The impact of design on the decay heat removal capabilities of a modular pebble bed HTR

The decay heat removal capabilities are an important safety feature of the modular pebble bed HTR. It is designed in a way that also during loss of cooling accidents the decay heat can be removed purely by passive means without exceeding predefined temperature limits for fuel and structures. Such a plant design, however, yields limitations on the power output. Thus, from the thermal hydraulic point of view a reactor with maximum power which still obeys the temperature limits of fuel and components, represents an optimal design of a modular pebble bed HTR.In this paper, design options for a modular pebble bed HTR are discussed with respect to their capabilities of decay heat removal.Both pressurized and depressurized loss of coolant accidents are investigated. Optimization of design features is considered with reference not only to the maximum fuel temperature during the accidents, but also to the temperature of structures, mainly that of the reactor pressure vessel. It is pointed out that annular cores can produce higher power without exceeding fuel temperature limits, especially during depressurization accidents. This is mainly due to geometrical effects. Heat storage effects of the inner column also have an influence on the maximum fuel temperature by increasing the time at which this temperature is reached. While a thermal insulation of the core and the reflector increases the fuel temperature, maximum temperature of the pressure vessel and the core barrel is decreased. Thus, carbon blocks represent an important element for optimization of the design.