Nuclear Engine Research Articles

High-resolution thermal analysis of nuclear thermal propulsion fuel element using OpenFOAM

This paper conducts high-resolution thermal hydraulic analysis of a Nuclear Thermal Propulsion (NTP) Fuel Element (FE) using a finite volume approach. Recent NTP design efforts have focused on using low-enriched uranium (LEU) fuel as an alternative to the historical highly-enriched uranium (HEU) fuel. In addition to changing materials’ properties, variations in dimensions and flow characteristics are also considered. This makes the applicability of legacy experimental data questionable for the new LEU designs, as the data was generated for a specific range of design characteristics. Nowadays, there is limited accessibility to experimental capability under realistic hot hydrogen conditions. However, some experimental setups could be replaced or complemented by higher-order numerical analysis, which is the end objective of the computational framework developed here. This study implements a 3D conjugate heat transfer (CHT) numerical solver between solid and fluid regions using OpenFOAM Computational Fluid Dynamic (CFD) toolbox. The hydrogen flow in the fluid region is simulated using Reynolds-averaged Naiver-Stokes (RANS) model, while the solid region is modeled using a conduction heat transfer solver. The 3D simulation results are validated against experimental data obtained from the Nuclear Engine for Rocket Vehicle Application (NERVA) Nuclear Rocket Experimental (NRX) A6 program. The latter shares similar design parameters with modern NTP systems, such as dimensions and flow conditions. In addition, legacy heat transfer correlations were implanted in a reduced-order 1.5D semi-analytic solution, and the results were compared against the OpenFOAM solution. The results presented in this paper conclude that OpenFOAM can serve as a high-resolution thermal hydraulic code both for reproducing legacy experiments and educating reduced-order models.

Numerical Simulation and Experimental Study on Residual Stress in the Curved Surface Forming of 12CrNi2 Alloy Steel by Laser Melting Deposition.

The performance and service life of the nuclear emergency diesel engine shaft made of 12CrNi2 alloy steel is very important for the safety of nuclear power. Laser melting deposition (LMD) is a challenging camshaft-forming technology due to its high precision, rapid prototyping, and excellent parts performance. However, LMD is an unsteady process under the local action of laser, especially for curved surface forming, which is more likely to generate large residual stress on components, resulting in cracks and other defects. At present, the stress research on LMD curved surface forming is relatively insufficient. In the present paper, material parameter testing, high-temperature mechanical properties analysis, single-track sample preparation, and heat source checks are conducted. At the same time, the ABAQUS software and the DFLUX heat source subroutine are used to compile the curved double-ellipsoidal moving heat source, and the effects of the temperature-dependent thermophysical parameters and phase change latent heat on the temperature field are considered. A three-dimensional finite element model is established to analyze the thermal stress evolution and residual stress distribution of multi-track multi-layer on a curved surface by LMD, and the effect of the scanning method and interlayer cooling time on the residual stress of the formed components is studied. The results show that with the increase in temperature, the strength of the material reduces, and the fracture morphology of the material gradually transitions from ductile fracture to creep fracture. The material parameters provide a guarantee for the simulation, and the errors of the width and depth of the melt pool are 4% and 9.6%, respectively. The simulation and experiment fit well. After cooling, the maximum equivalent stress is 686 MPa, which appears at the junction of the substrate and the deposited layer. The larger residual stress is mainly concentrated in the lower part of the deposited layer, where the maximum circumferential stress and axial stress are the tensile stress. Compared with the axial parallel lap scanning method, the arc copying lap scanning method has a relatively smaller maximum thermal stress and residual stress after cooling. The residual stress in the deposited layer is increased to some extent with the increase in the interlayer cooling time.

