**3. Design of DFW cutout for upper port irradiation end**

NAS is supposed to provide reliable and robust measurement data because it will be used for the calibration of other neutron diagnostics. From the point of reliability and robustness of the measurement, optimum location of the irradiation end is where the activation coefficient is insensitive to any environmental changes during the plasma operation and measurement, such as geometrical change of the surrounding material, plasma movement, and slight movement of irradiation end location. The geometrical changes of the irradiation end surrounding material can be happened due to the thermal expansion, vibration, distortion, and so on. Thus, location far away from plasma without any scattering material can be the best place for the irradiation end.

However, materials between the plasma and irradiation end cannot be avoided in real situation. If the location of the irradiation end is far away from the plasma, too much material inbetween will increase the measurement uncertainty. On the other hand, if the location of the irradiation end is very close to the plasma, plasma movement will increase the measurement uncertainty as well. So we should find a location where the effect of the plasma movement and the effect of the material are the modest. Normally, an irradiation end without any surrounding material nearby is chosen as the location in given position (by the port location, for example). If the effect of the plasma movement is very significant, compensation of the measurement can be necessary: (1) by using plasma location information from other diagnostics or (2) by measuring simultaneously in the opposite location vertically or radially.

However in ITER, where the radiation environment is extremely harsh, it is very difficult to avoid material around the irradiation end. Instead, we will try to find geometry of the surrounding material, whose impact on the measurement is minimized, with the help of neutron transport calculation.

The irradiation end in the upper port is selected as the object of the investigation because it is one of the locations inside the port plug, where the effect of the geometry change of the surrounding material is less severe than other locations. Most of the in-vessel irradiation ends are located between the blanket shields, where is vulnerable to the geometrical change. The activation coefficients of various samples with and without DFW material have been compared around the irradiation end (see **Figure 4** for instance). The effect of the geometry of the cutout in DFW was investigated to find a design: (1) whose absolute value of the activation coefficient is similar with the one without DFW material and (2) whose response to the plasma movement is not so severe.

Activation coefficients of three samples, that is, silicon, copper, and titanium at the upper port irradiation end were calculated using FISPACT and MCNP code. Objective nuclear reactions are 28Si(n,p)28Al, 63Cu(n,2n)62Cu, and 48Ti(n,p)48Sc.

**Figure 3** shows the MCNP model for the calculation. The cutout of DFW was designed to have a toroidal and poloidal angle of view as large as possible, while minimizing the amount of material in front of the irradiation end to the plasma direction, in order to minimize errors from the plasma movement and neutron transport calculation. Initial values for each dimension are:

• Depth: 130 mm.

the activated samples. The required parameters for the NAS such as neutron fluence will be evaluated from the gamma spectrum considering the location of the irradiation end, sample

NAS is supposed to provide reliable and robust measurement data because it will be used for the calibration of other neutron diagnostics. From the point of reliability and robustness of the measurement, optimum location of the irradiation end is where the activation coefficient is insensitive to any environmental changes during the plasma operation and measurement, such as geometrical change of the surrounding material, plasma movement, and slight movement of irradiation end location. The geometrical changes of the irradiation end surrounding material can be happened due to the thermal expansion, vibration, distortion, and so on. Thus, location far away from plasma without any scattering material can be the best place for the irradiation end. However, materials between the plasma and irradiation end cannot be avoided in real situation. If the location of the irradiation end is far away from the plasma, too much material inbetween will increase the measurement uncertainty. On the other hand, if the location of the irradiation end is very close to the plasma, plasma movement will increase the measurement uncertainty as well. So we should find a location where the effect of the plasma movement and the effect of the material are the modest. Normally, an irradiation end without any surrounding material nearby is chosen as the location in given position (by the port location, for example). If the effect of the plasma movement is very significant, compensation of the measurement can be necessary: (1) by using plasma location information from other diagnostics

or (2) by measuring simultaneously in the opposite location vertically or radially.

transport calculation.

movement is not so severe.

are 28Si(n,p)28Al, 63Cu(n,2n)62Cu, and 48Ti(n,p)48Sc.

However in ITER, where the radiation environment is extremely harsh, it is very difficult to avoid material around the irradiation end. Instead, we will try to find geometry of the surrounding material, whose impact on the measurement is minimized, with the help of neutron

The irradiation end in the upper port is selected as the object of the investigation because it is one of the locations inside the port plug, where the effect of the geometry change of the surrounding material is less severe than other locations. Most of the in-vessel irradiation ends are located between the blanket shields, where is vulnerable to the geometrical change. The activation coefficients of various samples with and without DFW material have been compared around the irradiation end (see **Figure 4** for instance). The effect of the geometry of the cutout in DFW was investigated to find a design: (1) whose absolute value of the activation coefficient is similar with the one without DFW material and (2) whose response to the plasma

Activation coefficients of three samples, that is, silicon, copper, and titanium at the upper port irradiation end were calculated using FISPACT and MCNP code. Objective nuclear reactions

material and its mass, and irradiation and cooling time.

74 Advanced Technologies and Applications of Neutron Activation Analysis

**3. Design of DFW cutout for upper port irradiation end**


Calculated activation coefficients are shown in **Figure 4**. When there is no DFW material (upper line) and when there is a cutout in DFW material (lower line). Absolute values of the activation coefficient are reduced by about 10% when the irradiation end is surrounded by

**Figure 3.** MCNP model for calculation: (left) side view and (right) front view.

**Figure 4.** Comparison of plasma movement effect with and without DFW.

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 ±10 cm radial movement of plasma.

movement is improved by increasing the width. It is easily identified the response of the irradiation end become more insensitive to the plasma movement as the size of the width increases. Slops of the linearly fit equations of the calculated activation coefficients are 9.2, 3.8, and 2.2 (×10−34) per 1 cm plasma movement, when the widths are 30, 100, and 170 mm, respectively. Calculated maximum errors according to this equation are 0.8 and 1.4%, when

Neutron Activation System for ITER Tokamak http://dx.doi.org/10.5772/intechopen.75966 77

The effect of the DFW cutout design on the measurement accuracy was investigated. The initial design values are proved to be proper except the toroidal width. It is recommended the toroidal width of the cutout to be as large as possible. The recommended design of the DFW cutout is shown in **Figure 6**. By making a cutout according to the design recommended by this calculation, we can imitate as much as possible the response of the ideal irradiation end,

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

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

the plasma movement values are ±5 cm and ± 10 cm, respectively.

where there is no surrounding material nearby.

**4. Evaluation of measurement accuracy**

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

profile of the neutron source (see **Figure 7**).

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.

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

movement is improved by increasing the width. It is easily identified the response of the irradiation end become more insensitive to the plasma movement as the size of the width increases. Slops of the linearly fit equations of the calculated activation coefficients are 9.2, 3.8, and 2.2 (×10−34) per 1 cm plasma movement, when the widths are 30, 100, and 170 mm, respectively. Calculated maximum errors according to this equation are 0.8 and 1.4%, when the plasma movement values are ±5 cm and ± 10 cm, respectively.

The effect of the DFW cutout design on the measurement accuracy was investigated. The initial design values are proved to be proper except the toroidal width. It is recommended the toroidal width of the cutout to be as large as possible. The recommended design of the DFW cutout is shown in **Figure 6**. By making a cutout according to the design recommended by this calculation, we can imitate as much as possible the response of the ideal irradiation end, where there is no surrounding material nearby.
