Directly below the Can is the Fuelsalt Drain Tank (FDT), shown in green. At the bottom of the Pot is a freeze valve shown in gray. The freeze valve is an insulated low point in the drain line. This valve is cooled by a flow of helium which freezes the fuelsalt in the valve creating a plug. The helium flow is controlled by a thermal switch which opens passively if the temperature at the top of the loop exceeds 750C. The plug then thaws and the fuelsalt drains to the FDT. Fission cannot take place in the drain tank since it has no moderator. This drain is totally passive. There is nothing an operator can do to prevent it.

A critically important feature of ThorCon is the silo cooling wall or cold-wall. The cold-wall is made up of two concentric steel cylinders, shown in blue. The annulus between these two cylinders is filled with water. The top of this annulus is connected to a condenser in a decay heat pond located at the forward end of the hull. The outlet of this condenser is connected to the basement in which the Can silos are located. This basement is flooded. Openings in the bottom of the outer cold-wall allow the basement water into the bottom of the annulus.

The Can is cooled by thermal radiation to the cold-wall. This heat converts a portion of the water in the wall annulus to steam. This steam/water mixture rises by natural circulation to the cooling pond, where the steam is condensed, and returned to the bottom of the cold-wall via the basement. In this process, some of the water in the pond is evaporated. The decay heat cooling towers keep the pond close to wet bulb temperature. If the pond cooling line is lost, there is enough water in the basement to handle the first 354 days of decay heat.

The cold-wall also cools the Fuelsalt Drain Tank (FDT). The drain tank is divided into a circle of cylinders. This arrangement provides sufficient radiating area to keep the peak tank temperature after a drain within the limits of the tank material. This cooling process is totally passive, requiring no operator intervention nor any outside power.