Exploration of trans-Neptunian objects using the Direct Fusion Drive

The Direct Fusion Drive (DFD) is a nuclear fusion engine that will provide thrust and electrical power for any spacecraft. It is a compact engine, based on the D -3He aneutronic fusion reaction that uses the Princeton field reversed configuration for the plasma confinement and an odd parity rotating magnetic field as heating method to achieve nuclear fusion (Cohen et al., 2019), which will heat the deuterium, also used as propellant.In this work we present possibilities to explore the solar system outer border using the DFD. The objective is to reach some trans-Neptunian object, such as the dwarf planets Makemake, Eris and Haumea in less than 10 years with a payload mass of at least of 1500 kg, so that it would enable all kind of missions, from scientific observation to in-situ operations. For each mission a thrust-coast-thrust profile is considered. For this reason, each mission is divided into 3 phases: i. the spiral trajectory to escape Earth gravity; ii. the interplanetary travel, from the exit of Earth sphere of influence to the end of the coasting phase; iii. maneuvers to rendezvous with the dwarf planet. Propellant mass consumption, initial and final masses, velocities and ΔV for each maneuver are presented. Calculations to reach a vicinity at 125 AU for the study of Sun magnetosphere as well as Eris via flyby are also presented, with interest on the influence of different acceleration phases.Our calculations show that a spacecraft propelled by DFD will open unprecedented possibilities to explore the border of the solar system, in a limited amount of time and with a very high payload to propellant masses ratio.

Assessment of near-term fuel screening and qualification needs for nuclear thermal propulsion systems

Nuclear thermal propulsion (NTP) is an in-space propulsion technology capable of both high specific impulse (850–1000 s) and thrust (44–1112 kN), which can help reduce trip times for crewed missions beyond low Earth orbit. NTP technology has been demonstrated during historic programs. Over 20 ground test reactor experiments were performed, which demonstrated the prototypic reactor operations, during the Nuclear Engine for Rocket Vehicle Application (NERVA)/Rover program (1955–1972). Although historical programs have shown that NTP is a viable in-space propulsion technology, developing NTP in modern programs is contingent on the development and qualification of ultrahigh-temperature nuclear fuel technologies that can withstand engine operating conditions. In historical NTP development programs such as NERVA/Rover, prototypic reactor/engine schemes were ground tested to assess the overall system feasibility and to qualify the reactor fuel forms for eventual flight systems. Although this approach is effective to verify fuel performance under prototypic conditions, relying solely on full-scale NTP reactor tests as the pathway for verifying or qualifying fuel is inefficient and cost prohibitive today. Additionally, modern nuclear licensing requirements state that before test reactor approval, reactor components and fuel elements should be qualified via non-nuclear (out-of-pile) and nuclear (in-pile) testing under representative operating conditions. Using this methodology, fuel matures as production scale fabrication methods are established, and as produced fuel performance is demonstrated. This paper provides an overview of historical approaches to NTP fuel performance maturation, including fuel screening and qualification needs, and provides insight for establishing an efficient testing paradigm that can be implemented to rapidly and affordably develop NTP fuel forms for eventual qualification.

The martensitic strengthening of 12CrNi2 low-alloy steel using a novel scanning strategy during direct laser deposition

Nuclear power emergency diesel engine crankshafts are commonly fabricated using 12CrNi2 low-alloy steel. Direct laser deposition (DLD) may address conventional manufacturing issues associated with cost, long lead time, and design complexity. The idle time between deposited layers significantly affects the mechanical properties of the deposited materials. To date, the influence of inter-layer idle time on microstructural evolution and mechanical properties of DLD 12CrNi2 steel has not been studied in detail. In this study, we have designed different idle times between deposited layers or tracks to investigate the effect of idle time on the microstructural evolution and mechanical properties of DLD 12CrNi2 samples. The results indicate that sample S3 fabricated using a scanning strategy consisting of 10 s dwelling between tracks with 2 min between layers presented the most optimum combination of ultimate tensile strength (1007 MPa) and elongation (19.3%). This is mainly because sample S3 had a higher proportion of martensite due to the higher cooling rate of the upper layers than other samples. This paper provides experimental data and analysis to understand how the microstructural evolution and mechanical properties of DLD 12CrNi2 steel are impacted by different idle times between layers or tracks.

Seeded Hydrogen in Mars Transfer Vehicles Using Nuclear Thermal Propulsion Engines

Using the NASA Design Reference Architecture, an Aerojet Rocketdyne vehicle architecture, and a University of Alabama in Huntsville nuclear thermal propulsion engine model, the performance of a Mars transfer vehicle using hydrogen propellant seeded with argon was analyzed. Seeded hydrogen, up to a maximum seed mass concentration of 55.85%, increased engine and vehicle performance by reducing pressure losses, decreasing reactor power, and increasing the overall while assuming constant propellant volume and vehicle dry mass. The tradeoff for using seeded hydrogen was lowered specific impulse and increased net propellant mass, resulting in increased vehicle wetted mass. Vehicle performance monotonically increased with seed mass concentration and provided a best-case 32-day reduction in round-trip transit time to Mars versus pure hydrogen.

