A Phazon Nutrient Vat is a processing vat featured in Metroid Prime in the room Elite Control of the Phazon Mines. It is a large tube of Phazon with spinning lights on either side. Scanning it seems to imply that it is processing Elite Nutrient Mix. There are two of these in the room.
"Phazon Nutrient Vat. Elite Pirate ration processing in progress."