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Calculations on UMSICHT Water Hammer Benchmark (Experiment 329) using TRACE and RELAP5

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## Calculations on UMSICHT Water Hammer Benchmark (Experiment 329) using TRACE and RELAP5

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### Calculations on UMSICHT Water Hammer Benchmark (Experiment 329) using TRACE and RELAP5

W. Barten, A. Jasiulevicius, O. Zerkak, R. Macian-Juan

LRS-Seminar, PSI, 6 April 2006

Structure

- Introduction
- UMSICHT water hammer experiments
- TRACE and RELAP5 models of UMSICHT PPP experiment 329
- Results of Base Case
- Enhanced Condensation
- Conclusions and Outlook

Context, Relevance and Recent Developments:It is not the pressure (wave/front propagation) alone …

- Relevance of Pressure and Flow Rate Wave, entering the vessel from the steam line during BWR turbine trip, on the reactivity of the core (e.g. Barten, Coddington, Ferroukhi, 2004,2006)
- Pilot Study on Mechanical Modelling aiming for Fluid-Structure Interaction (FSI) performed 2004-2005.
- Mechanical Modeling (1-D, 2-D, 3-D) using the FEAT code of cladding deformations during burst tests; relevance of mechanical properties (Barten, Hermann, Wallin, 2005)
- FSI Seed Proposal for NES (Barten, 2005)
- HSK On-Call on effect of 0.1x (SB) LOCA on PWR reactor internals: calculations of time-dependent pressure distributions (decompression wave) and thereby involved mechanical loads (O. Zerkak, LRS seminar 2006; Jasiulevicius, Zerkak, Barten, 2005)
- NURETH11 benchmark workshop on UMSICHT cavitation water hammer experiments (Barten, Jasiulevicius, Zerkak, Macian, 2005) + submitted manuscripts 2006

UMSICHT exp. 329: piping system for water hammer studies

- UMSICHT experiment 329 is NURETH11 Benchmark 2
- length ~160 m, diameter ~0.11m, vertical bridge and vertical/horizontal bridge, supports FP1, FP2, FP3
- Steady state flow (~4m/s) in piping system driven by pump
- Water Reservoir in pressure tank (~147 oC, ~10 bar)
- fast closing valve (0.03–0.1 s)

- Pressure measurements along pipe (P1 … P23)
- Wire Mesh Sensor (WM from FZR) at P03: temp., void fraction

UMSICHT exp. 329: steady state initial conditions

- Wall friction resistance coefficient selected as 0.0217 (means wall roughness 14510-6m)
- homogeneous losses are much more important than losses at bends (~6 times)
- Specified pressure loss at valve (0.63) evidently too small
- Measured pressure drop between P15 and P18 is about 0.1 bar, although P18 is about 2 m higher than P15
- Positions P04, P06 ?

- Herning correlation used for pressure losses at pipe bends, with correction for non-orthogonal bends

Boundary Conditions

- Boundary Condition Model (tests with full system model showed similar results)
- Inlet: constant pressure and temperature boundary condition at inlet of pump
- Outlet: nearly constant pressure (10.0-10.5 bar) and temperature in container (~147 oC)
- Closure of valve at 0.1s within 0.03 to 0.1s. Reduction of container inlet flow rate starts about 0.1s later.
- Backflow from container into pipe after 0.9s
- Flowrate oscillations to-and-from container

Base Case Calculations

- Timing of first pressure peak OK, second pressure peaks are slightly shifted, thus the flow rate behavior is qualitatively OK
- Amplitude of first pressure peak considerably under-predicted and dispersion considerably over-predicted
- Problem: Too much void is remaining adjacent to the valve at time of pressure excursion
- Amplitude agreement of second pressure peak is an effect of compensating errors: damping by void cushion is over-estimated and damping by FSI is not taken into account (not contained in code capabilities)

Enhanced condensation

- In an ad hoc approach in order to reduce the amount of void adjacent to the valve at the time of pressure excursion the condensation rate is increased.
- Heat Transfer coefficient has to be increased by more than a factor of 10.
- With factor of 100 at time of pressure excursion all void condenses.
- Then very close agreement of calculated first pressure excursion with measurement is attained, including the spiky shape.

