Thermonuclear Fusion Reactors Research Articles

The use of liquid metals in porous materials for divertor applications

Features of using capillary porous materials (stainless steel, tungsten, graphite) cooled by liquid lithium for divertor module of thermonuclear fusion reactor have been considered. Temperature distribution in divertor plate under the power density of 10 MW/m 2 was calculated for steady state and pulsed heat influences. The surface temperature as a function of power density and dependence of lithium evaporation yield on temperature was evaluated. Thickness of cover lithium layer with safe porous material against direct pulse heat influence type of plasma disruption (power density in the range of (1–5)10 3 MW/m 2) was estimated. Concerning the problem under consideration, features of hydrodynamic and capillary effects as well as chemical interaction between lithium and investigated materials have been discussed.

A vacuum ultraviolet study of rare-earth phosphor screens with application as diagnostics for hot plasmas (abstract)

In a previous work [Zurro et al., Rev. Sci. Instrum. 68, 680 (1997)], we outlined designs for several detector systems, based on rare-earth phosphor powders, with application as broadband VUV plasma radiation monitors in the TJ-II flexible heliac device. In addition to their immunity to microwave power, to ground loops, and to any kind of EMI, the phosphor powders considered as suitable candidates can withstand the high temperatures of the TJ-II vacuum chamber (150 °C), while possessing good sensitivity across the x-ray to UV range and fast response times. Among those selected was Y3Al5O15:Ce (P-46), which emits at 530 nm and when examined, it was found to be particularly suitable for our application due to its very fast response time (∼0.1 μs). We report on absolute efficiency measurements performed between 40 and 200 nm using a calibrated VUV source, and made on thin P-46 phosphor screens, having thicknesses between 1 and 20 mg/cm2. We combine these results with measurements made at visible wavelengths and in the x ray [A. Baciero et al., in Diagnostics for Experimental Thermonuclear Fusion Reactors 2, edited by P. E. Stott et al. (Plenum, New York, 1998)] to obtain fitting parameters for models used to predict phosphor response efficiencies in transmission and reflection modes. These models can be used to tailor the phosphor screen thickness to the radiation range of interest while reducing background noise. Such screens are currently used on the TJ-II.

Thermal fatigue behavior of low activation ferrite–martensite steels

Components like test blankets of the thermonuclear fusion reactor ITER or like DEMO-blankets are subjected during service to alternating thermal and mechanical stresses as a consequence of the pulsed reactor operation. Of particular concern is the fatigue endurance of reduced activation ferrite-martensite steels like the Japanese F82H mod. and the German OPTIFER IV, compared to martensitic steel for the next European torus (MANET II), as reference material, under cyclic strains and stresses produced by typical temperature changes. Into self developed thermal cycling fatigue test rigs variable strain rates are applied to these materials, due to the constant heating rate and variable temperature changes. Thermal fatigue behavior of ferrite-martensite MANET II, including dwell times, will be reported. On the other hand F82H mod. as well as OPTIFER IV samples are thermal cyclically tested in different as received conditions and the the results are compared to those of MANET II samples after the three-step reference annealing.

Integrating artificial neural networks and empirical correlations for the prediction of water-subcooled critical heat flux

The critical heat flux ( CHF) is an important parameter for the design of nuclear reactors, heat exchangers and other boiling heat transfer units. Recently, the CHF in water-subcooled flow boiling at high mass flux and subcooling has been thoroughly studied in relation to the cooling of high-heat-flux components in thermonuclear fusion reactors. Due to the specific thermal-hydraulic situation, very few of the existing correlations, originally developed for operating conditions typical of pressurized water reactors, are able to provide consistent predictions of water-subcooled-flow-boiling CHF at high heat fluxes. Therefore, alternative predicting techniques are being investigated. Among these, artificial neural networks (ANN) have the advantage of not requiring a formal model structure to fit the experimental data; however, their main drawbacks are the loss of model transparency (‘black-box’ character) and the lack of any indicator for evaluating the accuracy and reliability of the ANN answer when ‘never-seen’ patterns are presented. In the present work, the prediction of CHF is approached by a hybrid system which couples a heuristic correlation with a neural network. The ANN role is to predict a datum-dependent parameter required by the analytical correlation; this parameter was instead set to a constant value obtained by usual best-fitting techniques when a pure analytical approach was adopted. Upper and lower boundaries can be possibly assigned to the parameter value, thus avoiding the case of unexpected and unpredictable answer failure. The present approach maintains the advantage of the analytical model analysis, and it partially overcomes the ‘black-box’ character typical of the straight application of ANNs because the neural network role is limited to the correlation tuning. The proposed methodology allows us to achieve accurate results and it is likely to be suitable for thermal-hydraulic and heat transfer data processing.

