Effect of Fast Neutron Irradiation on the Properties of Boron Carbide Pellet

Boron carbide pellets were irradiated in the experimental fast reactor “JOYO” to ¹⁰B burnup of up to 170x10²⁶cap/m³, fluences of 2x10²⁶/m²(E>0.1MeV), and maximum temperatures of about 1,200°C. Post irradiation examinations were made of microstructural changes, helium release, swelling, and thermal conductivity.

Boron carbide pellets irradiated to high burnups developed extensive cracking. Helium release from the pellets was initially low, but enhanced helium release was observed at high burnups and high temperatures. The swelling linearly increased with burnup, and when boron carbide was irradiated at high temperatures, the swelling rate began to decrease corresponding to the beginning of enhanced helium release. The correlation between swelling and the helium release was studied and the swelling was interpreted in terms of accumulation of helium in the boron carbide pellet. The thermal conductivity of the boron carbide pellets decreased rapidly by neutron irradiation accompanied with loss of temperature dependence.     

Dependence of vacancy concentration on morphology of helium bubbles in oxide ceramics

Since the formation of helium bubbles degrades swelling property and thermal conductivity of minor actinide-containing mixed oxide (MA-MOX) fuel, it is essential to understand the conditions of the bubble formation. In order to examine the dependence of vacancy concentration on morphology of helium bubbles, helium was infused into (Zr,Fe)O_2?x. The oxygen vacancy concentration was controlled by addition of solute Fe³⁺ into ZrO₂. Helium was infused by hot isostatic pressing. The helium-infused specimens were observed by field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (FE-TEM). In addition, X-ray fluorescence, X-ray diffraction analysis, conversion electron yield–X-ray absorption near-edge structure and FE-SEM/EDX (energy dispersive X-ray) analyses were also made to interpret the results of microstructure observations. As a result of the helium infusion treatment, numerous 0.5–10 nm bubbles were observed and its number density clearly depended on oxygen vacancy concentration. On the other hand, sizes of the helium nano-bubbles in all specimens were almost constant.     

Helium bubbles and trace of lithium in B4C control rod pellets used in JOYO experimental fast reactor

B₄C pellets used in the control rod of the experimental fast reactor ‘JOYO’ with different ¹⁰B burnups from lower than 10 × 10²⁰ captures/cm³ to 80 × 10²⁰ captures/cm³ and irradiated at less than 800 °C were examined by transmission electron microscopy (TEM). In a B₄C pellet irradiated in an irradiation capsule of ‘JOYO’ at 800 °C up to 30 × 10²⁰ captures/cm³, intragranular helium bubbles appeared in flat plate-shapes with the plane of the plate parallel to the (111) rhombohedral plane. However, in the other specimens that were taken from an actual control rod, the helium gas formed very tiny spherical intergranular bubbles with a diameter of a few nanometers . These tiny bubbles make wavy arrays roughly parallel to the (111) plane. The B₄C specimens were heated on a TEM in situ heating holder up to 1040 °C for 10 min. Clustering of tiny bubbles was observed, but did not extend to the plate-shaped bubbles. In high burnup specimens, large bubbles/cracks were rarely found along the {100} planes, which may correspond to the amorphous bands caused by the slip. While heating the specimens in TEM over 800 °C, liquid phases of lithium-bearing compoundsappeared on the surface of specimen.     

The effect of tensile stress on the growth of helium bubbles in an austenitic stainless steel

Foils of the ternary alloy, Fe-17 wt% Cr-17 wt% Ni, were cyclotron-injected with ~160 at ppm helium and annealed at 1023 K for times up to 1.74 Ms (482 h). Some foils were subjected to tensile stresses which ranged from 9.8 to 27.5 MPa during annealing while others were unstressed. Helium bubbles grew in unstressed specimens at different rates depending on their location in the microstructure; the largest bubbles were found at triple grain junctions, smaller ones on the grain boundaries, and the smallest in the grain matrices. Bubble size in the matrix was approximately proportional to $\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ for annealing times greater than 2.88 × 10⁴s (8 h). Applied tensile stress accelerated the growth of bubbles, especially at triple grain junctions and grain boundaries. The enhanced growth in these areas appear to be associated with grain-boundary sliding. The number of bubbles in the matrix and on grain boundaries generally decreased with annealing time, suggesting that a migration-coalescence mechanism was also operating.     