Enhanced condensation, x50, example of TRACE (1)

- Timing of calculated pressure peaks coincides with measurements (timing of second and later peaks sensitively depends on condensation rate and other parameters)
- Amplitude and shape of first pressure peak is very well captured by calculation
- Experimental damping and dispersion of second pressure peak considerably underestimated by model result amplitude about halved and much broader, now “Gaussian” shape Dispersion yet stronger for 3rd peak

Enhanced condensation, x50, example of TRACE (2)

- FSI of fluid movement with vibrations of the piping structure (exchange of momentum and energy), which is not contained in the code capabilities, leads to experimental damping / dispersion
- Vapor generation after valve closure at 0.1s is well captured by both codes
- Considerable vapor reduction after 0.9 s (being possibly a 3D flow effect) is not represented in the code results
- Vapor generation at 1.9 s after first pressure peak is considerably over-predic-ted by codes ( effect at 3s is smaller)

Condensation models in TRACE and RELAP5

- TRACE interfacial mass transfer rate:
- Interface-to-liquid and interface-to-gas heat transfer rates:
- RELAP5 interfacial heat transfer between liquid and gas phases involves both heat and mass transfer.
- Individual correlations are different for different flow regimes.

After closure of valve, with decreasing pressure the saturation temperature reduces to and below the liquid temperature and vapor is generated.

When the water flows back to the valve the vapor is compressed, and the pressure rises.

The vapor temperature increases above the liquid temperature and even beyond the (with increasing pressure) also increasing saturation temperature.

This leads to condensation of vapor, however for base case parameters not as fast as observed in measurements.

After reflection of the pressure wave at the valve, the pressure and saturation temperature decrease again.

With the water flowing back to the valve, the vapor condenses again, the fluid hits the valve, and the second pressure and steam temperature peaks are encountered.

Temperature, condensation and flashing (1)For the sensitivity study, the heat transfer coefficients Halve in TRACE and Hil in RELAP5 were increased considerably.

With respect to this the pressure peak considerably increases, since the damping effect by the “void cushion” on the pressure amplitude is reduced.

Moreover the experimentally observed spiky shape of the first peak is now well represented.

At ~2s both codes yield a large vapor production as a result of the increased vaporization rate, which however is not observed in the measurements.

At that time, TRACE predicts the saturation temperature to fall below the liquid temperature.

The high void fraction for RELAP5 is due to the combined effects of the evaporation in the calculation volume and the convection of vapor from neighboring volumes.

Temperature, condensation and flashing (2)At ~ 3s the saturation temperature for the case with increased saturation drops only slightly below the liquid temperature and consequently less vapor is generated than after the first pressure peak for TRACE, while nearly no vapor is predicted at P03 by RELAP5.

Since no direct measurements of volume averaged liquid temperatures are available, we compare the code-calculated liquid temperatures with a local measurement from the wire mesh sensor.

As in the measurements, the TRACE and RELAP5-calculated liquid temperatures vary within a few degrees;

they reflect qualitatively the pressure decrease at ~0.2s and the temperature increase with the first pressure peak.

The temperature spike of 10 to 30 degrees before the first pressure peak in RELAP5 might be regarded as a model deficiency.

Temperature, condensation and flashing (3)Void fraction dynamics downstream the valve

- Expansion and compression of a two-phase “bubble” downstream the valve with size of up to 5m
- Problems with distribution of vapor in the two-phase region (only limited “flow”)
- RELAP5 over-predicts void content at P03, since it predicts that all liquid flows out of this region (between 0.3 and 1.5 s)

Pressure distribution along the pipe

- Pressure measurements at different positions along the pipe
- Considered are the first two pressure peaks
- Compression wave and decompression wave as calculated by both codes are similar to the measurements
- As mentioned above, the measured damping and broadening of the second pressure peak is not reflected in the measurements

The best estimate system codes TRACE and RELAP5 are potentially useful tools for the analysis of a cavitation water hammer in a pipe.

Improvements of the two-phase flow and the condensation and flashing models are necessary.

To capture effects of pressure wave propagation on the piping structures can only be correctly simulated by FSI techniques not available in modern nuclear system codes.

Continue analysis of UMSICHT PPP water hammer experiment 329 (and later exp. 135) using TRACE and RELAP5.

Use 3-D capability of TRACE to analyze e.g. Cold Water Hammer Test Facility experiments (CWHTF at FZR).

Couple a mechanical code, ANSYS, to TRACE to account for FSI and analyze further UMSICHT and CWHTF experiments.

Apply TRACE-ANSYS to LOCA (and possibly RIA) for accurate prediction of pressure wave propagation and FSI.

Conclusions and Possible Outlook:It is not the pressure (wave/front propagation) alone …
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