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Verification and Validation of Corrected Versions of RELAP5 for ATR Reactivity Analyses. Author: Cliff B. Davis. Introduction. RELAP5/MOD2.5 and RELAP5/MOD3 Version 3.2.1.2 are used to support safety analyses of the Advanced Test Reactor (ATR)
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Verification and Validation of Corrected Versions of RELAP5 for ATR Reactivity Analyses Author: Cliff B. Davis
Introduction • RELAP5/MOD2.5 and RELAP5/MOD3 Version 3.2.1.2 are used to support safety analyses of the Advanced Test Reactor (ATR) • The codes were used to simulate power excursion transients • Previous verification and validation calculations of the reactor point kinetics model showed excellent agreement with exact solutions or calculations from another code • However, errors in the point reactor kinetics model were reported in 2007 that have the potential to affect the ATR safety analyses • Corrected code versions were created • Verification and validation calculations were performed and are described here
The objectives of this presentation are to • Summarize the verification and validation of the point kinetics model for ATR applications • Inform code users of the effects of the code errors on representative transients
Two errors were reported in 2007 • The first error was associated with indices used in the calculation of terms associated with the delayed neutron groups • The second error was related to logic that determines when the code switches from the transient to the quasi-steady forms of the power equation to prevent a loss of precision • The switch is not related to stdy-st and transnt options • A re-evaluation of the coding in 2008 showed that the first “error” was not really an error; the original coding was correct • The first “correction” caused slightly worse results for a step reactivity insertion
Four codes were used in this analysis • RELAP5/MOD2.5 (original) • RELAP5/MOD2.5 with 2007 updates • RELAP5/MOD2.5 with 2008 updates • Uses a subset of the 2007 updates • Details are given in the paper • PTKIN3 • Was used for reactivity analyses of the ATR Critical (ATRC) facility before RELAP5
Four verification and validation cases were performed • A theoretical 0.10$/s reactivity ramp • A theoretical 0.15$ step reactivity insertion • A filler piece drop accident in the ATRC • A 0.45$/s reactivity ramp accident in the ATRC • Exact solutions are available for the first two cases • The validation for the latter two cases was accomplished via comparisons with PTKIN3 • No reactivity feedback was modeled
A wide range of reactivity was investigated • The two ATRC accidents were terminated by scram
Results for the 0.1$/s reactivity ramp • Input model obtained from ISL • Exact solution based on a high-fidelity numerical solution • RELAP5 results generated with a small (0.0001 s) time step
Results for the 0.1$/s reactivity ramp • Original code was in good agreement with the exact solution with large time steps, but in poor agreement with small ones • Agreement with the 2007 and 2008 versions improved as the time step decreased • The 2008 version was better than the 2007 version at the largest time step
Results for the 0.15$ step insertion • Power did not change during first time step with 2007 version • This result prompted the re-evaluation of the 2007 updates
Results for the 0.15$ step insertion • Slight (0.2%) artificial power increase with original version near 0.10 s • Caused by switching from transient to steady-state logic
Results for the 0.15$ step insertion • All versions were in excellent agreement with the exact solution • Results nearly independent of time step size • The maximum error was 0.1% at 1 s, 0.3% at 70 s
Results for the ATRC drop accident • Average reactivity insertion rate of 4.6$/s • These results are with a time step of 0.002 s • Results were similar with all codes, but the peak power was about 2% higher with the 2007 version
Results for the ATRC drop accident • The 2007 and 2008 versions were in excellent agreement with PTKIN3 for time steps of 0.001 s or less • Original code was generally in good agreement with the other codes, except at the smallest time step
Results for the ATRC ramp accident • 0.45$/s reactivity ramp • Peak powers were nearly identical with all codes at a time step of 0.002 s
Results for the ATRC ramp accident • The 2008 version and PTKIN3 gave nearly identical answers at all time steps and were reasonably converged for time steps ≤ 0.002 s • The 2007 and 2008 versions were similar except at the largest time step • Although the original version produced an accurate solution at relatively large time steps, it diverged at small time steps
Conclusions • The original version of RELAP5/MOD2.5 produced: • Excellent results for the step insertion and ATRC drop accidents • Good results for the reactivity ramps with large time steps • Poor results for the reactivity ramps with small time steps • The original version did not always converge at small time steps
Conclusions (cont’d) • The 2007 and 2008 versions converged to the correct solutions at small time steps • The 2008 version produced better results at large time steps than the 2007 version • Largest difference in peak power was about 6% • RELAP5 users from different organizations should use updates equivalent to those generated in 2008 rather than 2007 • The 2008 version of RELAP5/MOD2.5 should be used to perform future reactivity analyses of the ATR and ATRC