The purpose of this report is analyse the operating range of the permanent refueling water level indication system during mid-loop operation for UCN 3 and 4. This analysis shows that the operating range becomes 2.4 inches. Besides, if the instrument errors, flow dynamics errors are taken into account, the operating range of PRWLIS for UCN 3 and 4 is about 1 inch. Therefore, the mid-loop operation for UCN 3 and 4 has a potential operating problem because of little margin in operating range. (Author).