B4C Pellets Research Articles

Nickel Precipitation on the Surface of 15-15Ti Stainless Steel with B4C Exposed to Sodium

In order to evaluate the application performance of B4C pellets as the neutron absorbing material for fast reactor, compatibility experiments were carried out at elevated temperatures for 15-15Ti stainless steel-sodium-B4C ternary system. Experiments results show that the nickel (Ni) precipitates on the inner surface of the 15-15Ti stainless steel (SS) tube. The Ni precipitates on the surface as island distribution, and with the increase of experiment time, the precipitations grow up. Experiment analysis show that the Ni precipitation process is caused by boronation of the Fe and Cr elements of the 15-15Ti SS tube.

Thinning behavior of solid boron carbide immersed in molten stainless steel for core disruptive accident of sodium-cooled fast reactor

ABSTRACT Boron carbide (B4C)–stainless steel (SS) eutectic reaction behavior is one of the most important issues in the core disruptive accidents (CDAs) of sodium-cooled fast reactors (SFRs). In this study, the immersion experiments using B4C pellets with molten SS were conducted to evaluate the CDA sequences such as contact event of solid B4C with degraded core materials including SS at very high temperature. The immersion experiment aims at understanding the kinetic behavior of solid B4C–liquid SS reaction based on the reduced thickness of B4C pellet after the experiment in the temperature ranges from 1763 to 1943 K, which is higher than the temperature of solid B4C–solid SS reaction. Based on the kinetic consideration of the reaction rate constants for solid B4C–liquid SS reaction, it was found that similar temperature dependency was identified between solid B4C–liquid SS and solid B4C–solid SS. Besides, the reaction rate constants of solid B4C–liquid SS were smaller than those of solid B4C–solid SS extrapolated in higher temperature region by two or more orders of magnitude due to two different evaluation method for B4C side/SS side. It was confirmed that this difference was reasonable through the consideration of previous reaction tests in solid–solid contact for B4C side/SS side.

Fabrication of textured B4C ceramics with oriented tubal pores by strong magnetic field-assisted colloidal processing

Highly structure-controlled B4C ceramics were prepared via strong magnetic field-assisted slip casting of a slurry, containing B4C base particles and pore-forming agents with a fiber shape. To achieve gas release at a lower porosity for maintaining its mechanical strength, these B4C ceramics had a structure in which a large number of oriented tubal pores were dispersed in a crystallographically-aligned and dense B4C matrix phase. The B4C microstructure, such as structuration and orientation degree distributions of the B4C grains and tubal pores, was characterized by SEM observation, EBSD analysis, and X-ray CT. Among the investigations, it was found that the oxidic impurities, as an inhibitor of sintering, which existed on the B4C surface, can be removed by ethylation and azeotropy due to an ethanol treatment followed by vacuum drying. Thus, an ethanol treatment of a green compact before sintering was significantly effective for the fabrication of the B4C ceramics, including the microstructure that coexisted with a dense matrix phase with tubal pores. The resultant ceramic specimens showed the remarkable three-point bending strength of 459−554 MPa, which is two times higher when compared to conventional B4C pellets with a similar porosity.

High sphericity and diameter controllable B4C ceramic pellets prepared via simple low-cost PVA assisted planet-type rotation method

B4C ceramic pellets with diameter smaller than 0.7 mm have been simply fabricated via polyvinyl alcohol (PVA) assisted planet-type rotation method. Only commercial B4C powders and tiny amounts of PVA are needed in the experiment. The PVA solution is used to modify the commercial B4C powders. And the powders with adhesive properties are spheroidized to form green pellets during the rotation process. Results show that the green B4C pellets exhibit uniform size distribution and high sphericity. The appropriate diameter of 0.61 mm and high sphericity of 97.15% can be obtained by controlling the rotation time and the concentration of PVA solution. This fabrication method provides a low-cost and simple way to prepare B4C ceramic pellets with various diameters and high sphericity, and also sheds light on the mass production of other ceramic microspheres.

Initial investigation of B4C–TiB2 composites as neutron absorption material for nuclear reactors

In this study, a specifically designed B4C–TiB2 composite with the typical microstructural feature of a TiB2 network (cages) that encapsulates a B4C matrix was fabricated by the molten-salt and spark plasma sintering (SPS) method. The finite-element (FE) calculation results show that the connected TiB2 cages constitute a thermally conductive network, which effectively improves the overall thermal conductivity of the composite; these results agree well with the experimental results. Moreover, the Vickers indentation results reveal that the TiB2 network (cages) can effectively impinge/block the propagation of cracks, which increases the composite toughness. The composite was subjected to helium (He) ion irradiation to simulate the situation in which the B4C–TiB2 composites serve as neutron absorption material, and for which case a high quantity of He atoms is produced by the B10 (n, α) Li7 nuclear reaction. According to the transmission electron microscopy (TEM) results, the interfaces between TiB2 and B4C act as effective sinks for He atoms, and are preferential nucleation sites for He bubbles. The theoretical and experimental results show that when the B4C–TiB2 composites serve as neutron absorption pellets in nuclear reactors, they exhibit a better resistance to their disintegration than pure B4C pellets. Consequently, the performance of the control rods of nuclear reactors can be improved.