The microstructure of neutron irradiated 20Cr/25Ni/TiN austenitic stainless steel

A nitrided, titanium-stabilised, 20Cr/25Ni austenitic stainless steel was examined in the electron microscope, after 1000 h irradiation at 783 K to doses of 5.0× 10²⁴ n/m² (thermal) and 2.5× 10²⁴ n/m² (fast), in the PLUTO reactor. Microstructures were compared with those of as-received and thermal control samples. The austenitic matrix and M₂₃C₆ particles were free of irradiation-induced damage, while the TiN particles contained loops 2 nm in diameter which coarsened into a network on annealing at 1083 K. Annealing also resulted in a low density of transmutation-induced helium bubbles, ~ 4 nm in diameter, located in precipitate-free regions of grain boundaries. We conclude that 20/25/TiN is relatively unaffected by irradiation at these dose levels and that helium bubble embrittlement is unlikely under normal stresses.     

Formation of Gas Pores in Nickel Alloys and Structural Steel under Irradiation by Helium Ions

Transmission electron microscopy is used to study the development of helium porosity in binary alloys of nickel with elements possessing a different dimensional atomic mismatch with nickel – from negative (beryllium and silicon) to positive (molybdenum, tungsten, aluminum, titanium, tantalum, tin, and zirconium), in structural steels ChS-68, ÉP-150, and the nickel alloy KhNM. The gas pores were produced by irradiation with 40 keV He⁺ up to fluence 5·10²⁰ m^–2 at 650 and 20°C followed by annealing at 650°C for 1 h. It is shown that under high-temperature annealing beryllium and silicon, relative to nickel, give rise to the formation of larger bubbles, while elements with a larger positive size mismatch with nickel atoms substantially decrease the size and increase the density of the bubbles. On the whole, as atomic radius and the concentration of the alloying element increases in alloys, the gas swelling of the irradiated layer decreases. Under post-irradiation annealing, bubbles with the largest diameter and the lowest density develop in nickel. Any alloying used decreases the size and increases the density of bubbles. The data obtained are discussed from the standpoint of the formation of various vacancy complexes of helium and their thermal stability.     

Model of the Formation and Growth of a Gas-Filled Cavity in Boron Carbide under Irradiation

A model of the formation and growth of gas-filled pores in the cores of absorbing elements under neutron irradiation is described. The model is based on a definite similarity between the mechanisms of radiation damage in boron carbide and oxide fuel. For boron carbide, the key mechanism is radiation-stimulated coalescence of helium bubbles, which gives rise to the much larger swelling of boron carbide as compared with nuclear fuel. Regimes of the coalescence of helium bubbles as a function of the irradiation intensity, neutron spectrum, and radial position are investigated. It is shown that the model describes qualitatively well the basic development of gas-filled porosity under irradiation in fast and thermal reactors.     

The influence of temperature on the blister behavior of vanadium and binary V Ti-alloys during 200- and 2000-keV helium irradiation

The formation and growth of gas bubbles is one of the major problems for the integrity of the first wall in a fusion reactor. The helium-induced surface damage as well as the helium-induced bulk damage beneath the surface has been investigated by scanning and transmission electron microscopy techniques. Samples of vanadium, V-3 wt.%-Ti and V-20 wt.%-Ti were implanted with 200-keV and 2000-keV helium ions at temperatures between 450 and 700° C to fluences above the threshold dose for blistering (2 × 10¹⁷He⁺/cm²). The dependence of surface damage phenomena of blistering, exfoliation and perforation on dose, temperature, ion energy and yield strength is discussed. The results can be explained by the helium behavior in the bulk. This behavior is found to be characterized by the formation of large gas bubbles grown by a coalescence process and by the formation of small helium clusters at high dose implantation.     

