the oecd-nea sfp projectan experimental programme and related analyses forthe characterization of...

7/31/2019 The Oecd-nea Sfp Projectan Experimental Programme and Related Analyses Forthe Characterization of Hydraulic

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THE OECD-NEA SFP PROJECT

AN EXPERIMENTAL PROGRAMME AND RELATED ANALYSES FORTHE CHARACTERIZATION OF HYDRAULIC AND IGNITION

PHENOMENA OF PROTOTYPIC WATER

REACTOR FUEL ASSEMBLIES


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THE OECD-NEA SFP PROJECT

AN EXPERIMENTAL PROGRAMME AND RELATED ANALYSES FOR THECHARACTERIZATION OF HYDRAULIC AND IGNITION PHENOMENA OF PROTOTYPIC

WATERREACTOR FUEL ASSEMBLIES

1. Program Background and Experiment Description

1.1 Objective

The objective of the proposed project to is to perform a highly detailed thermal-hydrauliccharacterization of full length, commercial 1717 pressurized water reactor (PWR) fuelassembly mock-ups to provide data for the direct validation of MELCOR or other appropriatesevere accident codes. MELCOR model predictions based on extrapolations from theresults of a previously conducted boiling water reactor (BWR) study indicate that PWRassemblies will ignite and radially propagate in a spent fuel pool complete-loss-of-coolantaccident. The proposed PWR characterization will be similar to that successfully conducted

for the BWR study and will lead to two full-scale PWR fire tests where the zirconium alloycladding is heated in air to ignition. The PWR experimental design and data analysis will beclosely coupled with MELCOR modeling as was done in the previous BWR study. Theprevious BWR results are not directly applicable to a PWR assembly due to significantgeometric differences in the assembly designs (see Section 1.3 for details). The mostsignificant difference is the absence of the Zircaloy channel box on the PWR assembly. Theannular flow between the tube bundle and storage cell wall is fundamentally different in thePWR leading to unique hydraulic and convective heat transfer characteristics. The absenceof the Zircaloy channel box in the PWR assembly also reduces the relative inventory ofzirconium and may significantly alter the nature of the axial and radial burn frontpropagation.

The technical expertise gained during the BWR ignition tests allows a number of

experimental improvements to be incorporated into the proposed PWR study. An improvedthermocouple layout will minimize the number of sensors lost when ignition occurs, allowingmore information to be collected on the nature of the burn front. The exclusive use of fulllength assemblies will permit the direct measurement of the naturally induced buoyancydriven flow in all assemblies. Finally, the incorporation of prototypically pressurized rods insome assemblies will allow the measurement of the effect of ballooning on the thermalresponse and ignition characteristics of the PWR assembly. These and other experimentalimprovements will greatly increase the realism of the experimental results obtained in theproposed PWR study.

1.2 Testing Outline

The study will be conducted in two phases. Phase 1 will focus on axial heating and burnpropagation. A single full length test assembly will be constructed with zirconium alloy cladheater rods. As demonstrated in the previous BWR study, the thermal mass of thecompacted MgO powder will be used to make the electric heated rods an excellent match tospent fuel. The assembly thermal-hydraulic characteristics will be measured in two differentsized storage cells and conclude with an ignition test. The ignition test will determine wherein the assembly ignition first occurs and the nature of the burn along the axis of theassembly. The insulated boundary conditions will experimentally represent a hot neighborsituation which isa bounding scenario.


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Phase 2 will address radial heating and burn propagation and will include effects of fuel rodballooning. Bench scale ballooning tests will be conducted to help finalize the ballooning rodtest. Five full length assemblies will be constructed and tested in a 14 configuration. Thecenter assembly will be of the same heated design as used in Phase 1. The four peripheralassemblies will be unheated but highly prototypic, incorporating prototypic fuel rods and endplugs. These boundary conditions experimentally represent a cold neighbor situation,which complements the bounding scenario covered by Phase 1. The peripheral fuel rodswill be filled with high density MgO ceramic sized to precisely match the thermal mass ofspent fuel. Studies using this test assembly will conclude with a fire test where the centerassembly is heated to ignition, which eventually propagates radially to the peripheralassemblies. All of the fuel rods in two of the four peripheral assemblies will be pressurizedwith helium so that these fuel rods will balloon when the zirconium alloy cladding reacheshigh enough temperature. The two peripheral assemblies without pressurized rods wouldserve as control for evaluating the effect of ballooning.

