Neutronics analysis of a subcritical blanket system driven by a gas dynamic trap-based fusion neutron source for ^(99)Mo production

Gamma-emitting radionuclide ^(99m)Tc is globally used for the diagnosis of various pathological conditions owing to its ideal single-photon emission computed tomography (SPECT) characteristics.However,the short half-life of ^(99m)Tc (T_(1/2)=6 h)makes it difficult to store or transport.Thus,the production of ^(99m)Tc is tied to its parent radionuclide ^(99)Mo (T_(1/2)=66 h).The major production paths are based on accelerators and research reactors.The reactor process presents the potential for nuclear proliferation owing to its use of highly enriched uranium (HEU).Accelerator-based methods tend to use deuterium–tritium(D–T) neutron sources but are hindered by the high cost of tritium and its challenging operation.In this study,a new ^(99)Mo production design was developed based on a deuterium–deuterium (D–D) gas dynamic trap fusion neutron source (GDT-FNS) and a subcritical blanket system (SBS) assembly with a low-enriched uranium (LEU) solution.GDT-FNS can provide a relatively high-neutron intensity,which is one of the advantages of ^(99)Mo production.We provide a Monte Carlo-based neutronics analysis covering the calculation of the subcritical multiplication factor (k_(s)) of the SBS,optimization design for the reflector,shielding layer,and ^(99)Mo production capacity.Other calculations,including the neutron flux and nuclear heating distributions,are also provided for an overall evaluation of the production system.The results demonstrated that the SBS meets the nuclear critical safety design requirement (k_(s)<0.97) and maintained a high ^(99)Mo production capacity.The proposed system can generate approximately 157 Ci ^(99)Mo for a stable 24 h operation with a neutron intensity of 1×10^(14) n/s,which can meet 50%of China’s demand in 2025.

Slip boundary effect on the critical Reynolds number of subcritical transition in channel flow

In this letter,the effect of slip boundary on the origin of subcritical transition in two-dimensional chan-nel flows is studied numerically and theoretically.It is shown that both the positive and the negative slip lengths will increase the critical Reynolds number of localized wave packet and hence postpone the transition.By applying a variable transformation and expanding the variables about a small slip length,it is illustrated that the slip boundary effect only exists in the second and higher order modulations of the no-slip solution,and hence explains the power law found in simulations,i.e.the relative increment of the critical Reynolds number due to the slip boundary is proportional to the square of the slip length.

Effect of acid-associated mechanical pretreatment on the hydrolysis behavior of pine sawdust in subcritical water

The effects of sulfuric acid-associated mechanical pretreatment on the hydrolysis behavior of pine sawdust were investigated in this study.Sulfuric acid could act as an acidic catalyst to depolymerize holocellulose through cleavage of the glycosidic bonds,the dissociation energies of which were supplied by the impact of a ball on pine sawdust,during milling.The destruction of glycosidic and hydrogen bonds in pine sawdust resulted in a decrease of crystallinity and an increase of water solubility.The sulfuric acid could promote the hydrolysis of holocellulose and its hydrolysis products.It also destroyed the chemical linkages between holocellulose and lignin during ball milling.The cleavage of chemical linkages with holocellulose made lignin more difficult to hydrolyze in subcritical water,and higher activation energy was needed to hydrolyze pretreated pine sawdust at higher reaction temperatures.It also led to the formation of glucose char and aromatic-linked polymer char from the hydrolysis products of holocellulose.

Physical studies of minor actinide transmutation in the accelerator-driven subcritical system

The accelerator-driven subcritical system (ADS) with a hard neutron energy spectrum was used to study transmutation of minor actinides (MAs). The aim of the study was to improve the efficiency of MA transmutation while ensuring that variations in the effective multiplication factor (keff) remained within safe margins during reactor operation. All calculations were completed using code COUPLE3.0. The subcritical reactor was operated at a thermal power level of 800 MW, and a mixture of mononitrides of MAs and plutonium (Pu) was used as fuel. Zirconium nitride (ZrN) was used as an inert matrix in the fuel elements. The initial mass composition in terms of weight percentages in the heavy metal component (IHM) was 30.6% Pu/IHM and 69.4% MA/IHM. To verify the feasibility of this MA loading scheme, variations in keff, the amplification factor of the core, maximum power density and the content of MAs and Pu were calculated over six refueling cycles. Each cycle was of 600 days duration, and therefore, there were 3600 effective full power days. Results demonstrated that the effective transmutation support ratio of MAs was approximately 28, and the ADS was able to efficiently transmute MAs. The changes in other physical parameters were also within their normal ranges.It is concluded that the proposed MA transmutation scheme for an ADS core is reasonable.

