Download
1 / 43
430 likes | 592 Views
The fusion process. Nuclear fusion involves the bringing together of atomic nuclei. The atom's nucleus consists of protons (p) with a single positive charge and neutrons (n) of similar mass and no charge. A strong nuclear force holds these "nucleons" together against the repulsive effect of the pro
E N D
1. 1
2. The fusion process Nuclear fusion involves the bringing together of atomic nuclei. The atom's nucleus consists of protons (p) with a single positive charge and neutrons (n) of similar mass and no charge. A strong nuclear force holds these "nucleons" together against the repulsive effect of the proton's charge. The same number of negatively charged electrons as protons swarm around the nucleus to balance the proton charge. The mass of the atom lies almost totally in the nucleus.
The sum of the individual masses of the nucleons is greater than the mass of the whole nucleus. This means that the combined nucleus is in a lower energy state than the nucleons separately. The difference, the binding energy (?E=?m.c2 ), varies from one element to another.
When two light atomic nuclei are brought together to make a heavier one, the binding energy of the combined nucleus can be more than the sum of the binding energies of the component nuclei (i.e. it is in an even lower energy state). This energy difference is released in the "fusion" process (fusion reaction). See figure below)
A similar situation occurs when heavy nuclei split. Again the binding energies of the pieces can be more that of the whole and the excess energy can be released in the “fission” process
2
3. Binding energy released in fusion and fission reactions 3
4. In the Sun and stars a chain of fusion reactions occurs which converts hydrogen to helium. There are two chains both having the same effective results, and which dominates depends on the size of the star. For the Sun the proton cycle dominates. The overall reaction rate is extremely low, but it nevertheless drives the universe due to star sizes and huge masses. The particles are held together by gravity long enough for sufficient reactions to occur. For instance, in the core of the Sun the temperatures is 10 - 15 million °C. Along with the extreme pressure (a quarter of a trillion atmospheres) and density (eight times that of gold), this allows matter to be converted into large amounts of energy. 4
5.
To make fusion on a smaller scale on earth, more probable reactions have to be used. A figure of merit for a reaction is the product of the probability of reaction and the energy delivered per reaction. (specific reactivity)
The most attractive fusion reactions are the following (see next figure):
(1), D-T, (2) D-D, and (3) D-3He
A comparison of them in terms of specific reactivity is shown in the following figure.
D-T is the favorite reaction with a maximum reactivity at around 100 million °C. The next most reactive is D+D, about 40 times smaller, and D+3He, an isotope of helium, about 85 times smaller. The D+D reactivity value includes "side reactions" between D and the D+D reaction products, namely T and 3He.
5
6. Fusion reactions 6
7. Figure of merit for a fusion reaction (reactivity) 7
8. Deuteriom-Tritium fusion reaction 8
9. Plasma power balance D and T form a plasma already at few thousand degrees.
The reacting mix of D and T is a plasma at the temperatures of tens of million degrees needed to obtain adequate fusion reaction cross sections.
Three main parameters define the operation of a plasma in a confinement system:
n: plasma density;
?E: confinement time
T: plasma temperature;
?E = E/P: quality of plasma thermal insulation;
E = thermal energy in the plasma,
P = power needed to maintain the plasma at a constant temperature . 9
10. Plasma power balance (1) In any isolated system, a power balance can be established:
pfus = n2 <?v>QDT fusion power density
<?v> = average cross section-speed product for a maxwellian velocity distribution; QDT = energy released in fusion
Pfus= ? pfusdV total fusion power;
P? = ?1/5Pfus dV alpha particles power
Pheat heating power injected in plasma
Pa + Pheat = Plosses power balance
P losses = (?3nT dV)/ 10
11. Plasma losses Breemsstrahlung: emitted mainly by the electrons of the plasma accelerated by interaction with ions. Normally negligeable
Cyclotron radiation: emitted by the electrons gyrating in the magnetic field. The power associated is, in large measure, absorbed by the plasma which is optically thick at these frequencies
Impurity radiation: This is the major cause of losses. The impurities in the plasma enhance the losses by Breemsstrahlung and irradiate energy by atomic processes, line radiation and recombination, due to high Z impurities, not fully ionized at the plasma boundaries. With an Fe (atomic Z=26), content in the plasma of 0.2%, 10% of fusion power is lost by radiation not contributing therefore to heat the plasma
Conduction and convection: they can be reduced by shaping, in appropriate way, the magnetic field configuration .
