RSICC CODE PACKAGE PSR-465

1. NAME AND TITLE

EVNTRE: Code System for Event Progression Analysis for PRA.

2. CONTRIBUTORS

Sandia National Labs., Albuquerque, New Mexico through the Energy Science and Technology Software Center, Oak Ridge, Tennessee.

3. CODING LANGUAGE AND COMPUTER

FORTRAN 77; DEC VAX8650 (P00465D0VAX00).

4. NATURE OF PROBLEM SOLVED

EVNTRE is a generalized event tree processor that was developed for use in probabilistic risk analysis of severe accident progressions for nuclear power plants. The general nature of EVNTRE makes it applicable to a wide variety of analyses that involve the investigation of a progression of events which lead to a large number of sets of conditions or scenarios. EVNTRE efficiently processes large, complex event trees. It can assign probabilities to event tree branch points in several different ways, classify pathways or outcomes into user-specified groupings, and sample input distributions of probabilities and parameters. PSTEVNT, a postprocessor program used to sort and reclassify the 'binned' data output from EVNTRE and generate summary tables, is included.

5. METHOD OF SOLUTION

EVNTRE processes event trees that are cast in the form of questions or events, with multiple choice answers for each question. Split fractions (probabilities or frequencies that sum to unity) are either supplied or calculated for the branches of each question in a path-dependent manner. EVNTRE traverses the tree, enumerating the leaves of the tree and calculating their probabilities or frequencies based upon the initial probability or frequency and the split fractions for the branches taken along the corresponding path to an individual leaf. The questions in the event tree are usually grouped to address specific phases of time regimes in the progression of the scenario or severe accident. Grouping or binning of each path through the event tree in terms of a small number of characteristics or attributes is allowed. Boolean expressions of the branches taken are used to select the appropriate values of the characteristics of interest for the given path. Typically, the user specifies a cutoff tolerance for the frequency of a pathway to terminate further exploration. Multiple sets of input to an event tree can be processed by using Monte Carlo sampling to generate approximate mapping from input to output.

6. RESTRICTIONS OR LIMITATIONS

A preprocessor program supplied can be used to replace a PARAMETER statement if array sizes require adjustment.

7. TYPICAL RUNNING TIME

Central processing time can vary from a few minutes to several hours depending on the size and complexity of the problem. The size of the event tree, the amount of sampling done where uncertainties exist, and the extent of branching and calculation of branch probabilities in the event tree are some of the conditions which can add to the processing time. NESC executed the EVNTRE sample problem in approximately 45 CPU minutes, and the PSTEVNT sample problem in approximately 9 CPU minutes on a DEC VAX6220.

8. COMPUTER HARDWARE REQUIREMENTS

EVNTRE was developed for VAX computers. Memory requirements depend on the particular problems being solved. EVNTRE, as currently dimensioned, requires 850 Kbytes of virtual memory. PSTEVNT requires 1400 Kbytes of memory.

9. COMPUTER SOFTWARE REQUIREMENTS

This software runs under VMS 4.5; VMS 5.3 and requires a FORTRAN 77 compiler.

10. REFERENCES

J. M. Griesmeyer and L.N. Smith, "A Reference Manual for the Event Progression Analysis Code (EVNTRE)," NUREG/CR-5174, SAND88-1607 (September 1989).

S. J. Higgins, "A User's Manual for the Postprocessing Program PSTEVNT," NUREG/CR-5380, SAND88-2988 (November 1989).

L. Reed, "EVNTRE, NESC No. R1132, EVNTRE Tape Description and Implementation Information," NESC Note 91-66 (April 26, 1991).

11. CONTENTS OF CODE PACKAGE

Included in the package are the referenced documents and one 3.5" (1.44MB) DOS formatted diskette containing a self-extracting file. No executables are included with the package.

12. DATE OF ABSTRACT

January 2000.

KEYWORDS: PROBABILITY SAFETY ANALYSIS; RISK ASSESSMENT