The effect of α-irradiation from enriched uranium on

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Engineering Failure Analysis 74 (2017) 1–10

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The effect of α-irradiation from enriched uranium on the leaching properties of PTFE J.J. Badenhorst a,b,⁎, W.C.M.H. Meyer a, T.J. van Rooyen a, H.M. Krieg b a b

South African Nuclear Energy Corporation SOC Ltd (Necsa), Building 1900, P O Box 582, Pretoria 0001, South Africa Focus Area: Chemical Resource Beneficiation, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2531, South Africa

a r t i c l e

i n f o

Article history: Received 14 September 2016 Received in revised form 16 November 2016 Accepted 12 December 2016 Available online 18 December 2016 Keywords: MCNP Polytetrafluoroethylene Dose rate Degradation Enriched uranium α-irradiation

a b s t r a c t In the uranium enrichment plants of the South African Nuclear Energy Corporation SOC Ltd (Necsa), sintered polytetrafluoroethylene (PTFE) filters were used to remove gas entrained solid particles, in order to prevent blocking of the isotope separating elements. When these plants were decommissioned and dismantled, the filters which mostly contained solid uranium fluoride and uranium oxyfluoride compounds and compressor ring dust, were broken into pieces, put into metal drums, and stored. This waste contains enriched uranium and can't be disposed of at the current disposal site (Vaalputs), which only accepts low and intermediate level waste. Initial attempts, directly after removal from the enrichment plants, to decontaminate these filters using various aqueous solutions, were unsuccessful probably because of the known non-wettability of PTFE surfaces. However, recent attempts to leach the absorbed enriched uranium were successful. This resulted in a study to determine the effect of mainly α-irradiation from the enriched uranium on the morphology of PTFE. A Monte Carlo N-Particle transport code (MCNP), a general purpose radiation transport code modelling the interaction of radiation with materials that simulate nuclear processes, was used to determine the rate of PTFE radiolysis. The results confirmed that the dose rate received from mainly the α-particles (97%) during the storage period had caused significant structural damage to the PTFE depending on the enrichment grade and the amount of uranium on the filters. To confirm the modelling data, analytical techniques, including micro X-ray tomography, thermogravimetric analysis and X-ray diffraction were used to study the morphology changes in the PTFE structure of the research samples. Experimental results indicated that the crystallinity increased while the molecular weight of the PTFE decreased. This could be attributed to the radiation induced degradation of the PTFE by the absorbed enriched uranium. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Naturally occurring uranium consists of three major isotopes, 238U (99.2739–99.2752% natural abundance), 235U (0.7198– 0.7202%) and 234U (0.0050–0.0059%) [1]. All three isotopes are radioactive, creating a progeny cascade of radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683 × 109 years. 238U decays through a transition cascade (“decay-series”) featuring 16 prominent radionuclides. 235U transitions through a cascade featuring 14 prominent radionuclides

⁎ Corresponding author at: South African Nuclear Energy Corporation SOC Ltd (Necsa), Building 1900, P O Box 582, Pretoria 0001, South Africa. E-mail addresses: [email protected] (J.J. Badenhorst), [email protected] (W.C.M.H. Meyer), [email protected] (T.J. van Rooyen), [email protected] (H.M. Krieg).

http://dx.doi.org/10.1016/j.engfailanal.2016.12.003 1350-6307/© 2017 Elsevier Ltd. All rights reserved.

