EBR-II SP1 Experiments

construction:UN assessments under active modification

All nitride assessment cases are undergoing active modifications. At this time, the documenation here does not fully reflect the models in the input files. New documation that will be available soon will include improvements due to the new UNSifgrs model implementation.

Overview

The SP1 test was an accelerated irradiation test designed to demonstrate the irradiation performance of Nb-1%Zr clad UN fuel pins for the SP-100 space propulsion program. The UN pellets were loaded into Nb-1%Zr cladding tubes with a tungsten liner, irradiated in a fast-test reactor, EBR-II, and destructively examined after 0.8 at% burnup. Pin geometry, irradiation conditions, fuel swelling, fission-gas release, and microstructural data were published in (Dutt et al., 1984; Dutt et al., 1985; Matthews et al., 1988; Cowan et al., 1991).

Test Description

The SP1 test consisted of several helium-bonded UN and UO $var element = document.getElementById("moose-equation-1e8f6788-6d20-4c7c-a4f1-23f61234c2df");katex.render("_2", element, {displayMode:false,throwOnError:false});$ pins that were irradiated as part of the SP-100 program at high temperatures and relatively low burnup for space propulsion applications. Two of these pins with UN pellets were destructively examined and swelling and fission gas release were measured. These two pins, NBU-2 and NBU-3, are listed as assessment cases in BISON for UN fuel. The NBU-2 and NBU-3 pins had the same rod design and similar irradiation condition, with only the end burnup changing from 0.74 at% for NBU-2 to 0.81 at% for NBU-3.

Rod Design Specifications

The fuel was solid cylindrical pellets of UN. The pellet densities were around 87% TD, with calculated smear densities of around 80%. The pellets were encased in Nb-1%Zr clad with a chemical-vapor-deposited (CVD) tungsten liner. Table 1 lists the rod design specifications specifying if the value comes from a direct measurement or was unknown and had to be estimated or calculated.

Table 1: SP-1 Rod Geometry

Parameter	Value	Units	Source
Clad material	Nb-1%Zr		Matthews et al. (1988)
Bonding	He		Matthews et al. (1988)
Pin length	162.7	mm	Matthews et al. (1988)
Clad OD	7.62	mm	Matthews et al. (1988)
Clad thickness	0.635	mm	Matthews et al. (1988)
Liner length	142.2	mm	Matthews et al. (1988)
Liner OD	6.25	mm	Matthews et al. (1988)
Liner thickness	0.127	mm	Matthews et al. (1988)
Diametral gap	0.254	mm	Matthews et al. (1988)
Pellet diameter	5.842	mm	Matthews et al. (1988)
Fuel stack	76.2	mm	Matthews et al. (1988)
Top/bottom plug	4.8	mm	Estimated from model in Matthews et al. (1988)
Bottom gap under pellet	0.2	mm	Estimated
Plenum height	76.9	mm	Calculated (Pin length-Fuel stack-2*Top/bottom plug-Bottom gap under pellet)
Plenum pressure	12.4	MPa	Estimated (Blank (2006) max pressure allowed)
Smear density	80	%TD	Matthews et al. (1988)

Operating Conditions and Irradiation History

Being tested in EBR-II, an irradiation test vehicle was used for the SP-1 tests to increase the obtainable temperature of the fuel pin cladding (Dutt et al., 1984). Cladding temperatures of 1300 to 1500 K were therefore obtained to be relevant to space propulsion conditions.

The actual power history for specific EBR-II experiments is still being determined. Therefore, a simplified power history containing an initial ramp to power and hold for a given amount of time with a final power down is being used. The average burnup of the fuel at the end of the simulation is used as a check that the power history is reasonable. The parameters for the irradiation and temperature history are detailed in Table 2. The final burnup reached in the BISON simulations is 0.80 at.%.

The exact temperature history is also still being determined. In the meantime, the initial temperature is fixed to 298 K, and the temperature ramps up to the desired profile for the irradiation time before ramping down at the end of the simulation.