Evaluation of thermal shock fracture resistance of B4C/CNT composites with a high-frequency induction-heating furnace

Abstract Boron carbide (B4C) ceramics with high thermal stability and relatively large cross-section for fast neutron absorption have been used as neutron absorbers of the control rods for the fast breeder reactors (FBR). However, cracks induced in B4C pellets due to thermal stress and swelling during neutron irradiation have been one of the practical problems, which leads to short lifetime of the control rods. It is essential to improve the mechanical properties of B4C ceramics for suppressing the cracking of B4C pellets and to enhance the thermal conductivity for relaxation of thermal stress during neutron irradiation. According to our previous study, the fracture toughness and thermal conductivity of B4C ceramics were enhanced by carbon nanotube (CNT) addition. In the present work, we report the dependence of the thermal and mechanical properties on the CNT-orientation in the sintered body prepared by hot-pressing and the enhancement of thermal shock fracture resistance of the obtained B4C/CNT composites. The thermal shock fracture resistance of hot-pressed B4C/CNT composites was evaluated by means of thermal shock test with a high-frequency induction-heating furnace. The dependence of the thermal properties on the CNT-orientation and the enhancement of thermal shock resistance of B4C ceramics were confirmed.

First-principles investigation of neutron-irradiation-induced point defects in B4C, a neutron absorber for sodium-cooled fast nuclear reactors

Boron carbide (B4C) is a leading candidate neutron absorber material for sodium-cooled fast nuclear reactors owing to its excellent neutron-capture capability. The formation and migration energies of the neutron-irradiation-induced defects, including vacancies, neutron-capture reaction products, and knocked-out atoms were studied by density functional theory calculations. The vacancy-type defects tend to migrate to the C–B–C chains of B4C, which indicates that the icosahedral cage structures of B4C have strong resistance to neutron irradiation. We found that lithium and helium atoms had significantly lower migration barriers along the rhombohedral (111) plane of B4C than perpendicular to this plane. This implies that the helium and lithium interstitials tended to follow a two-dimensional diffusion regime in B4C at low temperatures which explains the formation of flat disk like helium bubbles experimentally observed in B4C pellets after neutron irradiation. The knocked-out atoms are considered to be annihilated by the recombination of the close pairs of self-interstitials and vacancies.

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

ABSTRACT 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.

Irradiation performance of sodium-bonded control rod for the fast breeder reactor

ABSTRACT The lifetime of control rods is limited by the absorber (B4C pellets)–cladding mechanical interaction (ACMI). Therefore, sodium (Na)-bonded control rods were developed for long-life control rods. Na-bonded control rods have been irradiated in the experimental fast breeder reactor, JOYO MK-III, and the diametrical changes of the Na-bonded absorber pins after the irradiation were measured in detail. In this paper, these detailed measurements were compared with the results obtained in helium (He)-bonded control rods with and without the shroud tube in a wide burn-up range. From the comparison, it was found that the diametric changes were smaller in the Na-bonded absorber pins than in the He-bonded ones. It was concluded that the Na-bonded absorber pins are very effective for achieving long-life control rods.

Synthesis, Densification and Characterization of Boron Carbide

This paper presents the experimental results on synthesis of boron carbide from its elements. Boron to carbon ratio of 4:1 and a synthesis temperature of 1850°C was found optimum to obtain stoichiometric B4C. Higher boron in charge resulted in the formation of boron rich phase B8C along with B4C but was not effective in eliminating free carbon in the product. Effect of hot pressing parameters on densification of boron carbide was investigated and the optimum operating parameters to obtain B4C pellets with a density of ≥95% TD were identified. Mechanical and thermal properties were characterized and compared with literature values.

Reduction in Degree of Absorber-Cladding Mechanical Interaction by Shroud Tube in Control Rods for the Fast Reactor

Research and development of a long-life control rod for fast reactors is being conducted at Joyo. One of the challenges in developing a long-life control rod is the restraint of absorber-cladding mechanical interaction (ACMI). First, a helium-bonding rod was selected as a control rod for the experimental fast reactor Joyo, which is the first liquid metal fast reactor in Japan. Its lifetime was limited by ACMI, which is induced by the swelling and relocation of B4C pellets. To restrain ACMI, a shroud tube was inserted into the gap between the B4C pellets and the cladding tube. However, once B4C pellets cracked and broke into small fragments, relocation occurred. After this, the narrow gap closed immediately as the degree of B4C pellet swelling increased. To solve this problem, the gap was widened during design, and sodium was selected as the bonding material instead of helium to restrain the increase in pellet temperature. Irradiation testing of the modified sodium-bonding control rod confirmed that ACMI would be restrained by the shroud tube regardless of the occurrence of B4C pellet relocation. As a result of these improvements, the estimated lifetime of the control rod at Joyo was doubled. In this paper, the results of postirradiation examination are reported.