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[2]. During the enrichment process for the manufacturing of reactor fuel, the percentage content of 235U needs to be increased and as a result the percentage content of U234, the lighter uranium isotope, is also increased. Prior to 1990, PTFE sintered candle filters were used in the enrichment facilities at Necsa to remove solid particles from UF6 to prevent the blocking of the separating units [3]. During operation and the final decommissioning of the enrichment facilities, these uranium-contaminated filters were removed and stored in metal drums, awaiting eventual disposal or possible recovery for re-use in the 99Mo manufacturing process [4]. The disposal of high concentrations of enriched uranium as waste at the disposal site, Vaalputs, are not allowed and therefore the enriched uranium must be removed before PTFE disposal would be allowed [5]. PTFE is extremely chemically stable against a variety of the most aggressive reagents, for example, elemental fluorine, uranium hexafluoride, molten alkali metal hydroxides and hot mineral acids. It has very low surface energy, leading to unusual advantages like low friction, anti-stick properties and non-wettability, thus the reason why PTFE seals and filters were used in the uranium enrichment plants [6]. Wettability of a solid surface is quantitatively measured in terms of contact angle: a small water contact angle (≪90°) corresponds to high wettability whereas large water contact angle (≫90°) corresponds to poor wettability. The water contact angle for PTFE is recorded between 100° and as high as 112° [7,8]. Leaching of uranium from the contaminated filters directly after decommissioning was unsuccessful but recent attempts after 20 years showed that N95% of the entrapped uranium could be leached from the PTFE using wet chemical techniques. PTFE suffers rapid molecular-mass degradation and evolution of low molecular-mass fluorocarbons when exposed to ionizing radiation caused by the alpha (α), beta (β), or gamma (γ) particles should the radiation energy be above the C\\F or C\\C covalent bonding energy. Radiation energy below this C\\F bonding energy has a possibility of generating excited states, thereby forming free radicals (unpaired electrons) and/or a number of other chemical species [9]. When ionization occurs, it causes a reduction in the molecular weight which primarily influences the mechanical properties of the PTFE, such as an increase in brittleness and a reduction in tensile strength, modulus and elongation [10]. While the minimum energy required to break the covalent bond of the main carbon chain of PTFE is in the range of 5–10 eV, the energy provided by beta and gamma photons (1.0–10 MeV) by many orders of magnitude surpasses this minimum value, implying that degradation is likely for PTFE [11]. The threshold dose from all sources of radiation, independent of the type of radioactive particle for PTFE damage and crumbling, was reported as 103 Gy and 104 Gy respectively. Threshold of damage is where the first change in physical properties can be detected and is indicative of the start of radiolysis [12]. The effect of radiation is correlated with the calculated absorbed dose which is the average amount of energy imparted to the medium per unit mass. In general, radiation exposure of polymers results in cross-linking, chain-scission and side-branch formation that can influence the properties of the polymer as follows [13]: Chain scission in PTFE will result in: ➢ ➢ ➢ ➢

brittleness, fracturing, gas generation, and sometimes de-polymerization.

PTFE is a partly crystalline polymer in which the crystallinity increases under irradiation, even at small doses [14]. This phenomenon is explained by assuming the formation of molecular fragments (during the radiation chain-scission), which can combine to form small crystallites. The thermal conductivity, λ, and density, ρ, under irradiation increase because the conductivity of polymers is defined by photon transfer and the crystallite density is higher than the density of the amorphous phase. Ionizing radiation produces excited ions, both directly and indirectly by neutralization of the initial ions that formed. If the excited state has sufficient energy, the covalent bond breaks to form free-radical fragments [15]. 

ðR−SÞ →R• þ S• The influence of α-radiation on the PTFE may either cause C\\C or C\\F-bond breaking which causes degradation of the PTFE, ultimately yielding tetra-fluoro-ethylene, C2F4, which further increases the brittleness of the PTFE [16]. PTFE nominally retains its innate chemical structure and morphology at doses b 103 kGy which refer to units of energy per unit mass. Cross-linking, fragmentation and un-saturation increases at doses above 104 kGy which include extreme structural degradation of the polymer, accompanied by conversion to oxygen functionalized and aliphatic compounds. At these high doses PTFE undergoes a 50% reduction of tensile strength. Cross-linking and side-branch formation as a result of high doses cause an increase in polymer crystallinity observed with FTIR and DSC. Radiation behaviour is very sensitive to the presence of oxygen [17]. The products obtained on irradiation of PTFE can be explained by the following mechanism (Fig. 1). As an initial step, radiation gives rise to the random formation of primary and secondary fluorocarbon radicals. These radicals could combine with fluorine radical or remain as radicals. In the presence of air (oxygen), it is postulated that initially both the primary and secondary radicals produced react quantitatively with oxygen to give perfluroalkylperoxy radicals which by abstraction of fluorine or interaction with each other, can give rise to perfloroalkoxy radicals. The COF groups hydrolyze to COOH groups during irradiation by atmospheric humidity. In this manner it is possible to produce PTFE micro-powders having near surface COOH groups [18]. The rate of radiation induced chemical reactions in PTFE is slow and can continue over long time periods. For instance, immediately after irradiation of PTFE an acid fluoride end group forms and in the presence of ambient air, additional free and bonded carbonyl structures of carboxylic acid end groups could form that influence the polarity of PTFE. The non-polar PTFE becomes more polar which increases the wettability of the PTFE. The result of chemical degradation could be ascribed as fragmentation,