Table 2: SP-1 Operating Conditions

Parameter	Value	Units	Source
Fast fluence	$var element = document.getElementById("moose-equation-54c3a1b1-762e-44b7-a99a-f6702ab9acf9");katex.render("1.2\\times 10^{22}", element, {displayMode:false,throwOnError:false});$	n cm $var element = document.getElementById("moose-equation-5afbf8e8-6b57-48dd-96f0-f9445c4061a7");katex.render("^{-2}", element, {displayMode:false,throwOnError:false});$	Matthews et al. (1988)
Peak power	80	W/g	Matthews et al. (1988)
Burnup (NBU-2)	0.74	at.%	Matthews et al. (1988)
Burnup (NBU-3)	0.81	at.%	Matthews et al. (1988)
Irradiation duration	96	Equivalent full power days	Matthews et al. (1988)
Coolant $var element = document.getElementById("moose-equation-4c5d95ea-8715-4f15-93d7-9edba96fb214");katex.render("T_{in}", element, {displayMode:false,throwOnError:false});$	1290	K	Cowan et al. (1991)
Coolant $var element = document.getElementById("moose-equation-aa4c40fd-646e-4a16-8128-7e226bcfb602");katex.render("T_{out}", element, {displayMode:false,throwOnError:false});$	1350	K	Cowan et al. (1991)
Average cladding temperature	1500	K	Matthews et al. (1988)
Average fuel temperature	1950	K	Matthews et al. (1988)

Model Description

The NBU-2 and NBU-3 pins from the SP1 tests are modeled in BISON in two different ways:

In one assessment case, only the fuel stack of the NBU-2 and NBU-3 cases is simulated. This case is called SP1_Fuel_focus.
In the other one, the cladding and the liner are also simulated. This case is called SP1_Pin.

The specifications and conditions for both cases are described below. Note that the irradiation test vehicle is not being modeled.

Temperature Profile

For the case SP1_Fuel_focus, the temperature at the surface of the fuel is fixed constant at the average fuel temperature calculated in (Matthews et al., 1988).

For the case SP1_Pin, since the exact temperature profile on the surface of the cladding is unknown, it is assumed to follow a second degree polynomial $var element = document.getElementById("moose-equation-9fc981e2-f2ad-4de2-94a3-d2c6783092af");katex.render("T(x) = ax^2 + bx + c", element, {displayMode:false,throwOnError:false});$ with $var element = document.getElementById("moose-equation-2ff8e0ed-1888-4051-9880-74c061d28fb9");katex.render("x", element, {displayMode:false,throwOnError:false});$ being the position along the cladding, and $var element = document.getElementById("moose-equation-9514b1ef-58e2-45ed-a4db-c094dc853fc2");katex.render("a", element, {displayMode:false,throwOnError:false});$ , $var element = document.getElementById("moose-equation-f56a1333-cb1d-4a9b-bb8b-f7eed44f62fd");katex.render("b", element, {displayMode:false,throwOnError:false});$ , and $var element = document.getElementById("moose-equation-394e07cd-2ea5-4d43-8955-6713c3fb9954");katex.render("c", element, {displayMode:false,throwOnError:false});$ constants defined to reproduce the provided inlet, outlet, and average temperature in Kelvins provided in Table 2.

Power Profile

Concerning the power profile, the SP1 tests were performed in EBR-II. However, since the exact power profile is still unknown, it is assumed to be homogeneous along the fuel stack for both SP1_Fuel_focus and SP1_Pin cases.

Pressure Profile

No pressure is accounted for on the surface of the fuel pellet in the SP1_Fuel_focus case.

The coolant pressure in the SP1_Pin case is unknown. The coolant is made out of Lithium, and the pressure is assumed to be equal to 15.1 MPa.

