SBLC system

The SBLC system is a redundant control system capable of shutting the reactor down from rated power operation to the cold condition in the postulated situation that the control rods cannot be inserted. The operation of this system is manually initiated from the reactor control room. 

The equipment for the SBLC system is located in the reactor building and consists of a stainless steel storage tank a pair of full capacity positive displacement pumps and injection valves a test tank and the necessary piping, valves, and instrumentation. 

The SBLC system is adequate to bring the reactor from the hot operating condition to cold shutdown and to hold the reactor shutdown with an adequate margin when considering temperature, voids, Doppler effect, equilibrium, xenon, and shutdown margin. It is assumed that the core is operating at normal xenon level when injection of liquid control chemical is needed. 

The liquid control chemical used is boron in the form of sodium pentaborate solution. It is injected into the bottom of the core where it mixes with the reactor coolant. The sodium pentaborate is stored in solution in the SBLC tank. Electric heaters automatically keep the solution above the saturation temperature. The system temperature and liquid level in the storage tank are monitored, and abnormal conditions are annunciated in the control room. 

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The remainder of the paper is organized as follows. In Section 2, the fault tree associated with the studied SBLC system is presented. In Section 3, the Bayesian and possibilistic approaches to fault tree analysis are briefly reviewed, and details on the hybrid approach are provided. In Section 4, the results of the application of the different approaches to the case study of Section 2 are presented, setting the stage for the discussion in Section 5. Section 6 concludes the work. 

The system considered is the Stand-By Liquid Control (SBLC) system analyzed by Wu (2007). The SBLC system provides an independent alternative method to the control rod drive system which is the primary system for shutting down the reactor. 

Note that the above relations in the hybrid approach concerns uncertainty about q, which is a property of the population to which the SBLC system belongs. 

The rmcertainty descriptions for the top event and top event probability (chance) presented in the previous subsection follow from the calculus that applies to each approach. It then remains to provide interpretations, as has been done in the previous subsection. In this subsection we discuss the use/appropriateness of the different approaches to analyze the fault tree for the SBLC system described in Section 2. 

Considering the expert statements in Section 3 for the fault tree of the SBLC system, the establishment of triangular and uniform possibility distributions provides a direct transformation from the given expert statements into distribution functions for the failure rates in question, with minimal interference from the fault tree analyst, although the choice of distribution is of course a judgment by the analyst. 

Standby liquid control (SBLC) system Reactor core isolation cooling (RQC) system RHR system... 

Let. 4 denote the top event SBLC failure on demand in the fault tree of Section 2.2, and q the probability (chance) of occurrence of A over a fixed mission time, Tm- The probability (chance) q depends on the logical structure of the fault tree and the probabih-ties (chances) of occurrence of component failures, or basic events S, / = 1,2,..., 22. The probabilities (chances) Pi(Xi) of occurrence of the basic events Bi in the fixed mission time are assumed to be unknown. Here X, is a parameter, possibly vector-valued, of the underlying failure time distribution of component i. In this work, an exponential failure time distribution is assumed for all components in the system, i.e., p(Xi) = -exp — XiTm), and we use = 31 days. 

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