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Fig. 1. Probable course of radiation-induced degradation of PTFE in the presence of oxygen.

unsaturation and functionalization of molecules in the PTFE matrix. Continuing irradiation causes evolution of volatile species resulting in further morphological roughening and mass loss [19]. In order to confirm the assumption that α-irradiation from enriched uranium changes the morphology of PTFE over time, a theoretical model was applied. The Monte Carlo radiation transport modelling code, MCNPX 2.7, used in this study, was developed and maintained by the Los Alamos National Laboratory (LANL) in the USA. MCNPX (recently integrated with MCNP5 and now known as MCNP6 is an internationally recognized code for studying the transport of neutrons, photons, alpha particles, electrons, protons and many other particles, in matter. The equations of radiation transport through matter can only be solved analytically for some specific configurations. However, since the interaction processes for photons and electrons are well-known and precise cross-section data are available, radiation transport is ideal to be simulated using Monte Carlo methods. In these simulations, a photon or electron (“particle”) is tracked from creation until termination, with all decisions (starting energy, location of interaction, scatter angle, etc.) based on the interaction physics, extensive cross-section tables and pseudo-random numbers [20]. The code can model geometric detail to an accuracy of 0.1 μm [21]. The use of the Monte Carlo method to simulate radiation transport has become the most accurate means of simulating medical imaging systems with the aim of optimizing the instrumentation design or improving the accuracy of quantitative analysis for solving specific problems. In a recent study a MCNP modelled phantom (human body) was compared to a PET/CT image. The

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dimensions of the MCNP simulated image is approximately the same as the real dimensions of the CT grey scale image of the phantom. Thus, the MCNP code successfully simulated a CT image [22]. To use the MCNP code, the user creates an input file that is subsequently read by MCNP. This file contains the geometry specification, the elemental or isotopic content of materials, the location and energy spectrum of the photon, α-particle and electron source, the type of answers desired and any variance reduction techniques to be used to improve computational efficiency. These calculations quantify the dose rate contributions of the different types of ionizing radiation emitted by the mixture of uranium isotopes and their ingrowth progeny, i.e. alpha-particles, electrons or photons. The spatial pattern of energy deposition around a hypothetical point source embedded in the PTFE material was also calculated using this code. As input into the model, the enrichment and characterization of the PTFE filters were measured using an IQ3 drum scanner, which has the capability of assaying 208-L drums using a six high-purity germanium (HP6e) detector configuration [23]. It is a non-destructive technique and results are reported as average uranium enrichment of the drum content [24]. For the purpose of this study, the research samples were selected from different stored drums containing a range of reported uranium enrichment grades on PTFE material. The aim was therefore to model the influence of alpha radiation (from the enriched uranium absorbed in the current PTFE matrix) on the properties of PTFE using an energy deposition calculation from the Monte Carlo code MCNPX and to confirm the modelled results experimentally. 2. Experimental 2.1. Monte Carlo modelling for enriched uranium contaminated PTFE 2.1.1. Influence of total dose rate from radioactive particles generated from a point source of U235 through a PTFE matrix Monte Carlo software was used to simulate the stochastic radiation quantum transport through the geometry of the PTFE matrix. The modelling was performed using a hypothetical PTFE matrix containing 200 mg of 23% enriched uranium as a hypothetical point source, placed at the centre of a 1.0 cm PTFE sphere. Although the research material had an average pore size of 1.49 μm which results in a bulk density of 1.1 g cm− 3 throughout the PTFE matrix, the MCNP model assumes a mass-density of 2.2 g cm−3 for the PTFE which entrapped the enriched uranium. The spatial region between 0 and 1 cm was subdivided into 1000 spatial cells having logarithmically spaced cell radii. One thousand energy deposition values (F6 tallies) were calculated for each of the emitted primary particles: alpha-particles, electrons (line-electrons and beta-electrons), photons and recoil atoms. A graph was plotted of dose rate versus radial distance from the point source into PTFE after 20 years of ingrowth. The aim of the modelling was to calculate the contribution of each radioactive particle from enriched uranium, in terms of energy deposition and distance travelled into the PTFE matrix. A spectroscopic multi-particle source term was calculated as follows for the scenario described above. The initial (t = 0) number of atoms of U234, U235 and U238 in the 200 mg sample of 23% enriched uranium, was calculated and then “grown in” for t = 20 years using the code FISPACT-2007. A 33-row {Nuclide; Activity} matrix, referred to as a {N;A}-matrix, was obtained from this code, and these values were then used in the utility code MCNP_SOURCE_APP, developed at Necsa, South Africa. The latter code reads the {N;A}-matrix and uses the International Commission on Radiological Protection (ICRP) spectroscopic radionuclide emission database to calculate a net spectroscopic source definition (SDEF source term for MCNP runs). Such spectroscopic modelling source terms can be many thousands of lines long. This can only be reduced by approximation and it was decided to ignore charged particle and photon emissions under 1 keV in energy for modelling purposes. 2.1.2. Influence of dose rate from or a distributed source of 235U through a PTFE matrix Monte Carlo software was used to model the stochastic radiation quantum transport through a hypothetical volume of 4 cm3 and a density of 2.2 g cm−3 contaminated with 10% enriched uranium absorbed into the pores of the PTFE (1.0 g enriched uranium per 10 g of PTFE). The aim was to determine the dose that the PTFE will receive using a series of different enrichment grades and the dose through the PTFE and whether the accumulated dose could result in a morphological change of the PTFE matrix. MCNP was used to calculate the dose rate for a fixed mass fraction of uranium (10%) and different enrichment grades (ξ) which ranged from 0 to 100% (hypothetically because 100% enrichment is impossible). A graph was plotted for uranium enrichment (x-axis) versus dose rate for the PTFE volume (y-axis). Results from the graphs indicate how the enriched uranium influences the degradation of the PTFE over time.

