Effect of grain size on gas bubble evolution in nuclear fuel:Phase-field investigations

Numerous irradiation-induced gas bubbles are created in the nuclear fuel during irradiation, leading to the change of microstructure and the degradation of mechanical and thermal properties. The grain size of fuel is one of the important factors affecting bubble evolution. In current study, we first predict the thermodynamic behaviors of point defects as well as the interplay between vacancy and gas atom in both UO_(2) and U_(3)Si_(2) according to ab initio approach. Then, we establish the irradiation-induced bubble phase-field model to investigate the formation and evolution of intra-and inter-granular gas bubbles. The effects of fission rate and temperature on the evolutions of bubble morphologies in UO_(2) and U_(3)Si_(2) have been revealed. Especially, a comparison of porosities under different grain sizes is examined and analyzed. To understand the thermal conductivity as functions of grain size and porosity, the heat transfer capability of U_(3)Si_(2) is evaluated.

Fretting Wear Characteristics of Nuclear Fuel Cladding in High-Temperature Pressurized Water

In pressurized water reactor(PWR),fretting wear is one of the main causes of fuel assembly failure.Moreover,the operation condition of cladding is complex and harsh.A unique fretting damage test equipment was developed and tested to simulate the fretting damage evolution process of cladding in the PWR environment.It can simulate the fretting wear experiment of PWR under different temperatures(maximum temperature is 350℃),displacement amplitude,vibration frequency,and normal force.The fretting wear behavior of Zr-4 alloy under different temperature environments was tested.In addition,the evolution of wear scar morphology,profile,and wear volume was studied using an optical microscope(OM),scanning electron microscopy(SEM),and a 3D white light interferometer.Results show that higher water temperature evidently decreased the cladding wear volume,the wear mechanism of Zr-4 cladding changed from abrasive wear to adhesive wear and the formation of an oxide layer on the wear scar reduced the wear volume and maximum wear depth.

Optimizing near-carbon-free nuclear energy systems:advances in reactor operation digital twin through hybrid machine learning algorithms for parameter identification and state estimation

Accurate and efficient online parameter identification and state estimation are crucial for leveraging digital twin simulations to optimize the operation of near-carbon-free nuclear energy systems.In previous studies,we developed a reactor operation digital twin(RODT).However,non-differentiabilities and discontinuities arise when employing machine learning-based surrogate forward models,challenging traditional gradient-based inverse methods and their variants.This study investigated deterministic and metaheuristic algorithms and developed hybrid algorithms to address these issues.An efficient modular RODT software framework that incorporates these methods into its post-evaluation module is presented for comprehensive comparison.The methods were rigorously assessed based on convergence profiles,stability with respect to noise,and computational performance.The numerical results show that the hybrid KNNLHS algorithm excels in real-time online applications,balancing accuracy and efficiency with a prediction error rate of only 1%and processing times of less than 0.1 s.Contrastingly,algorithms such as FSA,DE,and ADE,although slightly slower(approximately 1 s),demonstrated higher accuracy with a 0.3%relative L_2 error,which advances RODT methodologies to harness machine learning and system modeling for improved reactor monitoring,systematic diagnosis of off-normal events,and lifetime management strategies.The developed modular software and novel optimization methods presented offer pathways to realize the full potential of RODT for transforming energy engineering practices.

Atomistic study on the microscopic mechanism of grain boundary embrittlement induced by small dense helium bubbles in iron