Effect of Cryogenic Treatment on Property of 14Cr2Mn2V High Chromium Cast Iron Subjected to Subcritical Treatment

Effect of cryogenic treatment on the microstructure, hardening behavior and abrasion resistance of 14Cr2Mn2V high chromium cast iron （HCCI） subjected to subcritical treatment was investigated. The results show that cryogenic treatment after subcritical treatment can obviously improve the hardness and abrasion resistance of HCCI because abundant retained austenite is transformed into martensite and fine secondary carbides E（Fe, Cr）23 C6 ] precipitate. The amount of martensite and precipitated secondary carbide in HCCI experiencing subcritical treatment followed by cryogenic treatment was more than that experiencing the subcritical treatment followed by air cooling. When the abrasion resistance of HCCI reaches the maximum, its microstructure contains about 15 % retained austenite. Cryogenic treatment can further reduce the austenite content but the retained austenite cannot be transformed in to martensite completely.

Application of FLUKA and OpenMC in coupled physics calculation of target and subcritical reactor for ADS

The study of accelerator-driven subcritical reactor systems(ADSs) has been an important research topic in the field of nuclear energy for years. The main code applied in ADS research is MCNPX, which was developed by Los Alamos National Laboratory. We studied the application of the open-source Monte Carlo codes FLUKA and OpenMC to a coupled ADS calculation. The FLUKA code was used to simulate the reaction of highenergy protons with the nucleus of the target material in the ADS, which produces spallation neutrons. Information on the spallation neutrons, such as their energy, position,direction, and weight, can be recorded by a user-defined routine called FLUSCW provided by FLUKA. Then, the information was stored in an external neutron source file in HDF5 format by using a conversion code, as required by the OpenMC calculation. Finally, the fixed-source calculation function of OpenMC was applied to simulate the transport of spallation neutrons and obtain the distribution of the neutron flux in the core region. In the coupled calculation, the high-energy cross-section library JENDL4.0/HE in ACE format produced by NJOY2016 was applied in the OpenMC transport simulation. The OECD–ADS benchmark problem was calculated, and the results were compared with those obtained using MCNPX. It was found that the flux calculations performed by FLUKA–OpenMC and MCNPX were in agreement, so the coupling calculation method for ADS is reasonable and feasible.

Toluene oxidization to benzaldehyde in subcritical water

Effects of reaction parameter on yield of benzaldehyde produced from toluene oxidization using hydrogen peroxide in subcritical water are investigated. The experimental results show that if the molar ratio of hydrogen peroxide to toluene is controlled within a reasonable range, the by-products may be neglected. The optimum technology of toluene oxidization to benzaldehyde is reaction time 60 min, reaction temperature 350℃, molar ratio of hydrogen peroxide to toluene 3.5. The yield of benzaldehyde can reach 17.2 % under the optimum condition. Research results of chemical reaction kinetics show that the consecutive reaction consists of two first-order reaction, and activation energy of these two reactions are 89 kJ·mol^-1 and 76 kJ·mol^-1 respectively,

