Essay

The main components of fusion reactors are:

·         Blanket –

The blankets have the main functions of recovering and converting the fusion power into high grade heat, of entirely producing the required tritium fuel, and of producing the necessary shielding efficiency from high energy neutrons for steel vacuum vessel and external machine components. Besides these, power plant blankets must comply with safety and environmental requirements, especially concerning accidental sequence behaviour and low activation characteristics for the short and long term [8]. It is the most critical and challenging components of reactor, since it faces the hot plasma and neutrons directly. Coolant is always circulated through the blankets. Beryllium was used in JET as a cover to the first wall, but it breeds plutonium into the reactor. So, other materials are researched as an alternative.

For self-sustaining fusion to occur, the reaction must produce one tritium per reaction. For that purposes, the new material for breeder blanket is being developed known as Eurofer steel, which is a low-activation ferritic-martensitic steel. There are several of these blanket concepts has been developed, like helium-cooled led-lithium blanket (HCLL) and water-cooled lead-lithium blanket (WCLL) [7] [9].

 

·         Magnet System –

Figure 4 ITER Tokamak Cross-section [6]

The plasma is confined and shaped by a combination of magnetic fields from three main origins: toroidal field coils, poloidal field coils and plasma currents. The nested magnetic surfaces can confine, shape and control the plasma pressure equivalent to a few atmospheres, with a density 106 times smaller than in the atmosphere (n = 1020/m3, T ? 10 keV). Aiming in ITER at steady-state operation, all the coils are superconducting. ITER uses high-performance, internally cooled superconductors called “cable-in-conduit conductors”, in which bundled superconducting strands-mixed with copper-are cabled together and contained in a structural steel jacket. The niobium-tin (Nb3Sn) superconducting strands used in ITER’s toroidal field and central solenoid, while niobium-titanium (NbTi) in poloidal field coil [6].

 

 

 

·         Vacuum Vessels –

The whole fusion experiments are carried out in vacuum vessels, an airtight steel chamber which also acts as a first safety containment barrier. It is a component with a multiple function. It provides remote handling operations for diagnostics, heating systems, water circulation for cooling. And improves neutrons shielding with improving plasma stability. It is entirely enclosed by large vacuum chamber cryostat. The ITER vacuum vessel will measure 19.4 metres across (outer diameter), 11.4 metres high, and weigh approximately 5,200 tonnes (with the installation of the blanket and the divertor, the vacuum vessel will weigh 8,500 tonnes) [6] [10].

 

·         Divertor –

Divertor is the bottom of the vacuum vessel, it extracts heat and ash produced, minimizes plasma contamination, and protects surrounding walls from thermal and neutronic loads. Each divertors “cassette assemblies” has a supporting structure in stainless steel and three direct plasma-facing components: the inner, outer vertical targets and the dome [10].

Besides providing shielding of the vessel, the modular cassettes support the divertor target plates, a set of particularly high heat flux components, built with high conductivity armour of carbon fibre composite (CFC) and tungsten. These materials can be eroded by the plasma particles, mostly during short pulses of high heat loads, associated with edge localised modes (ELM) or plasma disruptions. This erosion process not only will call for replacement from time to time of the worn out divertor targets, but also may create carbon dust[6]. In ITER 54 ten-tonne cassette assemblies of divertor will be installed [10]. Many materials are being studied for the divertor component.

 

·         Cryostat –

Cryostat is the large stainless-steel high-vacuum pressure chamber, which gives high vacuum, ultra-cool environment for vacuum vessel and the superconducting magnets.  The tokamak assembly starts with the installation of the bottom lid of the cryostat. It also allows the access for maintenance for some parts of the reactor such as divertor, cooling systems, heating systems and blanket section.

 

4. Breeding Blanket

Before presenting the European candidate DEMO blanket concepts it is worth briefly discussing the basic consequences of the requirements on blanket design. One of the main objective is to breed tritium and act as neutron multiplier. Tritium is most-efficiently produced by the reaction –

 …………………………………………………………………………………. (5)

And therefore, in any blanket concept tritium breeding is made by irradiatingThe main components of fusion reactors are:

·         Blanket –

The blankets have the main functions of recovering and converting the fusion power into high grade heat, of entirely producing the required tritium fuel, and of producing the necessary shielding efficiency from high energy neutrons for steel vacuum vessel and external machine components. Besides these, power plant blankets must comply with safety and environmental requirements, especially concerning accidental sequence behaviour and low activation characteristics for the short and long term [8]. It is the most critical and challenging components of reactor, since it faces the hot plasma and neutrons directly. Coolant is always circulated through the blankets. Beryllium was used in JET as a cover to the first wall, but it breeds plutonium into the reactor. So, other materials are researched as an alternative.

For self-sustaining fusion to occur, the reaction must produce one tritium per reaction. For that purposes, the new material for breeder blanket is being developed known as Eurofer steel, which is a low-activation ferritic-martensitic steel. There are several of these blanket concepts has been developed, like helium-cooled led-lithium blanket (HCLL) and water-cooled lead-lithium blanket (WCLL) [7] [9].

