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Review of non-tokamak steady-stateFusion Neutron Sources
1st RCM on CRP F1.30.15 on “Conceptual Development of Steady-State Compact Fusion Neutron Sources”
November 2012, IAEA, Vienna.
Open traps and stellarators
The need to drive the toroidalplasma current in axisymmetric toroidal devices is a serious obstacle to finally develop a steady-state fusion neutron source in such devices. In particular, the power required to drive such current could turn out to be too high for a reactor realisation. The need for a current drive is removed in open systems (mirror machines etc) or toroidal systems with a rotational transform (stellarator or more preferably a stellarator-mirror systemswhere the fusion neutron generation is localized to a mirror machine section of the toroidal device), allowing steady-state operations. Open geometry provides certain flexibility in technical solutions (for divertor, diagnostics, etc), where axisymmetric mirror machines have the simplest magnet. In recent years, a rather dramatic improvement in plasma performance in axisymmetric mirrors have been demonstrated (in the single cell GDT and the multimirror GOL-3 experiments at the Budker Institute). Quadrupolar fields are considered in the SFLM (straight field line mirror) concept for gross plasma stability.A reminding concern is the longitudinalplasma loss in open systems. However, experiments on GDT device in Novosibirsk have already demonstrated, in agreement with the theory, that strong expansion of the magnetic field in the end tanks beyond the mirrors inhibited the high heat losses connected to thermal electron conduction. According to the experiments, the end losses are determined by the plasma exhaust through the mirrors, i.e. by the plasma directed velocity and plasma density in the mirrors, as predicted by the theory. The stellarator-mirror concept eliminates the axial heat losses completely, although there is a certain price in added complexity.
One of the major advantages of mirrors is the high achievable beta value, which can be as high as 0.6. This has been successfully demonstrated in the GDT experiment. Also, since the high neutron flux can be localized in the certain regions by skew injection of the neutral beams, it allows for flexible arrangement with sensitive equipment outside regions of high neutron flux. Therefore, a sufficient neutron shielding can be arranged, and the device components can operate without replacement of systems for decades. Power loads on the expander plates (which plays the role of “divertor plates”) and the load on the first wall are some other examples where the open geometry provides solutions.
Plasma confinement (end loss in open systems, neoclassical banana effects for quadrupolar mirrors and radial loss in stellarators) are of some concern. In many other aspects, the challenges seem less severe than for tokamaks, but insufficient plasma confinement is a real threat. Material load seems tolerable. Plasma heating in steady state could be a challenge, but ion cyclotron heating (and probably NBI, possibly also ECRH) are candidates.
Institute/Country:Budker Institute Novosibirsk, Russia
The Budker Institute is developing axisymmetric mirrors. Key questions have been to overcome gross instability in axisymmetric mirror systems and to increase the electron temperature. Expanders beyond the confinement region have beendemonstrated experimentally to provide high temperature plasmas with plasma betas ranging up to 0.6. Magnetic divertor is considered as a tool to further improve of the gross plasma stability. Potential plates have been installed on the GDT device, and it is demonstrated that a rotational shear is beneficial for confinement with an electron temperature reaching 260 eV. In 2002, the GOL-3 was modified to a multimirror system, which resulted in a record electron temperature above 2.5 keV. The heating in this device is provided with a relativistic electron beam along the axis of the system. The density in the GOL-3 device is high, in the range 1021-1022 particles per m3. Construction of the new GDM device, where the purpose is to combine favourable features of GDT and GOL-3, has recently been initiated. A long central cell in GDM is surrounded by extra mirror cells with the purpose to enhance longitudinal confinement. One end sections is already under construction. This endsection is planned to be used to test various heating methods (including electron beam injection with extended pulse length and ECRH) and for material tests.
A development of DC electron beams for plasma heating and electric field control in the plasma was started for the next step facility, which should be operated in long pulses or continuously. The first experiments with the electron beams with power of 1-10MW and pulse duration ~1ms have been done on GDT and GOL-3 devices successfully.
The results achieved in the GDT device are already of interest for a neutron source with sufficiently strong neutron intensity per area, equal to 0.5 MW/m2. The research at Budker Institute also considers hybrid reactors, incineration of minor actinides and even a pure fusion power reactor scenario for the GDM device.
Investigations ofGDT as a neutron source for hybrid reactors.
Institute/Country:Institute of Nuclear Energy Safety Technology
Chinese Academy of Sciences (INEST), Hefei, China.
The new Institute at Hefei is studying a variety of options for hybrid reactors. Software have been developed which make the studies more flexible. Several concept studies have previously been made on tokamaks and spherical tokamaks. Recently, the scope of studies has been broadened to include a concept study based on the GDT device.