Seeded Hydrogen in Nuclear Thermal Propulsion Engines

Using a NASA Design Reference Architecture, a nuclear thermal propulsion engine model was developed to numerically examine the effects on engine performance by adding heavy noble gases into the hydrogen propellant stream (seeding) before the reactor inlet. Seeded hydrogen, up to a maximum seed mass concentration of 55.85%, increased engine performance by reducing pressure losses, decreasing reactor power, and improving convective heat transfer. Seeding hydrogen also has the potential to increase the engine thrust by up to 13.75% by using the decreased pressure losses. The decreased reactor power reduces the cool down propellant mass of the vehicle, whereas the increased bulk density of the propellant can provide more overall. The tradeoff for introducing a seed into the hydrogen stream is lowered specific impulse.

Flow boiling heat transfer characteristics using the modified eulerian and wall heat balance model

Flow boiling heat transfer is routinely encountered in nuclear reactors, steam engines and other engineering applications. Although several researchers have carried out different numerical and experimental investigations on flow boiling, the underlying physics of the interfacial interaction is still a complex phenomenon to understand in detail. Hence, the numerical simulation and optimisation regarding the adoption of engineering flow boiling parameters have been conducted in this study using the Modified Eulerian-Eulerian Model (MEEM) and Wall Heat Balance Model (WHBM). To predict interpenetrating flow fields and to provide detailed relevant information on the flow behaviour, this study considered a uniform axial heating profile for a cylindrical flow channel. The Raynolds Average Navier Stokes (RANS) equation with an appropriate turbulence model are used to predict the effect of turbulence on the mean flow field, while the MEEM multiphase sub-models are employed to predict the temperature distribution along the wall, the average void fraction, tracking of the single bubble detachment diameter, heat balance at the wall, effect of surface roughness on heat transfer, the effect of aspect ratio, and the critical heat flux. The results obtained from this study are compared with the selected numerical investigation and experimental data presented in the open literature. The present study shows a better approximate prediction (with minimal uncertainties) of both the subcooled boiling heat transfer and the saturated boiling heat transfer. In summary, this study agrees with extant theories and experimental predictions. Thus, it has provided more profound insights into flow boiling heat transfer particularly for flow in a vertical pipe.

Remaining Useful Strength (RUS) Prediction of SiCf-SiCm Composite Materials Using Deep Learning and Acoustic Emission

Prognosis techniques for prediction of remaining useful life (RUL) are of crucial importance to the management of complex systems for they can lead to appropriate maintenance interventions and improvements in reliability. While various data-driven methods have been introduced to predict the remaining useful life (RUL) of machinery systems or batteries, no research has been reported on the remaining useful strength (RUS) prediction of silicon carbide fiber reinforced silicon carbide matrix (SiCf-SiCm) materials with pivotal role in its potential usage as a structural material in nuclear reactors and turbine engines. Knowledge of its degradation process is of the utmost importance to the manufacturers. For this purpose, two approaches based on the machine-learning techniques of random-forest (RF) and convolutional neural network (CNN) are proposed to predict the RUS of SiCf-SiCm using only acoustic emission (AE) signals generated during the material’s stress applying process. Experimental results show that the CNN models achieved better predictive performance than the RF models but the latter with expert-engineered features achieves better prediction for AE signals in the early stage of degradation. Additionally, our results demonstrate that both models can correctly predict the SiCf-SiCm RUS as evaluated by our robust testing method from which the best average root mean square error (RMSE) and Pearson correlation coefficient of 3.55 ksi units and 0.85 were obtained.

