Fabrication and Thermal Conductivity of Boron Carbide/Copper Cermet

Studies on fabrication and thermal conductivity of B₄C/Cu cermet were made to obtain high performance neutron absorber materials for Liquid Metal-cooled Fast Breeder Reactor (LMFBR). A mixed powder of B₄C and Cu was mechanically blended at high speed thereby a coating layer of Cu was formed on the surface of B₄C powder. Then the B₄C powder with Cu coating was hot pressed at temperatures from 950 to 1,050°C to form a B₄C cermet. A high density B₄C/CU cermet with 70 vol% of B₄C and relative density higher than 90% was successfully fabricated. In spite of the low volume fraction of Cu. the B₄C/Cu cermet exhibited high thermal conductivity which originated from the existence of continuous metallic phase Cu in B₄C/Cu cermet.     

Degradation and oxidation of B4C control rod segments at high temperatures

Extensive series of test were performed of the degradation of boron carbide absorber rods and the oxidation of the resultant absorber melts. Various types of control rod segments made of commercial materials used in French 1300 MW PWRs were investigated in the temperature range between 800 °C and 1700 °C in a steam atmosphere. The gaseous reaction products were analyzed quantitatively by mass spectroscopy for evaluation of the oxidation rates. Extensive post-test examinations were performed by light microscopy, scanning electron microscopy as well as EDX and Auger spectroscopy. Rapid melt formation due to eutectic interactions of stainless steel (cladding tube) and B₄C, on the one hand, and steel and Zircaloy-4 (guide tube), on the other hand, was observed at temperatures above 1250 °C. Complex multi-component, multi-phase melts were produced. ZrO₂ oxide scale on the outside kept the melt within the guide tube, thus preventing its early relocation and oxidation. Rapid oxidation of the absorber melts and remaining boron carbide pellets took place after failure of the protective oxide shell above 1450 °C. Only very little methane was produced in these tests which is of interest in fission product gas chemistry because of the production of organic iodine.     

Interaction between B4C, 304 stainless steel,and Zircaloy-4 in H2O/H2 atmosphere at 1473 K

The control blade and the fuel rod installed in channel box melt in steam during severe accident of a boiling water reactor. In order to clarify the melting phenomena and relocation of the structural material in the core of the reactor, interaction and melting behavior among B₄C, 304 grade of stainless steel (SST), and Zircaloy-4 in atmosphere containing H₂/H₂O at 1473 K are investigated. The results showed that the reaction at the interface between B₄C and SST under H₂O atmosphere was slow and an oxidation was observed after 3600 s. Under H₂O/H₂ atmosphere, the concentration of B and C in the SST increased and the SST melted. Despite the atmosphere, an oxide layer formed on the surface of Zircaloy-4, and thus the reactions proceeded slowly when the Zircaloy-4 was contacted with B₄C and SST. Under H₂O atmosphere, continuous oxidation happened to SST, and SST was partially melted. Under H₂ atmosphere, the SST was also melted due to the diffusion of B and C from the B₄C. In addition, the oxidation of B₄C affected the oxidation behavior of SST and Zircaloy, and thus the oxidation and the hydroxylation of B₄C in a severe accident was discussed thermodynamically.     

Recovery of Neptunium by Electrolysis of NpN in LiCl-KCl Eutectic Melts

The electrochemical behavior of neptunium nitride, NpN, in the LiCl-KCl eutectic melt containing NpCl₃ at 450, 500 and 550°C was investigated from the viewpoint of the application of electrochemical refining in a fused salt to nitride fuel cycle. The electrochemical dissolution of NpN began nearly at the potential theoretically evaluated, though this reaction was irreversible owing to small partial pressure of N₂ in the salt and the reaction rate was slow. Under the electrolysis in the NpCl₃-LiCl-KCl eutectic melt, NpN was dissolved into the salt as Np³⁺ at the anode, and Np metal was deposited at the cathode. About 0.5 g of Np metal was obtained by heating the deposit containing the salt at 800°C for 3.6 ks.     

