MODELING OF INTERACTION LAYER GROWTH BETWEEN U-Mo PARTICLES AND AN Al MATRIX

(1)

MODELING OF INTERACTION LAYER GROWTH BETWEEN U-Mo PARTICLES AND AN Al MATRIX

YEON SOO KIM^1*, G.L. HOFMAN¹, HO JIN RYU², JONG MAN PARK³, A.B. ROBINSON⁴, and D.M. WACHS⁴

1Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 USA

2Dept. of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon 305-701 Republic of Korea

3Korea Atomic Energy Research Institute, 989-111 Daedeokdaero, Yuseong-gu, Daejeon 305-353 Republic of Korea

4Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-6188 USA

*Corresponding author. E-mail : yskim@anl.gov

Received October 31, 2013

1. INTRODUCTION

U-Mo alloy fuel is a prime candidate for conversion of high-power research and test reactors, from HEU cores to LEU cores, thanks to its high density and advantageous irradiation performance. Two forms of U-Mo alloy fuel have been studied in the U.S. GTRI-CONVERT program (formerly known as RERTR): One is U-Mo particle dispersion in an Al matrix (U-Mo/Al dispersion fuel); the other is direct bonding of U-Mo thin foil to Al-alloy cladding (U-Mo monolithic fuel) (Fig. 1). In this paper, because modeling the interaction layer growth between the U-Mo particles and Al matrix was concerned, we focused only on the dispersion fuel form.

One of the intrinsic characteristics of the U-Mo/Al dispersion fuel is perhaps the interaction layer (IL) growth between the U-Mo fuel particles and matrix Al. Interaction layer growth is due to interdiffusion between the fuel particle (U and Mo) and matrix (Al). Hofman and co-workers [1]

reported the first observation of IL growth in U-Mo/Al from an irradiation test, where they also proposed a corre-

Interaction layer growth between U-Mo alloy fuel particles and Al in a dispersion fuel is a concern due to the volume expansion and other unfavorable irradiation behavior of the interaction product. To reduce interaction layer (IL) growth, a small amount of Si is added to the Al. As a result, IL growth is affected by the Si content in the Al matrix. In order to predict IL growth during fabrication and irradiation, empirical models were developed. For IL growth prediction during fabrication and any follow-on heating process before irradiation, out-of-pile heating test data were used to develop kinetic correlations. Two out-of-pile correlations, one for the pure Al matrix and the other for the Al matrix with Si addition, respectively, were developed, which are Arrhenius equations that include temperature and time. For IL growth predictions during irradiation, the out-of-pile correlations were modified to include a fission-rate term to consider fission enhanced diffusion, and multiplication factors to incorporate the Si addition effect and the effect of the Mo content. The in-pile correlation is applicable for a pure Al matrix and an Al matrix with the Si content up to 8 wt%, for fuel temperatures up to 200 ºC, and for Mo content in the range of 6 – 10wt%. In order to cover these ranges, in-pile data were included in modeling from various tests, such as the US RERTR-4, -5, -6, -7 and -9 tests and Korea’s KOMO-4 test, that were designed to systematically examine the effects of the fission rate, temperature, Si content in Al matrix, and Mo content in U-Mo particles. A model converting the IL thickness to the IL volume fraction in the meat was also developed.

KEYWORDS : U-Mo, U-Mo/Al, Dispersion Fuel, Interaction Layer, IL Growth, Modeling, In-pile Data

Fig. 1. Schematic Illustration of Cross Section Views of Dispersion Fuel and Monolithic Fuel Forms

(2)

lation for IL growth expressed as a function of the fission rate, temperature, and time. Hayes [2] later incorporated this model in a computer model PLATE. Ryu and co- workers [3] systematically measured the kinetics of IL growth and obtained the activation energy for IL growth for out-of-pile heating tests.

U-Mo/Al dispersion fuel performed well in irradiation tests at lower power and burnup, so its qualification appeared near [4]. However, later tests adopting higher power, i.e., higher fission rate and higher burnup, showed enhanced fuel-meat swelling, which was caused by massive IL growth, and in some cases, by large pore formation in the fuel meat [5],[6]. The pores were distinguishable from the fission gas bubbles in the fuel particles, in that they mostly formed outside of fuel particles, i.e., in the ILs, although both were commonly filled with fission gases. The formation of IL results in a net volume increase and a net thermal conductivity decrease in the meat. The low thermal conductivity of IL provides adverse feedback for higher IL growth by further increasing the fuel meat temperature. While U- Mo particles and Al are crystalline, ILs formed between U-Mo and Al are amorphous during irradiation [7],[8]. The amorphous nature of the ILs is believed to be the reason for large pores formed outside of fuel particles [9]. Fission gas diffusivity in the amorphous ILs is higher than in its crystalline state, and viscosity is lower, both of which promote fission-gas-pore formation in the ILs.

A remedy was needed to reduce the IL growth rate and strengthen the IL to resist against pore formation and growth. Noticing the low IL growth between U3Si2/Al dispersion that did not form pores in the IL until full LEU burnup (see [10],[11]), it was proposed at ANL that a small amount of silicon be added to the aluminum matrix [12],[13]. An ample number of studies, both out-of-pile ([14]-[25]) and in-pile ([26]-[42]), have been conducted to examine the effectiveness and better application of the remedy. The common findings were that Si addition to Al not only reduced IL growth, but also delayed pore formation and growth in the IL. Hence, overall performance of this fuel was improved.

Since IL growth is one of the deciding performance variables for U-Mo/Al dispersion fuel, an accurate model is also required. The studies for interaction layer (IL) growth modeling for in-pile applications includes two cases: one for the pure Al matrix case [1],[2],[43],[45], and the other for the Si-added matrix case [28],[48]. The most up-to-date correlation for the pure Al matrix case was by Wachs [45], which was confirmed as accurate by Kim [48]. The correlation for the Si-added matrix case, developed mostly on low-temperature miniplate test data, was applied for all Si contents larger than 2 wt%. There was no need to provide a separate factor to consider the effect as a function of the Si content, because in the low temperature tests the addition of 2 wt% Si virtually suppresses IL growth to the lowest possible level. However, it has been noticed that the modeling under-predicts the high temperature

test results from the RERTR-9 and KOMO-4. The high temperature test KOMO-4 showed that a Si content higher than 2 wt% had different IL growth kinetics [39],[49].

Therefore, a model applicable to the high temperature regime and a Si content higher than 2 wt% was necessary.

Another objective of the present work is to unify the existing separate models for the pure Al matrix and the Al matrix with Si addition.

An interaction layer can also grow during fabrication and the follow-on blister test when the fuel plates undergo heat processes. An IL growth model in this step was also required.

2. MODELS FOR OUT-OF-PILE INTERACTION LAYER GROWTH

2.1 Al Matrix with no Si Addition

IL growth during fuel fabrication and any follow-on heating test is modeled in this section. The interaction layer growth between U-Mo and Al is a typical interdiffusion process between U and Mo atoms versus Al atoms. Hence, in this paper, interaction and interdiffusion are used inter- changeably. In addition, the rate-controlling process in interaction layer growth is interdiffusion, not the reaction product formation [11]. The out-of-pile IL growth takes the following modified Arrhenius correlation because interdiffusion is due to thermal activation:

where YAlis the average IL thickness (µm) in a pure Al matrix, i.e., Al matrix without any intentional Si addition, T is the fuel meat temperature in K, and t is the time in s.

