ML19170A227

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Proceedings of the 17th International Symposium on the Packaging and Transportation of Radioactive Materials Patram 2013, August 18-23, 2013, San Francisco, CA, USA - Criticality Safety of Enriched UF6 Cylinders
ML19170A227
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Issue date: 06/19/2019
From: Begue L, Caplin G, Evo S, Milin M
Daher Nuclear Technologies GmbH, Govt of France, Institute for Radiological Protection & Nuclear Safety (IRSN)
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Proceedings of the 17th International Symposium on the Packaging and Transportation of Radioactive Materials PATRAM 2013 August 18-23, 2013, San Francisco, CA, USA CRITICALITY SAFETY OF ENRICHED UF6 CYLINDERS L. Begue IRSN - France M. Milin IRSN - France G. Caplin IRSN - France S. Evo IRSN - France ABSTRACT The transport of enriched hexafluoride uranium (UF6) is carried out in large steel cylinders. In filled cylinders, a maximum of 0.5wt. % of impurities is assumed to be present in UF6. Usually, for nuclear criticality safety studies, these impurities are supposed to be HF and the cylinders content is then modeled by a UF6-0.1HF medium completely filling the cavity. The aim of this paper is to highlight parameters which have a significant and non-intuitive impact on reactivity and to present an evolution of the nuclear criticality safety assessment by taking into account uranium residues like UO2F2 in filled UF6 cylinders.

The first part of this paper focuses on three main parameters (uranium mass, interstitial moderation between cylinders, cylinders steel wall thickness) which have an impact on reactivity. The calculations are performed for an infinite array of 30B cylinders (30 inches diameter) containing enriched UF6 up to 5wt. % of 235U. The main conclusions of this part are:

 the maximum of reactivity is reached when cylinders are not entirely filled with UF6,

 the interstitial moderation combined with a gap between cylinders lead to increase reactivity.

Moreover, some air in-leakage in cylinders is possible and its moisture content results in the hydrolysis of UF6 and creation of non-volatile uranium residues, in particular UO2F2 complexes with H2O and HF. Because cylinders are not washed before each transport, the accumulation of such residues is possible in the cylinders.

So, the second part of the study deals with the conservative composition of these residues in regard to reactivity and compares the reactivity of the packaging with and without their presence.

According to scientific literature, the ratio H/U in UO2F2 complexes is variable. Furthermore, the mass of uranium residues depends on the amount of air moisture in-leakage in a cylinder which is difficult to quantify. The approach presented in this paper considers calculations with various spherical masses of UO2F2 per package with various H/U ratios surrounded by UF6-0.1HF. The results of these calculations show that the presence of uranium residues can increase considerably the reactivity of the packaging (depending on the H/U ratio in the complex residues).

INTRODUCTION The packaging and transport of UF6 are performed in accordance with the standards ISO 7195 [1]

and ANSI N14.1 [2] which provide safety requirements and assist all stakeholders in the nuclear industry. These standards present various designs of UF6 cylinders. This article deals with the most used type of cylinders, the 30 inches cylinder containing UF6 enriched up to 5wt.% of 235U. As shown on figure 1, the 30 inches cylinder (cylinder 30B for example) is a right circular cylinder and

is made of carbon steel. Generally, it is transported in an overpack (shielding) to protect the cylinder (and in particular the drain plug) against mechanical and thermal effects in case of an accident during the transport.

UF6 is shipped in solid phase and is transformed in gas or liquid phase to be processed in nuclear facilities. The mass limit in a cylinder, according to the requirements of the standard [1], is based on the UF6 liquid density at 394K (3257 kg/m3 compared to the solid density which is 5090 kg/m3 at 294K), on the minimum internal volume of the cylinder (0,736 m3) and on a minimum required free space in the cylinder of 5 %. So, the maximum fill limit for a 30 inches cylinder is 2277 kg of UF6.

The minimum purity of UF6 has to be equal to 99.5 % according to the standards. All the remaining compounds (0.5 % in mass) which are allowed in the content (for product quality concerns) are volatile compounds, such as the hydrofluoric acid gas (HF). However, during the use of UF6 cylinders, non volatile products of uranium reactions may be created (the so-called residues), in particular uranyl fluoride compounds (UO2F2) which are one of the products of UF6 hydrolysis.

The nuclear criticality safety of UF6 enriched cylinders is based on the geometrical dimensions and on a limited moderation of the content.