The helium bubbles induced by 14 MeV neutron irradiation can cause intergranular fractures in reduced activation ferritic martensitic steel,which is a candidate structural material for fusion reactors.In order to elucidate the susceptibility of different grain boundaries(GBs)to helium-induced embrittlement,the tensile fracture processes of 10 types of GBs with and without helium bubbles in body-centered cubic(bcc)iron at the relevant service temperature of 600 K were investigated via molecular dynamics methods.The results indicate that in the absence of helium bubbles,the GBs studied here can be classified into two distinct categories:brittle GBs and ductile GBs.The atomic scale analysis shows that the plastic deformation of ductile GB at high temperatures originates from complex plastic deformation mechanisms,including the Bain/Burgers path phase transition and deformation twinning,in which the Bain path phase transition is the most dominant plastic deformation mechanism.However,the presence of helium bubbles severely inhibits the plastic deformation channels of the GBs,resulting in a significant decrease in elongation at fractures.For bubble-decorated GBs,the ultimate tensile strength increases with the increase in the misorientation angle.Interestingly,the coherent twin boundary∑3{112}was found to maintain relatively high fracture strength and maximum failure strain under the influence of helium bubbles.

Effect of Different Laser Energy Nitriding on the Fretting Wear Performance of Zr Alloy

The zirconium(Zr)alloy fuel cladding is one of the key structural components of a nuclear reactor and the first and most important line of defense for accommodating fission products.During the operation of nuclear reactors,Zr alloy fuel cladding is subjected to extreme harsh environments,such as high temperature,high pressure and high flow rate for a long period of time.The wear and corrosion resistance of Zr alloys is important for the safe operation of nuclear reactors.Surface modification can effectively improve the corrosion and wear resistance of fuel cladding.Compared with coating technology,nitriding technology does not have problems for bonding between the coating and the substrate.Current research on surface nitriding of Zr alloys mainly focuses on plasma nitriding and ion implantation techniques.Research on laser nitriding of Zr alloy surfaces and their fretting wear characteristics is scarce.In this study,the surface of Zr alloy was treated with laser nitriding at different laser energies.The microstructure of Zr alloy treated with different laser energies and its fretting wear performance were studied.The results showed that after nitriding with different laser energies,the surface of the Zr alloy showed a typical molten state after melting,vaporizing and cooling under the thermal effect of the laser,and this state was more obvious with the increase of the laser energy.At the same time,doping of N atoms and formation of the ZrN phase led to different cooling rates in the molten zone that produced large tensile stresses after cooling.This led to cracks on the surface of Zr alloys after laser nitriding at different energies,and the crack density increased with increasing laser energy.This also led to an increase in the surface roughness of the Zr alloy with increasing laser energy after laser nitriding treatment.Due to the presence of water in the industrial nitrogen,nitrides were generated on the surface of the sample along with some oxides.When the laser energy was 100 mJ,there was no ZrN generation,and N existed mainly as a diffusion layer within the Zr alloy substrate.ZrN generated when the laser energy reached 200 mJ and above,which increased with the increase of laser energy.Due to the generation of ZrN phase and the presence of some oxides,the surface Vickers hardness of Zr alloys after laser nitriding treatment at different energies increased by 37.5%compared to Zr alloys.After laser nitriding treatment,the wear mechanism of Zr alloys changed.For the untreated Zr alloys,the wear mechanism was dominated by delamination and spalling wear,accompanied by oxidative and abrasive wear.The phenomenon of delamination and peeling decreased with the increase of laser energy.Wear mechanisms changed to predominantly abrasive wear with oxidative wear and delamination spalling.The wear volume of sample nitriding with laser energy 400 mJ was reduced by 46.5%compared with that of untreated Zr alloy.

Improving the ductility and toughness of nano-TiC/AZ61 composite by optimizing bimodal grain microstructure via extrusion speed