The pressurized rods in two peripheral assemblies are expected to balloon before theyignite. The ballooning will restrict the buoyancy driven flow in the assembly which in turn willincrease the heating rate and result in a more rapid progression to ignition. Comparison ofthe induced flow and thermal response of the two peripheral assemblies with ballooned rodsand the two peripheral assemblies without ballooned rods will provide a direct measure ofthe effect of ballooning.

As was done in the previous BWR study, MELCOR modeling results will be utilized in allstages of testing. Pre-test MELCOR modeling results will be used to guide the experimentaltest assembly design and instrumentation. For example, the location and routing ofthermocouples as well as the sizing of hot wire flow sensors will be based on the initialMELCOR results. MELCOR modeling results will also be used to choose experimentaloperating parameters such as the applied assembly power. Post-test data analysis will beperformed using the MELCOR model. At each step in the testing, improvements will bemade to the MELCOR model such that confidence in the modeling validity will continuallyimprove.

1.3 Technical MotivationA detailed hydraulic characterization of a full scale, highly prototypic PWR assembly mockuphas been conducted. This study was a vital first step for characterizing a PWR assembly buta detailed thermal-hydraulic characterization is also needed. A similar characterization wasperformed on a full scale, highly prototypic BWR assembly in a previous study but theseresults are not applicable to a PWR. The geometry of a PWR assembly differs significantlyfrom the BWR assembly in a number of important ways:

1) The water rods in the center of the BWR assembly carry a significant fraction of thetotal induce flow and aid in cooling. Water rods are not present in the PWRassembly.

2) Eight of the seventy-four rods in the 99 BWR assembly are partial length and end

1.32 m (52 in.) below the top of the assembly. The increased void space in thisupper portion of the BWR greatly reduces flow resistance in this upper region.Partial length rods are not present in the PWR assembly.

3) The tube bundle of the BWR is contained inside of a Zircaloy canister that provides awell-defined, confining flow boundary. The absence of a canister on a PWRassembly means that the walls of the storage cell provide the flow boundary and anappreciable annular gap between the wall and the tube bundle exists. This annular


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flow path complicates the thermal-hydraulic coupling in the PWR assembly. Theabsence of the Zircaloy channel box on the PWR assembly also reduces the relativeinventory of zirconium in the assembly, which is expected to significantly alter thenature of the axial and radial burn front propagation.

The presently ongoing PWR hydraulic characterization is providing vital parameters requiredfor calculating the buoyancy driven flow that is established in a heated assembly. This flow

must be calculated accurately in order to predict the thermal response of a spent fuelassembly in a complete loss-of-coolant accident. While the hydraulic characterization inPhase I includes the effects of the annular gap, the ability of severe accident codes to modelthe multi-dimensional flow paths and the magnitude of the corresponding heat transfer tothe annular flow will be determined. This can only be accomplished through highly prototypicheated experimentation as proposed in this study.

Fuel rod ballooning is an important phenomena expected to occur prior to ignition. Rodballooning has been shown to occur in the temperature range of 950 K to 1150 K. In theBWR 14 ignition test a peak clad temperature of 1050K was reached at 2.75 hrs and therapid escalation in temperature to ignition began at 4.75 hrs at a clad temperature of 1200 K.Thus fuel rod ballooning is expected to occur during the crucial period prior to ignition andcould be expected to decrease the time to ignition by an hour or more.

1.4 Current Studies

In an ongoing study, the hydraulic characterization of a full scale, highly prototypic PWRassembly mockup is being conducted using three state-of-the-art quartz crystal differentialpressure gauges and a laser Doppler anemometer (LDA) apparatus. The pressure gaugesallow accurate measurement of the pressure drop across individual spacers at very lowReynolds numbers (Re = 12 to 1000). The LDA can accurately measure velocities as low as0.001 m/s. A clear plastic window running the entire length of the assembly allows probingthe flow field at any point in the assembly.

2. Phased Experimental Approach

2.1 Phase 1

The testing in Phase 1 will focus on axial heating and burn propagation. The test assemblywill prototypically represent commercial 1717 PWR fuel bundle. The various componentscomprising a typical 1717 PWR assembly are illustrated in Figure 1. The main structuralcomponent of the assembly is the core skeleton which consists of eleven spacerspermanently attached to twenty-five guide tubes. The 264 fuel rods pass through thespacers and are held in place in the assembly by the top and bottom nozzles.

Figure 1 Various components in a typical 1717 PWR fuel assembly.

