Check to see if your Nikon uses the same flash metering algorithm as Canon - when set tot aperture-priority but with the flash on, the camera will sett he aperture & shutter speed to correctly expose the background (the outside of the tunnel in your case, but it might get confused if too much of the dark tunnel fills the scene) and uses the flash to expose the subject.
Note that this can cause long expose times if the background is dark, so a tripod can be required.
This is basically the answer that Alaska man gave - set the exposure for the background and set the flash power for the model - you jut might be able to get the camera to do it for you.