The GDT device, with its very high mirror ratio and sloshing ions embedding in the gas dynamic regime warm plasma, has great potential to serve as a fusion neutron source with neutron flux density 1-4MW/m2, but tradeoff studies show that this kind of gas dynamic trap regime have limitations caused by its own principles, besides, all kinds of possible instabilities may set a limit to sloshing ions density, so firstly the gas dynamic regime operated plasma itself has a restricted operation space, then instabilities may set some further limitations on the operation space. Further optimization will analyse these uncertainties to give a more reasonable design scheme of compact fusion neutron source, especially, optimization should emphasize production of neutron rather than fusion energy. Conceptual design work ondifferent blankets (especially fission blanket with enriched uranium or spent fuel etc.) based on GDT geometry is underway, 3-D neutronics simulations will be performed. A general comparison study of different concepts (tokamak, stellarator, GDT and so on) can be performed to give recommendation for choice of China’s CFNS. Whether other devices like multi-mirror or min-B cell could improve the GDT performance needs more investigation, especially experimentally (just as the GDM exploration). China welcomes international collaboration.
The SFLM concept study
Institute/Country:UppsalaUniversity, Ångström laboratory, Sweden.
The SFLM concept study is aiming at a neutron source for a hybrid reactor concept to produce power from spent nuclear fuel. The studies performed so far includes plasma theory (plasma stability, magnetic field design, particle orbits), plasma heating with ion cyclotron heating or NBI, neutron kinetic computations and superconducting coil designs.
It is well known that a quadrupolar field is favourable for stabilizing gross plasma perturbations in a mirror machine. A drawback is the strong ellipticity of the flux tube created by the quadrpolar field. It is found that straight, nonpararallell magnetic field lines have certain optimal features. The ellipticity is comparively low, there is no guiding center drift in the vacuum field and the configuration is marginally stable to flutes at zero plasma pressure. Each guiding center bounces back and forth on a single magnetic field line in the vacuum field and two new constants of motion can be found from this property. Finite beta corrections have also been considered. A new constant of motion, a radial drift invariant, has been shown to exist in symmetric quadrupolar fields. This invariant ensures confinement of the collision free motion to the region of a magnetic flux tube.
Expander tanks beyond the confinement region have a sufficiently large area to withstand the heat from leaking plasma. In the SFLM with its quadrupolar stabilization, the expanders are not required for stability, and the stability does not rely on a plasma flow into the expander tanks. There is then a possibility to decrease the expander tank plasma density substantially from that of the confinement region. This may have a positive effect on the thermal isolation of the confinement region, and this can increase the electron temperature beyond typical values for mirror machines.
RF heating studies indicate that the plasma could be efficiently heated in a scenario with the antennas located in a region without a strong neutron flux. The studies will be extended further to explore different alternatives in more depth. Calculations predict that a scenario with both hot deuterons and hottritons is energetically favourable. The tritium ions are to be heated by secondary harmonic heating, which is only efficient when the tritons have a sufficiently high energy and large gyro radii. The heating of minority (40%) deuterons by fundamental ion cyclotron heating is expected to be robust.
Superconducting 3D coils have been designed for the SFLM concept. These would be made of standard low cost materials. The outer radius is only 3m, providing a compact design. The inner radius of the coils is 2m, which is sufficiently wide to provide space for the vacuum chamber, a 15 cm buffer region to protect the first wall from a too strong neutron bombardment, neutron shielding for the magnetic coils and other sensitive equipmentas well as a tritium breeding region.
Neutron kinetic studies have been performed to compute the power amplification in the blanket, which could be as high as 150 due to an almost complete coverage of the fusion device by a fission mantle. This amplification corresponds to a neutron multiplication of keff=0.97. Studies have been on several accident scenarios (loss of coolant, void of coolant, partial replacement of the lead-bismuth coolant with water) have been carried out, and the results are so far favourable. One required geometrical arrangement is to connect coolant pipes around the fuel and in the buffer region to avoid overcriticality in case water would replace the lead-bismuth coolant to a certain level. Natural circulation is predicted to be sufficient to remove decay heat with a vertical orientation of the device. The safety studies need to be extended to include other scenarios not yet covered, for instance bending of the fuel rods or avoidance of release of fission product from the long fuel rods (expansion regions in the fuel rods have to be arranged for this).The tritium breeding ratio is around 1.8 (it could easily be adjusted to both lower and higher values. Monitoring of fusion neutrons is through the top or bottom of the device and neutron monitoring in the fission mantle region can be done with standard techniques. The degree of subcriticalitry needs to be monitored. Most of the results from the neutron kinetic studies can also be applied to axisymmetric devices.
The predictions for material loads are very favourable. A 4m expander tank radius provides a 100 m2 surface which can withstand the heat from leaking plasma (the heat load is expected to be below 1 MW/m2). Plasma load on the lateral surface is expected to be much smaller, since plasma loss is typically longitudinal. The buffer is important for protection from neutron bombardment on the first wall. The 200 dpa limit for the first wall is more than 30 years for a 1.5 GWth case. No sensitive equipment is placed in neutron rich regions. Diagnostics, refuelling, power feeding etc has to be carried out through the end openings to the confinement region.