In this study,the nano-TiC/AZ61 composites with different heterogeneous bimodal grain(HBG)structures and uniform structure are obtained by regulating the extrusion speed.The effect of HBG structure on the mechanical properties of the composites is investigated.The increasing ductility and toughening mechanism of HBG magnesium matrix composites are carefully discussed.When the extrusion speed increases from 0.75 mm/s to 2.5 mm/s or 3.5 mm/s,the microstructure transforms from uniform to HBG structure.Compared with Uniform-0.75 mm/s composite,Heterogeneous-3.5 mm/s composite achieves a 116.7%increase in ductility in the plastic deformation stage and almost no reduction in ultimate tensile strength.This is mainly because the lower plastic deformation inhomogeneity and higher strain hardening due to hetero-deformation induced(HDI)hardening.Moreover,Heterogeneous-3.5 mm/s composite achieves a 108.3%increase in toughness compared with the Uniform-0.75 mm/s composite.It is mainly because coarse grain(CG)bands can capture and blunt cracks,thereby increasing the energy dissipation for crack propagation and improving toughness.In addition,the CG band of the Heterogeneous-3.5 mm/s composite with larger grain size and lower dislocation density is more conducive to obtaining higher strain hardening and superior blunting crack capability.Thus,the increased ductility and toughness of the Heterogeneous-3.5 mm/s composite is more significant than that Heterogeneous-2.5 mm/s composite.

A novel approach for radionuclide diffusion in the enclosed environment of a marine nuclear reactor during a severe accident

A severe accident in a marine nuclear reactor leads to radionuclide leakage,which causes hidden dangers to workers and has adverse effects of environmental pollution.It is necessary to propose a novel approach to radionuclide diffusion in a confined environment after a severe accident in a marine nuclear reactor.Therefore,this study proposes a new method for the severe accident analysis program MELCOR coupled with computational fluid dynamics scSTREAM to study radioactive diffusion in severe accidents.The radionuclide release fraction and temperature calculated by MELCOR were combined with the scSTREAM calculations to study the radionuclide diffusion behavior and the phenomenon of radionuclide diffusion in different space environments of the reactor under the conditions of varying wind velocities of the ventilation system and diffusion speed.The results show that the wind velocity of the ventilation system is very small or zero,and the turbulent diffusion of radionuclides is not obvious and diffuses slowly in the form of condensation sedimentation and gravity settlement.When the wind speed of the ventilation system increases,the flow of radionuclides meets the wall and forms eddy currents,affecting the time variation of radionuclides diffusing into chamber 2.The wind velocity of the ventilation system and the diffusion speed has opposite effects on the time variation trend of radionuclide diffusion into the four chambers.

Wear Characteristics of the Nuclear Control Rod Drive Mechanism(CRDM)Movable Latch Serviced in High Temperature Water

The current research of nuclear control rod drive mechanism(CRDM)movable latch only makes a simple measurement of wear mass.The wear volume and difference in various claw surfaces are ignored and the degradation mechanism of each claw surface is not clear.In this paper,a detailed degradation analysis was carried out on each claw surface of movable latch combined with wear result and worn morphology.Results indicate that the boundary of carbide is preferred for corrosion because carbide presents a nobler Volta potential compared to the metal matrix or boundary region.Due to the oscillation of drive shaft between the claw surfaces of movable latch,the dominant wear mechanism on the upper surface of claw(USC)and lower surface of claw(LSC)is plastic deformation caused by impact wear.Mechanical impact wear will cause the fragmentation of carbides because of the high hardness and low ductility of carbides.Corrosion promotes the broken carbides to fall off from the metal matrix.The generated fine carbides(abrasive particles)cause extra abrasive wear on USC when the movable brings the drive shaft upward or downward.As a result,USC has a higher wear volume than LSC.This research proposes a method to evaluate the wear on the whole movable latches using a 3D full-size scanner.

In-situ LDHs growth on PEO coatings on AZ31 magnesium alloy for active protection:Roles of PEO composition and conversion solution

In this work,plasma electrolytic oxidation(PEO)coatings were produced on magnesium alloy AZ31 in aluminate,silicate and phosphate-based electrolytes,and followed by hydrothermal treatments in order to synthesis layered double hydroxides(LDHs)based nanocontainers.LDHs synthesis was done in three different growth solutions(deionized water,sodium nitrate and aluminum nitrate containing solution).In frame of this work it was shown,that it was difficult to form LDHs on Si-based PEO coating,due to more stable silicate phases in comparison with aluminate and phosphate phases in respective PEO coatings.The obtained hybrid LDH/PEO coatings were characterized using SEM,EDS and GDOES,and then the corrosion protection was further investigated by EIS.Based on the obtained results,it was confirmed that,the hydrothermal treatments in Al^(3+)containing solution played an important role on overall corrosion resistance for phosphate and silicate-based PEO coatings,but not for Al-based PEO coatings.

