactor core, the internal monitoring device including one or more internal sensor arrays each configured to take measurements of conditions of the reactor core at each of a plurality of different vertical regions within the reactor core to generate internal measurement data, the internal monitoring device being configured to provide the internal measurement data to one or more data signal lines of the nuclear reactor; in which the one or more internal sensor arrays includes a conductivity sensor array including a plurality of conductivity sensors (220A-C), each of the plurality of conductivity sensors extending to an outer surface of a first case, each of the plurality of conductivity sensors corresponding to one of the plurality of different vertical regions within the reactor core, and in which the first case includes a plurality of protrusions extending outwards above each of the plurality of conductivity sensors, an external monitoring device located in the reactor structure outside the reactor core, the external monitoring device including one or more external sensor arrays each configured to take measurements of conditions of the reactor core at positions outside the reactor core corresponding to each of 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 outside the nuclear reactor; and a receiver station configured to receive the external measurement data and the internal measurement data, and to determine a state of the reactor core based on the received external and internal measurement data.