11
12. 12
13. One approach to achieving the necessary conditions for fusion on earth is to exploit the inertia (mass) of the particles. Inertial fusion involves the firing many times per second of high energy particle or laser beams from all directions at tiny solid fuel pellets in a reaction chamber. Material sputtered off the pellet by the high energy beams drives a shock wave towards the pellet centre, raising its temperature and density. This implosion leads to sufficient fusion reactions occurring to overcome the losses, and a large amount of energy is released in a "micro-explosion". The resulting alpha particles, neutrons, and radiation flow radially out towards the reaction chamber walls. These are situated far enough (typically metres) away and built so as to be able to withstand the loads. 13
14. An alternative approach to achieve fusion in earth exploits the charge of the particles. In this case the plasma charged particles are deflected by a magnetic field and, if the field is strong enough, particles will orbit round a field line, gradually progressing along it if they have some longitudinal velocity..
The most attractive confinement schemes are based on toroidal plasma configurations . But unfortunately in a torus the magnetic field gets weaker across the minor diameter. Thus the particle orbit around the field line is tighter on the high field (inboard) side than on the low field (outboard) side. The result is a movement of the ions upwards and electrons downwards in the plasma, and the resulting electric field makes the plasma drift radially out of the torus.
To avoid this difficulty various systems have been proposed, namely:
- the tokamak
- the reversed field pinch
- the stellarator 14
15. The tokamak system The present preferred toroidal magnetic configuration is named Tokamak.
This system exploits the fact the plasma, even if it is globally neutral, can conduct electrical current due to the independently moving positively and negatively charged particles of which it is composed.
Then in tokamak a current pulse in a transformer primary winding (named central solenoid), placed in the hole of the torus, creates an electric field and hence drives a large current in the plasma ring, which serves as the sole secondary winding of the transformer. This plasma current provides a component of poloidal field in the plasma. In conjunction with the toroidal field provided by coils placed around the torus, this causes each field line to spiral round the plasma torus, generating a magnetic surface. Particles orbiting the field line are constrained near this surface, unless they collide with other particles. 15
16. Tokamak basic configuration The tokamak basic configuration involves three coil systems producing the following magnetic fields, see figure below :
Bt provided by the Toroidal Field (TF) coils.
B t(R) = Btox Ro /R (R= torus radius, Bto, Ro = values on the plasma axis)
BP, provided by the plasma current I induced through the Central
Solenoid (CS)
BP(a) = m0x I(a) /2p a (a= plasma radius)
Bv provided by Poloidal Field (PF) coils placed outside the TF-coils ring
16
17. Tokamak magnetic configuration 17
18. Safety factor (q) The combination of toroidal and poloidal magnetic field gives
helical field lines that in a regular situation lie on nested magnetic surfaces on which the plasma pressure is constant.
The general pattern of the helical field lines is described by the
safety factor (q )
q expresses the number of times a field line circles the major axis in circling once around the minor one:
For magnetic surfaces with circular cross section, approximately:
q = BT /BP x a/R
Elementary tokamak theory predicts stability against dangerous low order plasma kink modes (external kink modes) when:
q(r) >1 at all radia
(MHD stability Kruskal-Shafranov criterion ) 18
19. 19
20. Real magnetic configuration and plasma instabilities The real magnetic configuration of a Tokamak is for various reasons more complex ascompared to that created only by the TF coils and plasma current, namely:
Discrete number of the TF coils across the torus
Presence of the coils for field control
Presence of coils to “divert” the magnetic flux at the plasma boundry for extraction of unburned plasma particles and impurities
To these factors one has to ad to add the macroscopic plasma instabilities which can be classified at least according to four categories:
a) External kinks
b) Internal resistive kinks
c) Ballooning modes
d) Disruptive instabilities
In the assessment of the plasma parameters of a power station the capability to avoid these instabilities on the plasma confinement is assured by fixing un upper limit to bt., 20
21. 21
22. The poloidal magnetic divertor (1) A single null poloidal divertor, presently used in tokamaks, is the region of the machine between the cross point of the magnetic field lines and the walls used to exhaust thermal power (good part of the power associated with the helium ashes) and particles (helium nuclei in particular and impurities) (see figure.)