This article presents the main parameters which have an impact on the nuclear criticality safety analysis for an infinite number of UF6 filled 30 inches cylinders, in particular in presence of residues of uranium in the cylinders. In all the calculations, uranium content in 235U is 5 % by weight. Calculations are performed using the standard route APOLLO2-MORET 4 of the CRISTAL V1 package [3], associated with the JEF 2.2 nuclear data library.

ASSESSMENT OF FILLED CYLINDERS Criticality safety practice usually takes into account hydrofluoric acid (HF) as the only impurity in UF6 to assess nuclear criticality safety of filled cylinders. In the first part of this paper, only these volatile impurities are considered.

In order to account for the minimum UF6 purity of 99.5 %, a ratio H/U equal to 0.088, rounded to 0.1, is fixed in the homogeneous fissile mixture (UF6-0.1HF).

Impact of the UF6 mass and the interstitial moderation The assumptions for this first study are the following:

 cylinders are modeled by a cylindrical cavity made by a 7.94 mm steel thickness;

 the content is modeled by a homogeneous fissile mixture (UF6-0.1HF) at varying density in the entire internal cavity;

 the moderation between cylinders is modeled by a varying layer of water around each cylinder;

 the calculations are based on an infinite array of cylinders by applying a reflection directly on the water layer of one cylinder.

Figure 1. Picture and sketch of UF6 cylinder 30B (dimensions in cm and inches)

The calculation model is illustrated on the figure 2a below and covers an infinite array of cylinders (as represented on the figure 2b). The variation of the UF6 density in the entire internal cavity allows to make vary the UF6 mass in the cylinder. Moreover, this model can represent the variation of the UF6 density during the change of phases (solid or liquid). The modeling of a variable layer of water directly on the steel thickness and a reflection permits to cover the presence of water or miscellaneous hydrogenous products between cylinders in the array.

The results of the calculations, illustrated on figure 3, present the variation of the effective multiplication factor (k-eff) as a function of the mass of UF6 in each cylinder and the moderation between cylinders (varying thickness of a water layer around each cylinder). Cylinders are in contact when the layer of water is null and spaced out otherwise. The ratio between the UF6 density in the calculation and the maximal UF6 density is named filling ratio. When this ratio is equal to 1, the modeled mass of UF6 is equal to 3620 kg. The maximal allowed mass of UF6 specified by ISO 7195 (2277 kg) is obtained for a filling ratio of 0.63.

The previous graph shows that the highest reactivity is obtained when cylinders are surrounded by a layer of water and partially filled of UF6. In this example, the k-eff is under 0.80 without a layer of water around the cylinders and reaches 1 (critical(a)) when the water layer is 1 cm thick. In this case, corresponding to the optimal interstitial moderation between cylinders, the maximum of reactivity is obtained for an UF6 filling ratio close to 0.3. For a filling ratio of 0.63 (corresponding to the actual maximal amount of UF6 allowed per cylinder), the maximal value of Figure 2b. 3D schematic representation of the array UF6-0.1HF (variable density)

Reflection on the layer of water Steel thickness Layer of water (variable thickness)

Figure 2a. Calculation model Figure 3. Mass of UF6 and interstitial moderation impact on reactivity 0.7 0.75 0.8 0.85 0.9 0.95 1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

UF6 filling ratio k-eff 0 cm 0.2 cm 0.6 cm 1.0 cm 1.4 cm 2.0 cm 3.0 cm Série8 Thickness of the water layer 2277 kg d'UF6

reactivity is equal to ~0.96, that is 4 %(b) higher than the maximal reactivity for completely filled cylinders.

These results could be firstly explained by the following qualitative analysis: The increase of the water layer thickness between cylinders improves the neutron slowing down effect and the number of fission reaction in the fissile medium. This phenomenon is improved by increasing the neutron flow reaching the water layer by decreasing the density of the fissile medium.

This simple explanation could be verified by considering some main neutronic parameters. Indeed, the k-eff is the result of the ratio between the number of produced neutrons on absorbed neutrons in this infinite system (no neutron leakage). Figures 4 and 5 illustrate the variation of these two parameters in function of the UF6 filling ratio and the thickness of the water layer between the cylinders.