Effect of Neutron-Irradiation on Thermal Conductivity,Electric Resistivity and Thermal Expansion of Boron Carbide

Boron carbide possessing porosities of 91 and 77%T.D. were irradiated at about 300°C to 6.0x10¹⁸～ 3.3×10¹⁹ n/cm² in helium atmosphere. The thermal conductivity, the electric resistivity and the linear thermal expansion were determined. Upon irradiation to 3.3×10¹⁹ n/cm², cracking occurred in all of the 91%T.D. specimens and in two of the five 77%T.D. specimens. The thermal conductivity changed with irradiation more markedly in the case of the 77%T.D. specimen, which also increased its specific electric resistivity by a factor of 10⁴, while that of the 91%T.D. specimen increased by less than 10-fold. This significant difference in thermal and electric conductivity change by irradiation is attributed to the difference in microstructure. It was observed that the recovery of radiation damage began at about 300°C when the specimens were subjected to heating and the changes observed in respect of thermal conductivity, electric resistivity and linear thermal expansion.     

High temperature surface effects of He implantation in ICF fusion first wall materials

The first wall armor of the inertial confinement fusion reactor chambers must withstand high temperatures and significant radiation damage from target debris and neutrons. The resilience of multiple materials to one component of the target debris has been investigated using energetic (20-40 keV) helium ions generated in the inertial electrostatic confinement device at the University of Wisconsin. The materials studied include: single-crystalline, and polycrystalline tungsten, tungsten-coated tantalum-carbide ‘foams’, tungsten-rhenium alloy, silicon carbide, carbon-carbon velvet, and tungsten-coated carbon-carbon velvet. Steady-state irradiation temperatures ranged from 750 to 1250 °C with helium fluences between 5 × 10¹⁷ and 1 × 10²⁰ He⁺/cm². The crystalline, rhenium alloyed, carbide foam, and powder metallurgical tungsten specimens each experienced extensive pore formation after He⁺ irradiation. Flaking and pore formation occurred on silicon carbide samples. Individual fibers of carbon-carbon velvet specimens sustained erosion and corrugation, in addition to the roughening and rupturing of tungsten coatings after helium ion implantation.     

Helium and xenon behaviour in irradiated Am-containing MgAl2O4 (reactor experiment EFTTRA-T4)

Magnesium aluminate spinel (MgAl₂O₄) containing ²⁴¹Am was irradiated for 358 days in the High Flux Reactor in Petten (NL) with the objective of effectively transmuting this highly radiotoxic actinide. The feasibility of the process was demonstrated; however, a major drawback was observed as material swelling, probably due to the large quantity of helium emitted from formed short-lived actinides. The behaviour of helium and of fission gas in the matrix was investigated by post-irradiation thermal annealing experiments in a Knudsen-cell. The observed gas release was related to the microstructure of the matrix investigated by scanning and transmission electron microscopy. Helium was found to be present in large pores, whereas the less mobile xenon was distributed partly in the same pores and partly in small intragranular bubbles. During post-irradiation thermal annealing, almost complete helium release took place abruptly at a rather high temperature (∼1600 K), whilst only part of the xenon was released during this stage. The release of the remaining xenon was associated with a bulk thermal restructuring of the MgAl₂O₄ target at higher temperatures.     

Helium bubbles formation in aluminum: Bulk diffusion and near-surface diffusion using TEM observations

Experimental and analytical investigation of helium bubble formation and growth in aluminum is presented. A pure aluminum with 0.15 wt% of ¹⁰B was neutron-irradiated in the Soreq nuclear reactor to get homogeneous helium atoms in the metal according to the reaction . Formation and growth of helium bubbles was observed in situ by heating the post-irradiated metal to 470 °C in TEM with a hot stage holder. It was found that above 400 °C the change in the bubble shape takes less than a second. In other experiments the Al-¹⁰B was first heated in its bulk shape and then observed in TEM at room temperature. In this case the helium bubble formation takes hours. Analytical evaluation of the diffusion processes in both cases was done to explain the experimental results. The number of helium atoms in a bubble was calculated from the electron energy loss spectrum (EELS) measurements. These measurements confirmed the hard sphere equation of state (EOS) for inert gases that was used in the analytical diffusion calculations.     

