B747 Captain reports encountering severe turbulence at FL361 due to entering a build up that is not shown on weather RADAR. A WX SYS status message is received after the event.

Narrative

We were cruising at FL361 or 11;000 meters and N2044.3 E111.1. We were painting weather far left and right of course on the edges of our PFDs. I had ground contact and horizon to my left and was watching outside. It was a very black; starless night. Smooth ride all night. The First Officer was pilot flying and got up to get our hot meals out of the oven. I had O2 mask on and in position to do both pilot flying and non-flying pilot duties. As he stepped back into the cockpit with the tray it was as though we hit a wall. With no warning we were in severe turbulence and had a stick shaker. He managed to hang on to the observer seat (not the food) and remain safe. It took a few seconds to determine that we had hit a buildup that weather RADAR was not painting. The whole event lasted less than a minute and we deviated from altitude maximum of 800 FT. With aircraft back in control we made sure we were not hurt; then transferred to the L RADAR; which did not seem to [be] sweeping either or giving us reliable information. We then got a WX SYS status message. On the ground after landing it was discovered that a can had broken apart with the turbulence and was leaking a chemical. The contents of the can scattered about the main deck. Severe turbulence was reported; the proper inspection was done; and the weather RADAR [was] fixed.