Environmental radioactivity aspects of recent nuclear accidents associated with undeclared nuclear activities and suggestion for new monitoring strategies

Recently environmental radionuclide signals were observed in the atmosphere which could be associated with undeclared nuclear activities, not directly connected with development of nuclear weapons. Large-scale contamination of European air with Ru-106 observed in 2017 may represent such an accident, which was probably associated with the Mayak nuclear fuel reprocessing facility in the Chelyabinsk region of Russia. A recently announced nuclear accident at Nyonoska in the Archangelsk region may represent an undeclared nuclear activity associated with the development of a nuclear jet engine which could be based on radionuclide energy generator or a small nuclear reactor. It is concluded that environmental radioactivity impacts associated with recent nuclear activities create new challenges for fast and reliable national and international monitoring systems which would require development of new monitoring strategies.

SOLAR ROCKET ENGINES IN NEAR-EARTH SPACE

At the current stage of space exploration development, missions to the Moon and geostationary orbit are relevant. Enhancing of power supply level of such missions requires innovative technology development. Nuclear rocket engines and engines with use of solar energy are considered among such technologies. In a number of projects, such engines use intermediate heat energy accumulators, which complicates and increases the cost of construction. This embodiment uses a circuit that enables solar engine to operate with any orientation of the thrust direction towards the sun. The dependence of specific thrust of such engine on maximum heating temperature of working fluid (hydrogen) is shown. Such engines provide the ability to return to Earth at a speed of 8 km/s instead of 11 km/s, which increases flight safety and makes propulsion system reuse efficient.

Effects of Initial Crack Defect on Fatigue Crack Growth of Camshaft for Nuclear Emergency Diesel Engine

In this paper, the finite element analysis model of camshaft for nuclear emergency diesel engine with initial crack defect is established based on ABAQUS finite element analysis software. The effects of initial crack defect parameters on crack tip stress intensity factor are studied by a working condition discretization method. The numerical results show that the crack tip stress intensity factors of transverse crack and longitudinal crack are quite different in a load cycle of camshaft. The crack tip stress intensity factors of transverse crack fluctuate greatly, and the crack tip stress intensity factor range is much larger than that of longitudinal crack, which indicates that the fatigue fracture risk of transverse crack on the camshaft journal is higher. With the increase of initial crack depth from 1 mm to 4 mm, the crack tip stress intensity factor range increases from 410.84 MPa·mm1/2 to 720.24 MPa·mm1/2 by 75.3%, which indicates that the increase of initial crack depth can lead to a significant increase of the fatigue fracture risk. As the initial crack defect moves away from the cam, the maximum of the crack tip stress intensity factor initially increases and then decreases after reaching a maximum value. When the initial crack is 3 mm away from the end face of the cam, the crack tip stress intensity factor range is the largest (843.98 MPa·mm1/2), which indicates that the fatigue fracture risk of transverse crack at this position is the highest. The research results obtained in this paper may provide some reference for the fracture assessment and reliability analysis of camshaft for nuclear emergency diesel engine.

Modeling of corrosion-induced blockages on radiation heat transfer by using RELAP5-3D

Nuclear power propulsion for space applications is essential for long-term high payload missions. Several nuclear reactor types were investigated by the Nuclear Engine for Rocket Vehicle Application (NERVA) program of National Aeronautics and Space Administration (NASA). Due to the requirement of long-term missions, studies need to be conducted concerning the impact of planned/unplanned transients on nuclear thermal rockets. In this work, a system model based on RELAP5-3D is developed to simulate corrosion-induced blockages on the peripheral fuel elements of the Pewee I Test Reactor. In addition to the standard axial convective heat transfer, the model incorporates the radiation and conduction heat transfer across the radial direction. Steady-state conditions are achieved for the main thermal-hydraulic parameters. This work investigates the effect of radiation heat transfer across the gap between peripheral fuel elements and the inner core boundary wall caused by corrosion-induced fuel surface degradation of the coolant channel holes. Higher blockage ratios resulted in higher temperature rises across the coolant holes of the peripheral fuel elements. In addition, the pressure drop across the peripheral fuel element coolant holes was also larger at higher blockage ratios. For the channels undergoing blockages, their radiation heat transfer rate increases while their neighbors’ decreases. The temperature of the peripheral fuel elements will gradually get closer to the melting point of uranium carbide, especially when the coolant channels undergo the most severe blockage.