The constants have been fitted using the data taken from Ryu [3] (Table 1). Only data from dispersion fuel tests were used. The IL thickness was measured using optical micrographs of the cross section of a sample. From an optical micrograph, several IL thicknesses were measured, and the average value was used as the nominal value.

The method to obtain the true IL thickness is explained elsewhere [50]. Generally, in an optical micrograph, the common thinnest IL on the largest particle was taken to represent the true IL thickness.

The RERTR test-plates were fabricated using the hot rolling technology [51], in which the heat processes involved were much shorter, ~1 h, but at a much higher temperature, 500 ºC, than in-pile tests (Table 2, Table 3). For example, the test duration for the RERTR-4 test was 257 d, and the life-average fuel temperature was ~100 ºC.

IL growth during fabrication is hard to assess, so the correlation given in Eq. (1) is for an average value. The largest uncertainty during fabrication is due to the irregular contact pressure between particles with the matrix. For a (1)

(3)

typical rolling process used for RERTR test plates [51], the bonding between the U-Mo particles and matrix is poor during the pre-heating and first pass, so IL growth during these steps is virtually negligible. Meaningful bonding is achieved after the second pass and subsequent IL growth occurs during the last heating ~15 min (see [51] for detailed time and temperature for each step). The most IL growth takes place during the subsequent blister-annealing test at 485 ºC for an hour. Using Eq. (1), we estimated an average of ~1.5 µm of IL growth during fabrication. The test

samples of the KOMO-4 test from KAERI were produced using an extrusion process taking 0.5 hour at 400 ºC, for which Eq. (1) predicts negligible IL thickness. Table 2 compares the heat processes and IL growth involved during fabrication of some RERTR tests and the KOMO-4 test.

The Mo content of the U-Mo particles is in the range of 6 – 10 wt%. However, the most common Mo contents are 7 wt% and 10 wt% in the RERTR irradiation tests. In the Mo-content 10 wt%, the thermodynamic equilibrium phase of the U-Mo is the α-U + δ-U2Mo dual phase at

Ryu Ryu Ryu Park Iltis Iltis Iltis Iltis Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser Keiser

a: Fuel kernels are all U-7wt%Mo atomized particles except for Ryu’s U-10wt%Mo atomized particles b: Fabricated at 400 ºC by co-extrusion. IL during fabrication is negligible.

c: Fabricated at 425 ºC by hot rolling. IL ~0.2 µm forms during fabrication.

d: Fabricated at 425 ºC by hot rolling. IL ~0.2 µm forms during fabrication.

e: IL growth during fabrication (0.2 µm) is subtracted.

f: IL growth during fabrication (0.2 µm) is subtracted. Only IL thickness on the γ-phase U-Mo is considered.

Al Al Al Al-5Si Al-4Si Al-4Si Al-6Si Al-6Si Al-2Si Al-2Si Al-2Si Al-4Si Al-4Si Al-4Si Al-4Si Al-5Si Al-5Si Al-5Si Al-6Si Al-6Si Al-6Si Al-6Si

500 525 550 580 425 475 425 475 450 475 500 450 475 500 525 450 475 500 450 475 500 525

1 ^b 1 ^b 1 ^b 1 ^b 2 ^c 2 ^c 2 ^c 4 ^c 4 ^d 4 ^d 2 ^d 4 ^d 4 ^d 2 ^d 1 ^d 4 ^d 4 ^d 2 ^d 4 ^d 4 ^d 2 ^d 1 ^d

2.0 3.6 8.4 8 0.5 ê 0.6 ê 0.4 ê 1.1 ê 0.2 ^f 0.4 ^f 1.0 ^f 0.6 ^f 0.7 ^f 0.7 ^f 0.8 ^f 0.4 ^f 0.7 ^f 1.2 ^f 0.7 ^f 0.8 ^f 0.9 ^f 0.8 ^f

[3]

[39]

[23]

[24]

Source Matrix Heating

time (h) temp (ºC)

Average IL ^e

(µm) Ref.

Table 1. IL Growth between U-Mo ^aand Al from Out-of-pile Heating Tests, RERTR Tests and KOMO-4 Test during Fabrication and Follow-on Blister Test

(4)

temperatures below ~550 ºC. For other Mo content in the range of 6 – 10 wt%, the phase boundary is slightly higher and another phase (α+γ) exists. Therefore, during fabrication, some parts of the meta-stable γ-U particle transform to α-U.

The γ-U has a lower swelling rate during irradiation than the α-U (see [52]-[54]for details) and has a lower IL growth rate [17]. When the γ-U is partially transformed to the α-U during fabrication and the subsequent blister test, a thicker IL grows on the α-U while a thinner IL grows on the γ-U, which results in nodular type IL growth on the same U-Mo particle. In this situation, expressing IL in volume is more appropriate than in thickness.

2.2 Al Matrix with Si Addition

Measured data for U-Mo/Al-Si dispersion, i.e., U-Mo dispersion in a Si-added Al matrix, are scarce from out- of-pile tests and scatter with considerable uncertainties.

Iltis [23] measured the IL thicknesses of Si-added plates fabricated at a lower fabrication temperature than typical RERTR plates (Table 2) and then heated for two or four hours. As given in Table 2, 4 wt% Si and 6 wt% Si plates show no clear correlation between the Si content and IL thickness, implying that the effect of Si content on IL growth in the out-of-pile heating test is negligible. Keiser and co-workers [24] also reported an out-of-pile heating test result of plates with Si addition. Using images in Ref. [24], we measured IL thicknesses as given in Table 1. The Keiser data also confirm that there is no clear correlation between the Si content and IL thickness. This uncertainty is chiefly because of the irregular IL thickness, which is caused by the decomposition of the meta-stable γ-phase U-Mo particles.

It is typical that considerable parts of the U-Mo surfaces have extremely thin ILs while some parts have thicker ILs, including some nodular type ILs. As discussed in sect. 2.1, this nodular type of IL is known to form on the parts of the U-Mo surface transformed to α+γ' during the heating steps. Because of the much higher IL growth rate of the α phase [17], the fuel surface with γ-to-α+γ' phase transformation shows higher IL growth. Once formed, the thick IL on the α-U precludes Si diffusion to the fuel

surface and formation of a Si-barrier on the fuel surface to block Al diffusion. This further contributes to the formation of more irregular ILs in U-Mo/Al-Si dispersions than U- Mo/Al dispersions after in-pile tests. Hence, considerable uncertainty is inevitably involved in the IL thickness of a plate with Si addition.

