Availability increase evaluation of a CANDU-600 reactor based on the 2 out of 4 shutdown logic

Quality, reliability and maintenance are the three directly decisive factors for the availability of nuclear process and safety systems of a Nuclear Power Plant. Since the reliability of nuclear equipment and components based on the efforts performed for perfecting them can rapidly reach a 'saturation' point, the only way to improve a system availability is to find those possibilities to optimize its structure so that to strongly minimize its unavailability to work. Reliability analysis prove that very good results have been obtained by replacing the simple reduplicate schemes (1 out of 2; 2 out of 3) with more sophisticate once (2 out of 4; 2 x 2 out of 3). The paper reveals the advantages gained by a CANDU-600 reactor if the Shut Down System Number 1 is based on the 2 out of 4 logic instead of 2 out of 3. The investigation's framework is a new 2 out of 4 shutdown scheme, entailing only relay changes and aiming for identical design requirements and purposes as the initial one. The calculation use the classical logical block diagrams, reliability factors and equations and demonstrate the advantage of the proposed logic by computing and comparing the availability factors for 2 out of 4 and 2 out of 3 logic. The efficiency of the method is established by estimating in comparison with the initial 2 out of 3 logic the implicit investment owed to the additional 4 release and 1 measurement channel. The determined increase of the availability factor (5.86·10-6 year/years) and the subsequent rise of investment (8.6 millions lei) sustain the proposed method. (Author) 2 Tabs