Reduced Boron Concentration Research Articles

A realistic approach for the assessment of the consequences of heterogeneous boron dilution events in pressurized water reactors

In order to compensate the excess reactivity over the fuel cycle in pressurized water reactors, boric acid is added to the reactor coolant. During certain accident conditions, the formation and the subsequent transportation towards the reactor core of a slug of coolant with reduced boron concentration can lead to a positive reactivity insertion. In this paper, a new approach dealing with such heterogeneous boron dilution scenarios is presented. This approach is based on the use of computational fluid dynamics methods for the whole reactor pressure vessel with direct coupling to a 3D neutron kinetic model of the reactor core. The application of this approach is demonstrated on a main coolant pump start scenario for hot-shutdown conditions using the coupled ANSYS CFX/DYN3D code. The calculation results show the enormous influence of the coolant mixing on the reactivity behavior of the reactor core.

New Method for Reduction of the Capacitor Leakage Failure Rate Without Changing the Capacitor Structure or Materials in DRAM Mass Production

In the development of dynamic random access memory (DRAM) with a device size of 20 nm or less, the leakage current of a capacitor with high-k dielectrics is one of the main factors causing the failure of a device. To reduce the failure rate of the device, we conducted experiments to reduce the boron impurities, which form defect sites in the dielectrics of the capacitor. The boron source flux was reduced during the deposition process of the capping layer after the formation of the capacitor, and the actual boron concentration in the capping layer was measured by an atom probe tomography. The leakage current characteristics of the samples were confirmed through the dielectric leakage failure test, which is a mass production test technique. In addition, the resistance failure test was performed to measure the risk of increased resistance. From these results, it was confirmed that the failure rate due to the leakage current of the capacitor decreased linearly with reduced boron concentration and the increase in failure rate due to the cell node resistance was negligible. In addition, the results show that the failure rate of mass-produced DRAM products can be significantly reduced without any change in the structure or materials of the capacitor. This is expected to contribute greatly to the development of DRAM with dimensions on the scale of 20 nm or less.

Chitosan alleviates phytotoxicity caused by boron through augmented polyamine metabolism and antioxidant activities and reduced boron concentration in Cucumis sativus L.

The aim of this investigation was to study an association among boron-induced oxidative stress, antioxidant system and polyamine metabolism in cucumber Cucumis sativus (L) plants. Furthermore, it was also investigated whether chitosan would increase the antioxidant activity and polyamine concentration that could induce tolerance to boron-induced oxidative stress. Cucumber (cv. Sahil) plants were exposed to 0.5, 1.0, and 1.5 mM boron stress at the 16-day stage for one week while receiving nutrient solution. At the 23- and 27-day stages, plants were foliar-treated with chitosan solution (50 and 75 mg L−1), and harvested at the 34-day stage to investigate growth, photosynthesis, and biochemical characteristics. Boron stress significantly reduced the growth, total chlorophyll concentration, relative water contents, photosynthesis, stomatal conductance, internal CO2 concentration, stomatal size, and membrane stability index. However, boron stress elevated the enzymatic activities of various antioxidants, osmolytes, polyamine concentration, H2O2 concentration, lipid peroxidation and electrolyte leakage in leaves. The follow-up spray with chitosan to the plants subjected to boron stress improved growth, relative water contents, green pigments, photosynthetic activity and membrane stability index, and further enhanced the activity of antioxidant enzymes, osmolytes, leaf polyamine concentration, hydrogen peroxide concentration, lipid peroxidation, and electrolyte leakage in the leaves. The augmented level of antioxidant enzymes, root/shoot osmolytes and leaf polyamine concentration might have induced resistance to the boron stressed plants, causing an ameliorated growth, relative water contents, and photosynthesis related characteristics. It is also concluded that chitosan is an eco-friendly alleviator of toxicity caused by boron by enhancing polyamine concentration and strengthening antioxidant defensive system but reducing the boron concentration in plant tissues.

