According to the Mass Effect wikia, you only need to have completed the previous Priority mission, which is Priority: Sur'Kesh:
Prerequisite: Priority: Sur'Kesh
[Priority: Tuchanka] is automatically available after completing the prior plot mission.
According to the same source, it is a point of no return for several side missions, so if you want to complete everything, make sure you finish the following missions before doing Priority: Tuchanka:
- Benning: Evidence
- Citadel: Alien Medi-Gel Formula
- Citadel: Barla Von
- Citadel: Biotic Amp Interfaces
- Citadel: Hanar Diplomat
- Citadel: Improved Power Grid
- Ismar Frontier: Prototype Components
- Any interactions with Kelly Chambers (only available if she survived the Collector Base assault in Mass Effect 2). Also, if you visit and tell her to change her identity, she will remain available after the end of Priority: The Citadel II.
- Conversing with Matriarch Aethyta in the Apollo's Cafe.
List copied verbatim from source.