Indirect measurement of nuclear reaction cross sections at astrophysical energies

In order to extract the cross sections for thermonuclear fusion reactions in the region of astrophysical energies we have used a method based on a quasi-free reaction mechanism, called ‘Trojan horse’. In particular, information regarding the 7 Li( p, α) 4 He reaction is obtained from an analysis of the three-body 7 Li( d, 2 α) n process.

Performance of beryllium, carbon, and tungsten under intense thermal fluxes

Transient heat loads on a millisecond timescale with deposited energy densities beyond 1 MJ m −2 have been simulated in a plasma accelerator facility (VIKA) and in two high power electron beam teststands (JUDITH, JEBIS). Main objective of these experiments was to study and to compare the behaviour of different plasma facing materials (Be, CFC, W) under heat loads which occur during disruptions in future thermonuclear fusion reactors such as ITER. In these tests special attention was paid to the thermal shock resistance, the processes during melt layer formation, and the resulting material erosion. To perform these tests specific loading techniques and diagnostics have been developed and applied. Among these are high heat flux loading experiments at elevated temperatures ( T > DBTT) of the test coupons, fast surface pyrometry, and reliable techniques for the quantification of the absorbed energy.

Joining of Carbon‐Carbon Composites for Thermonuclear Fusion Applications

Carbon‐fiber‐reinforced carbon (CfC) composites have been joined by using different joining agents: metals (silicon, aluminum, and titanium), an intermetallic compound (magnesium silicide, Mg2Si), and glasses (borosilicates and zinc borates). These joining agents have been chosen by considering their possible use in a thermonuclear fusion reactor, as suggested by their low‐activation properties, reasonably high working temperature, industrial feasibility, and potential scale‐up of the joined carboncarbon structures using pressureless‐processing techniques. Each joined structure (CfC composite–joining material‐CfC composite) has been first morphologically characterized by scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and X‐ray diffractometry. Then, single‐lap shear tests have been performed on the most‐promising joined structures, i.e., silicon‐ and aluminum‐joined CfC‐composite samples, which have shown an average shear strength of 22 and 10 MPa, respectively.

Structure and composition of interlayers in joints between SiC bodies

Joints between SiC bodies (sintered α-SiC and SiC SiC f composites) which are intended for thermonuclear fusion reactor applications were produced by coating their surfaces with polycarbosilane (PCS) or a silicone resin (GE SR350) alone, or mixed with SiC or Ti powders, followed by heating in an argon flux at temperatures between 900 and 1200 °C applying no pressure or a very moderate one (< 0.1 MPa). The joint quality was determined by microstructural and microchemical analyses (TEM, AEM) and by fracture shear tests for comparative evaluations. For SR350, the joining material was a silicon oxycarbide glass, with no oxygen diffusion occurring between this and the SiC joined bodies. The flat interface structure and the lack of any reaction layer suggest that the joining mechanism involves the direct formation of chemical bonds between the SiC bodies and the joining material. Adding SiC or Ti powders does not improve the joining strength. Contrary to silicone resin, polycarbosilane does not serve as an effective joining material, possibly because of its different decomposition behaviour upon pyrolysls.

