Laser cutting of thick steel plates and simulated steel components using a 30 kW fiber laser

Laser cutting of thick steel plates and simulated steel components using a 30 kW fiber laser was studied for application to nuclear decommissioning. Successful cutting of carbon steel and stainless steel plates up to 300 mm in thickness was demonstrated, as was that of thick steel components such as simulated reactor vessel walls, a large pipe, and a gate valve. The results indicate that laser cutting applied to nuclear decommissioning is a promising technology.     

Experimental study of a single tube high RF power amplifier for ICRF heating system

A concept of a single tube high RF power amplifier was developed for ion cyclotron range of frequency (ICRF) plasma heating system. In the concept, a tetrode was used with a grounded cathode and input power to drive a control grid of the tetrode was provided by a switching circuit. As the new amplifier arrangement can eliminate a low power (10 kW level) and an intermediate power (100 kW level) tetrode amplifiers, their high voltage DC (HVDC) power supplies, and control and monitor system for these amplifiers and HVDC power supplies in a conventional high RF power source of the ICRF heating system, this new high RF power source is more flexible on frequency change and more mechanically reliable than the conventional one. A test amplifier composed of the tetrode and a field effect transistor (FET) switching circuit was constructed. The FET switching circuit was so compact that it could be mounted close to the tetrode socket. The maximum output RF power of 8.5 kW was obtained with a plate efficiency of 82% at 70 MHz. The feasibility of the single tube high RF power amplifier was experimentally proved. The plate efficiency of 82% could not be explained by the standard class-C amplification but by high efficiency amplification under assumptions of a flat-topped plate current pattern and double resonance of an output cavity at the fundamental frequency and the third higher harmonic frequency.     

Possible design improvements and a high power density fuel design for integral type small modular pressurized water reactors

The study evaluates potential weaknesses and possible improvements for integral type small modular pressurized water reactor designs. By taking International Reactor Innovative and Secure (IRIS) as the reference design and keeping the power output as the same, a new fuel and reactor design were proposed. The proposed design relocates the primary coolant pumps and the pressurizer outside the reactor pressure vessel (RPV). Three recirculation lines and jet pumps/centrifugal pumps are introduced to provide the coolant circulation similar to Boiling Water Reactor designs. The pressurizer component is expected to be similar to the AP600 design. It is located at one of the recirculation lines. The new fuel assembly adopts 264 solid cylindrical fuel pins with 10 mm diameter and 2.3 m height, arranged at a hexagonal tight lattice configuration. Large water rods are introduced to preserve the moderating power and to accommodate finger type control rods. The resulting fuel can operate with 104.5 kW/l power density while having substantially higher margin for boiling crisis compared to typical large PWRs. Full core neutronic analysis shows that 24-month cycle length and 50 MWd/kg burnup is achievable with a two-batch refueling scheme. Furthermore, the fuel behavior study shows that the new fuel with M5 type Zircaloy cladding show fairly acceptable steady state performance. A preliminary Loss of Coolant analysis shows that the new design could be advantageous over IRIS due to its low ratio of the water inventory below the top of the active fuel to total RPV water inventory. The proposed reactor pressure vessel height and the containment volume are 30% lower than the reference IRIS design.     

Calculation of the power distribution in the fuel rods of the low power research reactor using the MCNP4C code

The Monte Carlo method, using the MCNP4C code, was used in this paper to calculate the power distribution in 3-D geometry in the fuel rods of the Syrian Miniature Neutron Source Reactor (MNSR). To normalize the MCNP4C result to the steady state nominal thermal power, the appropriate scaling factor was defined to calculate the power distribution precisely. The maximum power of the individual rod was found in the fuel ring number 2 and was found to be 105 W. The minimum power was found in the fuel ring number 9 and was 79.9 W. The total power in the total fuel rods was 30.9 kW. This result agrees very well with nominal power reported in the reactor safety analysis report which equals 30 kW. Finally, the peak power factors, which are defined as the ratios between the maximum to the average and the maximum to the minimum powers were calculated to be 1.18 and 1.31 respectively.     