Park et al. [39] annealed a U-Mo/Al-5Si dispersion sample at 580 ºC for ~1 h and observed an average IL thickness of ~8 µm (see also Ref. [49]). Because the temperature was much higher, but still below the γ-phase boundary, it is probable that the decomposition of the meta-stable γ- phase U-Mo particles was higher in Park’s test, so his datum may be exaggerated. However, the extent of IL growth reported by Park is lower than those of U3Si/Al and U3Si2/Al dispersion fuels [11], suggesting that at the annealing temperature the U-Mo fuel surface forms an excellent barrier that effectively blocks Al diffusion into U-Mo. This result also implies that the effect of the presence of the Mo in U-Mo/Al-Si is to synergistically reduce IL growth as in U-Si intermetallic dispersion in Al with a small addition of Mo, as reported by Ugajin [55].

Some diffusion-couple test data are available in the literature [17],[18]. In a typical diffusion-couple test to measure interdiffusion between U-Mo and Al, a U-Mo block is welded with an Al block, and the metal couple, U-Mo versus Al, is isothermally heated. In the post-test analysis, the interaction layer thickness is measured. The contact pressure between U-Mo and Al in a diffusion- couple test is higher than that of a dispersion sample, so the IL growth rate is generally higher in the diffusion- couple test than in the out-of-pile test of dispersion fuel.

For this reason, the diffusion-couple test data cannot be used with the dispersion test data.

Using the data provided in Table 1, we performed a data fitting to the Arrhenius equation and the IL growth correlation is obtained as follows (R²=0.67):

where YAl-Si is the IL thickness (µm) in an Al with a Si addition, T in K and t in s.

RERTR-4, -5 RERTR-6, -7 RERTR-9 KOMO-4

Al Al-Si Al-Si Al-Si

500 500 500 400

~0.5 ^a

485 485 485 b

1 1 0.5

1.5 0.7 0.6 0.0 a: The time is estimated after bonding between U-Mo and matrix is established [51].

b: No blister test was performed.

Test Matrix Heating ^a

time (h)

temp (ºC) temp (ºC) time (h)

Blister annealing Predicted average IL (µm) Table 2. Heat Processes during Fabrication and Follow-on Blister Test and Predicted IL Growth before Irradiation for RERTR

Tests and KOMO-4 Test

(2)

(5)

Using Eqs. (1) and (2), the IL growth during fabrication and the follow-on blister test for the RERTR test plates, and KOMO-4 test were calculated and given in Table 2.

Once IL growth before irradiation is assessed by using Eq. (1) or Eq. (2), the total IL growth accumulating during fabrication and irradiation can be obtained by

where Y is the total IL thickness, Yfabis the fabrication IL thickness calculated by Eq. (1) or Eq. (2), and Yiis the IL thickness during irradiation. The model for IL thickness prediction during irradiation is described in the following section.

3. MODEL FOR INTERACTION LAYER GROWTH DURING IRRADIATION

3.1 In-pile Test Data

The irradiation test data used in the model development are summarized in Table 3. While the RERTR test samples are miniplates having the meat thickness of ~0.64 mm, KOMO-4 test samples have rod-type geometry with the meat diameter of 6.4 mm. Because of the much greater distance of heat transport and the higher heat flux, the KOMO-4 test samples have much higher temperatures than the RERTR test samples for the same power.

For the RERTR test plates, the fuel temperatures were

V6001M V6015G V6022M V6018G V6019G V8005B V0R010 R5R020 R1R010 R2R020 R2R010 R3R030 R0R010 R0R020 R2R040 R2R050 R3R040 R3R050 R2R088 R6R018 557MD3 6762S1 6765S1 6768S2

U-10Mo/Al U-10Mo/Al U-10Mo/Al U-10Mo/Al U-10Mo/Al U-10Mo/Al U-10Mo/Al U-7Mo/Al-0.2Si U-7Mo/Al-1Si U-7Mo/Al-2Si U-7Mo/Al-2Si U-7Mo/Al-5Si U-7Mo/Al U-7Mo/Al U-7Mo/Al-2Si U-7Mo/Al-2Si U-7Mo/Al-5Si U-7Mo/Al-5Si U-7Mo/Al-2Si U-7Mo/Al-3.5Si U-7Mo/Al ^C U-7Mo/Al ^MR U-7Mo/Al-2Si ^C U-7Mo/Al-2Si ^MR U-7Mo/Al-5Si ^C U-7Mo/Al-5Si ^MR U-7Mo/Al-8Si ^C U-7Mo/Al-8Si ^MR

6 6 6 6 6 8 6 6 6 6 6 6 6 6 6 6 6 6 8.5 8.5 4.5 4.5 5 5 5 5 5 5

19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.1 19.5 19.4 19.4 19.1 58.1 58.1 58.2 58.2 58.3 58.2 58.1 58.1 19.5 19.5 19.5 19.5 19.5 19.5 19.5 19.5

257 257 257 116 116 116 135 135 135 135 135 135 90 90 90 90 90 90 115 115 132 132 132 132 132 132 132 132

65 65 65 65 65 65 65 65 65 65 65 65 140 135 75 90 80 135

50 50 260 260 260 260 260 260 260 260

4.51 5.52 5.27 2.30 2.92 2.38 2.90 3.12 2.73 3.08 3.21 2.99 3.89 5.55 3.84 3.98 4.43 3.98 5.60 5.45 4.02 4.02 3.70 3.70 3.56 3.56 3.49 3.49

2.03 2.49 2.37 2.29 2.91 2.37 2.48 2.67 2.34 2.64 2.75 2.57 4.61 6.57 4.94 5.12 5.70 5.12 5.64 5.49 3.52 3.52 3.24 3.24 3.12 3.12 3.06 3.06

81 106 106 80 98 109 101 105 87 93 96 95 112 135 115 134 117 124 153 154 181 165 194 182 187 171 181 167

15 ± 1 18 ± 2.5 19 ± 3.5 9 ± 1 15 ± 3.5 13 ± 1.5 7 ± 2 11 ± 4 2.7 ± 0.5 2.6 ± 0.5 2.9 ± 0.5 2.2 ± 0.5 9 ± 4 16 ± 5 3.2 ± 1.5 10.2 ± 2.5

2.7 ± 1.5 3.2 ± 2.5 9.1 ± 3.5 7.1 ± 3 32 ± 3 28 ± 3 17 ± 2 15 ± 2 11 ± 1.5

8 ± 1.5 7 ± 1 6 ± 1

1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 0.7 0.7 0.7 0.7 1.5 1.5 0.7 0.7 0.7 0.7 0.6 0.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

11.6 17.4 17.2 8.0 10.9 12.1 11.8 11.0 3.3 2.5 2.7 2.5 14.1 20.4 3.7 5.4 3.4 3.7 9.4 7.0 34.0 29.1 23.7 17.9 10.6 7.6 7.6 6.5

3.4 0.6 1.8 1.0 4.1 0.9 -4.8 0.0 -0.6 0.1 0.2 -0.3 -5.1 -4.0 -0.5 4.8 -0.7 -0.5 -0.3 0.1 -2.0 -1.1 -4.7 -0.9 0.4 0.4 -0.6 -0.5 Test

RERTR-4 Miniplate

RERTR-5 Miniplate

RERTR-6 Miniplate

RERTR-7 Miniplate

RERTR-9 Miniplate KOMO-4 Rod

Plate ID Fuel/matrix Composition ^a

Nominal Meat U density gU/cm³

Enrich.