Fundamental experiments on phase stabilities of Fe–B–C ternary systems

To understanding the control blade degradation mechanism of a boiling-water reactor (BWR), a thermodynamic database for the fuel assembly materials is a useful tool. Although the iron, boron, and carbon ternary system is a dominant phase diagram, phase relation data are not sufficient for the region in which boron and carbon compositions are richer than the eutectic composition. The phase relations of three samples such as Fe_0.68B_0.06C_0.26 (at%), Fe_0.68B_0.16C_0.16 (at%), and Fe_0.76B_0.06C_0.18 (at%) were analyzed by X-ray diffraction, scanning electron microscopy, and energy–dispersive X-ray spectrometry. The results indicate that the Fe₃(B,C) phase exists only in the intermediate region at 1273 K and that the solidus temperature widely maintains at approximately 1400 K for all three samples; these results differ from the calculated data using the previous thermodynamic database. The difference might originate from the overestimation of the interaction parameter between boron and carbon in Fe₃(B,C). Proper titling was performed using the present data.     

Combustion of Na2B4O7 + Mg + C to synthesis B4C powders

Boron carbide powder was fabricated by combustion synthesis (CS) method directly from mixed powders of borax (Na₂B₄O₇), magnesium (Mg) and carbon. The adiabatic temperature of the combustion reaction of Na₂B₄O₇ + 6 Mg + C was calculated. The control of the reactions was achieved by selecting reactant composition, relative density of powder compact and gas pressure in CS reactor. The effects of these different influential factors on the composition and morphologies of combustion products were investigated. The results show that, it is advantageous for more Mg/Na₂B₄O₇ than stoichiometric ratio in Na₂B₄O₇ + Mg + C system and high atmosphere pressure in the CS reactor to increase the conversion degree of reactants to end product. The final product with the minimal impurities’ content could be fabricated at appropriate relative density of powder compact. At last, boron carbide without impurities could be obtained after the acid enrichment and distilled water washing.     

Reactions between SiC and Pd or CeO2 at High Temperatures

Mixtures of SiC and Pd or CeO₂ were heated in air and in vacuum, and the reaction products were determined by powder X-ray diffractometry. High temperature X-ray diffractometry was also conducted to analyze the reaction. The reaction between SiC and Pd began at about 1,000°C, and the products were found to be Pd₂Si and Pd₃Si. A mixture of SiC and CeO₂ produced no detectable amount of product when heated in air up to 1,500°C, but when heated in vacuum, reaction began above 1,100°C, which generated different products depending on the conditions. Two unknown phases appeared during the heating. These phases were considered to be Ce₄(SiO₄)₃ and cubic CeC₂, the latter compound being formed by the reaction between the carbon in SiC and CeSi₂ melt.     

Cathodic polarization curves of the oxygen reduction reaction on various structural materials of boiling water reactors in high temperature–high purity water

Cathodic polarization curves of the O₂ reduction reaction were measured by using electrodes made from typical structural materials of boiling water reactors (BWRs) to evaluate the effects of kind of material on the electrochemical corrosion potential (ECP) calculation. To estimate ECPs at any region in the BWRs on the basis of the BWR environmental conditions, anodic and cathodic polarization curves should be obtained in advance under relevant conditions. The concentration of oxidants such as O₂ and H₂O₂ in coolant changes depending on the region in which they exist. As well, reduction reaction rates might differ depending on the kind of materials. In this work, the cathodic polarization curves of type 316L stainless steel (316L SS) and Alloy 182 were measured in high purity water at 553 K with different O₂ concentrations and compared with those of type 304 SS (304 SS). The results showed that the cathodic polarization curves differed depending on the kind of materials at the activation-controlled region. But, the difference in the ECP vs. O₂ concentration relationship was small when the ECPs were calculated by using both anodic and cathodic polarization curves measured on the objective material.     

