**4. Evaluation of measurement accuracy**

Measurement accuracy of NAS with 12 irradiation ends is estimated using MCNP calculations. The response of each irradiation location is evaluated by changing the location and the profile of the neutron source (see **Figure 7**).

The evaluated result of the neutron source displacement effect (**Figure 8**) shows that the upper port is the best position for the irradiation due to its lowest sensitivity. The induced error due to the vertical displacement can be even lower when it is compensated with the measurement at divertor position, as long as the irradiation end at divertor is well characterized during the plasma operation. It is estimated that induced error from the neutron source displacement can be ~ ± 1% even without compensation from other diagnostics, from the simultaneous

**Figure 7.** Evaluation of the effect of neutron source position and broadening.

**Figure 6.** Image of DFW cutout for NAS.

±10 cm radial movement of plasma.

76 Advanced Technologies and Applications of Neutron Activation Analysis

DFW material. In spite of the DFW surrounding the response of the irradiation end to the vertical movement of plasma is almost the same with the one without DFW except for the absolute value shift. However, clear decrease of the activation coefficient can be identified when plasma moves outward radially. This can introduce additional error about 2.5% by

Effect of the toroidal width of the cutout was investigated, and the result is shown in **Figure 5**. The width was increased from the initial value (30 mm) up to the geometrical maximum (170 mm) and the activation coefficient of 63Cu(n,2n)62Cu reaction was investigated by moving the plasma position in the radial direction. The absolute values of the activation coefficients become closer as the width of the cutout increases. The differences between the 'No-DFW material' case are about 10, 2, and 0.8%, when the widths are 30, 100, and 170 mm, respectively, when the plasma is kept at its central place. Also the response to the plasma

**Figure 5.** Toroidal width effect on activation coefficients of 63Cu(n,2n)62Cu response by radial plasma movement.

measurement from the upper and divertor position, when the displacement range is within ±20 cm vertically and radially. The equatorial port position can be used for backup when the data are compensated from other diagnostics.

**5. Designs of the NAS components for ITER**

ant manifold.

Thermal analysis has made significant impact on the design of the NAS front-end components (**Figure 10**). All NAS components installed inside the vacuum vessel shall follow the design guideline SDC-IC (Structural Design Criteria for ITER In-vessel Components), which requires the maximum temperature of the components to be less than about 500°C.According to the simple thermal analysis on the irradiation end in the upper port, the temperature of the irradiation end is found to exceed 500°C when the irradiation end protrudes only by 6 cm from the actively cooled diagnostic shield module (DSM) inside (but not touching) the diagnostic first wall (DFW) that has a full depth of 60 cm. Similarly, all in-vessel irradiation ends located inboard side of the vacuum vessel are found to exceed 500°C, when there is no active cooling of the irradiation end structures. The temperature could be below 500°C only when the forced circulation of He gas with the velocity higher than 10 m/s is provided for the in-vessel transfer line during the plasma operation, which can be problematic when the gas blowing with such velocity fails, for example, when the capsule touches the irradiation location and plugs the hole for the gas circulation. In order to resolve the thermal issue, the design is updated to cool down all in-port irradiation ends by attaching the cooling jacket around the irradiation end structure, where coolant can be supplied from the in-port cool-

Port plug irradiation ends mainly consist of two transfer lines which are composed of coaxial or parallel tubes (**Figure 11**). Most components will be fabricated with SS316L except the capsule monitoring cabling, which consists of MgO mineral insulated (MI) cables and Al2

based electrical feedthrough. The front part of the irradiation end is enclosed with the coolant housing, which is connected with the coolant tubing. Two guiding rings are attached on the outside of the coolant housing for the smooth insertion of the irradiation end into the

DSM. The feedthroughs will be welded on the closure plate of the port plugs.

**Figure 10.** Calculated temperature of NAS irradiation ends.

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Neutron Activation System for ITER Tokamak http://dx.doi.org/10.5772/intechopen.75966

The effect of neutron source broadening (**Figure 9**) on the measurement, which cannot be estimated during the in-vessel calibration, was evaluated. The result also indicates that the upper port is the best position because it has the lowest effect from the neutron source broadening, and shows good characteristic of depending only on the vertical broadening. It is interesting to note that the equatorial port position shows symmetric measurement with the upper port position. Therefore, the simultaneous measurements from the upper and equatorial port position are expected to provide the total neutron production with the broadening error of ~1% without compensation from other diagnostics, when the profile peaking factor is in the range of 3 < α < 7.

The calculations show that with the combination of the measurements from the upper port, equatorial port, and divertor region can provide relatively good evaluation of the total neutron production in the plasma. In spite of the low reliability of the measurement from the inboard midplain position, it is reasonable to keep this irradiation ends, as they are the only ones capable of providing the absolute value of the neutron flux coming to the inboard side.

**Figure 8.** Evaluation of the effect of neutron source position.

**Figure 9.** Evaluation of the effect of neutron source broadening.