Thermonuclear Milestones: (1) The American Effort

The history of thermonuclear research traces its roots back to the year 1941. in a lecture delivered in May 1941, Tokutaro Hagiwara, a physicist at the University of Kyoto, postulated that a thermonuclear fusion reaction between hydrogen atoms could be triggered by the explosive fission chain reaction of uranium‐235. in September 1941, Enrico Fermi at Columbia University proposed a similar idea to Edward Teller. Discussions between Fermi and Teller ultimately suggested the feasibility of using an atomic explosion to initiate thermonuclear reactions in a deuterium medium. the conversations with Fermi sparked in Teller a decade‐long messianic obsession with the notion of building a thermonuclear superbomb.

Prospect for study of radiation damage at RADEX-15, radiation experiment facility, based on the beam stop of Moscow Meson Factory

The Moscow Meson Factory (MMF) of the INR RAS supplies a unique possibility for studying radiation behaviour of structural materials for thermonuclear fusion reactors in intense fluxes of primary and secondary protons and spallation neutrons. The first stage of MMF working at every 400 MeV and proton current up to 70 μA is now available for simulations of radiation damage in nuclear materials. Experiments can be carried out in conditions close to that of fusion facilities at the beam stop at the experimental area of the MMF INR RAS. The calculated value of the equivalent neutron flux averaged over the volume of 300 cm 3 is equal to 10 15 n/cm 2s at the total projected proton current of 1 mA. Some experimental, theoretical and calculational activities are proposed on the basis of RADEX-15 cryogenic facility at the MMF beam stop.

Low temperature irradiation experiments and material testing in Petten

JRC, Petten Establishment, and ECN combine the full range of facilities required for the investigation of irradiation damage of fusion reactor structural materials. The high flux reactor (HFR) Petten, owned by the Commission of the European Communities, is one of the few large materials test reactors still in operation in Western Europe. The HFR is particularly suited for research on materials for future thermonuclear fusion reactors. The ECN hot cell laboratory has dedicated facilities for post-irradiation mechanical testing (fracture mechanics, tensile, fatigue, impact testing, creep) of materials. In addition, thermophysical properties can be measured, the weldability of irradiated materials is investigated with laser and TIG welding equipment. For post-testing analysis, various techniques are used, including optical microscopy and scanning and transmission electron microscopy. Materials being investigated for current fusion programmes include ITER-grade (IG) austenitic steel type 316LN-IG (plate, electron beam- and TIG-welds), vanadium alloys (e.g. V—4Cr—4Ti) and ferritic-martensitic steels (e.g. MANET, F82H, JLF; plate, welds). Experimental details on irradiation and post-irradiation testing are presented in the paper.

Theoretical evaluation of the physical properties of copper—lead rheocast alloys

The use of the copper—lead rheocast alloy as a joining material is very attractive in the design of the plasma-facing components of a thermonuclear fusion reactor. Presently, experimental data on its main physical properties are not yet available and therefore they have been estimated theoretically. The evaluation of the following properties is performed at different alloy compositions: thermal conductivity, density, specific heat, elastic modulus, Poisson's ratio, coefficient of thermal expansion. Their use is suggested for preliminary calculations until the experimental data are obtained.

Fatigue Crack Growth Properties of a Cryogenic Structural Steel at Liquid Helium Temperature

The structural materials of the coils of superconducting magnets utilized in thermonuclear fusion reactors are used at liquid helium (4.2 K) temperatures and are subjected to repeated thermal stresses and electromagnetic forces. A high strength, high toughness austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N) has recently been developed for large, thick-walled components used in such environments. This material is non-magnetic even when subjected to processing and, because it is a forging material, it is advantageous as a structural material for large components. In the current research, a large forging of 12Cr-12Ni-10Mn-5Mo-0.2N austenitic stainless steel, was fabricated to a thickness of 250 mm, which is typical of section thicknesses encountered in actual equipment. The tensile fatigue crack growth properties of the forging were examined at liquid helium temperature as function of specimen location across the thickness of the forging. There was virtually no evidence of variation in tensile strength or fatigue crack growth properties attributable to different sampling locations in the thickness direction and no effect of thickness due to the forging or solution treatment associated with large forgings was observed. It has been clarified that there are cases in which small scale yielding (SSY) conditions are not fulfilled when stress ratios are large. ΔJ was introduced in order to achieve unified expression inclusive of these regions and, by expressing crack growth rate accordingly, the following formula was obtained at the second stage (middle range). da/dN = CJ ΔJmJ, CJ = AJ/(ΔJ0)mJ, where, AJ = 1.47 × 10−5 mm/cycle, ΔJ0 = 2.42 × 103N/m.