Phase equilibria in the Li4SiO4Li2SiO3 region of the pseudobinary Li2OSiO2 system

The constitution of the Li₄SiO₄Li₂SiO₃ region of the pseudobinary L₂OSiO₂ system was investigated between 1000 and 1300°C by isothermal heat treatment, differential thermal analysis, ceramography and X-ray diffraction. The two boundary phases of the sub-system, Li₄SiO₄ and Li₂SiO₃, melt congruently at 1258°C and 1209°C, resp. An intermediate high-temperature phase, Li₆Si₂O₇, forms peritectically at 1030°C and decomposes eutectoidally at 1020°C. A eutectic sub-system exists between Li₄SiO₄ and Li₆Si₂O₇ with the eutectic temperature and composition of 1025°C and 38.3 mol% SiO₂, resp. Li₆Si₂O₇ can be quenched to room temperature in a metastable state and crystallizes in a tetragonal system with the lattice parameters a = 770.9 pm and c = 486.0 pm.     

硼酸盐晶须增强聚乙烯复合材料及其热中子屏蔽性能研究

以硼酸镁(Mg2B2O5)和硼酸铝(Al4B2O9)晶须作为中子吸收体与高密度聚乙烯(HDPE)复合,制备了硼酸盐晶须/HDPE复合材料。讨论了影响材料力学性能及屏蔽性能的因素,并与常用的碳化硼(B4C)屏蔽材料进行了对比。实验结果表明:3种复合材料对热中子的屏蔽效果为B4CMg2B2O5Al4B2O9,复合材料对热中子的屏蔽率均随吸收体含量和材料厚度的增加而增大,当硼酸镁晶须/HDPE复合材料的厚度为15.76mm时,材料对热中子的屏蔽率可达86.58%。晶须/HDPE复合材料的拉伸强度随晶须含量的增加而增大,当硼酸镁晶须的含量为9.1%时,复合材料的拉伸强度可达24.39 MPa,和碳化硼/HDPE复合材料相比,硼酸盐晶须更能增强HDPE基屏蔽材料的力学性能。     

Characterization of solidified melt among materials of UO2 fuel and B4C control blade

To predict the fundamental phase relationships in the solidified core melt of the Fukushima Daiichi Nuclear Power Plant, solidified melt samples of the various core materials [B₄C, stainless steel, Zr, ZrO₂, (U,Zr)O₂] were prepared by arc melting. Phases and compositions in the samples were determined by means of X-ray diffraction, microscopy, and elemental analysis. With various compositions, the only oxide phase formed was (U,Zr)O₂. After annealing, the stable metallic phases were an Fe-Cr-Ni alloy and an Fe₂Zr-type (Fe,Cr,Ni)₂(Zr,U) intermetallic compound. The borides, ZrB₂ and Fe₂B-type (Fe,Cr,Ni)₂B, were solidified in the metallic part. Annealing at 1773 K under an oxidizing atmosphere (Ar-0.1%O₂) resulted in the oxidation of U and Zr in the alloy and in ZrB₂, and consequently the (Fe,Cr,Ni)₂B and Fe-Cr-Ni alloy became dominant in the metallic part. The experimental phase relationships in the metallic part agreed reasonably with the thermodynamic evaluation of equilibrium phases in a simplified B₄C–Fe–Zr system. The metallic Zr content in the melt was found to be a key factor in determining the phase relationships. As a basic mechanical property, the microhardness of each phase was measured. The borides, especially ZrB₂, showed notably higher hardness than any other oxide or metallic phases.     

Preselection of Ni-Cr(-Mo) alloys as potential canister materials for vitrified high active nuclear waste by electrochemical testing

Several Ni-Cr(-Mo) alloys (Hastelloy C4, Inconel 625, Sanicro 28, Incoloy 825, Inconel 690) were tested by electrochemical methods to characterize their corrosion behavior in chloride containing solutions at various temperatures and pH-values in respect to their application as canister materials for final radioactive waste storage. Especially, Hastelloy C4 was tested by potentiodynamic, potentiostatic and galvanostic measurements. As electrolytes H₂SO₄ solutions were used, as parameters temperature, chloride content and pH-value were varied.All tested alloys showed a clearly limited resistance against pitting corrosion phenomena; under severe conditions even crevice corrosion phenomena were observed. The best corrosion behavior, however, is shown by Hastelloy C4, which has the lowest passivation current density of all tested alloys and the largest potential region with protection against local corrosion phenomena.     

