Virtual Collaborator is a sort of virtual robot in 3d virtual space which has human-like appearance and behaves as plant operators do in plant control room. A prototype system of such a virtual collaborator has been developed as a virtual operator in a virtual control room of nuclear power plant, and it was demonstrated by an example simulation practice for plant emergency situation, that the prototype system by itself could monitor the control panel, diagnose the root cause of plant anomaly, and counteract the transient to safety shutdown state of nuclear power plant in accordance the operation manual. The implication of this virtual collaborator experiment is that the plant control can be fully automated even in the case of emergent situation so that there would be a possibility of excluding human operator in the control room in future.