室温および液体ヘリウム温度での極低温用オーステナイト・ステンレス鍛造材の引張特性と疲労き裂進展特性におよぼす熱履歴の影響

By using forging of a newly developed austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N), a candidate material for the support structure of superconducting magnets for thermonuclear fusion reactors, the effects of a sensitizing heat treatment (24 hours at 650°C, followed by water quenching) on tensile and fatigue crack growth properties (crack closure behavior) at room and liquid helium temperatures were clarified. As a result, it has been found that although the sensitizing heat treatment has virtually no effect on both tensile and fatigue crack growth properties at room temperature, a decrease in tensile total elongation and an increase in fatigue crack growth rate relative to the effective stress intensity factor range due to such heat treatment are evident at liquid helium temperature. This is believed to be due to the difference in microscopic cracking path. Fracture occurs transgranularly in a solution treated material, whereas fracture occurs mainly along the grain boundaries in a sensitizing heat treated material. Furthermore, it has been ascertained that, at liquid helium temperature, the fracture surface profiles of fatigue cracks are much more serrated and the crack-opening load increases, due to sensitizing heat treatment.

CuPb rheocast alloy as joining material for CFC composites

High heat flux components for future use in thermonuclear fusion reactors are designed as layered structures. The assembling of the different parts (armour, heat sink and external structure) requires a joint which could withstand large heat loads and thermal stresses. In this paper we examined a 50 wt% PbCu rheocast alloy (RCA) as joining material for the armour/heat sink joint. The alloy was prepared in vacuum in a rotational furnace and was characterized by SEM-EDS analysis and heating microscopy. The obtained microstructure was globular as foreseen and it remained after prolonged heating at 650°C. The alloy showed very good ductility: sheets of about 200 μm were rolled starting from about 1 × 1 × 1 cm 3 cubes. The alloy was successful in joining both the armour and the heat sink materials, respectively, carbon fibre reinforced composites and copper. Initial mechanical testing shows that the technique is viable for the foreseen applications in the field of thermonuclear fusion reactors.

Applicability of the UV-Stimulated Oxidation to Tritium Cleanup

The rate of the UV-stimulated HT oxidation was studied in H{sub 2}(HT)-O{sub 2}-O{sub 3} atmospheres with excess O{sub 3}. The concentration of HTO increased linearly with UV irradiation time. The formation rate of HTO was estimated to be 3.4 x 10{sup -2} Bq cm{sup -3} s{sup -1}, which was about 14000 times greater than that of the UV-stimulated HT oxidation in the H{sub 2}(HT)-O{sub 2} atmosphere. Namely the excess O{sub 3} greatly assisted the UV-stimulated HT oxidation. The HTO formation obeyed the half order kinetics to hydrogen pressure and 0.7 order with respect to photon flux. Computer simulation consisting of 33 elementary reactions was employed to make clear the mechanism of the HT oxidation. The computer simulation reproduced the same hydrogen pressure and photon flux dependences as the experimental results. It was revealed that the main path for HTO formation is as follows: (1) HT oxidation is initiated by photolysis of O{sub 3} to O({sup 1}D) radicals; (2) O({sup 1}D) radicals react with H{sub 2}(HT) to form OH(OT) radicals; and (3) OH(OT) radicals produce H{sub 2}O(HTO) by the reaction with H{sub 2}(HT). On the basis of computer analysis, it is concluded that the considerable increase in the rate of HTOmore » formation is due to the increase in O({sup 1}D) production in the presence of O{sub 3}. The present results suggest that the O{sub 3}-assisted UV-stimulated HT oxidation is expected to be applicable to non-catalytic oxidation of tritium in thermonuclear fusion reactors. 8 refs., 6 figs., 1 tab.« less

Irradiation devices and irradiation programmes at the high flux reactor (HFR) Petten for the investigation of the irradiation behaviour of stainless steels

