5.3.1 Heating power

The three ITER heating neutral injectors will be capable of injecting "only" 50 MW of D0 into ITER, and significantly higher power may be required on a reactor. If that is realised by adding more injectors, an even larger neutral beam cell will be required. However that might be avoided by increasing the power from each injector, which might be achieved by having a more uniform power density on the calorimeter. In principle, that might be possible as the geometric limitations associated with the gas neutraliser used in the ITER injector would not apply to a photon neutraliser-based system. However that requires detailed considerations of the injector design, and those are not possible with the present state of development of the photon neutraliser.

#### Figure 2.

Computer-generated cut-away view of an ITER HNB beamline. The scale below the beamline shows the distance along the beamline axis in metres from the last grid of the accelerator.

Research, Design, and Development Needed to Realise a Neutral Beam Injection System… DOI: http://dx.doi.org/10.5772/intechopen.88724

#### Figure 3.

Computer view of the ITER NB cell with three heating neutral beam injectors and a diagnostic neutral beam injector installed. The three heating neutral beam injectors should be capable of injecting 50 MW into the ITER plasma.

### 5.4 Operation in a nuclear environment

The injectors at ITER are expected to perform in a harsh nuclear environment which will not be better on a reactor. In the present scheme of things, the injectors are an extension of the vacuum vessel and therefore a part of the nuclear confinement barrier. It is therefore essential that their integrity is assured for all situations including normal and accidental events which can be initiated when the machine operates.

The design and operation of injectors in a nuclear environment need to be considered from various aspects such as the engineering design codes used, the injector materials, their layout, their operational response to normal and accidental scenarios under various load conditions, and maintenance, including remote handling, tooling, and the deployment of remote handling tools. All those have been addressed for the ITER injectors, and the impact on the design has been very significant. Some of the various aspects are discussed below in order to give an idea of the complexity involved in adapting the systems to nuclear environments.

#### 5.4.1 Materials

Operation in a radiation environment impacts the choice of materials used for components, such as to ensure that the essential material characteristics are not modified by irradiation and to reduce the degree of activation and the quantity of activated waste. This impacts on all parts of the injector, including, for example, the choice of vacuum seals, where, essentially, only all metal seals are accepted, the joining techniques for similar and dissimilar materials, etc. The definitions for reactors will be stringent because of the harsh environment and strict safety standards that will be applied. Radiation can damage electronics, components, and sensors, corrupt signals, and make optical fibres and windows become opaque. These effects can appear instantaneously or over a period of time due to an accumulation of, for example, atomic displacements. It is clear that the design of an injector designed to operate on a reactor will have to ensure that all electronics are situated in low radiation areas, as has been done for the ITER design, both to ensure their lifetime is adequate and to allow their maintenance.

#### 5.4.2 Injector layout

Designing a system for the environment of a reactor requires a serious consideration of the layout of the injectors, ensuring good access for the remote handling tooling and adequate shielding in case of the need for man access. Meeting these requirements requires sufficient space, but there is a conflicting requirement, which is to keep the "nuclear island" as small as possible. The latter means that the location of the inventory, the first confinement boundary, is confined to as small an area as possible. The vacuum boundary of the machine, in normal operation, defines the first confinement barrier, and any penetration through the barrier becomes part of the first confinement barrier. To ensure reliability and safety, it is likely that two metal valves will need to be installed, in series, upstream of any penetration of the barrier, as is the case on ITER. In that case, the vacuum boundary of the machine and the second valve, which is the last layer of the primary confinement barrier, defines the nuclear island. To minimise the size of the nuclear island, the valves and windows must be located as close as possible to the machine. This can lead to a complex design which must tolerate high radiation loads, heat loads, neutron fluxes, and electromagnetic loads whilst simultaneously fulfilling the safety function of primary confinement. An example of the impact on the injector design is that this means that any guide tubes for the laser light used for the photon neutraliser will need to incorporate a window in series with a valve close to the injector vessel. That window will have to meet all the requirement of the first confinement barrier and vacuum boundary as well as be able to transmit the laser light quasi-continuously. As some laser power will be absorbed by the window, it is probable that the window will need to be cooled by some type of cooling system. Also the window must be capable of withstanding any over pressure or heat load that might occur during the failure inside the reactor.

#### 5.4.3 Operational response to normal and accidental scenarios

To ensure safe operation of injectors on a reactor, all possible accidental events will have to be analysed under all possible load conditions including those arising from abnormal plasma events such as a "vertical displacement event" (VDE),<sup>4</sup> and under all conditions, i.e. when the system is operating, in stand-by or being maintained. In addition, nuclear analyses will have to be performed to confirm that radiation damage to the materials will be below the level at which the mechanical properties are compromised.

#### 5.4.4 Remote handling

Operation in a nuclear environment limits direct access and hands on maintenance as the injector will become radioactive, and there are then hazards related to exposure to radiation and from inhalation of airborne contamination. The expected level of radioactivity in the ITER NB cell and the defined radiation zoning of the facility lead to the need for remote handling to be in place. Human access is only allowed when the dose rates are kept beneath the allowed limits of <100 μSv/h.

<sup>4</sup> In some magnetic confinement devices loss of control of the plasma position can result in a rapid vertical movement of the plasma, leading to a violent disruption (sudden loss of the plasma), which is known as a vertical displacement event. The consequent release of the energy that was stored in the plasma can lead to very strong, potentially damaging, forces being applied to the machine.

Research, Design, and Development Needed to Realise a Neutral Beam Injection System… DOI: http://dx.doi.org/10.5772/intechopen.88724

Achieving the compliance with the radiation zoning is carried out by the use of effective shielding in the design of the components and reducing materials which are more activated, i.e. using low cobalt content stainless steel, but achieving a similar situation on a reactor will be difficult.