Transient fuel performance analysis of UO_(2)–BeO fuel with composite SiC coated with Cr cladding based on multiphysics method

The transient multiphysics models were updated in CAMPUS to evaluate the accident-tolerant fuel performance under accident conditions.CAMPUS is a fuel performance code developed based on COMSOL.The simulated results of the UO_(2)–Zircaloy fuel performance under accident conditions were compared with those of the FRAPTRAN code and the experimental data to verify the correctness of the updated CAMPUS.Subsequently,multiphysics models of the UO_(2)–BeO fuel and composite SiC coated with Cr(SiC_(f)/SiC-Cr)cladding were implemented in CAMPUS.Finally,the fuel performance of the three types of fuel cladding systems under Loss of Coolant Accident(LOCA)and Reactivity Insertion Accident(RIA)conditions was evaluated and compared,including the temperature distribution,stress distribution,pressure evolution,and cladding failure time.The results showed that the fuel temperature of the UO_(2) fuel under accident conditions without pre-irradiation was lower after being combined with SiC_(f)/SiC-Cr cladding.Moreover,the centerline and outer surface temperatures of the UO_(2)–BeO fuel combined with SiC_(f)/SiC-Cr cladding reduced further under accident conditions.The cladding temperature increased after the combination with the SiC_(f)/SiC-Cr cladding under accident conditions with pre-irradiation.In addition,the use of SiC_(f)/SiC-Cr cladding significantly reduced the cladding hoop strain and plenum pressure.

Applying multi-scale simulations to materials research of nuclear fuels:A review

Computational simulation is an important technical means in research of nuclear fuel materials.Since nuclear fuel issues are inherently multi-scopic,it is imperative to study them with multi-scale simulation scheme.At present,the development of multi-scale simulation for nuclear fuel materials calls for a more systematic approach,in which lies the main purpose of this article.The most important thing in multi-scale simulation is to accurately formulate the goals to be achieved and the types of methods to be used.In this regard,we first summarize the basic principles and applicability of the simulation methods which are commonly used in nuclear fuel research and are based on different scales ranging from micro to macro,i.e.First-Principles(FP),Molecular Dynamics(MD),Kinetic Monte Carlo(KMC),Phase Field(PF),Rate Theory(RT),and Finite Element Method(FEM).And then we discuss the major material issues in this field,also ranging from micro-scale to macro-scale and covering both pellets and claddings,with emphasis on what simulation method would be most suitable for solving each of the issues.Finally,we give our prospective analysis and understanding about the feasible ways of multi-scale integration and relevant handicaps and challenges.

A Peridynamic Approach for the Evaluation of Metal Ablation under High Temperature

In this paper,the evaluations of metal ablation processes under high temperature,i.e.,the Al plate ablated by a laser and a heat carrier and the reactor pressure vessel ablated by a core melt,are studied by a novel peridynamic method.Above all,the peridynamic formulation for the heat conduction problem is obtained by Taylor’s expansion technique.Then,a simple and efficient moving boundary model in the peridynamic framework is proposed to handle the variable geometries,in which the ablated states of material points are described by an additional scalar field.Next,due to the automatic non-interpenetration properties of peridynamic method,a contact algorithm is established to determine the contact relationship between the ablated system and the additional heat carrier.In addition,the corresponding computational procedure is listed in detail.Finally,several numerical examples are carried out and the results verify the validity and accuracy of the present method.