Figure 4a, obtained with a water layer of 1 cm, shows that the ratio between productions and absorptions of neutrons in the fissile medium (k-inf) increases with the UF6 density diminution in the cylinders. This UF6 mass diminution involves the increase of neutron leakage out of the cylinders and thus an improvement of the slowing down of neutrons in the water layer as shown on figure 4b with the decreasing curve of the energy corresponding to average lethargy of neutrons causing fission (EALF). This effect is obviously limited by the decrease of the absorption rate of neutron in the fissile medium (a part of which is causing fissions) and by the increase of the absorption rate (capture) in the water layer and in the steel shell, as shown on the figures 4a and 4c.

Thus, the combination of these effects involves an optimum of k-eff for an UF6 filling intermediate ratio (around 0.3).

Regarding now the effect of the water layer thickness (which provides the moderation of the neutrons, as seen previously), the figure 5a, obtained with a filling ratio of 0.3, shows that the ratio between productions and absorptions of neutrons in the fissile medium (k-inf) increases with the thickness of the water layer. Figure 5b below shows that for a small thickness of the water layer, the EALF is in the epithermal range whereas, for larger layers, the EALF is in the thermal range. This effect is compensated, as showed on figures 5a and 5c, by:

0 0.3 0.6 0.9 1.2 1.5 1.8 0.0 0.2 0.4 0.6 0.8 1.0 K-inf K-eff Absorption rate in fissile medium 0.1 1

10 100 0.0 0.2 0.4 0.6 0.8 1.0 Figure 4b. EAFL (eV) vs UF6 filling ratio Figure 4c. Absorption in materials (%)

vs UF6 filling ratio 1

10 100 0.0 0.2 0.4 0.6 0.8 1.0 Steel Water Figure 4a. Neutronic parameters vs UF6 filling ratio (thickness of water layer = 1 cm)

the increase of neutron absorptions in the water layer and in the steel shell as the slowing down of the neutrons is better due to the increase of the water quantity between cylinders; the absorption rate of the neutrons in the fissile medium (a part of which is causing fissions) which decreases in the same time.

Thus, the combination of these effects involves an optimum of k-eff around a thickness of the water layer of 1 cm (space between cylinders of 2 cm).

Impact of the modeling of UF6 in cylinders A second way is possible to represent the varying mass of UF6 in the cylinders, by varying the UF6 crust (at the maximum density) on the cylinders walls. This model is illustrated on the figures 6a and 6b below.

The results of this other modeling are illustrated on the figure 7 below and are compared with the previous case with a variable density of UF6 (i.e Figures 2a and 2b), as a function of the mass of UF6. This comparison is made for the optimal interstitial moderation (optimal water layer thickness between cylinders).

Figure 6a. Calculation model Crust of UF6-0.1HF (maximum density)

Reflection on the layer of water Layer of water (variable thickness)

Void in internal cavity Figure 6b. 3D schematic representation of the array 1

10 100 1000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 5b. EAFL (eV) vs the water layer thickness in cm Figure 5c. Absorption in materials (%)

vs the water layer thickness in cm Figure 5a. Neutronic parameters vs the water layer thickness in cm (UF6 filling ratio = 0.3) 0.4 0.6 0.8 1

1.2 1.4 1.6 0

0.5 1

1.5 2

2.5 3

K-inf K-eff Absorption rate in fissile medium 0

10 20 30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Steel Water

The maximum reactivity is higher for the model with a crust of UF6 than for the low UF6 density model. The difference between these two models is about 1.5 %(b) and the underlying hypotheses of the crust modeling could not be considered as incredible because UF6 is shipped and stored in solid phase. This last model should be part of the nuclear criticality safety assessment.

Impact of steel thickness on k-eff In the previous calculations, the cylinders wall is modeled by a 7.94 mm thickness of steel which is the minimum steel thickness which should be guaranteed according to ISO 7195 [1]. As showed on the figure 1, the nominal thickness in the design plans is 12.7 mm. Moreover, during transport, an overpack, made of foam enclosed by steel plates, is placed around the cylinder.

The impact of this parameter on reactivity is studied by plotting the variation of the reactivity in function of the UF6 filling ratio for three different steel thicknesses (7.94 mm, 10 mm and 12.7 mm). The results of this study are illustrated on the figure 8 below.

The results of the calculations show that the steel thickness around the fissile material has a significant impact on the reactivity. For example, the impact on reactivity between 7.94 mm and 10 mm of steel or between 10 mm and 12.7 mm for each cylinder is a decrease around 7 %(b).

Indeed, steel is an absorbent material for the neutrons. Consequently, the thickness of steel of UF6 cylinders is an important parameter allowing to decrease the reactivity. However, it should be kept in mind that the thickness of steel considered in the nuclear criticality safety assessment has to be guaranteed during lifecycle of the UF6 cylinders.