Stellarator-mirror concept
Institute/Country:Institute of Plasma Physic of the NationalScienceCenter “Kharkov Institute of Physics and Technology” / Ukraine
The main activity is on radio-frequency heating in stellarator plasma and study of magnetic mirror and stellarator based concepts of fusion neutron sources.
In the fission-fusion hybrid concept described in Ref. [V.E. Moiseenko et al J Fusion Energ29 (2010) 65] neutrons are generated in deuterium-tritium plasma confined magnetically in a stellarator-type system. The hot minority tritium ions are sustained locally at an embedded into the stellarator magnetic mirror with lower magnetic field by radio-frequency heating or neutral beam injection (NBI). The localization of the hot sloshing ions and the neutron generating zone to the mirror part enables to surround the neutron generating zone by a local fission mantle.
The stellarator with locally decreased toroidal field is considered as a model of the stellator-mirror hybrid. Using the Biot-Savart numerical code, the calculation of the magnetic configuration of the Uragan-2M stellarator (IPP, Kharkiv) is performed where the aim is to find a magnetic configuration in which closed magnetic surfaces occupies a substantial volume. Based on results of such calculations, initial experimental investigations of the magnetic configuration have beenperformed.
The particle motion in the paraxial open trap with an arbitrary quadrupolar field is investigated analytically having in mind to find a motional invariant determining a constancy in average of the particle radial coordinate.
The studies on particle motion at a magnetic mirror and the neutronic calculations for a hybrid were made in collaboration with Prof. Ågren group from Uppsala University, Sweden. Upgrade of the kinetic model for accounting the wave-particle interaction in SFLM concept is in progress. Kinetic plasma calculations are carried out for neutral beam injection made near the ends of the magnetic trap normally to the confining magnetic field. The results of kinetic calculations are used to describe the particle and power balance of the sloshing ions. The studies for the sub-critical fast nuclear reactor with the plasma neutron source are continued. Using the neutronic analysis, the neutron fluxes at the stellarator part of the stellarator mirror hybrid device are calculated aiming to find an appropriate neutron shield. The neutron damage of the ICRH antenna is calculated for different antenna materials.
The application to State Foundation for Fundamental Researches¸ Ukraine, for grant on joint research with the Prof. B. Kuteev group from Kurchatov Institute, Russia, is submitted for a topic ‘A study on characteristics of neutron yield and balance in nuclear fusion devices and development of concepts of neuron sources based on a spherical tokamak and a combination of a stellarator and a magnetic mirror’.
Compatibility of Kharkiv project with the CRP goals:
•The project investigates options for steady-state compact fusion neutron sources (CFNS) with typical fusion power in the range 1-100 MW (intensity 3.5x1017-1019 n/s), neutron wall loading in the range 0.1-1 MW/m2 based on magnetic confinement approached such as tokamaks, stellarators and mirror machines.
•The project explores plasma parameter spaces for optimizing core and edge plasma performance for neutron production at fusion energy gain value Q = 0.1-1.
•The project formulatesa concept for enabling technologies and associated materials: this will include the magnet systems, vacuum vessel, divertor, blankets, the heating and current drive systems, the pumping, cooling and fuelling systems, the tritium plant, diagnostics, the remote handling system. A challenge is the high power of NBI or RF compared to existing experiments. This should be assessed in the future.
The safety issues are expected to be similar to open trap safety since the fission mantle design is similar.
•The kinetic KNBIM code is developed. This code models hot ions in plasma of an open trap or a stellarator.
Plasma focus
Development of a Concept ofExtremely Bright Compact Fusion Neutron Source Based on Dense Plasma Focus Device for Applications
Institute/Country:The Inter-regional Public Organization“Moscow Physical Society”, Moscow, R.F.
Investigation of neutron fields surrounding the chambers of the main-stream fusion devices with magnetic plasma confinement such as ITER or with inertial plasma confinement of the type of NIF or Z-machine is of great importance. Indeed numerous elements of the chambers’ constructions as well as auxiliary equipment (plasma heating devices, diagnostics complex, etc.) represent neutron scatterers and absorbers. So they can produce in the 3-D neutron field around the fusion chambers “voids” and “hot spots”.
Thus it seems to be very fruitful to characterize the 3-D neutron field surrounding the nuclear fusion chamber (NFC) with a help of a bright “point” neutron source that has pulse duration (of the 14-MeV neutrons) in nanosecond range. This short neutron flash will allow using the time-of-flight (TOF) technique with flight bases of the order of real sizes of NFC.
In that case the spatial size of the neutron shell irradiated from the source will have a size of about tens of cm being much less compared with the main construction elements of NFC. Ideally such a procedure must be repeated step by step during all construction phases of a NFC. Such bright nanosecond neutron pulses can be generated by a Dense Plasma Focus.