Design of copper/carbon-coated fiber Bragg grating acoustic sensor net for integrated health monitoring of nuclear power plant

A nuclear power plant (NPP) is a harsh environment that gives rise to age-related degradation of the plant structures, and eventually leads to radiation leakage that threatens humans. Integrated structural health monitoring (ISHM) technology is a strong candidate for the prevention of the NPP accidents during operation. Prior studies have shown that fiber Bragg gratings (FBGs) and metal-coated fibers have good radiation and high temperature resistance. In this study, a FBG acoustic sensor using a metallic adhesive for installation and a relatively economical copper/carbon (Cu/C)-coated fiber is developed for ISHM of high temperature NPP structures. A chemical method is proposed to remove the Cu/C coating. A 5 mm FBG was successfully inscribed in a Ge-doped silica core through a 7 mm-long silica section with the coating removed. The Cu/C-coated fiber with the same core/clad structure as the standard SMF allowed no-loss fusion splicing, and showed good adaptability to the economical standard fiber, adaptor, connector, and instruments. It showed also good thermal resistance (<345 °C) with no degradation in optical power during the optical transmission. The metallic adhesive used to install the FBG in a one-end-free configuration showed superior bonding reliability during temperature cycles ranging from 25 °C to 345 °C. The FBG reflectivity was stabilized at a 58% drop from the initial reflectivity, and the Cu/C-coated FBG sensor using the metallic adhesive could successfully detect the acousto-ultrasonic waves generated by pencil lead breaking and laser beam excitation.     

Measurements of the isothermal, power and temperature reactivity coefficients of the IPR-R1 TRIGA reactor

The aim of this paper is to present the experimental results of the isothermal, power and temperature coefficients of reactivity of the IPR-R1 TRIGA reactor at the Nuclear Technology Development Center - CDTN in Brazil. The measured isothermal reactivity coefficient, in the temperature range measured, was −0.5 ¢/°C, and the reactivity measurements were performed at 10 W to eliminate nuclear heating. The reactor forced cooling system was turned off during the measurements. When the reactor is at zero power there is no sensible heat being released in the fuel, and the entire reactor core can be characterized by a single temperature. The power coefficient of reactivity obtained was approximately −0.63 ¢/kW, and the temperature reactivity coefficient of the reactor was −0.8 ¢/°C. It was noted that the rise in the coolant temperature has contributed only with a small fraction to the observed negative effect of the reactivity. The power defect, which is the change in reactivity taking place between zero power and full power (250 kW), was 1.6 $. Because of the prompt negative temperature coefficient, a significant amount of reactivity is needed to overcome temperature and allow the reactor to operate at the higher power levels in steady state.     

Design of a high power millimeter wave launcher for EC H&CD system on ITER

An optimization procedure of the quasi-optical system for a millimeter wave launcher is developed for the ITER electron cyclotron heating and current drive (EC H&CD) launcher. In the launcher, the radiated RF beams from eight corrugated waveguides are reflected sequentially by two mirrors and injected into a plasma through a small aperture in the blanket shield module on the top of the launcher. Using a steepest decent method, the heat load on the mirrors is successfully reduced to the acceptable level by flattening the RF power profile on the mirrors keeping the scattering of the RF power to a minimum from the mirrors. It is found that 20 MW injection will be acceptable even when the resistivity of 2.64 × 10⁻⁸ Ωm for the surface of the mirror (dispersion strengthened copper, 151 °C assumed) is increased by a factor of ∼10 with a contamination.     