IMPACT OF UO2F2 RESIDUES IN FILLED CYLINDERS UF6 cylinders have to be cleaned before undergoing various tests and controls, at least every five years. The purpose of cleaning is to remove all residues from the cylinders. These residues are grease, slag, oxides, dirt and other foreign matters. Many of them are insoluble and non volatile 0.9 0.925 0.95 0.975 1

1.025 0

500 1000 1500 2000 2500 3000 3500 UF6 mass (kg) k-eff UF6-0.1HF crust UF6-0.1HF at variable density Figure 8. Steel thickness impact on reactivity Steel thickness of cylinders 0.8 0.85 0.9 0.95 1

0 0.2 0.4 0.6 0.8 1

UF6 filling ratio k-eff 7.94 mm 10 mm 12.7 mm Figure 7. Impact of the UF6 mass on reactivity

reaction products of uranium, like UF5 (formed by the reaction between iron and UF6), UF4 (formed by the reaction 2 UF5  UF4 + UF6) and uranyl fluoride solids (UO2F2) formed by hydrolysis reaction between gaseous of UF6 and moisture introduced by air leackage (UF6 + 2H2O  UO2F2 +

4HF). From a nuclear criticality safety point of view, it is assumed that these last residues (UO2F2) bound all the others non volatile reaction products of uranium (UF5, UF4) because of the existence of hydrated compounds of UO2F2.

In current criticality studies, the presence of UO2F2 residues is taken into account for empty cylinders which contain a residual amount of UF6 and non volatile reaction products of uranium in quantities less than those specified in standards [1] and [2] (the so-called heel). In this case, the content of empty cylinders is modeled by a limited mass of UO2F2 hydrated by any quantity of water. Generally, these uranium residues are not taken into account in filled cylinders. To upgrade the safety approach, the second purpose of this article is to show in which conditions the presence of these residues could be reasonably neglected. For this, it is first necessary to found in chemical literature the possible compositions of UO2F2 residues products and then to make calculations of k-eff to estimate the impact of a varying mass and composition of hydrated compounds of UO2F2 in filled UF6 cylinders.

Origin and characteristics of uranium residues The hydrolysis reaction is due to small introduction of air moisture in UF6 cylinders (under the atmospheric pressure) during engagement (or accosting) and disengagement for the filling or the extraction of UF6 and during long storage periods. In this case, the available quantity of water in the cylinder is low (for the UF6 hydrolysis reaction) in comparison to the UF6 content. According to the scientific literature, experiments show that the products of hydrolysis reaction between UF6 (gaseous) and H2O (vapour) are gaseous HF and a solid complex UO2F2-xH2O-yHF in which x and y ratios are variable in function of experimental conditions. In criticality safety assessment, it could be assumed (by conserving the same H/U ratio) a bounding material UO2F2-(x+y/2)H2O to model these uranium residues. The main issue is to have a conservative approach to define the maximum credible ratio H/U (2x+y).

When looking in details at the scientific literature, it can be shown that only few results are dedicated to the composition of the direct products of the UF6 hydrolysis reaction (there are mainly either the hydratation of anhydrous UO2F2 by a vapour of H2O, either the reaction between hydrated UO2F2 and HF). For example, one of these [4], shows that the formula of the complex produced by the reaction between water and UF6 (gas) is UO2F2-1.3H2O-0.35HF (the ratios x and y are average values). It is difficult to conclude from these results only, especially knowing that the experimental conditions are not sufficiently detailed.

Regarding experiments about the hydratation of anhydrous UO2F2, the operating conditions are generally the reaction of air moisture on samples of anhydrous UO2F2. In the document [5], the hydrated complexes observed are from UO2F2-1.86H2O up to UO2F2-4H2O. The experiments show that this last complex, formed in case of high humidity content on small samples, loses water after a long time in ambient air, to become a more stable complex UO2F2-2H2O. A diagram of the different UO2F2-xH2O forms in function of the temperature and the H2O pressure results from these experiments. This diagram shows that, at 298K, four different hydrated complexes exist (UO2F2, UO2F2-2H2O, UO2F2-2.5H2O and UO2F2-4H2O), but at 308K and 350K, the highest hydrated complex is respectively UO2F2-2.5H2O and UO2F2-2H2O. The existence of the upper hydrate (UO2F2-4H2O) is also approached in the bibliographical study of the document [6] which references the results of Brooks investigations [7]. In the document [8], others experiments show that the hydratation of low thickness samples of anhydrous UO2F2 by air moisture (relative humidity inferior or equal to 90 %) could lead to the formation of a complex UO2F2-4H2O non stable which evolves towards the stable complex UO2F2-1.5H2O (after several hours). The hydratation of higher