CRUD, boron, and burnable absorber layer 2-D modeling requirements using MOC neutron transport

The method of characteristics (MOC) neutron transport modeling requirements of very thin CRUD and burnable absorber layers on pressurized water reactor (PWR) fuel rods are investigated. Ray tracing parameters, including spacing size and number of angles, and mesh refinement studies are performed for two-dimensional assembly simulations using the DeCART code. It is found that the presence of a 10μm thick Integral Fuel Burnable Absorber (IFBA) layer within the lattice model requires a ray spacing that is approximately five times smaller than modeling a lattice without IFBA. The modeling of a CRUD layer necessitates less stringent requirements, due to the fact that the boron-10 concentration in a CRUD layer is at least one order of magnitude lower than what is typically found in IFBA layers. Three dimensional CRUD distributions and compositions computed by the CRUD chemistry code MAMBA are extracted and utilized. The reactivity effects of radial and azimuthal mesh refinement, including homogenization strategies of the CRUD layer with the coolant, are investigated. Additionally, the reactivity effect of including the CRUD structure porosity is discussed. A fresh fuel lattice with 1300ppm is also compared with a reduced boron concentration of 0ppm to assess the reactivity response of the system to changes in the reactor environment.

Influence of interface on structure and magnetic properties of Fe50B50 nanoglass

In contrast to rapidly quenched metallic glasses, nanoglasses consist of two components, namely amorphous nanograins and interfacial regions with distinctively different properties. Various physical methods have been employed to obtain information on the atomistic and magnetic properties of such materials. For the case of a Fe50B50 nanoglass, using high-energy X-ray diffraction, it was found that the short-range order of the nanograins is similar to that of a crystalline FeB alloy. Magnetic Compton scattering shows that the total magnetic moment is the sum of the magnetic moment of the nanograins and the weak magnetic moment of the interfacial regions (μInterface = +0.08 μB). The measured moment of boron agrees (μBoron = −0.08 μB) with linear Muffin-Tin calculations. From the results of Mössbauer and magnetic Compton scattering, it can be concluded that the boron atoms segregate in the interfacial regions, resulting in a reduced boron concentration in the nanograins.

Differences in boron distribution and forms in four citrus scion–rootstock combinations with contrasting boron efficiency under boron-deficient conditions

Boron efficiency of scion ‘Fengjie-72' is related to its less reduced boron concentration and distribution in leaves, achieved by decreasing the ratio of available boron in roots under boron-deficient conditions. Boron (B) deficiency is widespread in citrus orchards. Previous studies have demonstrated that the B-efficient navel orange scion ‘Fengjie-72' (Fs) and rootstock Carrizo citrange (Cr) are more tolerant to B deficiency than the closely related B-inefficient scion ‘Newhall’ (Ns) and rootstock trifoliate orange (Tr), respectively. However, the mechanisms underlying such differences remain unclear. Here, we investigated the differences in B distribution and forms among four combinations (Fs/Cr, Fs/Tr, Ns/Cr, and Ns/Tr) under adequate (0.25 mg/L) or deficient (0.001 mg/L) B supply for 300 days in sand culture. The results showed that B concentrations in buds and leaves of Fs-grafted plants were significantly higher than the respective concentrations of Ns-grafted plants under B-deficient conditions. Moreover, B distribution of Fs-grafted plants due to B deficiency was reduced less in leaves, but more in roots as compared to that of Ns-grafted plants. However, Ns/Cr accumulated more B in the scion stem (24 %) than the other combinations (17–19 %) when B was limited. A correlation was established between B efficiency and the ratio of B concentration in the rootstock stem or buds to the scion stem. Under B-deficient conditions, the ratio of available B (free B and semi-bound B) was significantly higher in leaves in Cr-grafted (36 %) than Tr-grafted plants (29 %), but lower in roots of Fs-grafted (22 %) than Ns-grafted plants (28 %). These results suggest that, under B-deficient conditions, differential B efficiency arises probably because Cr transports more B into scion, Fs redistributes B more efficiently within the plant, or both.

