1. 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

2. 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-- CHF – The Groeneveld 1986 CHF treatment in RELAP5-3D is out of date and probably not very accurate at low pressures.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?

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

4. 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

5. 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

6. 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.

7. 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.

8. 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?

9. 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.

10. 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

11. 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?