Study on B4C decoupler with burn-up reduction aiming at 1-MW pulsed neutron source

In pulsed neutron sources, a neutron absorber called decoupler, which is characterized by a decoupling energy (Ed), is attached to the moderator to sharpen the neutron pulses for achieving good neutron energy resolutions. Cadmium and boron carbide (B₄C) are widely used as the decoupler materials. However, it is difficult to use B₄C in MW-class spallation neutron sources owing to high burn-up, which decreases Ed and increases helium gas swelling due to neutron absorption. To solve these issues concerning a B₄C decoupler, we introduce the concept of pre-decoupler to reduce neutron absorption in the B₄C decoupler, which is sandwiched by appropriate neutron absorption materials. Then, we study impacts of the pre-decouplers on B₄C decoupler in terms of burn-up by performing simplified model calculations. It is shown that neutron absorption in B₄C is reduced by 60% by using a Cd pre-decoupler without neutron intensity penalty. Moreover, helium gas swelling in B₄C is restrained to be one-third of the value when not using the pre-decoupler.     

Structure and electrophysical properties of liquid Pb83Mg17 and Pb83Li17 eutectics

Short range order structure of Pb₈₃Mg₁₇ and Pb₈₃Li₁₇ liquid alloys has been studied by means of X-ray diffraction method. The structure factors and pair correlation functions are analyzed. Experimental structure data were used to calculate the partial structure characteristics by means of Reverse Monte Carlo method. It is shown also that Li₄Pb significantly affects the structure of Pb₈₃Li₁₇ eutectic melt. For Pb₈₃Mg₁₇ eutectic melt the electrical resistivity and thermo-e.m.f. were measured in the temperature range of 550-1300 K. Their analysis confirms the diffraction data concluding the heterocoordinated atomic distribution Pb and Mg atoms.     

Measurements of Multiple Control Rods Effect in Graphite Moderated Reactors

Measurements on the effects of multiple control rods arranged in ring geometry were undertaken with application of the pulsed neutron technique in SHE, a heavily reflected graphite-moderated 20% enriched uranium critical assembly. The aggregate number of experimental cases exceeded 100, which systematically covered the four fundamental factors to determine the control rod effects:

1. The atomic ratio of ²³⁵U to C in the core region (C/²³⁵U = 2,226 and 5,378)

2. The neutron absorbing substances in the control rods and their densities (B₄C powder and Cd plate. Density of B₄C: 0.1 and 1.0 g/cm³)

3. Geometrical factors of the control rods (Cylindrical shape, 20 or 44 mm in diameter)

4. Number of control rods (One to six control rods inserted in circular geometry)

The experimental results are compared with calculations based essentially on a system of simple two group Nordheim-Scalettar method used as tool to solve the three-dimensional diffusion equation. Agreement within about 10% was obtained between measurement and calculation utilizing extrapolation distances for the fast neutron group, determined semi-experimentally.     

Effect of carbide on Al2O3/B4C burnable poison sintered in Ar

Al₂O₃–14 wt%B₄C composites with 0–1 wt% C addition were sintered in Ar at 1550–1650 °C. The influence of the C additive on the B loss, densification behavior, and microstructure of the Al₂O₃–B₄C composites were investigated. The results show that there are B₂O₃, H₃BO₃ and Al₁₈B₄O₃₃ exist between Al₂O₃ and B₄C interface, which result in B loss because of B₂O₃'s high vapor tension at above 1500 °C. The presence of Al₁₈B₄O₃₃ grains formed by chemical reaction of Al₂O₃ with surface oxides on B₄C inhibit the densification of pellets by reducing the specific free surface energy of the Al₂O₃. However, the added C eliminates those oxides to reduce B loss because of its higher activity than B₄C, and it also coarsens Al₂O₃ grain although the density of pellets is decreased by gas products.     