Reduction in Degree of Absorber-Cladding Mechanical Interaction by Shroud Tube in Control Rods for the Fast Reactor

Research and development of a long-life control rod for fast reactors is being conducted at Joyo. One of the challenges in developing a long-life control rod is the restraint of absorber-cladding mechanical interaction (ACMI). First, a helium-bonding rod was selected as a control rod for the experimental fast reactor Joyo, which is the first liquid metal fast reactor in Japan. Its lifetime was limited by ACMI, which is induced by the swelling and relocation of B4C pellets. To restrain ACMI, a shroud tube was inserted into the gap between the B4C pellets and the cladding tube. However, once B4C pellets cracked and broke into small fragments, relocation occurred. After this, the narrow gap closed immediately as the degree of B4C pellet swelling increased. To solve this problem, the gap was widened during design, and sodium was selected as the bonding material instead of helium to restrain the increase in pellet temperature. Irradiation testing of the modified sodium-bonding control rod confirmed that ACMI would be restrained by the shroud tube regardless of the occurrence of B4C pellet relocation. As a result of these improvements, the estimated lifetime of the control rod at Joyo was doubled. In this paper, the results of postirradiation examination are reported.

Performance Analysis of B4C Used as Shielding Material in China Experiment Fast Reactor

Hot-pressed boron carbide (B4C) pellet will be used as shielding material in China Experimental Fast Reactor (CEFR) which is the first fast reactor in China. In this paper, two types of B4C sample provided by two different units in China were investigated on out-reactor properties, and one type of sample was studied on irradiation performance. Finally, the evaluation of domestic B4C pellet as shielding material in CEFR was given: the B4C pellets produced in China could satisfy the requirements of CEFR.

Results of the QUENCH-09 Experiment Compared to QUENCH-07 with Incorporation of B4C Absorber

The purpose of the QUENCH experimental program at the Karlsruhe Research Center is to investigate the hydrogen source term that results from quenching an uncovered core, to examine the physicochemical behavior of overheated fuel elements under different flooding/cooling conditions, and to create a database for model development and code improvement. The QUENCH-07 and -09 test bundles consisted of 21 rods, 20 of which were electrically heated over a length of 1.024 m. The Zircaloy-4 rod cladding and the grid spacers were identical to those used in Western-type light water reactors (LWRs), whereas the fuel was represented by ZrO2 pellets. In both experiments the central rod was made of an absorber rod with B4C pellets and stainless steel cladding and of a Zircaloy-4 guide tube. Failure of the absorber rod cladding was detected at the same temperature in both experiments, i.e., at ~1555 to 1585 K. After a B4C oxidation phase at ~1720 to 1780 K and a subsequent transient test phase to well above 2000 K, cooling of the test bundle was accomplished by injecting saturated steam at the bottom of the test section. The presence of the B4C absorber material in the central rod triggers the formation of eutectic melts, i.e., melts that are formed far below the melting point of metallic Zircaloy (~2030 K), and the oxidation of boron/carbon/zirconium-containing melt can lead to increased amounts of hydrogen and to production of CO, CO2, and CH4 compared to a bundle without a control rod. The total amount of hydrogen released during the flooding, i.e., cooling, phase was, however, significantly larger in QUENCH-09 (~0.400 kg) than in QUENCH-07 (~0.120 kg). It is conjectured that it is mainly the period of steam starvation prior to the cooling phase of QUENCH-09 (steam flow reduction from 3.3 to 0.4 g/s for a duration of ~11 min) that caused the enhanced zirconium oxidation in the cooling phase of QUENCH-09. This is the revised and updated version of the paper that was presented at the 2004 International Meeting on LWR Fuel Performance in Orlando, Florida, on September 19-22, 2004, under the title “Results of the QUENCH-09 Experiment Compared to QUENCH-07 (LWR-Type Test Bundles with B4C Absorber).”

Multiple Recycling of Light Water Reactor Waste Plutonium in a Fast Spectrum Core

A fuel management scheme is proposed for a twofold purpose: incineration of light water reactor waste plutonium and electricity generation. The scheme is based on a fast spectrum core with lead as a coolant. The core is managed in a three-batch mode, 200 days per cycle. Enriched 10B, in B4C pellets, is used as burnable poison, reducing the criticality drop to just 2.8%/cycle. The latter can be handled easily with a few control rods. The core is flat, with a 400-cm diameter and 90-cm height, to ensure that core voiding results in a criticality decrease. Recycled fuel is recharged into the core after the fission products, but not actinides, have been removed. Equilibrium operation is reached within a short period of 2 yr. In a span of 27 yr, the core will incinerate the plutonium at an average rate of 730 kg/yr, while generating 1000 MW(electric).

Precise Nondestructive Determination of the Density of Porous Ceramics

A fast, accurate, inexpensive, and nondestructive method to determine the density of very porous ceramics has been found. This method is an extension of the water‐displacement technique with three weighings and the use of glycerol instead of water. For small (1.13 g), 29% porosity B4C pellets, the precision is 0.002 g/cm3 and the accuracy is better than 0.006 g/cm3 compared with the geometric density.