Experimental study of the behaviour of a prestressed concrete pressure vessel of a HTR at severe accident temperatures

For the risk of medium-sized High Temperature Reactors with prestressed concrete reactor vessel (PCRV) the behaviour of the vessel during a core heat-up accident is decisive. Tests up to extremely high accident temperatures with representative models are carried out. The fifth test in this series was carried out on a section model (dimensions 1.5 m x 1.0 m x 1.0 m) of the vessel top of a HTR. Additionally to the proceeding tests the model included two penetrations - one for a reflector rod and one for an incore rod. The test specimen was exposed to the temperature transients calculated for the hypothetical core heat-up accident, i.e. the maximum temperature at the cover plates was 1260 deg. C including a 10% margin. Since the accidents at HTR's develop very slowly this temperature will be reached after 6 weeks. The insulation remained intact at these temperatures. Only the edges of the cover plates were slightly lifted from the insulation but without dropping down. It was furthermore investigated up to which temperature of the cooling tubes and thus until which time a refeeding of the liner cooling system is possible after a complete failure of this system. From all five tests the following conclusions can be drawn: During a core heat-up accident caused by a total failure of the afterheat removal systems the prestressed concrete reactor pressure vessel can also be cooled with only one train of the liner cooling system. Refeeding of the system (i.e. by the fire brigade) is possible up to high temperatures. The insulation and the liner will not fail during long-term outage of the Linear Cooling System (LCS). The processes during refeeding of the LCS and the water release out of the concrete can be calculated with computer codes. (author)