Bottom

nozzle

Guide tubes

Top

nozzle


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The single full length heated PWR assembly will be fabricated using prototypic, commercial1717 PWR fuel assembly components and 0.375 heater rods made from 0.422 zirconiumalloy tubing supplied by an industrial vendor. The heater rods will be manufactured by acommercial vendor using the same fuel rod simulator design proven highly successful in theBWR study. The spent fuel rod simulators will have a linear power profile and a maximumoutput of 82 W/m (25 W/ft). An important attribute of the heater design is that the thermalmass of the compacted magnesium oxide (MgO) powder used to electrically insulate thecentral heating element from the cladding is virtually the same as the thermal mass of spentfuel over the entire temperature range of interest (as shown in Figure 2). These spent fuelsimulators will therefore heat at the same rate and store the same amount of thermal energyas spent fuel rods.

2.0E+06

2.2E+06

2.4E+06

2.6E+06

2.8E+06

3.0E+06

3.2E+06

3.4E+06

3.6E+06

3.8E+06

0 500 1000 1500

Temp (K)

rhoCp(J/m3/K)

Melcor spent fuel

MgO heater

Figure 2 Thermal mass comparison of spent fuel and MgO fuel rod simulators.

The experimental approach and instrumentation will be similar to that used in the previousBWR study. Figure 3 shows the test assembly used in the heated BWR study. The full

length electrically heated PWR test assembly will be positioned inside a storage cell that iscovered with a thick layer of high temperature insulation. The instrumentation will include hotwire flow meters, oxygen sensors, quartz light pipes, and a large number of thermocouples(TCs). Most of the TCs will be located within the bundle. The TCs in the top third of thebundle will be routed out the top of the assembly while those lower will be routed out thebottom in order to minimize the number of sensors lost when ignition occurs. Pre-testMELCOR modeling results will be used to determine the actual TC routing.


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Figure 3 Heated BWR test assembly and instrumentation.

The Phase 1 test plan will be very similar to the corresponding plan in the BWR study. Forthe pre-ignition testing, the assembly will be heated at a given power and the resultingsteady state temperatures and induce flow rate determined. The maximum temperaturesmust be kept below 900 K in order to avoid excessive oxidation of the zirconium

components. Figure 4, Figure 5, and Figure 6 show some of the corresponding datagenerated in the BWR study (in red) along with the MELCOR model calculations (in blue).The steady state buoyancy driven flow and resulting temperature profile are highly coupled.The thermal gradient inside the bundle creates the buoyancy that drives the flow. The flowin turn convectively cools the bundle such that the flow and the thermal gradients come intobalance. The resulting data set provides an excellent validation database for any dynamicthermal-hydraulic numerical model of the assembly.

In contrast to the previous BWR project, two series of heated pre-ignition PWR experimentswill be conducted using two different size storage cells that span those typically found incommercial storage casks and spent fuel pools. The two different sized storage cells willform two different sized annular gaps between the tube bundle and the storage cell wall.The parallel annular flow path between the bundle and the inner storage cell wall

complicates the thermal-hydraulic coupling in the PWR assembly. The partitioning of flowbetween the annular and bundle regions will be characterized by the laser Doppleranemometer (LDA) used in the recent PWR hydraulic characterization study. The ability andvalidity of severe accident codes to account for the magnitude of the heat transported to theannular flow will be determined.

Thermocouple

O2 Sensor

Light

pipe

Hot wire

Flow meter

High

temperature

insulation

Storage

cell


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0

25

50

75

100

125

150

0 500 1000 1500 2000 2500 3000

Assembly Power (Watts)

FlowRate(slp

m)

Figure 4 Volumetric flow rates as a function of assembly input power forexperiment (symbols) and MELCOR (line).

300

500

700

900

1100

0 500 1000 1500 2000 2500 3000

Assembly Power (Watts)

PCT(K)

Figure 5 Peak cladding temperature as a function of assembly input power forexperimental (symbols) and MELCOR (blue line).


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300

400

500

600

700

0 2 4 6 8 10 12

Time (hrs)

Temperature

(K)

Figure 6 Comparison of the experimentally measured thermal transient response(solid red) with the MELCOR code (dashed blue) for an assembly powerinput of 1370 W.