U-235

%

Time EFPD

Average particle size µm

FD at meat

center (x10²¹) f/cm³

Life average

FR (10¹⁴) f/cm³-s

Life average

Temp ºC

Measured IL thickness

µm

As- fabricated

IL thickness

µm

Predicted IL thickness

µm

Error ^b µm

Table 3. Test Data and Comparison of IL Thicknesses between the Measured and Predicted from RERTR-4, -5, -6, -7, -9 and KOMO-4 Tests

a: Numbers before Mo and Si mean the Mo content in fuel particles and the Si content in matrix in wt%, respectively.

b: error = measured – predicted C: meat center

MR: meat mid-radius

(3)

(6)

calculated at BOL and EOL, and arithmetic averages were taken to represent the test samples (Table 3). For the KOMO-4 test, the time-dependent fuel temperatures were calculated and life-average values were obtained.

The test-sample power, and hence the fission rate, decreases due to depletion of fissile atoms in the sample. On the other hand, fuel temperatures tend to increase as the meat thermal conductivity degrades due to IL growth and the formation of fission gas pores. These two factors somewhat compensate for each other, and as a result, the change in fuel temperature over test life is insignificant. In the RERTR miniplate tests, although the temperature and fission rate (i.e., power) are each independent variables to each other, they are still coupled. In this regard, the KOMO-4 test data are valuable because they have higher temperatures than the RERTR test data while the fission rates are similar.

IL thicknesses were measured at several locations in an optical micrograph and the average value was used as the nominal value. The method to obtain the representative IL thickness is illustrated in Fig. 2. Two cross sections are shown for illustration. The cut surface A is off of the equator B. Therefore, the IL thickness L1 measured in the cut surface A is thicker than L2 measured in the equator.

Generally, on a metallographic picture, the common thinnest IL is more likely to be the true IL thickness, because it is more likely to be on a plane close to the equator of the fuel particle.

In Fig. 3 and Fig. 4, posttest micrographs of RERTR- 6 and KOMO-4 samples are shown, respectively. The RERTR-6 is a test having relatively low temperatures and low fission rates. Although temperatures vary by ~18 ºC, all test plates consistently show that IL thickness decreases as the Si content increases. This trend is more noticeable in the KOMO-4 test, which has less temperature variability.

The ILs of the RERTR samples were measured at the plate transverse (width) center to exclude possible influence

of pores forming in the matrix, and fuel creep occurring in the fuel meat loaded closer to the ATR core center, a phenomenon reported previously [56],[57]. For the KOMO- 4 test, measurements were made at the meat center and meat mid-radius regions, by which two IL thicknesses at different temperatures with the same fission rate were obtained (Table 3).

The measurement errors, given in Table 3, stem largely from the irregular IL thicknesses. The errors tend to be larger for the samples with a Si-added matrix than the

Fig. 2. Schematic Illustration of the Measurement Method of Interaction Layer Thickness. A and B are Cross Section

Planes.

Fig. 3. Post Irradiation Micrographs Showing Microstructures of RERTR-6 plates. The Burnup is in Percent Fission Per

Initial Heavy Metal Atom (FIHMA).

Fig. 4. Post Irradiation Micrographs Showing Microstructures of KOMO-4 Samples. The Burnup is in Percent Fission Per

Initial Heavy Metal Atom (FIHMA).

(7)

pure Al matrix samples. PIE images also showed locally thick, nodular type, ILs, the cause for which originated from the α+γ' transformation during fabrication and the following blister test. On the other extreme, PIE images showed that ILs on some U-Mo particles were still thinner than the average IL thickness during fabrication. This indicates either that some parts of the fuel surface had poor contact from fabrication to the end of irradiation, or that large Si particles contacting the fuel particle effectively blocked Al diffusion. The as-fabricated IL thickness was subtracted from the post-irradiation IL, which means the predicted results by the new correlations are for those during irradiation.

3.2 Correlation Development for Irradiation 3.2.1 Basic Correlation

The IL growth correlation during irradiation is a modified Arrhenius equation from Eq. (1), and an additional term with the fission rate is multiplied to account for fission- enhanced-diffusion (FED):

where ˙f is the fission rate, and A and q are empirical constants. The power, p, to the fission rate was empirically determined as p=0.5 [11],[44].

There have been several other correlations that take a similar form to Eq. (4) but with different constants [44], [45],[48],[58]. In order to fit both the RERTR and KOMO data, the fitting constants here were revised from those of Refs. [44] and [45]. The best fit was obtained with A=

2.6x10^-8 and q=3850 K for plates with U-7Mo in a pure Al matrix. In Table 3, the predicted and measured IL thicknesses are compared. Therefore, the correlation for the samples with a pure Al matrix is:

where Y is in µm, ˙f in f/cm³-s, T in K, and t in s.

The effects of Si addition and Mo content are considered by multiplying additional factors to Eq. (5) as follows:

where fSiis the reduction factor by Si addition in the Al matrix and fMois the factor to assess the Mo content other than 7 wt% in the U-Mo alloy particles. The modeling of these factors is described in the following subsections.

3.2.2 Factor for Si-addition Effect, fSi

The addition of Si in the Al matrix results in an expo- nential reduction of IL thickness that varies with the Si content. Comparing the results from the lower temperature test RERTR-6 and the higher temperature test KOMO-4, it is noticeable that the Si addition effect on IL growth also

depends on temperature. For the RERTR-6 test, the Si effect is virtually saturated at 2 wt% Si (Fig. 5). However, the KOMO-4 test shows that the IL reduction effect continues up to 8 wt% Si without saturation. Fitting the IL reduction versus the Si content for two different temperatures given in Fig. 5, we obtained the IL reduction factor by Si addition in Al as

where T is the fuel meat temperature in K (≤ 473 K) and WSiis the Si content in the matrix in wt% (≤ 8 wt%). fsi is in the range 0.002 ≤ fSi ≤ 1. The minimum occurs at the Si content of 8 wt%, whereas the maximum occurs for the pure Al.

3.2.3 Factor for Mo Content Effect, fMo

It is known that as the Mo content increases, the IL growth rate decreases [2],[54],[14], so cases with a Mo content other than 7 wt% need to be considered. The current understanding is that the metastable γ-phase U-Mo more readily decomposes to the α+γ' if the Mo content in the alloy is reduced. Although the α-phase transforms back to the γ-phase soon after irradiation starts [59],[60],[61], once formed, the α-phase produces a thicker IL during fabrication and the subsequent blister test. The thicker IL formed on the a phase weakens the Si addition effect, and the result is an increase in the overall IL growth rate during irradiation. In the thick IL, as the Si concentration decreases, so does its activity [43]. Mirandou [19] also found a weak- ened Si effect in the thick IL from an elevated-temperature out-of-pile test and argued that it was owing to the formation

Fig. 5. Comparison of IL Reduction Factors Obtained for the Low-temperature RERTR-6 Test (at Life-avErage T ~100 ºC) and the High-temperature KOMO-4 Test (at Life-average T

~180 ºC), where the IL Reduction Factor is Defined as the Square of IL Thickness Divided by the Square of IL Thickness

for a Pure Al Matrix at the Same Irradiation Condition.