2.2. Characterization of reference PTFE (non-contaminated) and enriched uranium contaminated PTFE 2.2.1. PTFE research samples A PTFE reference sample (un-irradiated) and a PTFE sample irradiated from enriched uranium, sourced after 20 years of storage in a metal drum was analysed to determine the effect of the α-irradiation from the enriched uranium on the morphology of the PTFE. The measured bulk density of the PTFE filter material was 1.1 g cm−3 compared to that of virgin PTFE (2.2 g cm−3). A short summary of the analytical techniques namely: Micro X-ray tomography, X-ray diffraction (XRD), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) as well as analysis done on the research samples are given.

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2.2.2. Micro X-ray tomography Micro X-ray tomography imaging generates 3D images of a sample's morphology and internal microstructure with resolution down to the sub-micron level, supplying high quality micron-level information of the structure in a non-invasively and non-destructively manner [25]. In this study, a Nikon XTH 225 ST micro focus X-ray tomography system with a 100 kV X-ray source with spatial resolution of 0.08 mm was used to determine the position of the uranium absorbed in the PTFE matrix, thereby determining whether the enriched uranium had only been absorbed on the surface of the sample or impregnated throughout the PTFE matrix. 2.2.3. X-ray diffraction analysis (XRD) As a powder diffraction technique to determine the elemental composition and/or crystalline phase changes due to radiation in PTFE matrixes [26], a Bruker D8 Advance instrument consisting of a 2-Theta goniometer X-ray diffractometer with a copper target X-ray beam was used. Analyses were done on two PTFE samples, a reference PTFE sample (no exposure to radiation, non-contaminated), and a PTFE sample that was contaminated with enriched uranium and therefore exposed to mainly α-irradiation. For the XRD analysis, the measurement parameters were as follows: Diffractometer: D8 Advance Goniometer: θ-θ Target Tube: Cu Recording range: 15° to 120° Frame width: 0.04° Counts/frame: 1.5 s The 2007 PDF-2 database was used for chemical phase identification for enriched uranium overlaying the proposed stick patterns from the ICDD database with the XRD spectra to determine the uranium species in the PTFE matrix. 2.2.4. Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) analysis TGA measurements were performed on the PTFE samples to study the possible morphology changes (due to radiation) that would be reflected in the thermal decomposition signal. TGA spectra were generated using a Q600 TG/DSC Delta series analysis system in air with a temperature range of 30–600 °C at a rate of 20 °C min−1. The thermogram, i.e., a plot of wt% loss as a function of temperature, was used to study the variation in thermal stability of the PTFE [27]. DSC analysis on a Perkin Elmer Pyris 1 differential scanning calorimeter was done to determine if polymer degradation occurred by enriched uranium irradiation during the storage of the PTFE filters. Heat flow associated with the structures (amorphous and crystalline) and changes in the structure (transitions) of materials as a function of time were measured. The molecular weight calculated from Suwa's equation was used. 3. Results and discussion 3.1. Monte Carlo modelling 3.1.1. Influence of dose rate from or a point source of enriched uranium through PTFE The MCNP model indicated that for a point source of 200 mg of 23% enriched uranium per 1.0 cm3 sphere of PTFE, the total U235 primary particles namely, electrons (e), protons (p), alpha (α) and heavy-recoil particles generated, are in the order of 400 million particles after 20 years (the storage time of the PTFE). The total combined dose rate contributions of 2.346 × 103 Gy h−1 are summarized in Table 1. The modelled results regarding the influence of dose rate from a point source after 20 years indicate that the largest dose rate contribution was from the α-particles (2.29 × 103 Gy h−1), followed by that of the heavy recoil ion dose rate (3.99 × 101 Gy h−1), then followed by that of the electrons (1.31 × 101 Gy h−1), while the dose rate for the photons was negligible. The results indicated that the α-particles is responsible for the main dose rate (97.7%), which implies that should structural damage in the PTFE matrix occur, it is possible due to the α-irradiation from the enriched uranium entrapped in the PTFE matrix. For this reason, the radiation damage to PTFE from homogenously and uniformly distributed enriched uranium, may also be referred to as “alpha-particle damage”. Table 1 Fractional dose contributions from a point source (20% mass fraction uranium that was 23% enriched) in PTFE matrix by different radiation quanta after 20 years. Decay particle

