## 3 Modeling Approach
### Irradiation Unit Layout  
The production of radioactive isotopes in the equatorial port of EU DEMO is currently not foreseen but is studied here as a potential option that could be introduced in the design at a later point in time. The concept and layout of a DEMO irradiation port is, therefore, not available in the DEMO project, and to this end, a preliminary design of the irradiation units and their integration in the port was developed for this work. The possible structure  
Figure 1: Geometry hierarchy in the MCNP geometry model.  
of the unit assumes that the isotope production technique in DEMO could be similar to established techniques implemented in fission reactors. In most cases, the layout applied for the arrangement and the neutron irradiation of the precursor raw materials is close to conventional fuel pins or rods assembled in fuel bundles and enclosed in cassettes [6].  
The irradiation port is assumed to have a double-wall structure with internal cooling water with total thickness of 140 mm (steel/water mixture). This provides the volume for the integration of the irradiation units for the isotope production. The size of the irradiation unit matches the DEMO equatorial port, which has a size of 2800 \(\times\) 1080 \(\times\) 2000 mm (H \(\times\) B \(\times\) L).  
The principal dimensions of the irradiation rods come from the main design used for the \({}^{60}\)Co isotopes production in the CANDU reactor [6; 23]. Shown in Figure 2 are basic options of the pin design that can be used as a basis for the diverse geometry modifications that can be applied for the fabrication of different radioisotopes. There are two options to allocate the metals for irradiation in the pin: (a) a metal pencil of \(\sim\)6.5 mm diameter is placed in a SS316L steel container with a wall thickness of 0.8 mm, and (b) small metal pellets of 1 mm diameter and 1 mm length are allocated in an aluminum alloy Al-6061. The former option can also be used for the generation of various isotopes. Depending on the irradiation campaign duration, safety requirements for the rod design can be relaxed, that is, the rod can contain fewer protecting safety barriers or cylindrical layers against radioactivity leakage compared to the layout used for the cobalt irradiation (Figure 2c,d). In these cases, the material is enclosed in the capsule: (c)--aluminum and (d)--quartz. The outer diameter of the irradiation rods is 17 mm.  
The rods containing initial material for the irradiation are bundled together to make up an assembly. This assembly contains 30 rods that are arranged in a hexagonal matrix with a pitch (distance between rods) of 11 mm (Figure 3). The whole assembly is enclosed in the hexagonal cassette with the outer size of 136 mm and the wall thickness of 4 mm. The rods are fixed inside the cassette with spacing grids. The central rod in the cassette is made of steel to reinforce the whole structure. The cassettes are installed in a radial direction from the plasma with a gap in-between of 5 mm; they fill the whole space of the irradiation unit, and they are mounted between supporting grids: plasma-faced and back one.  
The irradiation port is separated from the plasma by a 25 mm thick first wall cooled by the pressurized water of 155 bar. The cooling of the irradiation assembly is assumed with low pressure water to enable the possibility of the cassettes' removal during the reactor operation, dwell, maintenance, or shut down. The design of the cell adjacent to the irradiation unit is out of the scope of the present work and it should be elaborated on separately to define an actuation system for the secure extraction of the cassettes. This  
Figure 2: The layouts of the irradiation rods for the fabrication of different radioactive isotopes.  
assumes the possibility to individually remove cassettes from the irradiation port, the remaining space in the irradiation unit being automatically filled with water.