A nuclear instrumentation system includes a source range channel, an intermediate range channel, and a power range channel. Each channel includes one detector installed around the pressure vessel. The detectors of the power range channel and the intermediate range channel both include several fission chambers. The detectors of the intermediate range channel and power range channel share several fission chambers. Since some detectors employ fission chambers, the Gamma radiation resistance property, anti-noise property, and anti-electromagnetic interference property are improved. Sharing fission chambers reduces the number of detectors to be installed, thus relieving the installation workload and the positioning of a follow-up detector. Further, the system increases the number of some channels, which increases redundancy and improves system reliability.