The Phase 1 testing will conclude with the ignition of the assembly much like that conductedfor the BWR study. Figure 7 and Figure 8 show the peak clad temperature and oxygenconcentration data (in red) collected during the ignition of a full length BWR assembly.MELCOR predictions agreed favorably with experimental data. The correspondingMELCOR calculations are shown in blue. The power used represented a 100 day oldassembly. The insulated boundary conditions experimentally represented a 100 day oldassembly surrounded by 100 day old neighbors. This hot neighbor scenario represents animportant bounding situation where the oldest, lowest power assembly can heat to ignition.In the BWR test, ignition initiated about two thirds the way up the heated length and thenburned downwards. Once ignition occurred the oxygen concentration dropped sharply tozero above the burn front. The downward advance of the burn front was driven by theupward flowing supply of oxygen. All of the zirconium was not consumed as the front moveddownward. Once the bottom was reached, the burn front reversed directions and movedback up the length of the assembly consuming the remaining zirconium. The assembly wascompletely destroyed by the fire as shown in Figure 9.

The burn front could not move upwards initially because the burning Zircaloy rods andchannel box consumed all the oxygen. The absence of a Zircaloy channel box on a PWRassembly could lead to different burn characteristics. If some oxygen can bypass the

burning rods in the annulus, the burn front may proceed further upwards initially, and theburning of the zirconium may be more complete as the burn front moves down. This burnbehavior would also likely affect the radial propagation characteristics, which is the focus ofthe testing in Phase 2.


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300

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0 2 4 6 8 10 12

Time (hr)

Temperature(K

)

Figure 7 PCT for both the ignition experiment (solid red) and the MELCOR model(dashed blue).

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5

10

15

20

25

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Time (hr)

O2

Concentratio

n(%)

Figure 8 Oxygen concentration within the bundle as measured in the ignitionexperiment (solid red) and predicted by MELCOR (dashed blue).


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Figure 9 Postmortem of the assembly after ignition.

2.2 Phase 2

The testing in Phase 2 will address radial heating and burn propagation. Five full length

assemblies that prototypically represent commercial 1717 PWR assemblies will beconstructed and tested in a 14 configuration as shown in Figure 10 and Figure 11. Thecenter assembly will be comprised of the same unpressurized heated design as used inPhase 1. The four peripheral assemblies will be unheated but highly prototypic incorporatingnot only the commercial structural skeleton but also commercial zirconium alloy fuel tubes,surrogate ceramic fuel pellets, and commercial end plugs. Two of the peripheral assemblieswill contain pressurized rods, which will balloon when a critical temperature is reached. Theinternal pressure of some of the pressurized rods will be monitored with strain gauges. The


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five assemblies will be positioned in a highly prototypic 33 commercial pool rack. The poolrack will be covered with a thick layer of high temperature insulation. The boundaryconditions experimentally represent a cold neighbor situation, which signifies an importantbounding situation and complements the bounding scenario covered by Phase 1. The useof all full length assemblies in the 14 configuration allows the naturally induced flowgenerated by each assembly to be monitored and represents a considerable experimentalenhancement over the short assemblies used in the previous BWR study.

A similar set of pre-ignition characterization experiments will conducted with the Phase 2 testapparatus. The center assembly will be heated at a given power with the resulting steadystate temperatures and induced flow rates determined in all five assemblies. The maximumtemperatures again must be kept below 900 K in order to avoid excessive oxidation of thezirconium components.

The Phase 2 testing will conclude with the ignition test of the test apparatus similar to thatconducted for the BWR study. The center assembly will simulate a younger, higher poweredassembly than that used in the Phase 1 ignition test. The cold neighbor boundaryconditions require a higher power in order to achieve ignition since these peripheralassemblies act as a heat sink. In the previous BWR test, a 15 day old assembly wassimulated. Figure 12 shows the thermal response of the center and peripheral assembliesin the BWR 14 ignition test. Figure 13 shows the oxygen response of the center assembly.The data is shown in red and the corresponding MELCOR simulation is shown in blue.Again, the BWR ignition corresponds to a steep drop in oxygen concentration. The inabilityof any oxygen to bypass the burn front may be an artifact of the Zircaloy canister burning.The absence of a Zircaloy channel box on a PWR assembly could lead to different burncharacteristics. If some oxygen can bypass the burning rods in the annulus, the burn frontmay proceed further upwards initially and the burning of the zirconium may be morecomplete on the way down. This burn behavior would likely affect the radial propagationcharacteristics. MELCOR predictions agreed favorably with the experimental data. Thisvalidation shows that MELCOR can be used to analyze the different mitigation strategies forthe safety of SFP.


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Figure 10 Side view of the 14 configuration.

Figure 11 Plan view of the 14 configuration.

0.075 Boral

composite instainless steelwrapper

Electrically heated centerassembly.

0.422 Zircaloy tubes madeinto 0.375 heaters

Unheated peripheralassemblies.0.374 Zircaloy tubes withMgO fill and capped withprototypic end plugs.