Effects of Substrate Preheating Temperatures on the Microstructure, Properties, and Residual Stress of 12CrNi2 Prepared by Laser Cladding Deposition Technique.

The 12CrNi2 alloy steel powder studied in the present paper is mainly used to manufacture camshafts for nuclear power emergency diesel engines. Laser cladding deposition is of great significance for the manufacture of nuclear power emergency diesel camshafts, which has the advantages of reducing material cost and shortening the manufacturing cycle. However, due to the extremely uneven heating of the components during the deposition process, a complex residual stress field occurs, resulting in crack defects and residual deformation of the components. In the present paper, 12CrNi2 bulk specimens were prepared on the Q460E high-strength structural steel substrate at different preheating temperatures by laser cladding deposition technique, and a finite element residual stress analysis model was established to investigate the effects of different preheating temperatures on the microstructure, properties, and residual stress of the specimens. The results of the experiments and finite element simulations show that with the increase of preheating temperature, the content of martensite/bainite in the deposited layer decreases, and the ferrite content increases. The proper preheating temperature (150 °C) has good mechanical properties. The residual stress on the surface of each specimen decreases with the increase of the preheating temperature. The longitudinal stress is greater at the rear-end deposition part, and the lateral residual stress is greater on both sides along the scanning direction.

Heat transfer characteristic analysis of a novel annular nuclear heat exchanger for propulsion system

In conventional nuclear propulsion systems, reactor with gas-cooled core is bulky and requires extra space to be placed. A new type of nuclear heat exchanger with multi-annular structure is directly put into the combustion chamber to heat compressed air. Nuclear fuel is evenly embedded into the annular wall, which guarantees a steady energy supply to meet the propulsion needs. Because of the strong heat transfer capacity, the reactor is smaller, more compact and flexible.In this paper, a numerical model of propulsion and heat transfer is established for the nuclear jet engine to verify the feasibility of the novel nuclear heat exchanger. To find the optimal structure for cruise and after-burning flight, flow and heat transfer characteristics of the new reactor system are studied by adjusting the annulus numbers in the finite space. The sensitivity of the external control variables to the improvement of propulsion performance is investigated under the new structure. Finally, reactor performance is analyzed by physical-thermal coupling calculation in core.The analysis results show that the multi-annular nuclear heat exchanger has the optimal structure at the intersection point of characteristic parameter normalization. The multi-annular structure with n = 64 has more uniform power distribution, which is selected as the optimal structure for the new system. It can meet the heat transfer and thrust requirements from cruise to after-burning flight. The new system reduces the additional reactor configuration and saves the afterburner space, which is of great importance to the miniaturization and performance improvement of the nuclear propulsion system.

Duty cycle of the radio galaxy B2 0258+35

Context. Radio loud active galactic nuclei (AGN) are episodic in nature, cycling through periods of activity and quiescence. The study of this duty cycle has recently gained new relevance because of the importance of AGN feedback for galaxy evolution.Aims. In this work we investigate the duty cycle of the radio galaxy B2 0258+35, which was previously suggested to be a restarted radio galaxy based on its morphology. The radio source consists of a pair of kpc-scale jets embedded in two large-scale lobes (∼240 kpc) with relaxed shape and very low surface brightness, which resemble remnants of a past AGN activity.Methods. We have combined new LOFAR data at 145 MHz and new Sardinia Radio Telescope data at 6600 MHz with available WSRT data at 1400 MHz to investigate the spectral properties of the outer lobes and derive their age.Results. Interestingly, the spectrum of both the outer northern and southern lobes is not ultra-steep as expected for an old ageing plasma with spectral index values equal to α1451400= 0.48 ± 0.11 and α14006600= 0.69 ± 0.20 in the outer northern lobe, and α1451400= 0.73 ± 0.07 in the outer southern lobe. Moreover, despite the wide frequency coverage available for the outer northern lobe (145–6600 MHz), we do not identify a significant spectral curvature (SPC ≃ 0.2 ± 0.2).Conclusions. While mechanisms such as in-situ particle reacceleration, mixing or compression can temporarily play a role in preventing the spectrum from steepening, in no case seem the outer lobes to be compatible with being very old remnants of past activity as previously suggested (with age ≳ 80 Myr). We conclude that either the large-scale lobes are still fuelled by the nuclear engine or the jets have switched off no more than a few tens of Myr ago, allowing us to observe both the inner and outer structure simultaneously. Our study shows the importance of combining morphological and spectral properties to reliably classify the evolutionary stage of low surface brightness, diffuse emission that low frequency observations are revealing around a growing number of radio sources.