Geometry and Mesh

For the case SP1_Fuel_focus, the 2D-RZ mesh for the fuel stack is generated with the internal rodlet meshing capability in BISON RodletMeshGenerator.

For the case SP1_Pin, The 2D-RZ mesh for the assessment case is generated with the internal smeared pellet meshing capability in BISON FuelPinMeshGenerator, which is able to model the Nb-1%Zr cladding with the tungsten liner.

All of the dimensions and meshing details are contained in the Mesh block.

Material and Behavioral Models

The following material and behavioral models for the UN fuel were used:

MNElasticityTensor: Computes Young's modulus and Poisson ratio for MN fuel
MNThermalExpansionEigenstrain: Computes an eigenstrain due to thermal expansion for MN fuel
MNCreepUpdate: Creep mechanical properties and deformation behavior for MN fuel
MNThermal: Calculates the thermal conductivity and specific heat for MN fuel
MNThermalExpansionEigenstrain: Calculates thermal expansion coefficient and isotropic expansion for MN fuel
MNVolumetricSwellingEigenstrain: Computes swelling due to solid and gaseous fission products for MN fuel

For the case SP1_Pin, the following material and behavioral models for the Nb-1%Zr (ASTM B391 Grade R04251) cladding were used:

ComputeIsotropicElasticityTensor: Computes isotropic elastic mechanical properties for generic material (elastic modulus of 68.9 GPa (ASM, 1993), Poisson's ratio of 0.4).
ComputeThermalExpansionEigenstrain: Computes eigenstrain due to thermal expansion with a constant coefficient (7.54 10 $var element = document.getElementById("moose-equation-ce9f2b43-1a42-4db5-afd3-e48d1808e380");katex.render("^{-6}", element, {displayMode:false,throwOnError:false});$ K $var element = document.getElementById("moose-equation-5aca1224-d66c-4773-937e-c6511b0ca31e");katex.render("^{-1}", element, {displayMode:false,throwOnError:false});$ for T $var element = document.getElementById("moose-equation-6e61daf3-0a4a-479f-896a-758a2ad66a76");katex.render("\\in", element, {displayMode:false,throwOnError:false});$ $var element = document.getElementById("moose-equation-d964390c-e511-42a1-828d-61ef5c4fcc4c");katex.render("[", element, {displayMode:false,throwOnError:false});$ 293.15, 673.15 $var element = document.getElementById("moose-equation-98f00a8d-0de3-4ec6-8972-df55bdc77727");katex.render("]", element, {displayMode:false,throwOnError:false});$ K (Cverna and Committee., 2002; ASM, 1993)).
StrainAdjustedDensity: Computes density for a generic material (8590 kg/m $var element = document.getElementById("moose-equation-7d28b7ef-23aa-4491-b26d-176248d72120");katex.render("^3", element, {displayMode:false,throwOnError:false});$ at room temperature (Robbins and Finger, 1991; Cverna and Committee., 2002; ASM, 1993)).
HeatConductionMaterial: Computes thermal conductivity and specific heat capacity for generic material. In the case of Nb%Zr, the thermal conductivity is set as a constant 41.9 W/(m.K) (measured at 298.15 K) (Cverna and Committee., 2002; ASM, 1993) and the specific heat capacity is defined as a constant 270 J/(kg.K) (measured at 293.15 K) (Cverna and Committee., 2002; ASM, 1993)).

For the case SP1_Pin the following material and behavioral models for the tungsten liner were used in these cases:

ComputeIsotropicElasticityTensor: Computes the elasticity tensor of tungsten
TungstenThermalExpansionEigenstrain: Computes eigenstrain due to thermal expansion of tungsten
TungstenThermal: Computes the thermal properties of tungsten
StrainAdjustedDensity: Computes density for a generic material (19300 kg/m $var element = document.getElementById("moose-equation-3e896ea7-1462-409f-8f10-b66543a6cd78");katex.render("^3", element, {displayMode:false,throwOnError:false});$ at 293.15 K (Cverna and Committee., 2002)).