Prototypic3x3 pool

rack

Ballooningperipheralassembly

Prototypic

Boral panelin steelwrapper

17x17 PWR

Electrically heated centerassembly.0.422 Zircaloy tubes madeinto 0.375 heaters

Unheated peripheralassemblies.0.374 Zircaloy tubespacked with MgO andcapped with prototypic endplugs

Prototypic3x3 poolrack


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300

700

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1500

1900

2300

0 2 4 6 8

Time (hrs)

Temperature(K

)

Figure 12 PCT of the center and peripheral assemblies as measured in theignition experiment (red lines) and as modeled in MELCOR (blue lines).

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5

10

15

20

25

0 2 4 6 8

Time (hrs)

O2Concentration(%)

Figure 13 Oxygen concentration within the center bundle as measured in theignition experiment (solid red) and predicted by MELCOR (dashed blue).

Center

Peripherals


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2.2.1 Ballooning Rod Design and Testing

Fuel rods balloon because the rods contain pressurized gas. At high enough temperature,the metallic walls soften and can no longer contain the pressure. Ballooning is a seriousissue not only because the rupture of the fuel rod wall is the first step in a radioactiverelease, but the ballooned rods restrict the flow path in the bundle leading to a fastertemperature rise toward ignition. The fuel rods in a commercial 1717 PWR assembly are

initially backfilled with about 17.2 bar (250 psi) helium at ambient temperature. After twoyears in the reactor, gaseous fission product release increases the internal pressure toabout 24.8 bar (360 psi) at ambient temperature. At an average rod temperature of 1050 K,the internal pressure reaches almost 89.6 bar (1300 psi). Rod ballooning has been shownto occur in the temperature range of 950 K to 1150 K [Nucl Sci. & Tech., Vol 42, No. 2 p.209-218 Feb 2005]. In the BWR 14 ignition test, a peak clad temperature of 1050 K wasreached at 2.75 hrs and the rapid escalation to ignition began at 4.75 hrs at a cladtemperature of 1200 K. Thus, fuel rod ballooning is expected to occur during the crucialperiod prior to ignition and could significantly decrease the time to ignition.

This study incorporates the phenomena of ballooning into the design of two of the fourperipheral assemblies. The rods in two of the peripheral assemblies would be pressurizedand the rods in the two other assemblies would be vented to atmosphere. The level of

pressurization is yet to be determined. The use of commercial fuel rod tubing, end plugs,and surrogate ceramic fuel pellets in the peripheral assemblies provides the opportunity tobackfill the rods with inert gas in prototypic commercial fashion. The end plugs will be laserwelded onto the tube. The pressurized rods are filled through a small hole in the top endplug, which is then laser welded closed. Vendors have been identified that have laboratorycapability for performing these fabrication procedures. If agreements cannot be institutedwith these vendors, in-house capability would need to be developed at SNL.

An important design issue for a ballooning rod is the choice of fill material that accuratelyrepresents the thermal mass of an actual spent fuel rod without altering the ballooningcharacteristics. MgO has been identified as having a higher thermal mass per volume (Cp)with a temperature dependence that parallels UO2 fuel pellets. Because the thermal mass ofMgO is greater it can be adjusted downward to accurately match UO2 by incorporating voids.

The packed MgO powder with 25% voids used in the heater rod design has been shown toaccurately match spent fuel. However, an issue with this approach is that the pore space inthe compacted MgO may be so small that ballooning would be inhibited.

Another possible design for the rods in the peripheral assemblies is shown in Figure 14.The ceramic fill is composed of high density magnesium oxide pellets. The pellets would becustom made so that the outer diameter allows easy installation into the tubes but the gasgap is not large enough to hinder thermal conduction into the pellet. The inner hole is sizedso that the thermal mass of the ceramic pellets matches the thermal mass of spent fuel asshown in Figure 2. Other design concerns such as the effect of the stored energy of the gaswill also need to be considered.

Single rod balloon testing will be required in order to sort out the best design. A tubular

muffle furnace will be used to heat sections of pressurized simulated fuel rods of variousdesigns to point of ballooning. A control design will incorporate solid high density alumina ormagnesia ceramic pellets so that the plenum volume is represented prototypically. Thenature of ballooning of the various test rods will be compared to the control rod and used asa factor in determining the best design for the Phase 2 experimental apparatus.


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Figure 14 Ballooning rod simulator design.

3. Time Schedule and Indicative Cost

The proposed project is expected to take thirty nine months to complete and is estimated tocost $5.2 M US. The proposal includes two full scale test apparatuses. Each test apparatuswill be subjected to a battery of pre-ignition testing and a final destructive ignition test. In thesecond test apparatus, two of the five assemblies will be constructed with pressurized rodsso as to incorporate rod ballooning phenomena.