Irradiation testing of stainless steels represents an important share in the utilization spectrum of the high flux reactor (HFR) Petten. These tests address in particular austenitic and martensitic steels for thermo-nuclear fusion reactors, austenitic steels for fast breeder reactors, and stainless steels and CrNi-alloys for light water reactors. A number of well-proven irradiation devices covering an extensive range of applications and operated by a vastly experienced group are available at the HFR. In addition, at the Petten hot cells, many diversified tools exist for irradiation testing of stainless steel. The irradiation devices are tailored to provide typical, reactor specific environment conditions to the samples. Such conditions are maintained within narrow specifications. In this respect, the long lasting operational experience originating from the many materials irradiation programmes performed at the HFR, the high and consistent availability of the HFR and its reliability with regard to provision of predictable and reproducible irradiation conditions, are the keys to these many demonstrable experiments. The adjustment of typical neutron spectrum at the irradiation samples is performed through an appropriate choice of structural materials and their arrangement within the irradiation device. Many irradiation devices are reloadable, being specially suited for intermediate unloading and hot cell measurement of the irradiated samples. Other devices are equipped with integral loading and measuring systems for in-pile measurement of characteristic irradiation parameters (e.g. the creep behaviour). The neutron fluence is calculated and then measured after irradiation from neutron fluence monitor sets which are integrated into the sample carriers. The post irradiation examination and testing of the irradiated samples is performed either at the customer's hot cells or at the Petten hot cells, in particular, when intermediate measurements are required. Dedicated and specialized testing equipment for tensile, creep, Charpy, fatigue and crack propagation testing, various microscopy systems (SEM, TEM) and other test equipment (gamma-scanning, chemical analysis, image analysis, etc.) are all available at the Petten hot cells.

High heat flux burnout in subcooled flow boiling

The paper reports the results of an experimental research carried out at the Heat Transfer Division of the Energy Department, C.R. Casaccia, on the thermal hydraulic characterization of subcooled flow boiling CHF under typical conditions of thermonuclear fusion reactors, i.e. high liquid velocity and subcooling.

A merging preaccelerator for high current H− ion beams

The high power ion beams used in the next generation thermonuclear fusion reactors require high current negative ion beams accelerated to high energy, with high efficiency. One way to meet these requirements is to merge multiple low current density H− beamlets into a single high current beam. The feasibility of a high current merging preaccelerator was demonstrated in this experiment by merging 19 beamlets of H− ions distributed over a circular area 80 mm in diameter from a Japan Atomic Energy Research Institute negative ion source. H− ions were extracted at a current density exceeding 10 mA/cm2 at the ion source which operates at 0.13 Pa (1 mTorr), with a low arc power density (70 V×250 A). Spherically curved grids (with built-in magnetic electron suppression) were used in the preaccelerator to focus the extracted beamlets into a single 104 mA, 100 keV beam. The merged beam has a diameter of 23 mm and a converging angle of ±30 mrad at the beam envelope. The rms emittance of the 104 mA merging beam was 1.00 π mrad cm, which is a condition acceptable to the electrostatic quadropole accelerator for further acceleration.

Detection of subcooled boiling heat transfer regimes up to critical heat flux by accelerometric equipment

The design of the plasma-facing components of the Next European Torus-International Thermonuclear Experimental Reactor fusion reactor has been addressing the highest requirement in the field of heat transfer thermal hydraulics. The more promising heat transfer technique, among those possible using water as coolant, is based on the subcooled boiling thermal hydraulics in the fully developed regime, with the highest heat transfer coefficient, but avoiding the reaching of the critical heat flux (CHF) and its consequent dangerous burn-out. To this aim an experimental activity was launched in order to optimize the material, the physical parameters and the structure geometry. Among others, an objective of this work is the development of an experimental system for the detection of the subcooled boiling phenomenon covering the whole heat transfer regime, on externally heated cylindrical channels, from the single phase up to the CHF. The principle is based on the recording, by using quartz accelerometers, of the bubble implosion noises transmitted by the structure in the proximity of the collapsing regions.