Oxidation behavior of Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2)C-M_(x)C(M=Ti,Zr,Hf,Nb,Ta) composite ceramic at high temperature

Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2)C-M_(t)C composite ceramic was prepared by hot press sintering,with the Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2)C high-entropy carbide as the main phase.Secondary phase M_(x)C(M=Ti,Zr,Hf,Nb,Ta) was found to be distributed relatively uniform in the composite ceramic.The oxidation behavior of the ceramic was examined after exposure to 923 K and 1173 K.Morphology of the surface and cross sections of all oxidation samples were observed.The characteristics of the oxidation behavior of the high-entropy carbide and the secondary phase M_(x)C were compared and analyzed.The secondary phases(such as Ti-rich carbide or Hf-rich carbide) in the material were seriously oxidized at 923 K and 1173 K,which reflects the superior oxidation performance of the high-entropy carbide.The nano high-entropy oxides with Ti,Zr,Hf,Nb,Ta,and O elements were discovered by oxidation of the composite ceramic.This research will help deepen the understanding of the oxidation mechanism of high-entropy carbide and composite ceramic.

High-resolution neutronics model for ^(238)Pu production in high-flux reactors

We proposed and compared three methods(filter burnup,single energy burnup,and burnup extremum analysis)to build a high-resolution neutronics model for 238Pu production in high-flux reactors.The filter burnup and single energy burnup methods have no theoretical approximation and can achieve a spectrum resolution of up to~1 eV,thereby constructing the importance curve and yield curve of the full energy range.The burnup extreme analysis method combines the importance and yield curves to consider the influence of irradiation time on production efficiency,thereby constructing extreme curves.The three curves,which quantify the transmutation rate of the nuclei in each energy region,are of physical significance because they have similar distributions.A high-resolution neutronics model for ^(238)Pu production was established based on these three curves,and its universality and feasibility were proven.The neutronics model can guide the neutron spectrum optimization and improve the yield of ^(238)Pu by up to 18.81%.The neutronics model revealed the law of nuclei transmutation in all energy regions with high spectrum resolution,thus providing theoretical support for high-flux reactor design and irradiation production of ^(238)Pu.

镍/钼复合中间层钎焊连接C/C复合材料和304不锈钢

本文使用在BNi-2或AgCuTi箔片钎料中添加金属中间层(Ni、Mo或Ni/Mo)的方法有效提高了C/C复合材料和304不锈钢真空钎焊接头的强度,并先后对接头的微观结构、连接机理和力学性能进行分析。分析表明,BNi-2+Ni/Mo接头的微观结构包括Cr_(3)C_(2)、Ni(s,s)、Cr_xNi_(y)+MoNi_(4)、MoNi_(4)+Cr_(x)M_(y)和Ni(s,s)+Cr-Fe;AgCuTi+Ni/Mo接头的界面则由TiC、Ag(s,s)+Cu(s,s)、Ni、Ag(s,s)+Cu(s,s)+MoTi、Mo和MoTi+Cu(s,s)+Ag(s,s)构成。有限元模拟结果表明,AgCuTi钎料加持下的镍/钼复合中间层能够有效缓解C/C母材与焊缝界面间的残余应力。室温下BNi-2+Ni/Mo、AgCuTi+Ni、AgCuTi+Mo和AgCuTi+Ni/Mo钎焊接头的平均剪切强度分别为2.57 MPa、10.91 MPa、23.81 MPa和26.77 MPa,有效验证了有限元结果的可靠性。

XPZLIB:an HDF5-format multi-group cross-section library

A multi-group cross-section library is fundamental for deterministic lattice physics calculations.Most existing multi-group cross-section libraries are customized for particular computer codes,as well as for particular types of nuclear reactors.This paper presents an HDF5-format multi-group cross-section library named XPZLIB.XPZLIB was produced using a selfdeveloped XPZR module integrated into the NJOY2016 code,and an in-house PyNjoy2022 system was developed for autoprocessing.XPZLIB contains detailed data content and well-organized data structures that are user-and developer-friendly.Three typical XPZLIBs with different numbers of energy groups,nuclides,and depletion reaction types were released via the Tsinghua cloud website.Furthermore,the applicability of the released XPZLIBs was investigated using HTGR and PWR lattice calculations,which can provide guidance for applying XPZLIB under different scenarios.

