ICONE11-36137 Simulation of a Strongly Asymmetric "Loss of Load" Event, Using the coupled 3D Code System R/P/C

Abstract

Framatome-ANP developed S-RELAP5,a RELAP5 based thermal hydraulic code and PANBOX, which is a 3D code for calculation of neutronic and thermal hydraulic core kinetics. For core thermal hydraulics the COBRA 3-CP code is introduced as integral part into PANBOX. The coupling of all three codes has provided the powerful code system R/P/C. The capability of the code system has been tested by application to several suitable transients. The measured values of the data recording system of the plant were kindly put at our disposal. For the purpose of this presentation a particularly interesting transient with "loss of load" combined with a temporary coast down of one main coolant pump is discussed. The cause of this transient was the accidental disconnection of a 400 kV power switch at an off site transformation station. As a consequence the change over of the turbine control from power to speed control was managed by resetting the power nominal value to station service power by dropping 5 (uneven) control rod pairs, which resulted in an azimuthal asymmetric neutron distribution over the core. The turbine valves were spontaneously closed in order to limit the overspeed at 110%. Due to the simultaneous voltage break down (83%) in one of the station power net bars and the consequent long duration switch towards the reserve net the main consumers on this bar suffered coast down (i.e. one (of four) main coolant pumps and one (of two) main feedwater pumps). After change to the reserve net the consumers were reactivated again according to the design procedure. The reconnection of the main coolant pump caused a cold water plug to enter the core and a distortion of the massflow therein resulting in a significant power peak. The key phenomena of the given transient, which have to be reproduced by the calculation are detection of "Loss of Load" by comparison of the reactor and generator power and the synchronous and asymmetric drop of 5 control rod pairs with the result of temperature decrease and following reactivity increase (check of the I&C model). coast down and reactivation of the main coolant pump in one coolant circuit and the formation of an accentuated low temperature profile up to the core region. The physical progress of the transient is dependent of a double asymmetry : the pattern of an a odd number (5) of fallen control rod pairs in the core. the temperature distribution, which causes interaction with the asymmetrically distributed reactivity sinks. Therefore a 3-D calculation is indispensable for a stringent simulation. The presented calculation was carried out as a part of the validation procedure for the R/P/C code system. Without any essential corrections of input values or code adjustments the represented results could be achieved. Even in details, the calculation and measurement results meet surprisingly well.