(4)

(5)

(6)

(7)

(8)

of the α-phase, on which U⁴Mo6Al43+UAl3 phases form, excluding Si.

In the RERTR experiment modeling [2], we observed that the IL growth rate decreased linearly with the Mo content in the range of 6 – 10 wt%. This effect was similarly formulated as follows:

where WMois the Mo content in fuel particles in wt%. fMo

varies between 0.85 (at 10 wt% Mo) and 1.05 (6 wt% Mo).

A Mo-content beyond 10 wt% is not considered in the US RERTR program.

4. DISCUSSION

One of the objectives of the development of the present model was to unify the two independent IL growth correlations applicable either to a pure Al matrix, or to an Al matrix with up to 8wt% Si and temperatures up to 200 ºC.

In addition, IL formation before irradiation is included, so the model can be used for fuels produced by various fabrication methods.

The observations from the KOMO-4 test [39] showed that Si tends to accumulate mostly at the IL-matrix interface due to Si diffusion from the matrix. It is known that Si contained only in the fission-fragment recoil zone produces Si diffusion flux. As the IL grows, the overlapping volume fraction of the recoil zones increases and, hence, there is a decrease in the effective amount of Si per unit-fuel-surface area available for diffusion.

For the RERTR-9 test, the fuel plates were made with smaller fuel particles and higher U-loading, which yielded more recoil-zone overlapping during the test, and in turn reduced the effective Si amount available per unit-fuel- surface area. Contrarily, the KOMO-4 test samples had larger fuel particles sizes and relatively lower U-loading in the meat (see Table 3). As a result, smaller recoil-zone overlapping was made, so the effective Si amount per unit-fuel-surface area was higher. As U-loading increases, not only the matrix volume decreases and the depletion of the matrix occurs earlier, but homogeneity in fuel particle spatial distribution also tends to decrease. Regions with higher U-Mo particle densities occur, where the effective Si amount per unit-fuel-surface area decreases. Hence, even for the same Si content in the matrix, the IL growth rate can be different.

In the meat, the homogeneity of fuel-particle spatial distribution may also affect the effective Si availability.

For example, a higher homogeneity in fuel-particle spatial distribution improves the effect of Si addition.

An additional consideration in the efficacy of Si is how Si is added to the matrix Al. The finer the Si precipitates, the more effective is the Si addition: Hence, a lower IL growth rate. At present, there are two viable methods of

adding Si to the matrix. One is adding Si to the Al as an alloying element, and the other is directly adding the Si powder to the Al powder during fuel fabrication. The former was considered the better method to provide finer Si particles in the matrix.

In order to have more accurate modeling, consideration of the parameters discussed above (U-loading, homogeneity in fuel-particle spatial distribution and Si precipitate size in the matrix) may also be necessary. In the present modeling, however, these parameters are not explicitly considered.

The Si concentration in the IL tends to decrease during irradiation. As the Si concentration decreases, the Si’s strength to reduce the IL growth rate also decreases. How- ever, the change in the Si effect during irradiation is not included in the present model.

The comparison between the measured data and predicted IL thicknesses are given in Table 3. Considering uncertainties in temperatures and measurement errors, a fair agreement was generally achieved, except for some cases.

The uncertainty in the meat temperatures is believed to be caused by the uncertainty in the meat thermal conductivity at high burnup. Measurement errors were primarily due to the irregular IL thickness.

5. CONVERSION OF IL THICKNESS TO IL VOLUME IL growth data given in thickness are often more convenient than those given in IL volume, because they do not need supplemental data such as U-loading and U- Mo particle size. However, for high IL growth cases, the thickness is not only difficult to measure, but it is also less meaningful. For these circumstances, data on IL volume fraction is a better metric. Consequently, a conversion model from IL thickness to IL volume was developed, assuming that fuel particles had a uniform size and were distributed in an FCC array.

Fig. 6 illustrates the configuration of two neighboring fuel particles and the IL formed around them. The distance,

Fig. 6. Schematic Showing Configuration of Two Neighboring Particles with IL Formed on the Surfaces and IL Overlapping.

(8)

(9)

d, between the surfaces of two neighboring particles is

where a is the FCC unit cell edge length, and r is the fuel particle radius. The fuel volume fraction in the meat is defined as

where Vmis the meat volume and Vfis the fuel volume in the unit cell, respectively. Because there are four fuel particles in the unit cell, Vfis given by

where r is the average fuel particle radius known from the fabrication data. From Eqs. (9) – (11), d can be found by

where the applicable range of vfis less than 0.74, which is the FCC packing fraction. The fuel volume fraction vfis usually calculated with known fabrication data (U-loading), because vf =wU/ρ^U where wU is U-loading, and ρ^U is U- density in the fuel particle. For example, vffor 8 gU/cm³ of U-7Mo is calculated by vf= wU/ρU= 8/16.3=0.49 where ρU = 17.5 * (1 – 0.07). The physical density of U-7Mo alloy is 17.5 g/cm³.

In Fig. 6, the overlapping part of the IL volume of the right-hand-side fuel particle, by the neighbor particle, is a partial sphere with height, h, of which the volume is

where h = Y – d/2 and R= r + Y. Here Y is the IL thickness.

When the IL overlaps with neighboring fuel particles, i.e., Y is greater than d, as marked by the darker gray region in Fig. 6, and this volume must be subtracted from the large cap volume given in Eq. (13). Because there are two symmetric caps, two of these small caps must be considered.

where g = (Y– d)/2.

The IL volume can be calculated considering only one fuel particle surrounded by 12 neighboring ones by

The IL volume fraction vIL in the fuel meat can be calculated by

where VILis given by Eq. (15) and Vfgiven in Eq. (11).

Fig. 7 shows the model predictions for IL volume fraction versus IL thickness for 6 gU/cm³ and 8 gU/cm³ U-loading cases with a fuel particle size of 70 µm. Also included is a case for the KOMO-4 test, for which fuel loading is 5 gU/cm³with fuel particle size of 150 µm. Fuel swelling and fuel consumption by the interdiffusion layer growth are not considered. However, these two factors somewhat tend to compensate for each other to give reasonably acceptable model predictions. A more detailed model is necessary for an accurate result, but the model described here is suitable for approximate estimates.

6. CONCLUSIONS

In the US GTRI-convert program (formerly known as RERTR), U-Mo alloy particle dispersion in Al is a primary fuel form being studied for use in converting HEU-fueled research and test reactors to LEU-fueled ones. In order to achieve sufficient total uranium densities, high fuel-volume loading is pursued. One of the largest obstacles to good fuel performance is interaction layer (IL) growth between the U-Mo particles and matrix Al, which needs to be minimized.

For an accurate fuel performance analysis, a prediction model for IL growth is therefore necessary. Two empirical IL growth correlations were developed for out-of-pile fabrication and possible subsequent testing: One is for the pure Al matrix, and the other for the matrix with Si addition. The IL growth correlations are Arrhenius equations.

Fig. 7. Prediction of IL Volume Fraction as a Function of IL Thickness for 6 gU/cm³and 8 gU/cm³U-density in the Meat (Average Fuel Particle size = 70 µm) and for 5 gU/cm³U- Density in the Meat (Average Fuel Particle Size = 150 µm).

