Inherent safe heat removal in advanced medium-sized high-temperature reactors

One of the main points for the inherent safety of a pebble bed high temperature reactor (HTR) is to guarantee the safe removal of the after-heat in case of a break-down of all active cooling systems like heat-exchangers or liner-cooling. This will be necessary because it is well known today that graphite pebble bed fuel elements stay intact, if the accident temperature is below 1600 deg. C. Therefore the heat must be taken out of the reactor system by passive, natural law heat-transfer mechanism so that the maximum fuel temperature stays below the specified limit. Today medium-sized HTRs with a power of 750 MWth and more (TGTR-300, HTR 500) reach temperatures of more than 2400 deg. C in small parts of the core in such hypothetical accidents. A possible way to realize the inherent safe heat removal in advanced medium-sized HTRs is to change the form of the core. Instead of employing the standard cylindrical geometry a plate shaped core should be preferred, described by the proportion of width to depth of approx. 6:1, causing short distances for heat transfer to the side reflectors within the core and due to the reduction of the mean core power density less than 3 MW/m3. A further improvement of the passive heat removal in case of a failure of the liner-cooling can be achieved by adding cast iron and/or metal granulated material to the concrete of the reactor vessel increasing the thermal conductivity. In case of a total failure of the liner-cooling, heat removal within the prestressed concrete vessel is also possible using installed vertical ducts which make it possible that fresh air runs through the ducts by natural convection cooling the reactor vessel. Even the total removal of the after-heat is possible by suitable arrangement, number and shape of such ducts. (author). 4 refs, 11 figs, 1 tab