Dose rate (Gy h−1)

Alpha (α) Recoil Electron Photon (γ)

2.29 3.99 1.31 9.38

× × × ×

103 101 101 101

Contribution to total dose rate (%) 97.70 1.70 0.56 0.04

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In terms of dose rate damage to the PTFE matrix (distance travelled), the model indicated that the α-particle, being a heavy ionizing particle, travelled only 0.05 mm into the PTFE whereas the photons and electrons travelled 1.0 cm into the PTFE (Fig. 1). The average pore size of the PTFE is 1.48 μm whereas the distance between the pores is approximately 6.0 μm. The assumption can be made that the radiation was not localized around the uranium but throughout the matrix. The graph represents only the material attenuation, where the dose rate was multiplied with 4πR2 on a logarithmic axis as an indication of relative energy values. 3.1.2. Influence of dose rate from or a distributed source of 235U through a PTFE matrix As previously mentioned, Monte Carlo methods can be used to simulate the stochastic particle migration through the geometry of a matrix [29]. To predict the effect of radiation from 235U as a distributed source throughout the PTFE volume (PTFE matrix), the probability relationships of radiation interaction with the PTFE was modelled for a hypothetical rectangular volume of PTFE sample (2 cm × 2 cm × 1 cm) containing different mass fractions of uranium per mass of PTFE. The results for dose rate vs. uranium enrichment obtained from the MCNP calculation for 10, 20 and 50% (m/m) fractions are presented in Fig. 2. Results yielded linear relationships for dose rate versus % uranium enrichment. Total dose rate (rad) in the presence of O2 causes degradation of which scission seems to be predominant as the molecular weight decreased. Polymer molecules could link together after scission to form large 3D-molecular networks resulting in hardening and embrittlement. For the MCNP model the initial onset of radiation on the PTFE structure from enriched uranium (starts with a small total dose of 0.5 kGy as well as the damage threshold per mass unit from the total dose received, reported as 10 kGy, and 100 kGy) is the accumulated dose value that could result in a morphological change of the polymer matrix [27]. These values were used as input parameters into the model to calculate the time for the PTFE to reach its radiation damage threshold as well as complete destruction for different ranges of mass fraction and uranium enrichment embedded inside the PTFE matrix. Calculated values derived from the MCNP output are represented in Fig. 3. The results indicated that for PTFE contaminated with 10% (mass fraction = 10%) natural uranium (no enrichment), it will take approximate 100 years for PTFE to reach the damage threshold and 1000 years for the accumulated dose to result in a morphological change. The results confirm the composition of natural uranium containing 0.7% 235U and 0.005% 234U which decay through α-radiation. PTFE contaminated with the same amount of uranium but hypothetically 100% enriched, will reach its threshold after only one year for accumulated dose to result in a morphological change. The higher the concentration and enrichment grade of the uranium, the faster the morphologic changes in the structure of the PTFE. 3.2. Characterization of reference (non-contaminated) and enriched uranium contaminated PTFE 3.2.1. PTFE samples In Fig. 4, photographs of both the reference PTFE filter and enriched uranium contaminated PTFE filter samples are presented. The difference in appearance (both in the thickness and the deposits on the PTFE) in the enriched uranium contaminated sample that has been exposed to enriched uranium is apparent. 3.2.2. Micro X-ray tomography To confirm that the U was present throughout the filter material and not only on the surface of the PTFE filters, micro-X-ray tomography as described in Section 2.2 was used to analyze the research samples before destruction. The image (Fig. 5) confirms that U was present throughout the filter material and was displayed as different colours (a function of density) in the 3-D image. The density of a chemical component as observed by micro-X-ray tomography as different colours in the 3-D image is a function of its molecular mass and therefore it can be assumed that the red and green areas probably indicate different U-F and U-O-F compounds in the PTFE matrix. The MCNP model was based on the fact that the enriched uranium was diffused throughout the