The cross point, where the two branches of the field separatrix intercept, is created by zeroing the poloidal field and through it the particles and the energy they transport enter the divertor region. SOL (scrape- off - flayer) is the region of the plasma beyond the separatrix. The region of the plasma beyond the separatrix is named SOL (scrape-off-layer); SOL is narrow, typically less than 10 mm
Thermal power densities on the divertor plates must be kept within the target materials temperature and erosion acceptable limits (15-20 MW7m2)
High neutral densities must be created in the divertor region to facilitate pumping out of the vacuum vessel the reaction ashes and the impurities 22
23. Magnetic divertor (2) Two ways to solve the problem of limiting the thermal load on the divertor targets.
Incline the divertor plates with respect to the separatrix in order to distribute the power and particles flow on a wider surface (see figure)
Inject impurities in the plasma edge to increase radiation from the scrape-off-layer to first wall and reduce the power ending inthe diveror region
The power deposited in the divertor will be 10-13% of the total thermal power for conversion to electricity, the remaining part being in blanket first wall (12-14%) in breeding blanket (70-75%) and few % in the shield. The largest part of power to divertor will come from neutrons, typically (60%), the remaining part coming from alpha and D-T ions and impurities. 23
24. 24
25. Divertor radial cross-section 25
26. 26
27. (D-T fuel, magnetic confinement, tokamak configuration)
In a fusion power plant the D-T fuel (plasma), contained in a vacuum vessel and magnetically confined, is heated up to a temperature which enables to obtain the number of fusion reactions to produce the required power.
The DT fusion reaction products are neutrons and alpha particles
D + T = a (3.52 MeV) + n (14. 06 MeV)
Alfa particles (ions) remain confined in the plasma volume and deliver , by slowing down, their energy to the electrons of the plasma and from these to the ions DT, so balancing the lost energy of plasma by electro-magnetic radiation and heat conduction and convection to the first wall.
Neutrons leave the plasma and by slowing down they transfer their energy to the plasma facing walls and to a further component named breeding blanket and to other more external systems (neutron shield, vacuum vessel ,magnets). The largest part of heat deposited by neutrons is recovered in the plasma facing walls first walls and breeding blanket (primary cooling circuit).
The breeding blanket contains Lithium based materials. Neutrons, reacting with its isotopes (Li-6 e Li-7), produce tritium which, once extracted, is reintroduced in the plasma chamber to balance the loss of tritium burnt by fusion reactions 27
28.
Fusion power plant (2)
First wall structures surrounding the plasma must face thermo-mechanical and electro-mechanical effects as well as the damage due to neutron and gamma radiation. This obliges to replace the first wall and breeding blanket during the lifetime of the plant. A neutron shield is placed outside the blanket in order to avoid the replacement of vacuum vessel and magnet system during the life of the plant.
The presence of tritium, which is a radioactive material, and of materials radiologically activated by neutrons, requires remote operation for maintenance and for the periodic replacements of the internal components of the machine.
The so named “ashes” from the combustion remaining inside the plasma, namely alpha particles, unburned D- T ions and impurities produced by the plasma-first wall interaction, are continuously removed from the vacuum vessel by openings in the magnetic field configuration (magnetic divertor). The divertor includes a target structure (plates) facing the plasma where the ions are neutralized and deposit their energy and a pumping channel for the exhausted gas extraction. 28
29. Fusion power plant (3)
Recovered Tritium from plasma exhaust is separated from the other elements (Deuterium, Protium, impurities) and reintroduced in the plasma chamber together with the tritium recovered from the breeding blanket (fuel cycle system)
The magnet system consists of various types of superconducting coils (Toroidal Field (TF) coils, a Central Solenoid (CS) coil and Poloidal Field (PF) coils (divertor, stability and correction coils). All coils are cooled by a supercritical helium flow maintained cryogenic temperature by circulation pumps
The tokamak vessel and superconducting magnets are located inside a thermally shielded cryostat to maintain the cryogenic temperatures needed for superconductivity.