Reactivity balance in the IPR-R1 TRIGA reactor

Considering that the power of the IPR-R1 TRIGA reactor, located at the Nuclear Technology Development Center, Brazil, will be increased from 100 kW to 250 kW, some experiments were done in order to evaluate the magnitude of the reactivity effects associated with the reactor operation. The core excess of reactivity obtained was 1.99 $, and the shutdown margin was 1.33 $. The reactivity needed to operate the IPR-R1 reactor at 100 kW was 0.72 $, mainly due to the prompt negative temperature coefficient. A significant amount of reactivity is needed to overcome temperature and allow the reactor to operate at the higher power levels. The loss of reactivity due to xenon poisoning after 8 h of operation at 100 kW was around 0.20 $, and the highest reactivity loss value caused by a void inserted in the central thimble was 0.22 $. From the results obtained, it was possible to balance all the determined reactivity losses with the reactivity excess available in the reactor, considering the present and the future reactor power operation.     

Laser photodetachment neutraliser for negative ion beams

We outline a speculative design for a photodetachment neutraliser for a negative ion neutral beam system, with neutralisation efficiency of 95% or more. The practical difficulties are enormous. The ion beam must pass through an optical cavity capable of reflecting the light many times. For 500 reflections, the laser optical power output ∼800 kW, giving circulating power ∼400 MW. All sources of light loss combined need to be kept to 0.2% or less per pass. The losses due to photodetachment itself, and due to Thomson scattering in the beam plasma are negligible. A key task is to maintain the reflectance of the mirrors above 99.97% for long periods of operation, protecting all the components from thermal and neutron damage, and from caesium, sputtered atoms and other contamination. A diode-pumped Nd-doped YAG laser can have overall electrical-to-light (“wall-plug”) efficiency up to 25%. A DEMO concept reactor such as the EU Power Plant Conceptual Study (PPCS) Model B requires 270 MW heating power. If this is all provided by neutral beams, then a laser neutraliser might reduce the electrical power consumption for this from 900 MW to 520 MW.     

Evaluation of precipitates used in strainer head loss testing: Part II. Precipitates by in situ aluminum alloy corrosion

Vertical loop head loss tests were performed with 6061 and 1100 aluminum (Al) alloy plates immersed in borated solution at pH = 9.3 at room temperature and 60 °C. The results suggest that the potential for corrosion of an Al alloy to result in increased head loss across a glass fiber bed may depend on its microstructure, i.e., the size distribution and number density of intermetallic particles that are present in Al matrix and FeSiAl ternary compounds, as well as its Al release rate. Per unit mass of Al removed from solution, the WCAP-16530 aluminum hydroxide (Al(OH)₃) surrogate was more effective in increasing head loss than the Al(OH)₃ precipitates formed in situ by corrosion of Al alloy. However, in choosing a representative amount of surrogate for plant specific testing, consideration should be given to the potential for additional head losses due to intermetallic particles and the apparent reduction in the effective solubility of Al(OH)₃ when intermetallic particles are present.     

Development of remote pipe cutting tool for divertor cassettes in JT-60SA

Remote pipe cutting tool accessing from inside pipe has been newly developed for JT-60SA. The tool head equips a disk-shaped cutter blade and four rollers which are subjected to the reaction force. The tool pushes out the cutter blade by decreasing the distance between two cams. The tool cuts a cooling pipe by both pushing out the cutter blade and rotating the tool head itself. The roller holder is not pushed out anymore after touching the inner wall of the pipe. In other words, only cutter blade is pushed out after bringing the tool axis into the pipe axis. Outer diameter of the cutting tool head is 44 mm. The cutting tool is able to push out the cutter blade up to 32.5 mm in radius, i.e. 65 mm in diameter, which is enough to cut the pipe having an outer diameter of 59.8 mm. The thickness and material of the cooling pipe are 2.8 mm and SUS316L, respectively. The length of the cutting tool head is about 1 m. The tool is able to cut a pipe locates about 480 mm in depth from the mounting surface on the divertor cassette. The pipe cutting system equips two cutting heads and they are able to cut two pipes at the same time in order to remove the inner target plate. Reproducibility of the cross-sectional shape of the cut pipe is required for re-welding. The degree of reproducibility is inside 0.1 mm except for burr at outside of the pipe, which is enough to re-weld the cut pipe. Some swarf is generated during cutting the double-layered pipe assuming a plug located on the top of the pipe. The swarf is deposited on the bottom of the plug and collected by pulling out the plug in the actual equipment.     