Fig. 2. Dose rate versus distance into PTFE matrix for a point source of

235

U after 20 years.

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Fig. 3. Dose rate as a function of uranium enrichment for different mass fractions of uranium.

PTFE matrix. For the same reason, the research samples were “grinded” to ensure a more homogeneous PTFE sample for leaching the enriched uranium from the contaminated PTFE. 3.2.3. X-ray diffraction analysis (XRD) The results presented in Fig. 6 demonstrate the difference in the diffraction pattern between the reference PTFE sample (dotted curve) and the irradiated PTFE sample (solid curve). The diffraction pattern of the contaminated PTFE sample overlaid with the stick pattern of uranyl fluoride (UO2F2·H2O) and uranyl fluoride hydrate (UF4.1.5 H2O) which confirmed the presence of these uranium species/compounds in the PTFE filter material. Although the uranium peaks were identified, the peak intensity

Fig. 4. Time versus enrichment until PTFE begins to fail (threshold) and complete destruction (crumble).

Fig. 5. Research samples: From left to right: Reference (un-irradiated) PTFE candle filter and irradiated PTFE sample from metal drum.

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Fig. 6. Micro X-ray tomography image of irradiated PTFE sample contaminated with enriched uranium (green and red) in the pores of the sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

is very low as the uranium is embedded in the PTFE matrix (Fig. 7). Initially filter material contained trapped UF6 at the onset of storage and the presence of hydrolysis products indicates that H2O has penetrated into the PTFE pores during storage, thereby hydrolyzing the UF6 to UO2F2 and intermediate uranium fluorides to UO2F2·H2O and UF4·1.5 H2O as air stable species [29].

3.2.4. Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) analysis By comparing the TG curve of the reference PTFE to that of material that has been exposed to radiation for 20 years, several differences are seen (Fig. 8). The irradiated sample gradually releases volatile material up to the onset of thermal degradation. The onset of degradation for the irradiated sample is approximately 100 °C higher than that for the reference (un-irradiated) material. This could be indicative of changes in the amorphous content, thermal history, side chains, polymer-backbone or co-polymer composition in the PTFE structure that was caused by irradiation during storage [28]. The DSC results (Table 2) indicate an increase in the heat of fusion, ΔHf, of a sample which has the heat absorbed when the crystalline portion of the material melts. This is proportional to the amount of crystalline material and hence the increase of crystallinity of the irradiated PTFE sample which confirms the linear relationship between heat of fusion and crystallinity from literature [18]. This increase in crystallinity was also confirmed with XRD and showed a change in the morphology of PTFE due to irradiation. The slight increase in melting temperature was observed from the DSC thermal analysis for the irradiated sample. However, these differences are so small that no conclusion in terms of thermal degradation could be made. The results further indicate a significant decrease in the molecular weight which is in line with the increase in crystallinity. The assumption is that radiation produced primary and secondary free radicals that reacted with air to form stable end groups (acid fluoride and carboxyl groups) resulted in the lowering of molecular weight [18].

140000

R eference PTFE Irradiated PTFE

Lin (counts)

120000 100000

U F 4 .1.5 H O 2

80000

U O F .H O 2 2 2

60000 40000 20000 0 15

20

25

30

35

40

45

50

55

60

2 theta Fig. 7. XRD diffraction pattern of the Reference PTFE sample (dotted curve) and Irradiated PTFE sample (solid curve).

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9

40 100 30 80

HeatFlow /mW

20

Wt. %

60

40

20

10 0 -10 -20

Reference Irradiated

0

Reference Irradiated

-30 -40

0

200

400

600

800

0

Temperature /°C

200

400

600

800

Temperature /°C

Fig. 8. TGA curves for reference PTFE (un-irradiated) and the irradiated PTFE sample.