Feasibility of Reduced Boron Concentration Operation in Pressurized Water Reactor Plants

The negative impact of a boron dilution accident on the safety of a current pressurized water reactor (PWR) initiated investigations with the aim of checking the feasibility of reduced boron concentration operation. In addition, reduction of the maximum boron concentration in a PWR is a practical and feasible means to substantially reduce the radiation dose to operators and to minimize corrosion damage. Four types of integral burnable absorbers have been considered: gadolinium, integral fuel burnable absorber (IFBA), erbia, and alumina boron carbide. Under consideration of four different kinds of fuel assemblies (FA), four core design candidates were developed by applying current PWR OPR-1000 technology and by keeping major engineering design constraints and the equivalent fuel enrichment level used in the reference core (REF) design. However, an optimal design was targeted to achieve comparable discharge burnup as well as favorable design safety parameters. The comparative analysis between the REF and the optimal core designs is presented here. One of the designs is suggested as the most promising and favorable low boron core (LBC) design in this framework. The proper combination of axial and radial enrichment zoning patterns plus a mixture of fresh FAs with depleted assemblies in an LBC design candidate with an IFBA-bearing FA at equilibrium cycle could bring a two times narrower axial offset variation than that of the REF design, maintain an acceptable power peaking factor ˜23% lower than the design limit, and achieve higher fuel burnup. It was observed that this optimal LBC design could comply with current OPR-1000 reactor acceptance criteria associated with smooth reactivity swing, more flattened power distribution, and desired limiting safety parameters despite an 18% loss of shutdown reactivity worth at beginning of cycle when compared to the REF design.

Feasibility of Reduced Boron Concentration Operation in Pressurized Water Reactor Plants

Feasibility of Reduced Boron Concentration Operation in Pressurized Water Reactor Plants

Experimental determination of the boron concentration distribution in the primary circuit of a PWR after a postulated cold leg small break loss-of-coolant-accident with cold leg safety injection

It is known that under-borated coolant can accumulate in the loops and that it can be transported towards the reactor core during a loss-of-coolant-accident. Therefore, the mixing of weakly borated water inside the reactor pressure vessel was investigated using the ROCOM test facility. Wire-mesh sensors based on electrical conductivity measurement are used to measure in detail the spreading of a tracer solution in the facility. The mixing in the downcomer was observed with a measuring grid of 64 azimuthal and 32 vertical positions. The resulting distribution of the boron concentration at the core inlet was measured with a sensor integrated into the lower core support plate providing one measurement position at the entry into each fuel assembly. The boundary conditions for this mixing experiment are taken from an experiment at the thermal hydraulic test facility PKL operated by AREVA Germany. The slugs, which have a lower density, accumulate in the upper part of the downcomer after entering the vessel. The ECC water injected into the reactor pressure vessel falls almost straight down through this weakly borated water layer and accelerates as it drops over the height of the downcomer. On the outer sides of the ECC streak, lower borated coolant admixes and flows together with the ECC water downwards. This has been found to be the only mechanism of transporting the lower borated water into the lower plenum. In the core inlet plane, a reduced boron concentration is detected only in the outer reaches of the core inlet. The minimum instantaneous boron concentration that was measured at a single fuel element inlet was found to be 66.3% of the initial 2500 ppm.

Influence of boron related defects on activation of silicon implanted into undoped semi-insulating GaAs

The dependence of sheet carrier concentration of a Si-implanted layer on lattice parameters of undoped liquid encapsulated Czochralski GaAs was investigated. Crystals were grown from near-stoichiometric melts with varying boron concentrations. Sheet carrier concentration increases with an increase in lattice parameters due to the reduced boron concentration. An anomalous reduction in the lattice parameter suggests that native defects such as BGa VAs are responsible for the increase in the occupancy of implanted Si atoms on As sites.