(9)

(10)

(12)

(13)

(14)

(15)

(16) (11)

(10)

The in-pile growth correlation, developed using the RERTR-4, -5, -6, -7 and -9 data, as well as KOMO-4 data, takes a form similar to the out-of-pile correlations. For the in-pile correlation, the out-of-pile IL growth was augmented by fission-enhanced diffusion (FED). The FED effect was modeled by the square-root of the fuel-particle fission rate.

The in-pile correlation was further enhanced to include the effect of Si addition in the Al and the effect of the Mo content. The model is applicable for Si contents in the matrix up to 8 wt%, for fuel temperatures up to 200 ºC, and for Mo content in fuel particles in the range of 6 – 10 wt%.

An approximate analytical method to convert IL thickness to IL volume fraction in the fuel meat was proposed.

For a more accurate conversion, the volume changes in fuel and matrix caused by their consumption by IL growth must be considered together with the volume expansion by fission-product-induced fuel swelling, which was not considered in the present work.

ACKNOWLEDGMENTS

This paper contains information obtained from the reduced-size plate tests ~RERTR-6, -7, and -9 irradiated in the ATR, in the Global Threat Reduction Initiative- Conversion program ~formerly known as the RERTR program. For design and fabrication of the test samples, Mrs. T. Wiencek, C. Clark, and G. Moore are acknowledged.

The physics data were made available by Dr. G. Chang and Ms. M. Lillo at INL. The operation staff at ATR is also acknowledged for the RERTR irradiation tests. The authors appreciate the hands-on PIEs performed at the Materials and Fuels Complex of INL. The authors also thank Dr. J. S. Cheon of KAERI for providing the drawing shown in Fig. 2. This work was supported by the U.S.

Department of Energy, Office of Global Threat Reduction (NA-21), National Nuclear Security Administration, under Contract No. DE -AC-02-06CH11357 between UChicago Argonne, LLC and the Department of Energy.

REFERENCES_______________________________

[ 1 ] G. L. Hofman, M. K. Meyer, J. L. Snelgrove, M. L. Dietz, R. V. Strain, and K-H. Kim, “Initial assessment of radiation behavior of very-high-density low-enriched-uranium fuels,”

Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Budapest, Hungary, October 3-8, 1999. <http://www.rertr.anl.gov>

[ 2 ] S.L. Hayes, M.K. Meyer, G.L. Hofman, J.L. Snelgrove, R.A.

Brazener, “Modeling RERTR experimental fuel plates using the PLATE code,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Chicago, IL, USA, Oct. 5 - 10, 2003. <http://www.rertr.anl.gov>

[ 3 ] H. J. Ryu, Y. S. Han, J. M. Park, S. D. Park, and C. K. Kim,

“Reaction layer growth and reaction heat of U-Mo/Al dispersion fuels using centrifugally atomized powders,” J.

Nucl. Mater., vol. 321, pp. 210-220 (2003).

[ 4 ] G.L. Hofman, M.K. Meyer, J.M. Park, “Observation on the irradiation behavior of U-Mo alloy dispersion fuel,”

Proc. Int. Meeting on Reduced Enrichment for Research

and Test Reactors (RERTR), Las Vegas, Nevada, USA, October 1-6, 2000. <http://www.rertr.anl.gov>

[ 5 ] G. L. Hofman, Yeon Soo Kim, M.R. Finlay, J.L. Snelgrove, S.L. Hayes, M.K. Meyer, C.R. Clark, “Recent Observations at the Postirradiation Examination of Low-Enriched U-Mo Miniplates Irradiated to High Burnup,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Chicago, Illinois, October 5-10, 2003.

<http://www.rertr.anl.gov>

[ 6 ] P. Lemoine, J. L. Snelgrove, N. Arkangelsky and L. Alvarez,

“UMo dispersion fuel results and status of qualification programs,” Trans. RRFM 2004, Munich, Germany, March 21-24, 2004.

http://www.euronuclear.org0meetings/rrfm2004/index.htm [ 7 ] H.J. Ryu, Yeon Soo Kim, G.L. Hofman, “Amorphization of

the interaction products in U-Mo/Al dispersion fuel during irradiation,” J. Nucl. Mater., vol. 385, pp. 623-628 (2009).

[ 8 ] S. Van den Berghe, W. Van Renterghem, A. Leenaers,

“Transmission electron microscopy investigation of irradiated U-7 wt%Mo dispersion fuel,” J. Nucl. Mater. vol. 375, pp.

340-346 (2008).

[ 9 ] Yeon Soo Kim and G.L. Hofman, “Irradiation behavior of the interaction product of U-Mo fuel particle dispersion in an Al matrix,” J. Nucl. Mater., vol. 425, pp. 181-187 (2012).

[ 10 ] G.L. Hofman, J. Rest, J.L. Snelgrove, T. Wiencek, S.K.V.

Groos, “Aluminum-U3Si2 Interdiffusion and Its Implications for the Performance of Fuel Operating at Higher Temper- atures and Fission Rates,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactor (RERTR), Seoul, Korea, 1996. <http://www.rertr.anl.gov>.

[ 11 ] Yeon Soo Kim and G.L. Hofman, “Interdiffusion in U3Si-Al, U3Si2-Al, and USi-Al dispersion fuels during irradiation,” J.

Nucl. Mater., vol. 410, p. 1-9 (2011).

[ 12 ] G. L. Hofman, M. R. Finlay and Yeon Soo Kim, “Post- Irradiation Analysis of Low Enriched U-Mo/Al Dispersion Fuel Miniplate Tests, RERTR 4 and 5,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Vienna, Austria, November 7-12, 2004.

[ 13 ] Yeon Soo Kim, G.L. Hofman, H.J. Ryu, J. Rest, “Thermo- dynamic and metallurgical considerations to stabilizing the interaction layers of U-Mo/Al dispersion fuel,” Proc.

Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Boston, USA, Nov. 6-10, 2005.

[ 14 ] J.M. Park, H. Ryu, G. Lee, H. Kim, Y. Lee, C. Kim, Yeon Soo Kim and G.L. Hofman, “Phase stability and diffusion characteristics of U-Mo-X (X = Si, Al, Zr or Ti) alloys,”

Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Boston, USA, Nov. 6-10, 2005. http://www.rertr.anl.gov

[ 15 ] M. Mirandou, S. Arico, L. Gribaudo, and S. Balart, “Out- of-pile diffusion studies between U-7wt%Mo and Al-Si alloys,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Boston, USA, Nov.

6-10, 2005. http://www.rertr.anl.gov

[ 16 ] M. Cornen, M. Rodier, X. Iltis, S. Dubois, P. Lemoine,

“About the effects of Si and/or Ti additions on the UMo/Al interactions,” Trans. RRFM 2008, Hamburg, Germany, Mar.

2-5, 2008.

http://www.euronuclear.org/meetings/rrfm2008/index.htm

(11)

[ 17 ] J.M. Park, H.J. Ryu, S.J. Oh, D.B. Lee, C.K. Kim, Yeon Soo Kim, G. L. Hofman, “Effects of Si and Zr on the interdiffusion of U-Mo alloy and Al,” J. Nucl. Mater., vol.

