Abstract: Coolant fluid is sampled from a substantially horizontal part of the hot leg (16) of the primary circuit at at least three points (19, 20, 21) distributed at the periphery of a straight section of the hot leg (16), in such a way that at least one of the sampling points, or lower sampling point, is situated beneath the axis (17) of the hot leg (16). The temperature of the coolant water sampled at each of the sampling points (19, 20, 21) is measured at its outlet from the hot leg (16) and the coolant fluid is reintroduced into the hot leg (16) at a point situated in a position substantially diametrically opposite, with respect to one of the lower sampling points (20, 21), on the straight section of the hot leg (16). The sampling devices (19, 20, 21) connected by pipes (24, 25, 26) to a reintroduction element (22) are arranged at the periphery of the hot leg (16).