Laser cutting performances for thick steel specimens studied by molten metal removal conditions

Laser cutting performances for thick carbon steel and stainless steel specimens up to 300 mm in thickness were studied to dismantle large steel objects. The cutting performances were summarized based on the assist gas flow rate and the front kerf width, and the range for appropriate cutting conditions was shown. Gas pressure in the kerf region required for molten metal removal was estimated from the pressure loss on the kerf surface, which depended on the gas flow rate and the kerf width. The relation to keep sufficient gas pressure in the kerf well corresponded to the experimental relations for appropriate cutting. Drag force to the molten metal on the kerf surface was also estimated, which varied by the structures and materials. The behaviors such as cavity formation and its expansion in the kerf region at the unsuccessful cutting trials were well explained. The results are informative for the development of the laser cutting technology applied to the thick steel specimen for the nuclear decommissioning.     

The present status of R&D for the muon target at J-PARC: The development of silver-brazing method for graphite

At the J-PARC muon science facility, the muon target was made of an isotropic graphite (IG-43). The energy deposited by the proton beam is estimated to be 3.3 kW on graphite and 600 W on the copper frame. To alleviate the thermal stress, a titanium stress absorber is inserted between the graphite and the copper. Although graphite is known to be difficult to be brazed, the titanium is attached to the graphite through silver-brazing. In this report, we will describe the development of a silver-brazing method for graphite in the fabrication of the J-PARC muon target. A capillary test between the graphite and the titanium was performed to determine the optimal brazing conditions. The test involved bonding graphite and titanium plates while varying the gap between them in order to determine the brazing material and the optimal surface treatment of graphite. Subsequently, a trial muon-production target was fabricated using this optimized brazing method. Specimens were cut from the trial target, and bending test experiments were performed to determine the tensile and shear strength of the interface. As a result, it was confirmed that graphite could be bonded adequately through the silver-brazing.     

Characteristics of the local bubble parameters of a subcooled boiling flow in an annulus

An experimental study on the subcooled boiling phenomena was carried out in the SUBO (SUbcooled BOiling) test facility under steam-water flow condition. The test section is a vertical annulus of which axial length is 4.165 m with a heater rod at the center of a channel. The inner and outer diameters of the test section and the heater rod are 35.5 mm and 9.98 mm, respectively. For the measurement of the local bubble parameters, double sensor optical fiber probes were applied at six elevations along the test channel. Among them, one is installed in the unheated region which is located downstream of the heated section for the measurement of bubble condensation. A total of six test cases was chosen for the parametric study of the heat flux of 370-563 kW/m², mass flux of 1110-2100 kg/(m² s) and inlet subcooling of 19-31 K at pressure condition of 0.15-0.2 MPa. From the test, local void fraction, interfacial area concentration, Sauter mean diameter and bubble velocity were measured at 11 radial locations at each elevation. The measured data shows well development and propagation of the bubble parameters along the test channel. The present data is expected to be suitable for a benchmark, validation and model development of the CFD codes or existing safety analysis codes.     

Comparative study on Charpy specimen reconstitution techniques

Reconstitution techniques are often used to allow material from previously fractured Charpy-V specimens to be reused for additional experiments. This paper presents a comparative experimental study of various reconstitution techniques and evaluates the feasibility of these methods for future use in shielded cells. The following techniques were investigated: arc stud welding, 6.0 kW CO₂ continuous wave laser welding, 4.5 kW YAG continuous wave laser welding and friction welding. Subsize Charpy specimens were reconstituted using a 400 W YAG pulsed wave laser. The best result was obtained with arc stud welding; the resilience of the reconstituted specimens and the load-displacement curves agreed well with the reference specimens, and the temperature elevation caused by the welding process was limited to the vicinity of the weld. Good results were also obtained with friction welding; this process led to the best quality welds. Laser welding seems to have affected the central part of the specimens, thus leading to different resilience values and load-displacement curves.     