374, pp. 422-430 (2008).

[ 18 ] M. Mirandou, S. Arico, M. Rosenbusch, M. Ortiz, S. Balart, L. Gribaudo, “Characterization of the interdiffusion layer grown by interdiffusion between U-7wt%Mo and Al A356 alloy at 550 and 340 ºC,” J. Nucl. Mater., vol. 384, pp. 268 -273 (2009).

[ 19 ] M. Mirandou, S. Arico, S. Balart, L. Gribaudo, “Character- ization of the interaction layer in diffusion couples U-7wt

%Mo/Al 6061 alloy at 550 ºC and 340 ºC. Effect of the U(Mo) cellular decomposition,” Mater. Charact. Vol. 60, pp. 888-893 (2009).

[ 20 ] E. Perez, B. Yao, Y.H. Sohn, D.D. Keiser, Jr., “Interdiffusion in Diffusion Couples: U-Mo vs. Al and Al-Si,” Proc. Int.

Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), November 1-5, 2009, Beijing, China.

http://www.rertr.anl.gov

[ 21 ] J.H. Yang, H.J. Ryu, J.M. Park, “In-Situ High Temperature Neutron Diffraction Study of Phase Evolution in U-7wt%

Mo/Al and U-7wt% Mo/Al-5wt% Si Fuel Rods,” Proc. Int.

Meeting on Reduced Enrichment for research Test Reactors (RERTR), Beijing, China, Nov. 1-6, 2009.

[ 22 ] J.H. Allenou, H. Palancher, X. Iltis, M. Cornen, O. Tougait, R. Tucoulou, E. Welcomme, Ph. Martin, C. Valot, F.

Charollais, M. C. Anselmet, P. Lemoine, “U-Mo/Al-Si interaction: Interdiffusion of Si concentration,” J. Nucl.

Mater., vol. 399, pp. 189-199 (2010).

[ 23 ] X. Iltis, F. Charollais, M.C. Anselmet, P. Lemoine, A, Leenaers, S. Van den Berghe, E. Koonen, C. Jarousse, D.

Geslin, “Microstructural characterization of the E-FUTURE fresh fuel plates,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Lisbon, Portugal, Oct. 10 - 14, 2010. http://www.rertr.anl.gov

[ 24 ] D.D. Keiser, J. Jue, N. Woolstenhulme, A. Ewh, “Potential annealing treatments for tailoring the starting microstructure of low-enriched U-Mo dispersion fuels to optimize performance during irradiation,” J. Nucl. Mater., v. 419, pp. 226 -234 (2011).

[ 25 ] D.D. Keiser, J. Jue, B. Yao, E. Perez, Y. Sohn, C.R. Clark,

“Microstructural characterization of U-7Mo/Al-Si alloy matrix dispersion fuel plates fabricated at 500 ºC,” J. Nucl.

Mater., vol. 412, pp. 90-99 (2011).

[ 26 ] G.L. Hofman, Yeon Soo Kim, J. Rest, M.R. Finlay, “Attempts to solve the instability in the irradiation behavior of low enriched U-Mo/Al dispersion fuel,” Trans. RRFM 2006, Sofia, Bulgaria, April 30-May 3, 2006.

http://www.euronuclear.org/meetings/rrfm2006/index.htm [ 27 ] G.L. Hofman, Yeon Soo Kim, H.J. Ryu, D. Wachs, M.R.

Finlay, “Results of low enriched U-Mo dispersion fuel miniplates from irradiation test, RERTR-6,” Proc. Int.

Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Cape Town, South Africa, Oct. 29 - Nov. 2, 2006. http://www.rertr.anl.gov

[ 28 ] Yeon Soo Kim, H.J. Ryu, G.L. Hofman, S.L. Hayes, M.R.

Finlay, D. Wachs, G. Chang, “Interaction layer growth correlations for (U-Mo)/Al and Si-added (U-Mo)/Al dispersion fuels,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Cape Town, South

Africa, Oct. 29 - Nov. 2, 2006. http://www.rertr.anl.gov [ 29 ] G. L. Hofman, Yeon Soo Kim, H.J. Ryu, M.R. Finlay,

“Improved irradiation behavior of uranium-molybdenum /aluminum dispersion fuel,” Trans. RRFM 2007, Lyon, France, Mar. 11-15, 2007.

http://www.euronuclear.org/meetings/rrfm2007/index.htm [ 30 ] G. L. Hofman, Yeon Soo Kim, J. Rest, A.B. Robinson, D.M. Wachs, “Postirradiation analysis of the latest high uranium density miniplate test: RERTR-8,” Trans. RRFM 2008, Hamburg, Germany, Mar. 2-5, 2008.

http://www.euronuclear.org/meetings/rrfm2008/index.htm [ 31 ] D.D. Keiser, A.B. Robinson, D.E. Janney, J. F. Jue, “Results

of recent microstructural characterization of irradiated U- Mo dispersion fuels with Al alloy matrices that contain Si,” Trans. RRFM 2008, Hamburg, Germany, Mar. 2-5, 2008.

http://www.euronuclear.org/meetings/rrfm2008/index.htm [ 32 ] D.D. Keiser Jr., A.B. Robinson, J.F. Jue, P. Medvedev, D.

M. Wachs, M.R. Finlay, “Microstructural development in irradiated U-7Mo/6061 Al alloy matrix dispersion fuel,” J.

Nucl. Mater., vol. 393, pp. 311-320 (2009).

[ 33 ] D.D. Keiser, J.F. Jue, A.B. Robinson, “Results of SEM Characterization of a Dispersion Fuel Plate with Al-2Si Matrix Tested in the RERTR-7 Experiment,” Proc. Int.

Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Lisbon, Portugal, Oct. 10 - 14, 2010.

[ 34 ] J.M. Park, H.J. Ryu, Y.S. Lee, B.O. Yoo, Y.H. Jung, C.K.

Kim, Yeon Soo Kim, “PIE results of the KOMO-3 irradiation test,” Trans. RRFM 2008, Hamburg, Germany, Mar. 2-5, 2008.

http://www.euronuclear.org/meetings/rrfm2008/index.htm [ 35 ] A. Leenaers, S. Van den Berghe, S. Dubois, J. Noirot, M.

Ripert, P. Lemoine, “Microstructural analysis of irradiated atomized U(Mo) dispersion fuel in an Al matrix with Si addition,” Trans. RRFM 2008, Hamburg, Germany, Mar.

2-5, 2008.

<http://www.euronuclear.org/meetings/rrfm2008/index.htm>

[ 36 ] A. Leenaers, C. Detavernier, S. Van den Berghe, “The effect of silicon on the interaction between metallic uranium and aluminum: A 50 year long diffusion experiment,” J. Nucl.

Mater., vol. 381, pp. 242-248 (2008).

[ 37 ] A. Leenaers, S. Van de Berghe, F. Charollais, P. Lemoine, C. Jarousse, A. Röhrmoser, W. Petry, “Microstructural Analysis of Ground UMo Fuel With and Without Si Added to the Matrix, Irradiated to High Burn Up,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Beijing, China, November 1-5, 2009.