Effect of ECAP process on liquid distribution of AZ80M alloy during semi-solid isothermal heat treatment

Two kinds of semi-solid samples of AZ80−0.2Y−0.15Ca(wt.%)(AZ80M)magnesium alloy were prepared by semi-solid isothermal heat treatment of materials with and without equal channel angular pressing(ECAP)process.The microstructures of initial and semi-solid treated samples were compared and analyzed.The results showed a significant difference in the liquid phase distribution between three-pass ECAP processed(3P)and as-received samples during the isothermal heating process.The semi-solid 3P sample showed a more uniform liquid distribution due to its smaller dihedral angle.Besides,the coarsening processes of solid grains of as-received and 3P samples were dominated by the coalescence and Ostwald ripening mechanism,respectively.The difference of coarsening processes was mainly related to the proportion of the high-angle grain boundaries in materials,which further affected the evolution behavior of the liquid pools.

Relationship of microstructure transformation and hardening behavior of type 17-4 PH stainless steel

The relationship between the microstructure transformation of type 17-4 PH stainless steel and the aging hardening behavior was investigated. The results showed that, when 17-4 PH stainless steel aging at 595℃, the bulk hardness of samples attains its peak value （42.5 HRC） for about 20 min, and then decreases at all time. TEM revealed the microstructure corresponding with peak hardness is that the fine spheroid-shape copper with the fcc crystal structure and the fiber-shape secondary carbide M23C6 precipitated from the lath martensite matrix. Both precipitations of copper and M23C6 are the reasons for strengthening of the alloy at this temperature. With the extension of holding time at this temperature, the copper and secondary carbide grow and lose the coherent relationship with the matrix, so the bulk hardness of samples decreases.

MC simulation of a PGNAA system for on-line cement analysis

A prompt gamma neutron activation analysis system with a 252Cf neutron source for on-line cement analysis has been simulated with the MCNP code.The results indicate that the optimum arrangement is a Bi shield of 20-mm thickness,a polyethylene moderator of 50-mm thickness,a source-to-sample distance of 70 mm,and cement samples of 1200 mm×600 mm×170 mm.To absorb thermal neutrons and suppress low-energy γ-rays,the optimum-sized sheets are 150 mm×7 mm Cd,and 150 mm×15 mm Pb.

High temperature formability of graphene nanoplatelets-AZ31 composites fabricated by stir-casting method

Outstanding mechanical properties of graphene nanoplatelets(GNPs)make them ideal reinforcement for mass production of composites.In this research,the composites were fabricated by stir-casting method.GNPs were added in 1.5 and 3.0 wt.%into Mg–3wt.%Al–1wt.%Zn(AZ31)magnesium alloy.As cast ingots were preheated for one hour and extruded at 350℃ with extrusion ratio of 5.2:1.As extruded AZ31-GNPs composites were micro-structurally characterized with X-ray diffraction,optical microscopy and scanning electron microscopy.Vickers microhardness of synthesized materials was investigated both in parallel and perpendicular to extrusion directions.Room temperature mechanical testing revealed that with increasing GNP’s content,tensile fracture strain was remarkably increased without significant compromise in tensile strength.Furthermore,as extruded AZ31-3GNPs composites were subjected to tensile testing at temperatures ranging from 75℃ to 300℃ with initial strain rate of 2×10^(−3)s^(−1) to evaluate high temperature formability of composite.It was found that like CNTs,GNPs also have the potential to sustain tensile strength at high temperatures.