Table 2 DSC analysis results for reference (un-irradiated) and irradiated PTFE sample. PTFE sample

ΔHf (J g−1)

Melting point (°C)

Crystallinity (%)

Mn (g mol−1)

Reference (Un-irradiated) Irradiated sample

20.3 49.4

327.9 331.7

29.0 72.2

5.9 × 106 3.9 × 104

4. Conclusion Initial leaching experiments to decontaminate entrapped enriched uranium in PTFE were unsuccessful; however, after 20 years in storage, leaching was achieved. The increase in leachability was assumed to be due to radiation damage caused by the trapped enriched uranium in the PTFE. MCNP software was used to construct a model to determine the influence of the dose rate from enriched uranium on the degradation of PTFE. The results from the model indicated that 97% of the total dose rate emitted by enriched uranium could be attributed to α-particles generation. The model indicated that the enriched uranium trapped in the pores of the PTFE caused irradiation above the damage threshold of PTFE resulting in morphologic changes. The model also indicated that the higher the mass of uranium and enrichment grade, the faster the PTFE will undergo morphologic changes in its structure. TGA, DCS as well as XRD results confirm that the current research material (enriched uranium trapped in the pores of the PTFE for 20 years – irradiated PTFE) did undergo morphological changes (increase in crystallinity and decrease in molecular weight) thereby confirming the theoretical model. Based on literature, the model and the confirmed experiments, it is clear that the physical and molecular properties of the PTFE filter material has been mainly altered due to long term alpha irradiation of the PTFE from the entrapped uranium particles. The primary result from the interaction between PTFE and ionizing radiation (from α-particles) is chain scission which leads to a reduction in molecular weight. The reduction in molecular weight can affect such physical properties such as crystallinity, density and permeability [30]. Hence, in spite of the mandated use of PTFE in radiation environments there is a concern on the influence of radiation exposure on the engineering properties of PTFE. References [1] U. S. Department of Energy Office of Environmental Management-Depleted Uranium Hexafluoride Management Program, Characteristics of uranium and its compounds. Depleted uranium hexafluoride fact sheet, http://www.tis.eh.doe.gov/nepaFall 2001 (Accessed: March 2016). [2] D.R. Lide, Handbook of Chemistry and Physics, 81st ed., 2000–2001. [3] B. Leuner, Personal Interview, Necsa, 2014. [4] IAEA (International Atomic Energy Agency), Management of radioactive waste from Mo-99 production, Technical Report IAEA-TECDOC-1051, 1998 (http:// www-pub.iaea.org/MTCD/publications/PDF/te_1051_prn.pdf (Accessed: 28 August 2015)). [5] Upgrading of near surface repositories for radioactive waste, Technical Reports: Series No. 433, International Atomic Energy Agency, Vienna, 2005. [6] S. Ebnesajjad, Fluoroplastics, technology and engineering, Melt Processible Fluorpolymers 2 (2015). [7] S. Paria, N.R. Biswal, R.G. Chauhuri, Surface tension, adsoption, and wetting behaviors of natural surfactants on a PTFE surface, Soft Matter Synth. Process. Prod. 61 (2) (2015) 655–663. [8] R.G. Chaudhuri, S. Paria, Dynamic contact angles of PTFE surface by aqueous surfactant solution in the absence and presence of electrolytes, J. Colloid Interface Sci. 15 (2) (2009) 555–562. [9] D.L. Pugmire, C.J. Wetteland, W.S. Duncan, R.E. Lakis, D.S. Schwartz, Crosslinking of Polytetrafluoroethylene During Room Temperature Irradiation, Materials Science and Technology Division, Los Alamos National Laboratory, NM 87545, 2008. [10] B.A. Briskman, K.B. Tlebaev, Radiation effects on thermal properties of polymers, an analytical review, Nucl. Inst. Methods Phys. Res. B 185 (1–4) (2007) 116–122.

