WinNUPRA is a software
package for performing probabilistic risk/safety assessments (PRAs/PSAs). The system
consists of six major analysis modules designed to generate and analyze minimal cutset
solutions of various fault trees and cutset equations for accident sequences. Operations
are provided for direct solution of fault trees, for their minimum cutsets, and for
Boolean manipulation (merging) of cutset equations. The basic functions of each of the
modules are presented below.
Event Tree Editor The Event Tree Module allows you to create, edit, quantify, and print Event Trees. An event tree is a graphical representation of a sequence of functional states or events and their consequences. It is a technique for tracing an accident sequence following an initiating event in terms of the success or failure of significant subsequent actions. These actions include both accident-mitigating actions and normal operating actions; they can be either system actions or operator actions. The purpose of event tree analysis is to identify those event sequences leading to the defined undesirable event.
Fault Tree Editor The Fault Tree Module works primarily with fault tree files (file type LGC). A major portion of the functions in this module are devoted to creating and editing fault tree files that can be displayed in WinNUPRA's fault tree graphic format. Additional options allow you to:
In WinNUPRA, three basic event groups are used: Basic Events, Gates, and Transfers. A Basic Event is an event having no other events contributing to its failure. A Gate is a logic gate to which other events are logically connected. Gates can further be represented by three logical types: AND gates, OR gates, and K out of N gates. Transfers represent a connection between separate parts of a fault tree model (internal transfer) or may connect different fault trees (external transfer).
In WinNUPRA, fault trees are developed in virtual "pages". Each "page" of a fault tree contains a grid of 12 rows by 6 columns. Transfers are used to connect the fault tree logic from one virtual page to the next virtual page within a fault tree. In this format, gates play an important role in the graphical structure references. Each gate is uniquely named according to the fault tree name, page number, and page position.
Six different types of basic events are supported in WinNUPRA. These are:
Display and Edit operations can be performed on records in all the file types: Basic Event Data Files (file type BED), Parameter Files (file type PRM), and Sequence Files (file type SEQ). The module also contains several options to automatically update one type of data file from another or to recalculate values in the records of the file being edited. Finally, new BED files can be created by copying an existing BED file or selecting a subgroup of events from an existing BED file.
Link Fault Tree
Since fault trees may contain external transfers, to be solved they must be connected or "Linked" together. This function links the desired fault trees and saves the resulting "linked" tree in a file with the extension LKC.
Solve Fault Tree for Cutsets
This function is used to generate the minimal cutset equation for the desired fault tree logic model. Both unlinked (LGC) and linked fault trees may be solved. The resulting cutset equation is stored in a file with the extension EQN.
Some function cutset equations or sequence cutset equations may require further analyst input before they are used to quantify or merge event tree sequence equations. These changes generally come under the heading of cutset editing. Typical reasons for this are the absence of "NOT" gate modeling, operator actions or common cause failures that may have been modeled inconsistently. Therefore a cutset editor is provided to allow the user to manually edit a cutset equation (EQN) file. If a large number of systematic changes are desired, this feature can be used in an automated mode.
Merge Cutset Equations
When minimal cutset solutions are available for the individual functional events in an event tree file, the sequence cutset equations for the accident sequences in that event tree can be created. The minimal cutset solutions for the challenged events in each accident sequence are combined with Boolean operators to produce the sequence cutset equation. Merging performs this process, as well as the deletion of cutset equations that represent impossible combinations of events.
Concatenate Cutset Equations
A series of cutset equations can be joined together. Concatenation is the ANDing together of independent cutset equations. This operation concatenates all of the core melt sequence equations into a single plant damage equation. The resulting equation can be truncated, if desired.
Quantify a Cutset Equation
This operation substitutes the point estimates for the frequency of basic events into the cutset equation and calculates the numerical value for the failure probability or event frequency represented by that cutset equation.
Minimalize a Cutset Equation
Occasionally, non-minimal cutsets may be generated or there may be doubt about whether the cutsets obtained are minimal. This can occur when the results of several types of analyses (internal, fire, flood, seismic, etc.) are combined. This operation processes a cutset equation and performs a Boolean reduction, resulting in a minimalized cutset equation.
All of the above listed functions can be performed together in a single batch operation. This completely automates the solution process. If minor changes to a quantification are desired, the instructions can be easily modified to perform only the operations necessary to incorporate the change. The Batch Process in WinNUPRA 2.1 also includes Importance Analysis, Sensitivity Analysis, and Uncertainty Analysis.
Results Module As its name implies the WinNUPRA Results Module works with the results from operations in other modules, particularly with results from the computational operations in the Calculation Module.
The Integrated Results options are used to display and print summary reports of the accident sequence data produced in the Calculation Module. The results can be sorted by Initiator, Sequence, Status Class, Component Failure, or Component Success. Selected groups of results can be charted and graphed.
Variational analyses on concatenated sequence cutset equations representing a Level 1 PSA, can be performed to obtain measures of Importance, Sensitivity, and Uncertainty. Importance is calculated using the following measures: Risk Achievement Worth, Risk Reduction Worth, Generalized Importance, and Joint Importance. Sensitivity analyses can easily be performed with respect to variation in parameter values. The user can specify values for particular individual events and the outcome frequency is re-computed for comparison with the frequency without the substitutions. The Uncertainty Analysis option performs a Monte Carlo simulation, in a series of trials, on the basic events, in the cutset equation for the outcome of interest. In each trial, values of the parameter with defined distributions are chosen randomly from the parameter distributions. The resulting basic event values are substituted into the cutset equation, and the outcome frequency is computed. The results of many hundreds of trials are then ordered by the outcome values. The fifth percentile, for example, in this distribution of outcome values represents the 95% level of confidence that the actual values lies above that outcome value.
The user can also view and print any of the viewable files in the current model. He can display and interactively edit any WinNUPRA cutset equation file with the Cutset Editor, as an alternative to automated editing of cutset equations.
Tools Module The Tools Module is a collection of useful Tools, which supports creating and maintaining WinNUPRA models. The following tools are available:
WinNUPRA is a
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