The moderator fluid will be injected into the PRTR calandrin through injectors located between the shroud tubes and at the bottom of the calandrin. It is important that the size and arrangement of the injectors be such that complete mixing of the moderator will occur and prevent hot sports from forming in the moderator. Such hot spots could lead to undesired changes in the moderating characteristics due to boiling within the moderator. Also of importance is the requirement that the injector should not produce excessive turbulence at the moderator surface thereby complicating moderator level control. To determine the extent of moderator mixing within the calandrin, experimental studies were made employing a full scale PRTR calandrin mockup.