K. Okuno

Reversible disproportionation of ZrCo under high temperature and hydrogen pressure

Abstract Chemical behavior of an intermetallic compound ZrCo with hydrogen at high temperature region was studied with respect to application utilizing the reversible hydrogenation. While ZrCo compound reacts with hydrogen to form ZrCoH (0–3) below 400°C, the disproportionation reaction, 2ZrCo + H 2 → ZrH 2 + ZrCo 2 occurs above 400°C under hydrogen pressure higher than the equilibrium decomposition pressure of ZrCoH (0–3) . X-ray diffraction analysis showed that the reaction is completed in 5 h at 500°C and it takes more than 40 h at 400°C. No sign of generation of ZrH 2 + ZrCo 2 phase was observed by the X-ray diffraction in 10 h at 400°C, however. The disproportionated product completely returned to single-phase ZrCo when evacuated at or above 500°C for several hours, thus this reaction is reversible. Kinetics of the disproportionation reaction was expressed by an Avrami equation R = 1 − exp{−( t / π ) n }. The reaction speed was expressed by an Arrhenius form. For practical application of ZrCo, the results suggest some operational precaution and limitation, and recovery treatment when the material is suspected to have lost some capacity by disproportionation reaction.

Kinetic studies of the trttium release process in neutron-irradiated Li2O and LiOH

Tritium produced in Li2O(s) and LiOH(s) by the 6Li(n,α)T reaction was released mostly in the form of HTO(g) through heat treatment under vacuum. From rate measurements of the HTO(g) release process in the temperature range 530–700 K, however, a difference was found in the kinetic data between the two materials. The HTO(g) release from LiOH(s) was revealed to be controlled by desorption of HTO molecules in analogy with the thermal decomposition reaction of LiOH(s). On the other hand, the rate-determining step of HTO(g) release from Li2O(s) was found to be diffusion of tritium in the solid. The diffusion coefficients determined for Li2O samples, different in neutron fluence (nvt), were D = 1.9 × 10−2exp{−(24.8 ± 2.5) × 103RT} cm2/s at nvt = 5.4× 1015/cm15, D = 1.2× 10 −2exp{−(24.9 ± 1.7) × 103RT} cm2/s at nvt = 8.1 × 1016/cm2, and D = 5.1 × 10t-3exp{−(23.9 ± 1.7) × 103RT} cm2/s at nvt = 8.9 × 1017/cm2.

ITER CS model coil and CS insert test results

The inner and outer modules of the central solenoid model coil (CSMC) were built by US and Japanese home teams in collaboration with European and Russian teams to demonstrate the feasibility of a superconducting central solenoid for ITER and other large tokamak reactors. The CSMC mass is about 120 t; OD is about 3.6 m and the stored energy is 640 MJ at 36 kA and peak field of 13 T. Testing of the CSMC and the CS insert took place at Japan Atomic Energy Research Institute (JAERI) from mid March until mid August 2000. This paper presents the main results of the tests performed,.

Development of low carbon and boron added 22Mn-13Cr-9Ni-1Mo-0.24N steel (JK2LB) for jacket which undergoes Nb/sub 3/Sn heat treatment

Japan Atomic Energy Research Institute has developed a low carbon and boron added 0.03C-22Mn-13Cr-9Ni-1Mo-0.24N-0.003B steel (JK2LB) for a conductor jacket of the central solenoid for the International Thermonuclear Experimental Reactor in collaboration with Kobe Steel Ltd. The feature of JK2LB is to have enough ductility and toughness after reaction heat treatment to produce Nb/sub 3/Sn. Tensile properties, fracture toughness, and fatigue crack propagation rate were measured at 4 K by using samples taken from an intermediate billet and the final jacket of JK2LB, which were produced with a mass production line. The average elongation and fracture toughness at 4 K after the heat treatment are 33% and 93 MPa/spl radic/m for the final jacket, and 30% and 121MPa/spl radic/m for the intermediate billet, respectively. The mechanical properties of weld metals were also evaluated and all data satisfied the ITER targets. It was also clarified that improvement of ductility and toughness were caused by reduction of chromium carbide precipitations due to low carbon and boron addition.