Underwater Cutting of Reactor Core Internals by CO Laser Using Local-Dry-Zone Creating Nozzle

With a view to practical application of the CO laser to underwater cutting of thick steel plates, a nozzle for creating a local dry zone on the workpiece has been developed and tested. The nozzle directed against the workpiece surface discharges a jet of air, which forms the local dry zone, bounded by a cone of high-speed water jet discharged from a concentric annular outlet. Preliminary tests were performed to optimize the nozzle shape and operating conditions. The resulting nozzle was used with a 5 kW CO laser for actual underwater cutting tests on stainless steel plates: Entirely satisfactory cutting performance was confirmed on various workpiece geometries and working positions.     

Determination of Tehran Research Reactor power by N gamma detection

The radioisotope ¹⁶N is produced by the interaction of fast neutrons with ¹⁶O in water reactor coolant. This radioisotope emits at the two major gamma ray energies of 6.13 MeV and 7.1 MeV. Exploiting the linear relation between the number of gamma particles versus the reactor power change, the reactor power is determined by detecting and counting the emitted gammas. In this work, for the detection of gammas to measure the reactor power, two different methods are employed. First, by NaI(Tl) scintillator detector and second, by assembly of ten GM detectors. The obtained results confirm that the number of emitted gammas is proportional to the change in reactor power as shown by different monitoring systems such as UIC, CIC, FC, Cherenkov and thermal power. Both of the applied methods are shown to give reliable results for reactor power above 20 kW. Both systems, having been calibrated, are being used as monitoring systems of power in Tehran Research Reactor. These systems are usable in other research reactors and possibly in power reactors as well.     

Deep drawing of tungsten plates for structural divertor applications in future fusion devices

The reference design of a helium cooled divertor for future fusion reactors makes use of hundreds of thousands of finger units consisting of a pressurized structural part called a thimble. Due to the high number of parts needed, the thimble has to be fabricated by mass production techniques like deep drawing. As the thimble is a pressurized part exposed to an internal pressure of 100 bar, the demands for the material are high, which means that it requires the best available tungsten material. Former work has shown that pure tungsten material has the best impact properties and has to be preferred over other commercially available tungsten materials, such as that doped with potassium or strengthened with oxides like lanthanum oxide.Furthermore the inherent weakness of the grain boundaries has to be taken into account, which requires the need for grains that are aligned to the contour of the part (grain boundary alignment).This paper describes the successful deep drawing of a 1 mm tungsten plate in high vacuum at 600 °C. In doing this, a thimble can be machined with grains that follow the contour. Furthermore the characterization of a 1 mm tungsten plate is conducted by tensile tests at room temperature and at 600 °C, as well as by Charpy tests taking into account the anisotropic material behaviour.     

Numerical study on pressure wave propagation in a mercury loop

On-beam tests were carried out at the Los Alamos Neutron Science Center-Weapons Neutron Research (LANSCE-WNR) facility in June 2005 to investigate pressure wave mitigation in mercury targets for the MW-class spallation neutron sources under international collaboration between US Spallation Neutron Source (SNS) and Japanese Spallation Neutron Source (JSNS). A mercury loop was used for the target, a so-called In-Beam Bubbling Test Loop (IBBTL). The loop consists of the rectangular pipe of 25 mm × 50 mm² in cross section, 1.5 mm in wall thickness and 2 m in total length approximately. The SNS team set 8 strain sensors on the pipe wall to measure the strain propagation caused by the pressure wave. The maximum strain appeared at 350 mm apart from the proton-bombarded point at 5.5 ms after the proton bombardment. It is known that the propagation velocity of the pressure wave in mercury is ca. 1500 m/s and that of the stress wave in stainless steel is ca. 5000 m/s. However, the apparent wave propagation velocity in the IBBTL was lower than those velocities and was observed to be 65 m/s. Numerical analysis was carried out to understand the strain propagation in the pipe wall of the IBBTL. Numerical results showed that the maximum strain at 350 mm apart from the beam spot appeared at 5.5 ms after proton bombardment in good agreement with experimental results.