Amino Acids Production from Fish Proteins Hydrolysis in Subcritical Water

The hydrolysis technology and reaction kinetics for amino acids production from fish proteins in subcritical water reactor without catalysts were investigated in a reactor with volume of 400 ml under the conditions of reaction temperature from 180-320℃, pressure from 5-26 MPa, and time from 5-60 rain. The quality and quantity of amino acids in hydrolysate were determined by bioLiquid chromatography, and 17 kinds of amino acids were obtained. For the important 8 amino acids, the experiments were conducted to examine the effects of reaction temperature, pressure and time on amino acids yield. The optimum conditions for high yield are obtained from the experimental results. It is found that the nitrogen and carbon dioxide atmosphere should be used for leucine, isoleucine and histidine production while the air atmosphere might be used for other amino acids. The reaction time of 30 rain and the experimental temperature of 220℃, 240℃ and 260℃ were adopted for reaction kinetic research. The total yield of amino acids versus reaction time have been examined experimentally. According to these experimental data and under the condition of water excess, the macroscopic reaction kinetic equation of fish proteins hydrolysis was obtained with the hydrolysis reaction order of 1.615 and the rate constants being 0.0017, 0.0045 and 0.0097 at 220℃, 240℃ and 260℃ respectively. The activation energy is 145.1 kJ·mol^- 1.

Extraction of Astaxanthin from Euphausia pacific Using Subcritical 1,1,1,2-tetrafluoroethane

Euphausia pacific is an important source of natural astaxanthin. Studies were carried out to assess the extractability of astaxanthin from E. pacific using subcritical 1, 1, 1,2-tetrafluoroethane （R134a）. To examine the effects of multiple process variables on the extraction yield, astaxanthin was extracted under various conditions of pressure （30-150bar）, temperature （303-343 K）, time （10-50rain）, flow rate （2-10gmin-1）, moisture content （5.5%-63.61%）, and particle size （0.25-0.109mm）. The results showed that the extraction yield increased with temperature, pressure, time and flow rate, but decreased with moisture content and particle size. A maximum yield of 87.74% was obtained under conditions of 100bar, 333K, and 30min with a flow rate of 6gmin-1 and a moisture content of 5.5%. The substantial astaxanthin yield obtained under low-pressure conditions demonstrates that subcritical R134a is a good alternative to CO2 for extraction of astaxanthin from E. pacific.

Comparative evaluation for catalytic gasification of petroleum coke and asphaltene in subcritical and supercritical water

Subcritical and supercritical water gasification of petroleum coke and asphaltene was performed at variable temperatures(350–650°C),feed concentrations(15–30 wt%)and reaction times(15–60 min).Nickel-impregnated activated carbon(Ni/AC)was synthesized as a catalyst for enhancing syngas yields at optimal gasification conditions(650°C,15 wt%and 60 min).Structural chemistry of precursors and chars developed at different gasification temperatures was studied using physicochemical and synchrotronbased approaches such as carbon–hydrogen–nitrogen–sulfur(CHNS)analysis,thermogravimetric and differential thermogravimetric analysis(TGA/DTA),scanning electron microscopy(SEM),Fourier-Transform Infrared spectroscopy(FTIR),Raman spectroscopy,X-ray diffraction(XRD)and X-ray absorption spectroscopy(XAS).Asphaltene testified to be a better precursor for catalytic hydrothermal gasification leading to 11.97 mmol/g of total gas yield compared to petroleum coke(8.04 mmol/g).In particular,supercritical water gasification using 5 wt%Ni/AC at 650°C with 15 wt%feed concentration for 60 min resulted in 4.17 and 2.98 mmol/g of H_2from asphaltene and petroleum coke,respectively.Under the same conditions,the respective CH_4yields from catalytic gasification of asphaltene and petroleum coke were 2.54and 1.07 mmol/g.Nonetheless,asphaltene also seemed to an attractive feedstock for the production of highly aromatic chars through hydrothermal gasification.

Generalized Nonlinear Subcritical Symmetric Instability

Starting from nonlinear equations on the F-plane containing frictional dissipation under the Boussinesq approximation, a new kind of generalized energy is proposed as the Lyapunov function, and averages are taken as any functions of (x, z) instead of the commonly-used means of bilinear functions of (x, z), thereby resulting in a new criterion of generalized nonlinear symmetric stability. It shows that not only must the dissipative coefficient be greater than a certain critical value but the initial disturbance amplitude must be synchronously smaller than another marginal value as well. It follows that the latter imposes a crucial constraint on the former, thus leading to the fact that when the amplitude is bigger compared to another critical value, generalized nonlinear subcritical symmetrical instability may occur. The new criterion contributes greatly to the improvement of the previous results of its kind.