 

·         Magnet System –

Figure 4 ITER Tokamak Cross-section [6]

The plasma is confined and shaped by a combination of magnetic fields from three main origins: toroidal field coils, poloidal field coils and plasma currents. The nested magnetic surfaces can confine, shape and control the plasma pressure equivalent to a few atmospheres, with a density 106 times smaller than in the atmosphere (n = 1020/m3, T ? 10 keV). Aiming in ITER at steady-state operation, all the coils are superconducting. ITER uses high-performance, internally cooled superconductors called “cable-in-conduit conductors”, in which bundled superconducting strands-mixed with copper-are cabled together and contained in a structural steel jacket. The niobium-tin (Nb3Sn) superconducting strands used in ITER’s toroidal field and central solenoid, while niobium-titanium (NbTi) in poloidal field coil [6].

 

 

 

·         Vacuum Vessels –

The whole fusion experiments are carried out in vacuum vessels, an airtight steel chamber which also acts as a first safety containment barrier. It is a component with a multiple function. It provides remote handling operations for diagnostics, heating systems, water circulation for cooling. And improves neutrons shielding with improving plasma stability. It is entirely enclosed by large vacuum chamber cryostat. The ITER vacuum vessel will measure 19.4 metres across (outer diameter), 11.4 metres high, and weigh approximately 5,200 tonnes (with the installation of the blanket and the divertor, the vacuum vessel will weigh 8,500 tonnes) [6] [10].

 

·         Divertor –

Divertor is the bottom of the vacuum vessel, it extracts heat and ash produced, minimizes plasma contamination, and protects surrounding walls from thermal and neutronic loads. Each divertors “cassette assemblies” has a supporting structure in stainless steel and three direct plasma-facing components: the inner, outer vertical targets and the dome [10].

Besides providing shielding of the vessel, the modular cassettes support the divertor target plates, a set of particularly high heat flux components, built with high conductivity armour of carbon fibre composite (CFC) and tungsten. These materials can be eroded by the plasma particles, mostly during short pulses of high heat loads, associated with edge localised modes (ELM) or plasma disruptions. This erosion process not only will call for replacement from time to time of the worn out divertor targets, but also may create carbon dust[6]. In ITER 54 ten-tonne cassette assemblies of divertor will be installed [10]. Many materials are being studied for the divertor component.

 

·         Cryostat –

Cryostat is the large stainless-steel high-vacuum pressure chamber, which gives high vacuum, ultra-cool environment for vacuum vessel and the superconducting magnets.  The tokamak assembly starts with the installation of the bottom lid of the cryostat. It also allows the access for maintenance for some parts of the reactor such as divertor, cooling systems, heating systems and blanket section.

 

4. Breeding Blanket

Before presenting the European candidate DEMO blanket concepts it is worth briefly discussing the basic consequences of the requirements on blanket design. One of the main objective is to breed tritium and act as neutron multiplier. Tritium is most-efficiently produced by the reaction –

 …………………………………………………………………………………. (5)

And therefore, in any blanket concept tritium breeding is made by irradiating a lithium compound with the neutrons created by fusion reactions. The production of one tritium atom by the 6Li (n, ?) consumes one neutron, while its fusion with a deuterium generates only one neutron which exhibits a significant probability (30-35%) of not being available for tritium production (because of parasitic absorptions in blanket structures or streaming through the blanket openings). These neutron losses must therefore be compensated for, usually by also incorporating in the blanket, in addition to a 6Li-rich compound, a material should be multiplying neutrons by (n,2n) reactions, like Be (Beryllium) or Pb (Lead) [11]. The major candidate breeding materials consist of liquid breeders, mainly liquid metals although recently some attention has been given to FLiBe, and lithium ceramic breeders [12]. A variety of breeding blanket concepts has been considered, ranging from more conservative concepts to higher-risk higher-payoff concepts for future reactors. In general, there are three types – Ceramic breeder blanket; Self cooled liquid metal blanket and liquid metal with helium cooling blanket. a lithium compound with the neutrons created by fusion reactions. The production of one tritium atom by the 6Li (n, ?) consumes one neutron, while its fusion with a deuterium generates only one neutron which exhibits a significant probability (30-35%) of not being available for tritium production (because of parasitic absorptions in blanket structures or streaming through the blanket openings). These neutron losses must therefore be compensated for, usually by also incorporating in the blanket, in addition to a 6Li-rich compound, a material should be multiplying neutrons by (n,2n) reactions, like Be (Beryllium) or Pb (Lead) [11]. The major candidate breeding materials consist of liquid breeders, mainly liquid metals although recently some attention has been given to FLiBe, and lithium ceramic breeders [12].

A variety of breeding blanket concepts has been considered, ranging from more conservative concepts to higher-risk higher-payoff concepts for future reactors. In general, there are three types – Ceramic breeder blanket; Self cooled liquid metal blanket and liquid metal with helium cooling blanket.