29
30. Layout of a Fusion Power Plant 30
31. 31
32.
Plasma and vacuum vessel
First wall, breeding blanket and neutron shielding
Divertor and plasma exhaust extraction systems
Magnet and cryogenic systems
Plasma heating systems
Heat recovery and electricity conversion sytems
Tritium and fuel cycle systems
Control and safety systems
Remote handling systems for maintenance and repair
Auxiliary power plant systems 32
33. The design plasma parameters
Fusion power density ( pfus)
pfus = n2 < ?v > e
for 1 < T < 10 keV ? < ?v> ~T2 then
pfus~ n2T2;
?t = nT/Bt2/2?o ; nT = 2 ?o ?t / Bt2
then
pfus ~ ?t2Bt4
Limits on bt and Bt are fixed from equilibrium/stability and technological reasons, respectively 33
34. 34
35. Selection of plant design parameters
The selection of the plant parameters implies modeling of the interactions among and between the plasma physics and technology requirements. This leads to a set of equations , as those of the plant power and plasma balance, where the various terms have a complex dependence on the key parameters which determine the plasma, engineering and economic performance of a power station. Starting from these equations a system code has to be set up which enables to evaluate in a self consistent manner the parameters of the fusion power plant which have to satisfy an input requirement, as the minimization of the cost of electricity..
The key parameters for which a limit has been identified up to now for plasma performance of the plant are:
Energy confinement time tE , (see figure)
Plasma beta bt , see Fig.
Plasma density (limit n-Greenwald = I/pa2 , but no firm basis)
Safety factor q (q > 1)
Neutron wall loading
Power density at the divertor target
Toroidal field at the TF conductor 35
36. A simplified model for plasma power balance evaluation 36
37. 37
38. ? Neoclassical tearing modes
?t = ?N x I/aB (?N = beta normalized to ideal MHD)
? Resistive Wall Modes (coupling to the wall)
Stabilizing external magnetic fields feedback controlled
? Alfven Eigenmodes 38
39. 39
40. Current drive and power station operation (2)
An important parameter (Q) which characterizes the mode of operation of a power station is the ratio of the fusion power (PFus) to the auxiliary heating power (PHeat ), named Plasma Gain (Q)
Q = PFus / PHeat
Ignition (PHeat = 0) would correspond to Q =infinite.
Power plants will possibly operate steady-state with a continuous additional external heating power supply without reaching ignition conditions. In this case a fraction of the electrical power produced by the fusion plant will be ricirculated for the auxiliary heating in order to create the non inductive plasma current operation. This will imply an economic penalty as compared to the case of operation at ignition conditions, due to the necessity of circulating part of the electric fusion power for the additional power supply. 40
41. Bootstrap current and power station operation (3)
Plasma particles transport in a Tokamak is characterized by the presence of trapped particles and by their driftt (neoclassical diffusion).
Bootstrap current (BS) is a a self generated plasma current parallel to the magnetic field, proportional to the plasma ßp i.e. to the plasma pressure nT and driven by the plasma pressure gradient:
Ibs /Iplasma = 0.6 (r/R)0.5 ßp ;
Ibs (r) = - 1/Bp dp/dr
BS will be essential in a thermonuclear plasma of power plant because the generation of this current will allow to reduce the ricirculated electric power needed for generating the current drive. For this reason at present plasma physics efforts are aimed to identify plasma confinement regimes at high bootstrap current fraction (ex. Reversed Shear regime)
80% bootstrap current has been demonstrated in JT-60, 70% in JET 41
42. Reactor Power Flow Diagram 42
43. References
http://www.efda.erg/fusion_energy
R.Andreani (EFDA): The fusion reactor, Master Course on “Plasma techology and fusion”, Padova 2006 43
