Within the Main Cooling System the cooling fluid circulates in a single or in several cycles that are driven either by gravity (natural circulation) or by support of pumps (forced circulation). At least in the first of the above cases it is useful to subdivide the flow paths into two categories. An upward flow direction and a rising temperature with rising level of the fluid characterize the first of the latter, called henceforth risers. In the second category, called henceforth downcomers, the opposite is true. A downcomer can be placed concentrically within every riser, else both number and sizes of risers and downcomers can be different and downcomers shall be preferably not externally heated. In the latter case the downcomers can include a volume that is sufficiently large to store the heat from fire source(s) during time period(s) as prolonged as deemed necessary. Almost unlimited capacity for fire heat withdrawal can be secured even without any addition of volume if an adequate and reliable external source of cold water is available.

If both inside cross section and volume of every channel is adequately large then it may be adequate to fill such channels by a cooling fluid (e.g. water). A natural convection would form in this case a rising path near the channel's wall and a downcomer path around the channel's center. For a given maximum heat input to the outside of the channel's wall the cooling of the latter may be adequate even in such a simple system. But both fluid distributor at the bottom and fluid collector at the top are still necessary to equilibrate the temperature among all parallel tubes at all levels and hence to distribute uniformly the total heat input among all tubes. The total heat absorption capacity of such simple system and the time during which it can be heated between any given temperature limits, can be determined from its total volume and the expected maximum heat input.