thickness samples of anhydrous UO2F2 by air moisture shows the direct formation of the stable complex UO2F2-1.5H2O, without higher hydrated composition (in average for the sample). Thus, the results of the main experiments about UO2F2 complexes show that the hydratation of UO2F2 in presence of air moisture should not be higher that UO2F2-4H2O, with a stable form around UO2F2-2H2O.

Regarding experiments carried out on the action of gaseous HF on different hydrated UO2F2 solids, results presented in the document [5] show that this type of reaction is slow. The reaction of gaseous HF on the UO2F2-2H2O (at 298K) gives the complex UO2F2-2H2O-2HF, which is similar to the product obtained by Brooks experiments [7]. The experiments presented in the document [5],

carried out on two others hydrated complexes UO2F2-3H2O and UO2F2-4H2O, show the formation respectively of UO2F2-3H2O-2.44HF and UO2F2-4H2O-3HF complexes. The author observed that these last complexes are not stable (quick decomposition). So, when the hydrolysis reaction of UF6 is due to the introduction of a small quantity of air moisture in a UF6 cylinder, the formation of a limited hydrated complex of UO2F2 is observed by different experiments. The ratio H/U in the complex UO2F2-xH2O-yHF depends on the conditions of the use of the UF6 cylinders in the fuel cycle facilities and during the transport. Without specific arrangements to guarantee a maximum quantities of air moisture in-leakage in the cylinders and their conditions of use during their lifecycle (temperature, pressure), it is difficult to know the maximum H/U ratio (2x+y). However, regarding the current experimental knowledge, it can be assumed that this ratio is surely lower than 11 in UO2F2-xH2O-yHF residues.

In order to cover uranium residues in filled UF6 cylinders, calculations are carried out to study the impact of these residues in function of their composition and mass.

Impact of uranium residues on reactivity The mass and the moderation ratio H/U of uranium residues depending strongly on the way they are created, it is interesting to study their impact on reactivity. This comparison is performed for 10 mm steel thick cylinders filled with UF6-0.1HF at variable density. Interstitial moderation between cylinders is modeled by a layer of water around the cylinder (the thickness of the layer is chosen in order to reach the maximum k-eff). Uranium residues are modeled by a sphere of UO2F2-(x+y/2)H2O in one corner of the cylinder, which is the most reactive configuration (maximization of neutron interactions). The figures 9a and 9b show a sketch of the calculation model and a 3D representation of the associated array.

Figure 10 shows the increase of the maximum reactivity whatever the UF6 filling ratio is, due to uranium residues mass and ratio H/U in the UO2F2-(x+y/2)H2O medium (i.e. H/U=2x+y). keff is the difference between the k-eff of an array of cylinders with uranium residues and an array of cylinders without uranium residues.

Figure 9a. Calculation model UF6-0.1HF (variable density)

Uranium residues Reflection on the layer of water Figure 9b. 3D Representation of the array

From this figure, it can be noted that:

impact of uranium residues on the reactivity of UF6 cylinders depends on the mass and on the moderation ratio (H/U) of residues; a mass of uranium residues lower than 5 kg per cylinder or a moderation ratio H/U (i.e (2x+y) in UO2F2-(x+y/2)H2O) in uranium residues lower than 3 have a limited impact on reactivity (less than 1 %);

for a mass of uranium residues per cylinder lower than 11.4 kg (which corresponds to the allowed limit of heel for the 30 inches empty cylinder transport), the impact on reactivity of residues is higher than 1 % when the moderation ratio (H/U) in uranium residues is higher than 5; for a moderation ratio (H/U) in uranium residues lower than 11, the impact on reactivity of residues is higher than 1 % when the mass of uranium residues per cylinder is higher than 6 kg.

Figure 11 below shows the impact of uranium residues on reactivity of UF6 cylinders for various masses and moderation ratios in function of the UF6 filling ratio in cylinders.

