Calculation device for reactor core performance

The present invention provides a device for monitoring the power distribution of a BWR type reactor at a reduced cost and calculating reactor core characteristics. Namely, a reactor core is usually designed and operated so as to be 90deg mirror image symmetry or 90deg rotational symmetry. Accordingly, four objective positions can generally be measured at the same time by one detector assembly by utilizing the symmetricalness. As a result, the number of detectors, for example, neutron detectors or γ ray detectors disposed to the detector assembly or the number of all detectors can be reduced. Each of the detectors in the detector assembly is secured to the reactor core. As a result, mechanisms for moving the detectors can be made unnecessary. Then, the device is reduced in the cost, simplified in the structure, and facilitated in the operation thereby enabling to improve reliability of the device. (I.S.)