1 ROD BUNDLE HEAT TRANSFER TEST RESULTS; Spacer Grid Effects and Potential Impacts on LOCA Evaluation Models Stephen M. Bajorek, Ph.D. Senior Technical.

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ROD BUNDLE HEAT TRANSFER TEST RESULTS;Spacer Grid Effects and Potential Impacts on

LOCA Evaluation Models

Stephen M. Bajorek, Ph.D.Senior Technical Advisor for Thermal-Hydraulics

Office of Nuclear Regulatory ResearchUnited States Nuclear Regulatory CommissionPh.: (301) 415-7561 / Stephen.Bajorek@nrc.gov

Presentation to the2009 Regulatory Information Conference

March 12, 2009

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Spacer Grid Effects

The effect of spacer grids on reflood heat transfer has been observed in several test series (FLECHT-SEASET, FLECHT, FEBA). Important effects are:

Convective enhancement (downstream of grid) Droplet breakup Grid rewet

The NRC has sponsored tests at the Penn State Rod Bundle Heat Transfer Test Facility (RBHT) to investigate reflood heat transfer and other LOCA processes.

Reflood Heat Transfer Steam Cooling Droplet Injection

The NRC has recently begun to evaluate the data and incorporate findings into analysis codes.

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RBHT Test Facility

Flow Housing Instrumentation

Rod Bundle

Window(s)

Detailed P Cells

Steam Probes

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RBHT Steam Probe Rake

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Digital Imaging System

Camera

Lens

Light SettingFocus

Focus (fine)

IR LaserAperture Power Indicator

Light

Droplets ScatteringSheet

TOP VIEWNOT TO SCALE

Shutter

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Convective Enhancement

Clad Temperature, Exp 1096 (2.54 cm/sec, 137.9 kPa, 760 C initial temperature, 1.312 kW/m)

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tc(66")

tc(73")

grid@ 69.25"

ct @ 66"

ct@ 73"

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Droplet Breakup Steam Temp, Exp 1096 (2.54 cm/sec, 137.9 kPa, 760 C initial temperature, 1.312 kW/m)

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sp(79.7")

sp(93.53")

grid@ 89.7

sp @ 79.7"

sp@ 93.53"

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Spacer Grid Effects

Local convective enhancement downstream of spacer grids is due to disruption of the boundary layer. The “entrance length effect” increases the heat transfer coefficient as the boundary layer reforms.

Droplet breakup increases the interfacial area of the droplet field. Rapid evaporation decreases the steam temperature, which increases the rod to fluid heat flux.

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Droplet Breakup – Dry Grids

The conventional “theory” has been that the primary mechanism for droplet breakup (and resulting downstream increase in evaporation) is due to drops interacting with a dry grid. Sharp obstructions shear the incoming droplet into two or more smaller droplets.

Grid Spacer

Rod Bundle

Outgoing Droplets

(Downstream)

Incoming Droplets

(Upstream)

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RBHT Test 1300 Results

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RBHT Test 1383 Results

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Droplet Size vs. Time

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Droplet Formation Processes

Dry Grid Wet Grid

May not persist except far from QF, or if VIN is small.

Drop formation by breakup at sharp interfaces.

Drop sizes found to be small.

For VIN > 1.0 in/s, most grids show quick rewet.

Drop formation may be due to entrainment from film on grid.

Drop sizes increase as quench front approaches grid.

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Summary & Conclusions

RBHT data from recently completed test series is now being evaluated and used in code assessment.

RBHT Reflood test data show grid rewet to occur everywhere in the bundle when VIN > 3 in/sec and on two or more grids ahead of the quench front for low flooding rates.

Drop sizes downstream of a wet grid are larger than those produced by a dry spacer grid, indicating that the mechanism for drop formation changes.

Consequence for Evaluation Models: Wet grids can act as a source for droplets high in a bundle. Drop formation mechanism in codes to be concerned may not be that at the

quench front, but from rewet grids just downstream of high power and PCT locations.