3.1 Time Schedule of Deliverables

Bottom

end plug

Top end

plug with

gas fill

hole

Spring

Zircaloy

tube

High density

MgO ceramic


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Table 1 Time schedule of deliverables.

Testing Phase Deliverable TypeTime from ProjectStart

Phase 1 (Test Plan) Letter Report 3 mo.Phase 1 (Test Report) SAND Report 12 mo.Phase 2 (Ballooning Rod

Design)Letter Report 18 mo.

Phase 2 (Test Plan) Letter Report 20 mo.Phase 2 (Test Report) SAND Report 39mo.

3.2 Cost Summary

Phase 1: Single full length 1717 PWR with Zirlo clad heater rods

Technical Summary Measure thermal response and naturally induced flow for various

assembly powers

Conclude with ignition experiment

Major Material Estimate Summary ($200k) Zircaloy tubing Heater fabrication PWR skeleton Instrumentation

Labor Estimate Summary ($1,300k) Two engineers Three technicians

Phase 2: 14 full length 1717 PWRs with one heated center and four unheatedperipheral assemblies. Two peripheral assemblies have pressurized rods.

Technical Summary Develop ballooning rod simulator Pressurize fuel rods in two peripheral assemblies to characterize

ballooning Measure thermal response and naturally induced flow for various

assembly powers Conclude with ignition experiment that includes rod ballooning in

peripheral assemblies

Major Material Estimate Summary ($1,400k)

Zircaloy tubing Heater fabrication Peripheral rod fabrication Peripheral rod pressurization for ballooning PWR skeletons Instrumentation

Labor Estimate Summary ($2,300k)


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Two engineers Three technicians

4. Addendum 1: Analysis Support

An objective of the thermal-hydraulic and ignition characterization of the PWR assemblies is

to provide prototypic data for code validation. The NRC has previously developed computercodes for the analysis of spent fuel response to a range of accident conditions. In theabsence of prototypic validation data, the inputs were specified using available geometricdata and textbook correlations. The codes were subsequently used for a variety of studiesto characterize the response of spent fuel in casks and spent fuel pools (SFPs) to accidentconditions. With the generation of the thermal-hydraulic data in the recent BWR SFP andthe proposed PWR experimental programs, a higher degree of confidence can be developedon the appropriateness of the models in the codes and the accuracy of the results.

In the previous BWR SFP experimental program, an analysis task was integrated into theprogram to support the experimental design with pre-test predictions and post-test analysis.The experimental design analysis was used to answer a range of design questions such asthe magnitude of the heat loss versus the insulation thickness and optical port size, the

impact of MgO as a surrogate for UO2, the impact of a linear power profile versus aprototypic axial profile, the impact of heater rod failure (i.e., the decay heat source) on thetest progression, the impact of reduced oxygen concentration on partial length tests, etc.The test program also allowed improvements to be made in the analytical model. Forexample, the use of the prototypical assembly and rack in the experimental program resultedin precise calibration of component weights, dimensions, and design for input to theanalysis code. The characterization of the hydraulic tests gave new insights into laminarflow losses and the role of flow in the water rods that was not previously available. Finally,the thermal testing gave invaluable insights into the radiative emissivity of oxidized surfaces,the effective thermal resistance of the canister and rack wall, three-dimensional effects, andthe appropriateness of the simplified correlation for breakaway oxidation derived fromisothermal thermogravimetric (TGA) tests conducted at SNL.1

A similar analysis effort is proposed for the PWR thermal-hydraulic and ignition testing. Theanalysis effort would include three components, (1) experimental design analysis, (2) pre-test predictions, and (3) post-test analysis. The NRC will use MELCOR code. TheMELCOR code is based on a building block design (i.e., a user-specified arrangement ofcontrol volumes, flow paths, and heat conductors). The code includes specialized models torepresent the primary thermal radiation pathways for SFP or cask geometry. MELCOR alsoincludes an empirical lifetime breakaway oxidation model that was developed from SNLtesting. The code currently does not include a thermal-mechanical model for rod ballooningbut does have provisions for variable inertial and viscous resistance as a function of othercalculated parameters. A simplified rod ballooning model is a desired modeling outcomefrom the testing program.

