Abstract: The moderator cell is of great importance in CNS system, which is a main part of Chinese Advanced Research Reactor （CARR）. Void fraction in the moderator cell has great influence on the moderation ability, safety of reactor and so on. In order to decrease the void fraction in the annular channel between the moderator cell and the inner cell, holes may be opened in the top of the inner cell, so that some hydrogen gas vaporized from liquid hydrogen under the inner cell can be discharged without flowing through the annular channel. After a series of measurement of the relation between pressure drop and mass flux in small diameter short pipes, a correlation was obtainded, according to which the optimized number and diameter of holes are calculated. Moreover, self-adjusting ability and stability of this method at variable reactor powers is analyzed.