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[11] A. Oshima, S. Ikeda, E. Katoh, Y. Tabata, Chemical structure and physical properties of radiation-induced crosslinking of polytetrafluoroethylene, Radiat. Phys. Chem. 62 (2001) 39–45. [12] G.L. Fisher, R.E. Lakis, C.C. Davis, C. Szakal, J.G. Swadener, C.J. Wetteland, N. Winograd, Mechanical Properties and the Evolution of Matrix Molecules in PTFE Upon Irradiation with MeV Alpha Particles, Nuclear Materials Technology (NMT-16), Los Alamos National Laboratory, Los Alamos, NM 87545, United States, Department of Chemistry, Materials Science and Technology (MST-8), Los Alamos National Laboratory, Los Alamos, NM 87545, United States, 2006. [13] W.K. Fisher, J.C. Corelli, Effect of ionizing radiation on the chemical composition, crystalline content and structure, and flow properties of polytetrafluoroethylene, J. Polym. Sci. Polym. Chem. Ed. 19 (1981) 2465–2493. [14] G.L. Fisher, E.L. Rollin, C.C. Davies, C. Szakal, J.G. Swadner, C.J. Wetteland, N. Winograd, Mechanical properties and the evolution of matrix molecules in PTFE upon irradiation with MeV alpha particles, Appl. Surf. Sci. 253 (3) (2006) 13–25. [15] M. Dole, The Radiation Chemistry of Macromolecules, first ed.vol. 2, 1972, ISBN 9780323152839. [16] E. Roland, A.W. Leo, Gamma irradiation of fluorocarbon polymers, J. Res. Natl. Bur. Stand. Phys. Chem. 65A (4) (1961) 375–387. [17] K. Makuuchi, S. Cheng, Radiation Processing of Polymer Materials and its Industrial Applications, Wiley, 2011. [18] H. Dorschner, U. Lappan, K. Lunkwitz, Electron beam facility in polymer research: radiation induced functionalization of polytetra¯uoroethylene, Nucl. Inst. Methods Phys. Res. B 139 (1998) 495–501. [19] A. Oshima, Y. Tabata, S. Ikeda, K. Otsuhata, H. Kudoh, T. Seguchi, Radiation induced crosslinking of polytetrafluoroethylene, Proceedings of the 6th Japan-China Bilateral Symposium on Radiation Chemistry, JAERI Conference, Tokyo, Japan, 1995. [20] T.J. Van Rooyen, Transport and Shielding of Ionizing Radiation, Radiation and Reactor Theory (RRT) Section, Necsa, Pretoria, South Africa, 2012. [21] X. Shi, An overview of Monte Carlo N-particle software, http://www.Scholar.lib.vt.edu/theses/available/etd-07252000/14.Appendix.pdf (Accessed: 4 June 2013). [22] Z. Rafidah, M.S. Jaafar, A. Shukri, M.A.A. Khader, A. Munem, Comparison of teflon phantom image from PET/CT scanner and Monte Carlo simulation, J. Nucl. Relat. Technol. 6 (2) (2009). [23] L. Hordijk, Characterizing and declaring uranium in waste drums is not just putting it through a machine that automatically generates results, Drum scanner results: KMP A-Inventory listing of facility AZN-Radio Chemistry (RC) P1600 E G, Necsa, Proceedings of the 7th International Conference on Facility OperationsSafeguards Interface, Charleston, South Carolina, USA, February 29–March 5 2004. [24] B. Rollen, G. Bosler, J. Tanaka, B. Gillespie, L. Hordijk, R.L. Mayer, Validation of IQ3 measurements for high-density low-enriched uranium waste drums at Pelindaba, Proceedings of the 7th International Conference on Facility Operations-Safeguards Interface, Charleston, South Carolina, USA, February 29 – March 5 2004. [25] J.W. Hoffman, F.C. De Beer, Characteristics of the Micro-Focus X-ray tomography facility (MIXRAD) at Necsa in South Africa, 18th World Conference on Non-destructive Testing, Durban, South Africa, April 16 - 20 2012. [26] TN 109 EaglabsSM, XRD provides unique information about crystalline structure of materials. X-ray diffraction (XRD) services, Tech. Note (21) (2009) 1 (May). [27] A. Pienaar, TG analysis of PTFE samples, internal report: AC-TAS-REP-13006. South African Nuclear Energy Corporation (SOC) Ltd., Pretoria, South, Africa (2013). [28] Rogers Corporation, RT/Duroid: the Effect of Nuclear Radiation Exposure to RT/Duroid PTFE-based Composites, Advanced circuit materials, U.S.A., 1982 -2002 [29] P.A.B. Carstens, The advancement of fluorine technology at Necsa and its predecessors, Presentation at XI ICFPAM, Pretoria, 2011. [30] W.M. Peffley, V.R. Honnold, D. Binder, X-ray and NMR measurement and irradiated polytetrafluoroethylene and polychlorotrifluoroethylene, J. Polym. Sci. Part A1 4 (1966) 977–983.