HTR-MODUL for process heat application 170 MW th

A probabilistic safety analysis has been conducted at the Research Center Jiilich for the process heat variant of the German modular HTGR with the purpose to identify differences compared with the electricity generating HTR-MODUL [55]. The process heat HTR-MODUL consists of a pebble bed core with 360,(KX) spherical fuel elements to produce a thermal power of 170 MW. Helium coolant gas inlet / outlet temperatures are 300 and 950 °C, respectively. The system pressure is 4 MPa. The connection to the secondary circuit is given by a He / He intermediate heat exchanger. 

The safety concept considers two nuclear shutdown systems, a set of six reflector rods for reactor scram and power control and a KLAK system of small absorber balls for cold and long-term shutdown. Decay heat removal is made via the heat exchanger, an auxiliary cooling system, and the panel cooling system inside the concrete cavern, or, in case of a failure of these systems, passively by heat transfer via the surface of the reactor vessel. 

The principal barriers against fission product release into the environment are the high quality TRISO fuel, the reactor pressure vessel, and the reactor building. The calculation of the fission product release during normal operation of the reactor (Fig. 3-5) which determines the contamination of the primary circuit and thus the source term in case of a depressurization or a water ingress accident has again identified silver to be the nuclide with the largest release fraction. 

The higher fuel temperatures under normal operating conditions compared with the electricity generating variant result in a higher fission product contamination of the primaiy circuit outside the active core, a factor of 3 for iodine and a factor of 4 for cesium. In the case of a depressurization through a large leak, it is the dust-borae cesium and 

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