Abstract: The present disclosure relates to an apparatus for detecting nuclear reactor coolant leaks that is capable of maintaining radiation detection reliability and the integrity of a radiation detector from high-dose background radiation inside a nuclear reactor when a coolant leaks from the primary coolant system of the nuclear reactor. The apparatus of the present disclosure includes a shield having an internal space into which sample air containing a coolant leaking from a primary coolant system pipe of a nuclear reactor is introduced, and configured to close the exposed internal space or expand the internal space through detachable coupling; a first sensor disposed in the internal space to obtain a radiation measurement signal from the sample air; and a discriminator for determining whether a coolant is leaking based on the radiation measurement signal. This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (No.