Test of subcritical crack growth and fracture toughness under water-rock interaction in three types of rocks

The subcritical crack growth and fracture toughness in peridotite, lherzolite and amphibolite were investigated with double torsion test. The results show that water-rock interaction has a significant influence on subcritical crack growth. With water-rock interaction, the crack velocity increases, while the stress intensity factor declines, which illustrates that water-rock interaction can decrease the strength of rocks and accelerate the subcritical crack growth. Based on Charlse theory and Hilling & Charlse theory, the test data were analyzed by regression and the correlation coefficients were all higher than 0.7, which shows the correlation is significant. This illustrates that both theories can explain the results of tests very well. Therefore, it is believed that the subcritical crack growth attributes to the breaking of chemical bond, which is caused by the combined effect of the tensile stress and the chemical reaction between the material at crack tip and the corrosive agent. Meanwhile, water-rock interaction has a vital effect on fracture toughness. The fracture toughness of samples under atmospheric environment is higher than that of samples immersed in water. And water-rock interaction has larger influence on fracture toughness in amphibolite than that in peridotite and lherzolite.

Testing study of subcritical crack growth velocity and fracture toughness of marble

Subcritical crack growth of double torsion specimens made of marble was studied using Instron1342 type electro hydraulic servo test machine. The relations of the mode-I stress intensity factor KI versus the subcritical crack growth velocity V and the fracture toughness KIC were obtained by the double torsion constant displacement load relaxation method. The behavior of subcritical crack growth was analyzed. The results show that lgKI-lgV relations of marble measured by this method accord with linear rule, i.e. the relations between subcritical crack growth velocity V and stress intensity factor KI have a power law, which is in good agreement with Charles theory. The testing results provide a basis for time-dependency of rock engineering stability.

Supercritical as well as subcritical Hopf bifurcation in nonlinear flutter systems

The Hopf bifurcations of an airfoil flutter system with a cubic nonlinearity are investigated, with the flow speed as the bifurcation parameter. The center manifold theory and complex normal form method are Used to obtain the bifurcation equation. Interestingly, for a certain linear pitching stiffness the Hopf bifurcation is both supercritical and subcritical. It is found, mathematically, this is caused by the fact that one coefficient in the bifurcation equation does not contain the first power of the bifurcation parameter. The solutions of the bifurcation equation are validated by the equivalent linearization method and incremental harmonic balance method.

Eigen solutions of internal waves over subcritical topography

Diapycnal mixing plays an important role in the ocean circulation.Internal waves are a kind of bridge relating the diapycnal mixing to external sources of mechanical energy.Difficulty in obtaining eigen solutions of internal waves over curved topography is a limitation for further theoretical study on the generation problem and scattering process.In this study,a kind of transform method is put forward to derive the eigen solutions of internal waves over subcritical topography in twodimensional and linear framework.The transform converts the curved topography in physical space to flat bottom in transform space while the governing equation of internal waves is still hyperbolic if proper transform function is selected.Thus,one can obtain eigen solutions of internal waves in the transform space.Several examples of transform functions,which convert the linear slope,the convex slope,and the concave slope to flat bottom,and the corresponding eigen solutions are illustrated.A method,using a polynomial to approximate the transform function and least squares method to estimate the undetermined coefficients in the polynomial,is introduced to calculate the approximate expression of the transform function for the given subcritical topography.

Stabilization and control of subcritical semilinear wave equation in bounded domain with Cauchy-Ventcel boundary conditions

We analyze the exponential decay property of solutions of the semilinear wave equation in bounded domain Ω of R^N with a damping term which is effective on the exterior of a ball and boundary conditions of the Cauchy-Ventcel type. Under suitable and natural assumptions on the nonlinearity, we prove that the exponential decay holds locally uniformly for finite energy solutions provided the nonlinearity is subcritical at infinity. Subcriticality means, roughly speaking, that the nonlinearity grows at infinity at most as a power p 〈 5. The results obtained in R^3 and RN by B. Dehman, G. Lebeau and E. Zuazua on the inequalities of the classical energy （which estimate the total energy of solutions in terms of the energy localized in the exterior of a ball） and on Strichartz＇s estimates, allow us to give an application to the stabilization controllability of the semilinear wave equation in a bounded domain of R^N with a subcritical nonlinearity on the domain and its boundary, and conditions on the boundary of Cauchy-Ventcel type.