Other severe accident codes are expected to be utilized by other project participants. Inorder to facilitate the comparison of these various codes, a detailed test plan for each testphase including boundary and input conditions will be made available to project participants.Likewise, the test results for each test phase will be shared with each project participant toallow comparison with the pre-test predictions. This coupling of prototypic testing and

1 Natesan, K., and Soppet, W.K. Air Oxidation Kinetics for Zr-Based Alloys,

NUREG/CR-6846, Argonne National Laboratories, June 2004.


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advanced modeling is expected to generate greater confidence in the ability to evaluatethese accident scenarios..

5. Addendum 2: Addressing CSNI Expert Recommendations

A CSNI Experts meeting was held in Paris to review this proposal on September 9 and 10,2008. The purpose of this addendum is to summarize how the recommendations will be

addressed in the project. These recommendations are documented in Section 9 of theSummary Record of the Experts Meeting on the Proposed OECD Sandia Fuel Project(SFP)Paris 9-10 September 2008. From an experimental implementation perspective themost pertinent recommendations are found in items 3, 4, 6, 7, 8, 9, 10, and 17 and are listedbelow:

3. The materials employed are appropriate. The option to use Zirlo or Zr-4 as cladding

material is to be discussed.

4. The material of the cell is important to keep for as long as possible the geometry of the

cell after ignition (for Phase 1 testing).

6. Some aspects of instrumentation will need further discussion. In particular:

a. The use of high-temperature thermocouples in addition to the K-type should be

explored.

b. Where feasible, the Nitrogen consumption/release should be traced.

7. The potential for using guide tubes as instrument carrier was encouraged.

8. SNL will check the possibility to expand the laser velocimetry measurements to the

thermal-hydraulic, pre-ignition test in Phase 1.

9. Options for in-rod pressure measurements (e.g. bellows/LVDT coupling) alternative to

strain gauges should be considered.

10. Some post-test analyses should complement the on-line experimental data.

17. The power level is an important parameter that will need to be discussed in the Project.

Two other recommendations are concerned with numerical modeling, items 13 and 15.

13. The programme is to be devised to support the validation of a variety of computer

codes of interest to participants (not only MELCOR).

15. The use of some sets of experimental data for CFD applications should be considered.

These items are addressed in turn below. Note that many of these items are interrelated.

Item 3.

Two zirconium alloys are under consideration for use in the pressurized ballooning roddesign, Zr-4 and ZIRLO. ZIRLO is a modern alloy incorporating niobium developed byWestinghouse and is similar to M5, the modern alloy developed by Areva. Of concern is thatthe niobium may lower the melting point of the ZIRLO and M5 alloys such that ballooning

occurs at a lower temperature than with Zr-4. Modeling of the ballooning phenomena willrequire material specific temperature dependent creep data. Creep data for Zr-4 is publicallyavailable. Creep data for ZIRLO is proprietary Westinghouse information. In order toconsider using ZIRLO Sandia would have to obtain the creep data from Westinghouse undera nondisclosure agreement. Under the nondisclosure agreement, Sandia may not be able toshare the data with other participants. At least one other participant already has access tothe data but also can not share it. If the ballooning models of participants cannot beadequately accommodated with ZIRLO data, full assembly construction using Zr-4 will beconsidered.


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Consideration will also be given to using both materials. One pressurized assembly wouldbe built with Zr-4 the other pressurized assembly would be built of ZIRLO. The twounpressurized control assemblies would be split by material, one Zr-4 the other ZIRLO. Bythis approach a direct comparison between Zr-4 and ZIRLO may be possible, althoughexperimental symmetry may be compromised.

Item 4.

The storage cells typically used in industry are made of stainless steel which melts at~1700K. The cells used in the BWR ignition tests were made of stainless steel. The meltingof the cell raises three experimental issues: 1) Relocation of the molten steel mayaccelerate the downward movement of the burn front. While this is a prototypicphenomenon, the recommendation is to consider cell material options that could possiblyeliminate molten metal relocation in order to simplify the modeling. 2) Once the storage cellmelts, air can be drawn radially into the assembly through the insulation. This is notprototypic and may have unpredictable consequences on the burn front behavior. 3) Theloss of the storage cell integrity contributes to the difficulty of extinguishing the burn at theend of the experiment. The full length BWR ignition test could not be extinguished byblocking the air inlet. More than enough air was drawn through the insulation to keep thezirconium burning for days afterwards. Other chromium/nickel steels (Hastelloy, Incoloy,

Inconel, Invar, or Kovar) all melt by ~1700 K. To be useful, the material must be viable attemperatures up to at least 2000 K. This requires a ceramic material.The feasibility of accommodating this feature will be evaluated as part of this project. Anumber of technical issues can be anticipated. High purity alumina can be fashioned intocustom shapes like the rounded corner square tube required but an alumina storage cellfabricated as a 4 m long single piece would not be possible. There would need to be jointsapproximately every 0.5 m that will need to be sealed. The implementation of axiallydistributed gas sample ports is complicated by the use of a ceramic material.Instrumentation of the storage cell with thermocouples is also more complicated. However,if successfully implemented, the use of a ceramic storage cell would likely mitigate the threeexperimental issues listed above.

