Available online at www.sciencedirect.com Nuclear Engineering and Design 237 (2007) 2151–2156 A procedure to optimize the timing of operator actions of accident management procedures Nikolaus Muellner a,∗ , Marco Cherubini b , Wolfgang Kromp a , Francesco D’Auria b , Gianni Petrangeli b a University of Vienna, IRF, Tuerkenschanzstr. 17/8, A-1180 Wien, Austria b University of Pisa, DIMNP, Via Diotisalvi 2, 56122 Pisa, Italy Received 5 May 2006; received in revised form 2 March 2007; accepted 20 March 2007 Abstract The analysis of beyond design basis accidents (BDBA) is an essential component of the safety concept of nuclear power plants (NPP). Goal of the analysis is to achieve a set of actions aimed to prevent the escalation into a severe accident, to mitigate consequences of a severe accident, and to achieve a long term controllable state of the NPP. This paper presents an analytical procedure to optimize the timing of operator interventions. The procedure is demonstrated based on four sets of parameters, first, parameters which define the operator actions are chosen. Second, parameters which define the system availability are chosen. Third, parameters which define in a continuous way the status of the plant are chosen. Finally, one looks for a functional dependency of the accident management (AM)-parameters and the parameters describing the plant status. Once a function could be found, this function is “optimized” in the sense that the AM-parameters are varied to find a optimal overall condition for the plant. In the first part, the paper presents the analytical procedure in a general way, in the second part, an initiating event is chosen. The procedure is applied to a station black out (SBO) transient, and as operator action secondary side bleed and feed, followed by primary side bleed and feed, is foreseen. As result, the optimal timing to initiate both actions is achieved. © 2007 Elsevier B.V. All rights reserved. 1. Introduction The concept of defence in depth in nuclear safety foresees also the consideration of highly unprobable accident sequences and the possible use of all equipment still available to terminate or at least mitigate the accident Misak (2004); Petrangeli (2005). Accident management can be divided into five levels. The first level is to prevent abnormal situation and failures, the second level to control abnormal situation and failures, the third level to control accidents within the design basis. The fourth level deals with the control of accidents beyond the design basis (beyond the design basis because of multiple failures or highly unlikely initial events), and the fifth level includes off-site emergency response measures. The fourth level usually consists of a set of strategies and procedures for the operator of a NPP which make use of the avail- able equipment to mitigate the accident. Two main approaches ∗ Corresponding author. Tel.: +39 050 22103 73; fax: +39 050 22103 84. E-mail address: nikolaus.muellner@univie.ac.at (N. Muellner). can be distinguished, event-base procedures and symptom-based procedures. This paper presents a procedure which optimizes the use of equipment by the operator in case of a BDBA in the fourth level mentioned before. The aim is to improve a general strat- egy assuming an iniating event with multiple failures, and then varying the operator actions to optimize the recovery of the plant. The procedure needs computer-simulation results which are sufficently reliable to be useful. Up to now, simulations of severe accidents are to be treated with care, due to the complexity of the phenomena. Therefore, the domain in which the procedure could be useful, are accidents which progress beyond the design basis, but the analysis is stopped before core damage. Acci- dents involving multiple failures, like a loss of off-site power with additional failure of all diesel generators, are such kind of accidents. The computer code which was used to analyse the transient was RELAP5 mod 3.3ef, which is proven to be suffi- ciently reliable. The aim of the method is to help in the design of accident management procedures. Knowing the optimal strat- egy for a given accident should help in defining set points for the symptom-based accident management procedures. 0029-5493/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2007.03.011