M=mass of UO2F2-(x+y/2)H2O H/U=2x+y 0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1

0 0.2 0.4 0.6 0.8 1

UF6 filling ratio k-eff M=15kg & H/U=14 M=15kg & H/U=10 M=10kg & H/U=20 M=10kg & H/U=14 M=10kg & H/U=10 M=10kg & H/U=6 M=5kg & H/U=10 M=0kg Figure 11. Uranium residues impact on reactivity Moderation ratio of the residues (H/U) 0 5 10 15 20 keff > 10 %

7 % < keff 10 %

5 % < keff 7 %

3 % < keff 5 %

2 % < keff 3 %

1 % < keff 2 %

keff 1 %

0 5 10 15 20 Figure 10. k-eff increase due to uranium residues Mass of UO2F2-(x+y/2)H2O residues (kg)

From this figure, it is interesting to note the following points:

for small masses and/or moderation ratios of uranium residues, the increase on reactivity due to these residues is limited and the maximum of reactivity is obtained for an intermediate UF6 filling ratio (around 0.3); in this case, the presence of uranium residues do not significantly change the overall behavior of the cylinders reactivity.

for high masses and moderation ratios of uranium residues, the maximum of reactivity shifts to the maximum UF6 filling ratio. However, this effect seems to be significant only for uranium residues masses higher than the allowed limit of heel for the 30 inches empty cylinders and higher than the credible H/U assumed (11).

These results highlight the need to have more knowledge on the composition and on the mass of hydrated uranium residues in filled or partially filled cylinders. However, presently a maximum mass of UO2F2 residues cant be guaranteed by a weighing of a filled cylinder which contained some metric tons of UF6. The current scientific literature and partially knowledge on these residues are not sufficient to conclude on a specific reasonably bounding composition of them. As shown by the calculations results, a specific area (a mass lower than 10 kg or a ratio H/U lower than 5) appears where the impact of residues on the reactivity of UF6 cylinders can be neglected if specific arrangements (a feedback of the use of the cylinders, additional experiments and analysis on the products of the hydrolysis reaction, specific conditions of use of the cylinders) are provided to guarantee the boundaries of this area.

CONCLUSIONS This article shows that the uranium mass per cylinder, the interstitial moderation between cylinders and the cylinders steel wall thickness are three main parameters which should be studied in the nuclear criticality safety assessment of filled UF6 cylinders. Moreover, an evolution of assessment is presented by taking into account hydrated uranium residues produced by the hydrolysis of UF6 due to air moisture in-leakage in cylinders. These residues should be assumed to be hydrated compounds of UO2F2. The results of calculations show that the presence of uranium residues can increase considerably the UF6 cylinders reactivity (depending on the H/U ratio in the residues and on their mass). The safety issues are on the control of the mass and the composition of hydrated uranium residues in filled or partially filled cylinders. A specific area (maximum mass or ratio H/U of hydrated residues) appears where the impact of residues on reactivity of UF6 cylinders can be neglected. Thus, it seems necessary to deepen the knowledge on the composition and on the mass of hydrated uranium residues in the UF6 enriched cylinders to justify the criticality safety margins.

END NOTES:

(a): corresponding to the infinite array of cylinders which is conservative from the nuclear criticality safety point of view.

(b): percent in keff.

REFERENCES

[1] INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Packaging of Uranium Hexafluoride (UF6) for Transport, ISO 7195:2005 (F), ISO, Geneva (2005).

[2] AMERICAN NATIONAL STANDARDS INSTITUTE, American National Standard for Nuclear Materials - Uranium Hexafluoride - Packaging for Transport, ANSI N14.1 - 2001, ANSI, USA (2001)

[3] GOMIT, J.M., et al., CRISTAL V1: Criticality package for burn up credit calculations, Proc.

Int. Conf. on Nuclear Criticality Safety ICNC2003, Tokai-Mura, Japan, Oct. 20-24 (2003).

[4] H.E. Eduljee, B.R.348A, ICI Ltd, The reaction between water and uranium hexafluoride and the action of heat on the reaction product (1943).

[5] G. NEVEU, Report CEA 2106, Etude des composes UO2F2-H2O-HF (1961).

[6] F. MORATO, Thse de luniversité de Montpellier II (1997), F. MORATO, Contribution létude de la réduction de luranium. De UF6 UO2 par pyrohydrolyse et action du dihydrogne (1997).

[7] BROOKS, GARNER, WHIEEHEAD, IGR-TN/CA-277, U.K.A.E.A. (1958).

[8] UKAEA DEG, Report 352 (CA), The hydrates of uranyl Fluoride (1963).