Argonne National Laboratory 2007 RELAP5 International User’s Seminar Experience Using RELAP5-3D for Reduced Enrichment Research and Test Reactor Analysis, the Good and the Not So Good Floyd E. Dunn RERTR Program Argonne National Laboratory 2007 RELAP5 International User’s Seminar Idaho Falls, Idaho November 7-9, 2007

Summary The good – single phase liquid and maybe sub-cooled boiling: -- Single phase liquid calculations in RELAP5-3D agreed well with experiment for Miniature Neutron Source Reactor constant power and reactivity insertion transients. -- For the Budapest Research Reactor, RELAP5-3D predicts a coolant flow instability at the same point predicted by the Whittle and Forgan correlation. The not so good – low pressure boiling: -- RELAP5-3D tends to bomb when it gets far enough into boiling to predict flow instability. It gives no information on the consequences of flow instability -- CHF – The Groeneveld 1986 CHF treatment in RELAP5-3D is out of date and probably not very accurate at low pressures. -- CHF – The 1 – 1.5 atmosphere pressures of interest in RERTR is outside the range of applicability of the optional PGCHF treatment in RELAP5-3D. Who knows? -- Is it real or is it RELAP?

THE GOOD -- SINGLE PHASE LIQUID Comparisons with experimental data for the Miniature Neutron Source Reactor (MNSR)

Core Regions Used for MNSR Reactivity Coefficient Calculations CORE AND FUEL PIN PARAMETERS HEU LEU (12.5%) LEU (12.0%) Uranium Enrich.,% 90 12.5 12.0 Number Pins in Core 347 Clad Material Al Zr-4 Clad OD, mm 5.5 5.1 Clad Thick., mm 0.6 0.45 Fuel Meat Material Alloy UO2 Fuel Meat OD, mm 4.3 4.2 Gap Thickness, mm 0.02 0.05 Core Moderator Heated Tank Water Cool Tank Water Heated Beryllium Cool Beryllium Heated Region near Core Cool Region below Core

NIRR-1: Coolant Inlet and Outlet Temperatures for MNSR Normal Operation at Constant Power NIRR-1 Operation at Constant Power of 15 kW NIRR-1 Operation at Constant Power of 15 kW, Extended Time Scale

NIRR-1: Measured and Calculated Results for 3 NIRR-1: Measured and Calculated Results for 3.77 mk Transient in HEU Core Using RELAP5-3D Measured and calculated data agree very well when all feedbacks (negative and positive) are included.

NIRR-1 3.77 mk Transient WITH NO POSITIVE FEEDBACK COMPONENTS Inclusion of the positive feedback components from the tank water and beryllium reflector is essential for agreement with the experiment data.

The Good and the Not So Good – the Budapest Research Reactor RELAP5-3D predicted the onset of flow instability almost exactly where it is predicted by the correlation of Forgan and Whittle for a $0.15 reactivity insertion in the Budapest Research Reactor. RELAP5-3D quit soon after the onset of flow instability with the message “Thermodynamic property error with minimum time step, transient being terminated.” Does this mean that the reactor will melt down, or does it just mean that RELAP quit?

The Not So Good – CHF Treatment The 2006 Groeneveld CHF treatment should be more accurate at low pressures than the 1986 treatment in RELAP5-3D.

MNSR Thermal Hydraulic Safety Margins, LEU Fuel Clad Surface Temperature Heat Flux and CHF Groenveld 2006 CHF is probably more accurate than RELAP CHF Flow instability may be more limiting than CHF At normal operation, a margin of a factor of 10 or more Peak Fuel Temperature

Who Knows? In sub-cooled boiling at natural circulation conditions, RELAP often predicts flow oscillations. Are they real, or are they a code artifact? Large scale flow oscillations and flow reversal are often predicted after the onset of significant voiding. Instantaneous low flow rates or flow reversal lead to instant switch to film boiling in the code. Should CHF be based on instantaneous conditions, or some kind of a time average?