To improve the efficiency of in-core wet sipping leaker detection, a warm water injection method was developed. The method was characterized by pouring warm water into the channel box through the sipper cap and replacing all the water originally present with the poured water. Basic experiments were performed to determine the efficiency of the method. Mockup experiments were undertaken to confirm this and to ascertain the effects of operational conditions on the efficiency. These were done by the sipping procedures by means of a facility that included a full-scale 8 × 8 simulated fuel assembly. It was demonstrated that (a) the efficiency of detection for bottom leaks increased about a hundred times over the commonly used method, and (b) the increase in efficiency came from flattening the temperature distribution along the axial direction and exciting the natural convection flow in the whole assembly to promote the fission product transfer. Optimal operational conditions for the method were also proposed as follows:

  1. temperature: 318 K (45°C) [when reactor water temperature was 313 K (40°C)]
  2. water volume for each assembly: 60 ℓ
  3. injecting flow rate for each assembly: 15 ℓ/min.