[ 38 ] Yeon Soo Kim, G.L. Hofman, A.B. Robinson, “Effect of Si addition in Al in U-Mo/Al dispersion plates: Observations from side-by-side irradiations from RERTR-6, -7A, -9A and -9B,” Trans. RRFM 2009, Vienna, Austria, Mar. 22-25, 2009.

http://www.euronuclear.org/meetings/rrfm2009/index.htm [ 39 ] J.M. Park, H.J. Ryu, J.H. Yang, Y.S. Lee, B.O. Yoo, Y.H.

Jung, H. Kim, C.K. Kim, Yeon Soo Kim, G.L. Hofman,

“KOMO-4 test PIE results,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Lisbon, Portugal, Oct. 10 - 14, 2010. http://www.rertr.anl.gov [ 40 ] S. Dubois, J. Noirot, J. Gatt, M. Ripert, P. Lemoine, P.

Boulcourt, “Comprehensive overview on IRIS program:

Irradiation tests and PIE on high density UMo/Al dispersion

(12)

fuel,” Trans. RRFM 2007, Lyon, France, Mar. 11-15, 2007.

http://www.euronuclear.org/meetings/rrfm2007/index.htm [ 41 ] A. Izhutov, V. Alexandrov, A. Novosyolov, V. Starkov,

A. Sheldyakov, V. Shishin, V. Yakovlev, I. Dobrikova, A.

Vatulin, V. Suprun, Ye. Kartashov, V. Lukichev, “The Status of LEU U-Mo Fuel Investigation in the MIR Reactor,” Proc.

Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Beijing, China, November 1-5, 2009.

[ 42 ] A. Izhutov, V. Alexandrov, A. Novosyolov, V. Starkov, A. Sheldyakov, V. Shishin, V. Iakovlev, I. Dobrikova, A.

Vatulin, G. Kulakov, V. Suprun, “The Main Results of Investigation of Modified Dispersion LEU U-Mo Fuel Tested in the MIR Reactor,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Lisbon, Portugal, Oct. 10 - 14, 2010. http://www.rertr.anl.gov [ 43 ] Yeon Soo Kim, J.M. Park, H.J. Ryu, Y.H. Jung, G.L. Hofman,

“Reduced interaction layer growth of U-Mo dispersion in Al-Si,” J. Nucl. Mater., 430 (2012) 50.

[ 44 ] Yeon Soo Kim, G.L. Hofman, H.J. Ryu, “Irradiation-enhanced interdiffusion in the diffusion zone of U-Mo dispersion fuel in Al,” J. Phase Equil. Diff., vol. 27, pp. 614-621 (2006).

[ 45 ] D.M. Wachs, D.E. Burkes, S.L. Hayes, W. Skerjanc, G.L.

Hofman, Yeon Soo Kim, H.J. Ryu, “Modeling the integrated performance of dispersion and monolithic U-Mo based fuels,”

Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Cape Town, South Africa, Oct.

29 - Nov. 2, 2006. <http://www.rertr.anl.gov>

[ 46 ] A. Soba, A. Denis, “An interdiffusional model for prediction of the interaction layer growth in the system uranium- molybdenum/aluminum,” J. Nucl. Mater., vol. 360, pp.

231-241 (2007).

[ 47 ] D. Olander, “Growth of the interaction layer around duel particles in dispersion fuel,” J. Nucl. Mater., vol. 383, pp.

201-208 (2009).

[ 48 ] Yeon Soo Kim, G.L. Hofman, P.G. Medvedev, G.V.

Shevlyakov, A.B. Robinson, H.J. Ryu, “Post irradiation analysis and performance modeling of dispersion and monolithic U-Mo fuels,” Proc. Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Prague, Czech Republic, Sept. 23-27, 2007.

<(http://www.rertr.anl.gov>

[ 49 ] Yeon Soo Kim, G.L. Hofman, J.M. Park, “Behavior of silicon on improved performance of U-Mo/Al-Si dispersion

- analysis and interpretation of KOMO-4 test results,” Proc.

Int. Meeting on Reduced Enrichment for Research and Test Reactors (RERTR), Lisbon, Portugal, Oct. 10 - 14, 2010.

[ 50 ] Yeon Soo Kim, G.L. Hofman, A.B. Robinson, D.M. Wachs,

“Improved irradiation performance of uraniummolybdenum 0aluminum dispersion fuel by silicon addition in aluminum,”

Nucl. Technol., vol. 184, pp. 42-53 (2013).

[ 51 ] T. Wiencek, “Summary report on fuel development and miniplate fabrication for the RERTR program,” ANL/

RERTR/TM-15, Argonne National Laboratory, 1995.

[ 52 ] Yeon Soo Kim, Uranium Intermetallic Fuels (U-Al, U-Si, U-Mo), In: R.J.M. Konings (ed.) Comprehensive Nuclear Materials, volume 3, pp. 391-422 Amsterdam, Elsevier, 2012.

[ 53 ] Yeon Soo Kim, G.L. Hofman, J.S. Cheon, “Recrystallization and fission-gas-bubble swelling of U-Mo fuel,” J. Nucl.

Mater., vol. 436, pp. 14-22 (2013).

[ 54 ] Yeon Soo Kim, G.L. Hofman, “Improved Performance of U-Mo Dispersion Fuel by Addition of Si in Al Matrix,”

Argonne National Laboratory report, ANL/RERTR/TM- 11-24, 2011.

[ 55 ] M. Ugajin, M. Akabori, A. Itoh, N. Ooka, Y. Nakakura,

“Behavior of neutron-irradiated U3Si,” J. Nucl. Mater., vol. 248, pp. 204-208 (1997).

[ 56 ] G.L. Hofman, Yeon Soo Kim, A.B. Robinson, “Fission induced swelling and creep of uranium-molybdenum alloy fuel,” Trans. RRFM 2009, Vienna, Austria, Mar. 22 - 25, 2009.

<http://www.euronuclear.org/meetings/rrfm2006/index.htm>

[ 57 ] Yeon Soo Kim, G.L. Hofman, J.S. Cheon, A.B. Robinson, D.M. Wachs, “Fission induced swelling and creep of U-Mo alloy fuel,” J. Nucl. Mater., vol. 437, pp. 37-46 (2013).

[ 58 ] H.J. Ryu, Yeon Soo Kim, J.M. Park, H.T. Chae, C.K. Kim,

“Performance evaluation of U-Mo/Al dispersion fuel by considering a fuel-matrix interaction,” Nucl. Engin. Technol., vol. 40, pp. 409-418 (2008).

[ 59 ] Yeon Soo Kim, G.L. Hofman, “Fission product induced swelling of U-Mo alloy fuel,” J. Nucl. Mater., vol. 419, pp. 291-301 (2011).

[ 60 ] M.L. Bleiberg, L.J. Jones, B. Lustman, “Phase changes in- pile irradiated uranium-base alloys,” J. Appl. Phys., vol. 27, pp. 1270-1283 (1956).

[ 61 ] M.L. Bleiberg, “Effects of fission rate and lamella spacing upon the irradiation-induced phase transformation of U- 9wt%Mo alloy,” J. Nucl. Mater., vol. 2, pp. 182-190 (1959).