For the case SP1_Pin, thermal and mechanical contacts are modeled using mortar contact.

Input files

The input files for both SP1_Fuel_focus and SP1_Pin cases are located at bison/assessment/nitride/EBRII/SP1/analysis.

Results Comparison

The post-experiment examinations of the NBU-2 and NBU-3 pins provide the measurements listed in Table 3 and Table 4, respectively.

Table 3: SP-1 Measurements for NBU-2

Quantity	Measurements	Units	Source
FGR Xe	5.7	%	Matthews et al. (1988)
FGR Kr	5	%	Matthews et al. (1988)
FGR total	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-3559713f-e938-4921-a2ed-6fc7c20f9cbb");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-7d2156c1-57ee-4e73-9226-360f72e132fd");katex.render("_0", element, {displayMode:false,throwOnError:false});$	3.5	%	Matthews et al. (1988)
Fuel swelling $var element = document.getElementById("moose-equation-b33dda9e-818e-472b-8bfe-dadf4fea5066");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-6febc9eb-97aa-4fd6-bc70-18cc068a456e");katex.render("_0", element, {displayMode:false,throwOnError:false});$	8.9	%	Matthews et al. (1988)
Cladding strain $var element = document.getElementById("moose-equation-b6859c90-ad6e-44bd-83c7-b800d87a1790");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-9522be3f-dba1-466e-bcb2-6366b04e3f29");katex.render("_0", element, {displayMode:false,throwOnError:false});$	0.23	%	Matthews et al. (1988)
Fuel density decrease	Unknown
Change in cladding length	1.2	%	Matthews et al. (1988)
Change in cladding OD	3.6	%	Matthews et al. (1988)

Table 4: SP-1 Measurements for NBU-3

Quantity	Measurements	Units	Source
FGR Xe	7.9	%	Matthews et al. (1988)
FGR Kr	7.1	%	Matthews et al. (1988)
FGR total	Unknown
Fuel swelling $var element = document.getElementById("moose-equation-d9f71c1a-b6fc-4815-b7a5-b28ad3547f55");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-9441a27f-03cc-4d51-9ba1-59c1816016c3");katex.render("_0", element, {displayMode:false,throwOnError:false});$	3.7	%	Matthews et al. (1988)
Fuel swelling $var element = document.getElementById("moose-equation-cc036263-1921-41e2-a5ec-1e30ca45a0c1");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ V/V $var element = document.getElementById("moose-equation-ccdcdb6c-8a2f-4a6e-ab3b-0ed1f8ae3e55");katex.render("_0", element, {displayMode:false,throwOnError:false});$	8.3	%	Matthews et al. (1988)
Cladding strain $var element = document.getElementById("moose-equation-717a4d72-1e10-44f3-8005-e7ca90cc8721");katex.render("\\Delta", element, {displayMode:false,throwOnError:false});$ D/D $var element = document.getElementById("moose-equation-3d9124e3-6d7e-4282-900e-3471f19fde18");katex.render("_0", element, {displayMode:false,throwOnError:false});$	0.13	%	Matthews et al. (1988)
Fuel density decrease	Unknown
Change in cladding length	1.2	%	Matthews et al. (1988)
Change in cladding OD	3.6	%	Matthews et al. (1988)

Discussion

The BISON simulation results have not yet been compared to the post-experiment examinations data.

References

ASM. ASM handbook Volume 2 - Properties and selection: Nonferrous alloys and special-purpose materials. Volume 2. ASM International, 1993. ISBN 978-0-87170-378-1.

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Hubert Blank. Nonoxide Ceramic Nuclear Fuels, chapter, pages. John Wiley & Sons, Ltd, 2006. URL: https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527603978.mst0108, doi:10.1002/9783527603978.mst0108.

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Overview
Test Description
Model Description
Input files
Results Comparison
Discussion
References