Experimental results on instability caused by non-uniform current distribution in the 30 kA NbTi demo poloidal coil (DPC-U) conductor

Abstract Two 30 kA, NbTi Demo Poloidal Coils, DPC-U1 and DPC-U2, were fabricated and tested in the Demo Poloidal Coil project at the Japan Atomic Energy Research Institute. DPC-U1 and -U2 have a large current, forced flow cooling, cable-in-conduit conductor, which is composed of 486 strands. The strand surfaces are insulated by formvar to reduce coupling losses between the strands. DPC-U1 and -U2 reached their design current, but exhibited instability during charge, in many cases resulting in a coil quench. Such a quench occurred even at a current one-tenth of the conductor critical current. To clarify the cause of the instability, a detailed investigation on the quench current and normal voltage behaviour was carried out by charging the coil in several ways to the coil quench, and by measuring the stability of the coil at a current of 16–21.5 kA. These experimental results revealed the existence of non-uniformity of current distribution among the strands in the conductor, even under slow charging. This non-uniformity of current distribution caused the instability of the coil. The time constant of current redistribution is very large due to the insulation between the strands. However, if part of the conductor can be forced to go normal without coil quench occurring, a redistribution of current takes place and the current distribution becomes more uniform. It was then demonstrated that the current distribution could become uniform by applying heat to the conductor to generate intentional normalcy. Consequently, the possibility of stable operation of the DPC-U was suggested.

Current imbalance due to induced circulation currents in a large cable-inconduit superconductor

Abstract It has been previously reported by the authors that 30 kA NbTi pulsed coils (Demo Poloidal Coils; DPC-U1 and -U2) exhibit instability such as quenching at much lower currents than their critical level as a result of current imbalance in the conductor. In this paper, a theoretical study for such an imbalance in a large cable-in-conduit (CIC) conductor consisting of insulated strands is presented. This study indicates that significant circulation currents are induced in large CIC conductors, such as the conductor of DPC-U1 and U2, and remain for a long time because of the superconductivity of the strands. A large current imbalance is produced by superimposing the induced circulation current onto the transport current. It is also shown that the existence of an external field induces larger circulation currents, resulting in the larger current imbalance. For justification of these indications, characteristics of current imbalance are investigated from the experimental results. The magnitude of the current imbalance is evaluated as the ratio of the maximum strand current to the average strand current. This ratio was estimated to be 7.1 when DPC-U1 was charged singly, and reached about 15 when DPC-U1 was subjected to an external field from DPC-U2 and a test coil was installed between DPC-U1 and U2. Also, the time decay constant of the induced circulation currents was estimated to be around 2 h. These figures are interpreted by calculating the current distribution in the DPC-U1 conductor based on the assumption of asymmetric strand transposition of about 0.1% deviation in self inductances. It seems impossible to control such small asymmetry of the strand transposition in a commercial manufacturing procedure. Therefore, such instability as a result of current imbalance is inevitable in large CIC superconductors consisting of insulated stands. A similar instability may be caused in a large CIC superconductor when strands are coated with highly resistive material.

Separation of hydrogen isotopes by the cryogenic-wall thermal diffusion column

In this paper experimental study for separation of hydrogen isotopes has been performed by using a cryogenic-wall thermal diffusion column refrigerated by liquid nitrogen. The column separated H-D system at total reflux and total recycle operational modes. The dependences of the separation factor on the column pressure and hot wire temperature were examined for the total reflux experiments. The optimum pressure observed was 30 kPa at 1273 K. The maximum separation factor at 473 K was larger than that at 1273 K since HD molecules were not produced on the hot wire by the isotope exchange reaction. The separation factor was exponentially proportion al to the hot wire temperature. In the total recycle experiments, the separation factor was measured under a variety of flow rates, positions and compositions of the feed stream. The increase in the feed flow rate deteriorated the separation factor appreciably.