Synergistic effect of helium and hydrogen for bubble swelling in reduced-activation ferritic/martensitic steel under sequential helium and hydrogen irradiation at different temperatures

In order to investigate the synergistic effect of helium and hydrogen on swelling in reduced-activation ferritic/martensitic (RAFM) steel, specimens were separately irradiated by single He⁺ beam and sequential He⁺ and H⁺ beams at different temperatures from 250 to 650 °C. Transmission electron microscope observation showed that implantation of hydrogen into the specimens pre-irradiated by helium can result in obvious enhancement of bubble size and swelling rate which can be regarded as a consequence of hydrogen being trapped by helium bubbles. But when temperature increased, Ostwald ripening mechanism would become dominant, besides, too large a bubble could become mobile and swallow many tiny bubbles on their way moving, reducing bubble number density. And these effects were most remarkable at 450 °C which was the peak bubble swelling temperature for RAMF steel. When temperature was high enough, say above 450, point defects would become mobile and annihilate at dislocations or surface. As a consequence, helium could no longer effectively diffuse and clustering in materials and bubble formation was suppressed. When temperature was above 500, helium bubbles would become unstable and decompose or migrate out of surface. Finally no bubble was observed at 650 °C.     

Formation of Helium Porosity in Different Materials during Post-Radiation Annealing

The characterisitics of the development of helium porosity in bcc and fcc alloys and structural steel after irradiation with 40-keV He⁺ up to dose 5⋅10²⁰ m⁻² at 20°C and subsequent annealing at 650°C for 1 h and 5 h are studied by transmission electron microscopy. It it found that under these conditions smaller bubbles with high density are formed in bcc than in fcc materials. It is shown that for an annealing time of 5 h higher porosity is formed in all materials, except nickel, than with 1 h annealing. This is due to the inflow of thermal vacancies from the free surface. The data obtained are discussed from the standpoint of the formation of various helium-containing complexes, their thermal stability, and the diffusion mobility of the matrix atoms. __________ Translated from Atomnaya Energiya, Vol. 99, No. 2, pp. 115–120, August 2005.     

Helium bubble formation in Cu,Ni and Cu-Ni alloys

Copper, nickel, and Cu-Ni alloys have been irradiated with 200–400 keV ³He ions at a constant homologous temperature of $\text{T}\text{T}{}_{\mathrm{m}}\text{= 0.65}$. The samples have been analyzed by TEM to compare helium bubble size and density. Visible bubbles were observed in all samples. The pure copper and the Cu-Ni alloys were found to contain similar bubble densities and sizes after irradiation. The pure nickel samples contained helium bubbles of smaller size and higher density as compared to the alloys.     

Thermal desorption of helium from defects in nickel

Helium atoms, introduced into materials by helium plasma or generated by the (n, α) nuclear reaction, have a strong tendency to accumulate at trapping sites such as vacancy clusters and dislocations. In this paper, the effects of dislocations, single vacancies and vacancy clusters on the retention and desorption of helium atoms in nickel were studied. Low energy (0.1-0.15 keV) helium atoms were implanted in nickel with vacancies or dislocations without causing any displacement damage. He atoms, interstitial-type dislocation loops, and vacancy clusters were also introduced with irradiation damage by 5.0 keV helium ions. Helium thermal desorption peaks from dislocations, helium-vacancy clusters and helium bubbles were obtained by thermal desorption spectroscopy at 940 K, in the range from 900 to 1370 K, and at 1500 K, respectively. In addition, a thermally quasi-stable state was found for helium-vacancy clusters.     

