Author: Cliff B. Davis

Verification and Validation of Corrected Versions of RELAP5 for ATR Reactivity Analyses

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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

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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

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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

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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

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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

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A wide range of reactivity was investigated

• The two ATRC accidents were terminated by scram

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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

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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

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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

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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

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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

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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

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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

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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

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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

Maximum power (MW) Time step (s) Original 2007 2008 PTKIN3

0.01 18.45 17.54 18.45 18.51 0.002 18.53 18.50 18.49 18.50 0.001 18.67 18.50 18.50 18.50

0.0001 37.64 18.50 18.50 18.49

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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

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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