Radioactive ray detection device and nuclide monitoring method in primary loop of reactor

The embodiment of the invention relates to the field of nuclear reactor ray detection, in particular to a radioactive ray detection device, which is used for detecting radioactive rays of a primary loop of a reactor, and comprises a radioactive ray detection part for detecting the radioactive rays from the primary loop of the reactor; the shielding layer is arranged to shield radioactive rays from the outside of the radioactive ray detection device; wherein the radioactive ray detection part is arranged in the shielding layer, and an opening is formed in a preset part of the shielding layer, so that radioactive rays can enter the radioactive ray detection device only from the opening and are detected by the radioactive ray detection part; the size of the opening at a position close to the primary circuit of the reactor is smaller than the size of the opening at a position close to the radioactive ray detection unit. The ray detection device provided by the embodiment of the invention can accurately detect ray