Effect of injection temperature, 5% pre-strain and long time aging on the high-temperature mechanical properties of helium injected 316 stainless steel

The helium embrittlement of 316 stainless steel, helium injected to concentrations of 3 ~ 10 at ppm at 573 ± 50 or 1073 ± 20 K, was measured by tensile tests at 1023 K and was completely suppressed by the 5% pre-straining. This improved effect was lost by a post-injection again of 1000 h at 1123 K or 3000 h at 1023 K. It was concluded from the TEM observations that in the solution treated material helium bubbles were concentrated on the grain boundaries and caused embrittlement, but in the 5% pre-strained material helium bubbles nucleated on the dislocations and grew large enough to be unable to be dragged to the grain boundaries by the dislocations during the tensile test. The embrittlement after the long time aging was due to the gathering of the large helium bubbles at the σ-phase boundaries by a sweepout mechanism due to dislocation climb during the long time thermal recovery process.     

The evolution of He+ irradiation-induced point defects and helium retention in nuclear graphite

The atomic-level study of point defect evolution in nuclear graphite is essential for a deep understanding of irradiation-induced property changes. The evolution of helium ion irradiation-induced point defects and helium retention in nuclear graphite ETU-10 and ETU-15 were studied by positron annihilation Doppler broadening (PADB) experiments and thermal desorption spectroscopy (TDS) measurements. The graphite samples were implanted with 10¹⁵, 10^16_, and 10¹⁷ cm^?2 of 200 keV He⁺ at operation temperatures below 373 K. Frenkel pairs were created during ion irradiation and they annihilated during annealing. Three stages of interstitial-monovacancy annihilation are suggested. At low temperatures, the initial annihilation would be refined only to the recombination of intimate metastable Frenkel pairs. When temperature increases, the annihilation would expand to a larger extent that isolate interstitials and vacancies annihilate with each other. In the case of high doses irradiation, vacancy clusters form at elevated temperatures. The retention and release of helium is tightly related to the evolution of the defects, especially the vacancies. The small over-pressured He-V clusters (He_nV) are thought to be the most possible form of helium retention under irradiation.     

Diffusion of radiogenic helium in natural uranium oxides

The issue of nuclear waste management – and especially spent fuel disposal – demands further research on the long-term behavior of helium and its impact on physical changes in UO₂ and (U,Pu)O₂ matrices subjected to self-irradiation. Helium produced by radioactive decay of the actinides concentrates in the grains or is trapped at the grain boundaries. Various scenarios can be considered, and can have a significant effect on the radionuclide source terms that will be accessible to water after the canisters have been breached. Helium production and matrix damage is generally simulated by external irradiation or with actinide-doped materials. A natural uranium oxide sample was studied to acquire data on the behavior of radiogenic helium and its diffusion under self-irradiation in spent fuel. The sample from the Pen Ar Ran deposit in the Vendée region of France dated at 320 ± 9 million of years was selected for its simple geological history, making it a suitable natural analog of spent fuel under repository conditions during the initial period in a closed system not subject to mass transfer with the surrounding environment. Helium outgassing measured by mass spectrometry to determine the He diffusion coefficients through the ore shows that: (i) a maximum of 5% (2.1% on average) of the helium produced during the last 320 Ma in this natural analog was conserved, (ii) about 33% of the residual helium is occluded in the matrix and vacancy defects (about 10⁻⁵ mol g⁻¹) and 67% in bubbles that were analyzed by HRTEM. A similar distribution has been observed in spent fuel and in (U_0.9,Pu_0.1)O₂. The results obtained for the natural Pen Ar Ran sample can be applied by analogy to spent fuel, especially in terms of the apparent solubility limit and the formation, characteristics and behavior of the helium bubbles.     

The collapse of helium platelets in molybdenum

Platelets of helium in molybdenum have been observed to collapse into several small helium bubbles rather than into a single bubble [10]. We show that the driving force for collapse into n bubbles increases as n decreases. However, kinetic factors associated with the nucleation of ledges on the flat faces of the platelets ensure that the frequency of nucleation of several small bubbles far exceeds that for a single bubble. The temperature at which this collapse is expected correlates well with the observed platelet behaviour.