Alternatively, a secondary containment around a stainless steel cell is being considered.

This containment could be fashioned from a pipe or duct set around the cell with insulationbetween the two. This solution would address the radial ingress of air and the ability toextinguish the fire but not the relocation of molten metal from the cell.

Items 6 and 7.

The guide tubes will make good access points for some high temperature thermocouples(TCs). A typical spent PWR assembly would likely have something located inside the guidetubes such as a burnable absorber assembly or a control rod assembly. The appropriateamount of ceramic will be placed in the guide tubes to simulate the thermal mass of a typicalassembly. Some of this ceramic can be used as insulators for high temperature platinum(Pt) type TCs. Platinum melts at ~1900 K which is much better than the type K (~1400K) butnot high enough to survive the 2000 K temperatures of the burn. This has serious

implications for the desired instrumentation. The 24 guide tubes are most easily accessedfrom the top. However, any TCs routed down a guide tube from the top with a sensingjunction located below the ignition point will be lost. This means TCs routed from the top areuseful for measuring the upward burn rate but not the downward burn rate. The PWR isexpected to have a better chance of an upward burn component than seen in the BWR sosome (~10) Pt type TCs at the top of the assembly would be desirable.

The TCs need to be routed up from the bottom of the assembly in order to measure thedownward burn rate. Unfortunately, space is more constrained at the bottom of the


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assembly because restrictions to natural draft flows must be minimized. The number of Pttype TCs routed up from the bottom of the assembly should be limited to 2 to 4.

Tracking nitrogen consumption will require simultaneous measurement of nitrogen andargon concentrations. Essentially argon is used as a nonreactive tracer. If nitrogen isconsumed, the ratio of argon to nitrogen will increase. This type analysis can be done byrelatively simple packed column gas chromatography in about six minutes as described

recently in the literature.2 Consideration will be given to setting up on-line instrumentation orthe collection of samples for off-site analysis.

Item 8.

The incorporation of a limited number of optical windows in the bottom portion of thestainless steel storage cell used in the first pre-ignition Phase 1 test should be feasible.

Item 9.

Alternatives to strain gauges, such as end-tube LVDTs, for in-rod pressure measurementswill be considered if strain gauges prove ineffective in the single rod balloon testing.

Item 10.

Post-test inspection of the burnt assemblies will be performed. Special attention will begiven to the Phase 2 test to determine the location and extent of ballooning. Detailed post-test chemical analyses will be of most value if the ignition test can be successfullyterminated (i.e. the burn extinguished) at desired point in time. Detailed post-test chemicalanalyses were not conducted on the BWR specimens in part because the burn could not bestopped and most instrumentation was lost days before the experiment extinguished. Basedon the experience of the BWR burn tests, consideration will be given to insuring that thePWR burn tests can be extinguished at a desired point in the experiment. Using a ceramicstorage cell or secondary containment in the Phase 1 ignition test should help withextinguishing the burn.

2Sutour, C., Stumpf, C., Kosinski, J-P. Surget, A., Hervou, G., Yardin, C., Madec T., and

Gosset, A., Determination of the argon concentration in ambient dry air for the calculation of

air density, Metrologia 44 (2007) 448452.


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Item 17.

The power level options under consideration will be provided to all participants andcomments will be encouraged. The ultimate decision on power level will be made by Sandiaand USNRC.

Item 13.

The same data set available for MELCOR modeling will be made available to all participantswith the caveat that any proprietary Westinghouse information will be protected according toapplicable non-disclosure agreements. Sandia would cooperate with providing additionaldata to support other participant modeling efforts. This would likely apply to the ZIRLOcreep data and geometric specifics of the spacer mixing vanes.

Item 15.

There is no specific plan or funding available for Sandia to perform CFD modeling.However, most available CFD codes are capable of modeling the separate effects to bestudied as part of the test programme. Sandia would cooperate with providing the data tosupport other participant CFD modeling efforts subject to applicable non-disclosureagreements.

the oecd-nea sfp projectan experimental programme and related analyses forthe characterization of...

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