Critical current test results of 13 T–46 kA Nb3Al cable-in-conduit conductor

In the framework of ITER-EDA, a 13 T-46 kA Nb3Al conductor with stainless steel jacket has been developed in order to demonstrate applicability of an Nb3Al conductor with react-and-wind technique to ITER-TF coils. Using a 3.5 m sample consisting of a pair of conductors with 0% and 0.4% bending strain, the critical current performances of the Nb3Al conductors were studied to verify that the conductor achieves the expected performance and the bending strain of 0.4% does not originate degradation. The critical currents were measured at background magnetic fields of 7, 9, 10 and 11 T at temperatures from 6 to 9 K. The expected critical currents were evaluated taking into account the variation of the strain in the cross-section due to the bending strain as well as self-field and non-uniform current distribution as results of an imbalance in the joint resistance and inductances. The calculation results indicate that the current distribution is almost uniform and the experimental results showed good agreement with the expected critical currents. Accordingly, we can conclude that the fabrication process of this conductor is appropriate and the react-and-wind technique using the Nb3Al conductor is applicable to ITER-TF coils. In addition, the critical current of the Nb3Al conductor is expected to be 108 kA at 13 T and 4.5 K, resulting in a sufficient margin against the nominal current of 46 kA. Furthermore, it was found that the decrease in the critical current by thermal strain can be made small by applying the bending strain to the conductor so as to reduce the compressive strain at higher fields, i.e. inner side of the coil, in the conductor cross-section

Stabilized operation of 30 kA NbTi Demo Poloidal Coil (DPC-U) with uniform current distribution in conductors

Abstract Two 30 kA NbTi Demo Poloidal Coils (DPC-U1, -U2) were fabricated and tested in the Demo Poloidal Coil Project. DPC-U1 and -U2, referred to collectively as DPC-U, exhibited instability in pulsed and even in d.c. charge, such as coils quenching at much lower current than the design current which itself is still below the conductor critical current. It was found from the previous paper published by the authors that non-uniform current distribution is established in a large cable-in-conduit conductor, such as the DPC-U conductor, whose strands are electrically insulated from one another, due to an imbalance in the inductances of the strands. Also, it was clarified that the instability of DPC-U is caused by the current imbalance. However, it was also shown that the imbalanced distribution could be made uniform by creating a small resistive zone in the conductor by applying inductive heating pulses. In this paper, it is shown that the coil exhibiting instability due to imbalanced current distribution could be operated stably using this effect. In addition, it is shown that the DPC-U strands exhibited no deterioration in their designed critical current. This result indicates that the instability of DPC-U is entirely attributable to the current imbalance.

Development of advanced Nb3Al superconductors for a fusion demo plant

A Nb3Al superconductor inherently has the outstanding features of a high critical field and an excellent strain tolerance in critical current performance. The Japan Atomic Energy Research Institute developed the worlds first large Nb3Al coil using jelly-roll processed Nb3Al strands. This coil was successfully operated up to the nominal current and field of 46 kA and 13 T, respectively. The test results demonstrated that a Nb3Al conductor is suitable for application in high field, large magnets, such as the toroidal field (TF) coils in a fusion reactor. In parallel with this work, a Nb3Al strand having a high critical current at high field has been developed by the National Institute of Materials Science. This strand cannot incorporate enough copper stabilizer, resulting in poor stability, since it is heat-treated at a much higher temperature than the melting temperature of copper. In order to find a solution to this issue, we performed an analytical study, and it showed that the externally incorporated copper after the high temperature heat treatment is effective for stabilization when the electric conductance and heat transfer coefficient between the Nb3Al strand and the external copper are more than 10 MS m−1 and 10 kW m−2 K−1, respectively. Since they seem to be attained by using present conductor technologies, the development of a TF coil operated at a high field around 16 T seems promising.