Nuclear Energy for New Europe 2009

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No: 308

Conference: Nuclear Energy for New Europe 2009

Title: Upgrade to NPP Krško Internal Flooding PSA

Theme: Probabilistic Safety Assessment

Author(s): Ivan Vrbanić, Ivica Bašić, Rudolf Prosen

Contact : Ivan Vrbanić

E-mail: ivan.vrbanic@zg.t-com.hr

Address: APoSS d.o.o.
49210 Zabok

Country: Croatia

The existing internal flooding probabilistic safety analysis (PSA) study of Krško NPP was upgraded in response to the Krsko Periodic Safety Review (PSR) issues concerning the screening criteria for internal floods, residual risk from screened out areas and propagation of scenarios to plant damage states and Level 2.
The area screening process was revisited in order to cover also the areas with relevant risk potential, although without automatic reactor trip equipment, which may have been screened out in the original study. The PSA input to the list of relevant mitigation equipment was revisited regarding the latest, referential, version of the PSA model. Through the screening process, the residual core damage frequency (CDF) from screened out areas was systematically estimated in order to demonstrate that its combined value is acceptably low as compared to the total estimated internal flooding CDF.
In the next step, the internal flooding PSA model was extended to Level 2 in order to address the corresponding PSR issue. All Level 1 core damage sequences from the internal flooding event trees in the PSA model were extended to Level 2. Bridge trees (containment systems event trees) were developed. All resulting plant damage states previously unconsidered in Krsko PSA studies were propagated to release categories in a manner consistent with Individual Plant Examination (IPE) studies. Frequencies of release categories were quantified. The plant area screening process was repeated at Level 2 in a manner equivalent to the screening at Level 1. Residual Level 2 risk was estimated in terms of large early release frequency (LERF).
The paper describes the overall process of upgrading the internal flooding PSA study and model, and presents its results.

No:	308
Conference:	Nuclear Energy for New Europe 2009
Title:	Upgrade to NPP Krško Internal Flooding PSA
Theme:	Probabilistic Safety Assessment
Author(s):	Ivan Vrbanić, Ivica Bašić, Rudolf Prosen
Contact :	Ivan Vrbanić
E-mail:	ivan.vrbanic@zg.t-com.hr
Address:	APoSS d.o.o. 49210 Zabok
Country:	Croatia

The existing internal flooding probabilistic safety analysis (PSA) study of Krško NPP was upgraded in response to the Krsko Periodic Safety Review (PSR) issues concerning the screening criteria for internal floods, residual risk from screened out areas and propagation of scenarios to plant damage states and Level 2. The area screening process was revisited in order to cover also the areas with relevant risk potential, although without automatic reactor trip equipment, which may have been screened out in the original study. The PSA input to the list of relevant mitigation equipment was revisited regarding the latest, referential, version of the PSA model. Through the screening process, the residual core damage frequency (CDF) from screened out areas was systematically estimated in order to demonstrate that its combined value is acceptably low as compared to the total estimated internal flooding CDF. In the next step, the internal flooding PSA model was extended to Level 2 in order to address the corresponding PSR issue. All Level 1 core damage sequences from the internal flooding event trees in the PSA model were extended to Level 2. Bridge trees (containment systems event trees) were developed. All resulting plant damage states previously unconsidered in Krsko PSA studies were propagated to release categories in a manner consistent with Individual Plant Examination (IPE) studies. Frequencies of release categories were quantified. The plant area screening process was repeated at Level 2 in a manner equivalent to the screening at Level 1. Residual Level 2 risk was estimated in terms of large early release frequency (LERF). The paper describes the overall process of upgrading the internal flooding PSA study and model, and presents its results.