A new physical interpretation of optical and infrared variability in quasars

Changing-look quasars are a recently identified class of active galaxies in which the strong UV continuum and/or broad optical hydrogen emission lines associated with unobscured quasars either appear or disappear on timescales of months to years. The physical processes responsible for this behaviour are still debated, but changes in the black hole accretion rate or accretion disk structure appear more likely than changes in obscuration. Here we report on four epochs of spectroscopy of SDSS J110057.70-005304.5, a quasar at a redshift of $z=0.378$ whose UV continuum and broad hydrogen emission lines have faded, and then returned over the past $\approx$20 years. The change in this quasar was initially identified in the infrared, and an archival spectrum from 2010 shows an intermediate phase of the transition during which the flux below rest-frame $\approx$3400\AA\ has decreased by close to an order of magnitude. This combination is unique compared to previously published examples of changing-look quasars, and is best explained by dramatic changes in the innermost regions of the accretion disk. The optical continuum has been rising since mid-2016, leading to a prediction of a rise in hydrogen emission line flux in the next year. Increases in the infrared flux are beginning to follow, delayed by a $\sim$3 year observed timescale. If our model is confirmed, the physics of changing-look quasars are governed by processes at the innermost stable circular orbit (ISCO) around the black hole, and the structure of the innermost disk. The easily identifiable and monitored changing-look quasars would then provide a new probe and laboratory of the nuclear central engine.

Effect of High Boron Additive Content on SPS-Consolidation and Properties of Commercial Silicon Carbide Powder

Consolidation of commercial silicon carbide powder was performed by means of spark plasma sintering. It is shown that mechanical preactivation is a promising method for introducing a high content of boron additives into silicon carbide. The effect of the boron-additive amount on sintering, microstructure, and properties of the ceramic material was studied. A ceramic showing promise for use as a structural material in nuclear reactors and gas-turbine engines was obtained on the basis of silicon carbide with boron weight content 10%, density 3.12 g/cm3, hardness 31.9 GPa, and crack resistance 5.65 MPa·m1/2.

Superconductors Applied in Electrodynamic Engine

In this article, a new field of application of superconductors and heavy-duty materials (for example, graphene) is considered. It is shown that the use of superconductors in space technology makes possible to create new propulsion systems (which can be more effective than chemical and nuclear rocket engines). The article proposes an electrodynamic engine, which consists of two main parts: an electrodynamic tether and an electric jet propulsor. In the process of movement of a spacecraft in a magnetic field, the electrodynamic tether generates an electric current that calls into operation the electric jet propulsor. It will be shown that there is a mode of operation, when the force of acceleration of electric jet propulsor exceeds the braking force of electrodynamic tether. As a result, the velocity of the spacecraft will increase (while its mass and kinetic energy will decrease, in accordance with the laws of conservation). The only source of energy for the engine is the spacecraft kinetic energy. The article considers an example of using a new engine for flights to the Jupiter system.

Low-enriched cermet-based fuel options for a nuclear thermal propulsion engine

Nuclear thermal propulsion is the high thrust, high specific impulse rocket engine technology of choice for future missions to Mars and beyond. Previous engines designed and built under NASA’s Rover program made use of highly enriched uranium, a significant barrier to development today due to the political climate. This paper focuses on developing a nuclear thermal rocket engine based on a low enriched uranium (LEU) tungsten cermet fuel. Generally, this design is based on the Pewee reactor built by NASA under the Rover program. However, multiple modifications are introduced to optimize the proposed LEU engine to produce the maximum efficiency while meeting NASA’s ground rules as defined in the latest Mars reference missions. This paper presents numerous neutronic and thermal-hydraulic tradeoff studies to approach a near-optimum design. The result is an engine design capable of meeting, and in many cases surpassing, NASA’s requirements of a 25 klbf thrust engine with a thrust-to-weight ratio greater than 3.5 and a specific impulse greater than 900 s.