Resonance in flow past oscillating cylinder under subcritical conditions

Flow around an oscillating cylinder in a subcritical region are numerically studied with a lattice Boltzmann method(LBM). The effects of the Reynolds number,oscillation amplitude and frequency on the vortex wake modes and hydrodynamics forces on the cylinder surface are systematically investigated. Special attention is paid to the phenomenon of resonance induced by the cylinder oscillation. The results demonstrate that vortex shedding can be excited extensively under subcritical conditions, and the response region of vibration frequency broadens with increasing Reynolds number and oscillation amplitude. Two distinct types of vortex shedding regimes are observed. The first type of vortex shedding regime(VSR I) is excited at low frequencies close to the intrinsic frequency of flow, and the second type of vortex shedding regime(VSR II)occurs at high frequencies with the Reynolds number close to the critical value. In the VSR I, a pair of alternately rotating vortices are shed in the wake per oscillation cycle,and lock-in/synchronization occurs, while in the VSR II, two alternately rotating vortices are shed for several oscillation cycles, and the vortex shedding frequency is close to that of a stationary cylinder under the critical condition. The excitation mechanisms of the two types of vortex shedding modes are analyzed separately.

Scattering Process of Internal Waves Propagating over a Subcritical Strait Slope onto a Shelf Region

The scattering process, which means the redistribution of energy fluy in modenumber space, is analyzed for internal waves propagating from the abyssal ocean onto a subcritical strait slope and then a shelf region. In light of Wunsch＇s work, the waves are analytically expressed as superimposition of eigensolutions distribution of energy flux in the shelf region： one is the ratio of water depth in and the other is the ratio of the slope of the internal tide rays to the topographic energy flux distribution： the energy flux is focused around one modenumber or case, the range of modenumbers where energy flux is distributed is narrow. Two parameters have evident effects on the the shelf region to that in the abyssal ocean slope. Generally, there are two patterns of focused around two modenumbers. In any case, the range of modenumbers where energy flux is distributed is narrow.

Neutronics analysis of a stacked structure for a subcritical system with LEU solution driven by a D-T neutron source for~(99)Mo production

The utilization of neutrons markedly affects the medical isotope yield of a subcritical system driven by an external D-T neutron source.The general methods to improve the utilization of neutrons include moderating multiplying,and reflecting neutrons,which ignores the use of neutrons that backscatter to the source direction.In this study,a stacked structure was formed by assembling the multiplier and the low-enriched uranium solution to enable the full use of neutrons that backscatter to the source direction and further improve the utilization of neutrons.A model based on SuperMC was used to evaluate the neutronics and safety behavior of the subcritical system,such as the neutron effective multiplication factor,neutron energy spectrum,medical isotope yield,and heat deposition.Based on the calculation results,when the intensity of the neutron source was 59×10^(13)n/s,the optimized design with a stacked structure could increase the yield of ^(99)Mo to182 Ci/day,which is approximately 16% higher than that obtained with a single-layer structure.The inlet H_(2)O coolant velocity of 1.0 m/s and initial temperature of 20℃ were also found to be sufficient to prevent boiling of the fuel solution.

Simulation of an Accelerator Driven Subcritical Core with Mixed Uranium-Thorium Fuel

During recent years, a new generation of nuclear reactors, known as “Accelerator Driven Subcritical Reactors”, has been developed. One of the new application aspects for such reactors (besides transmutation of High Level Waste and burning Minor Actinides) is usage of thorium as nuclear fuel. In this work a subcritical core in experimental scale is simulated by MCNPX code. The core contains two types of fuel assemblies: (85% ThO2 + 15% UO2) and MOX (U-Pu). In the first step, only the thorium-contained fuel assemblies are loaded into the core. Criticality calculations using MCNPX show that the keff is so low that the fuel assemblies cannot run the subcritical core. This implies that MOX (U-Pu) assemblies must be loaded as well. Neutronic parameters of the thorium- fueled Accelerator Driven Subcritical core are then calculated as well as some other parameters related to accelerator coupled with the core. The main objective of this simulation is to study the behavior of Accelerator Driven Subcritical core with thorium assemblies.