Investigation of systems of fused salts based on thorium fluoride

The system ThF₄-NaCl-KCL, which is significant in the selection of an electrolyte for the production of thorium, by electrolysis, was investigated by thermal and x-ray phase-analytical methods. The salts were melted in an atmosphere of argon. The following structural diagrams were constructed: the system NaCl-ThF₄-an eutectic type with complete insolubility in the solid state (the eutectic lies at 712 °C and 52 mole % ThF₄); the system KCl-ThF₄-the same type (eutectic at 704 °C and 23 mole % ThF₄); and a section of the ternary system NaCl-KCl-ThF₄ along the line (1NaCl: 1KCl)-ThF₄ with the point of intersection with the eutectic crystallization curve at 628 °C and 40% (by weight) ThF₄.     

Compatibility between several heat resistant alloys and sintered Li2O in static helium gas environment

The reaction of sintered Li₂O discs with several commercial heat resistant alloys has been investigated under the conditions of 3.3 × 10⁴Pa ($\text{1}\text{3}\text{atm}$) static He gas atmosphere in the temperature range of 500 and 750° C. Reaction products were identified by X-ray diffraction analysis to be two phases of Li₅FeO₄ and LiCrO₂. The former was dominant below 650° C and the latter was dominant above 650° C. The activation energies of the reaction were determined by the Arrhenius plots for weight changes and sub-scale thickness measurements. The reactivity of the four Fe-Ni-Cr alloys decreased according to the order of Incoloy 800, 316 SS, Hastelloy X-R and Inconel 600. Grain boundary penetration was observed above 500° C for Incoloy 800, 550° C for 316 SS and 600° C for Inconel 600. There was no grain boundary penetration in Hastelloy X-R.     

Manufacturing of Porous Boron Steels Potentially Useful as Nuclear Materials

B₄C is a good neutron absorber, commonly used together with light materials in panels. The objective of this work is to manufacture high boron steels, using B₄C additions, through mechanical alloying and sintering, to get a material potentially useful for nuclear waste management. The porosity of the material can help to the removal of helium bubbles. Iron and B₄C powders were mechanically alloyed for different times, following the process studying apparent density, morphology (SEM) and structure (XRD).

Powder was uniaxially compacted and sintered at different conditions. Specimens were analysed by SEM and physical and mechanical properties were evaluated (density, dimensional change and bending strength).

Microstructures are very different and therefore, they have different properties depending on sintering temperature. Although boride formation always takes place, only ferritic areas were found at 600°C, meanwhile ferritic and perlitic areas appeared at 900°C, and both of them disappeared at l,200°C.     

Preliminary study on nano- and micro-composite sol-gel based alumina coatings on structural components of lead-bismuth eutectic cooled fast breeder reactors

In order to protect the structural components of lead-bismuth eutectic cooled fast breeder reactors from liquid metal corrosion, Al₂O₃ nano- and micro-composite coatings were developed using an improved sol-gel process, which includes dipping specimens in a sol-gel solution dispersed with fine α-Al₂O₃ powders prepared by mechanical milling. Accelerated corrosion tests were conducted on coated specimens in liquid lead-bismuth eutectic at 500 °C under dynamic conditions. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses revealed that the coatings are composed of α-Al₂O₃ and they are about 10 μm thick. After the corrosion tests, no spallation occurred on the coatings, and neither Pb nor Bi penetrated into the coatings, which indicates that the coatings possess an enhanced dynamic LBE corrosion resistance to lead-bismuth eutectic corrosion. The nano-structured composite particles integrated into the coatings play an important role in achieving such superior lead-bismuth eutectic corrosion resistance.