Nuclearfusion: Why The Gee-Whizz Factor Costs So Much

The reason is that the ITER people have only recently woken up to the fact, gently pointed out by me to the JET boffins back in 1985, that though the giant magnet around the walls of the tokamak can contain the primary plasma, which is ionized, it cannot contain the neutrons that are the intended reaction products, because neutrons possess no electrical charge ex definitione.

One difficulty faced by ITER is that it is so much larger than all previous tokamaks.

As the late Professor R.V. Jones of Aberdeen University used to say, "In physics, scale matters." As the reactor ramps up from the warm-up or low-confinement regime to the operational or high-confinement regime, oscillations at the edges of the plasma occur.

To try to divert these outbursts of high-energy particles away from the walls, it is necessary to install a divertor plate at the bottom of the torus.

Position of the divertor system at the foot of the torus.

At the strike points where the plasma interacts with the divertor plate, even during normal operation the ITER torus will be subjected to plasma fluxes sufficient to vaporize the divertor plate at the strike point, where the initial flux incident upon the divertor plate is focused.

Secondly, the ITER divertor plates will be too close to the primary plasma.

Looking down a segment of the torus towards the too-small area for the missing divertor.

One cannot simply replace the damaged divertor plate, since the deposition of a large amount of energy at the strike point as currently designed would generate such intense radiation and scattering of plasma-particle fluxes from the dense secondary-plasma cloud that internal components in the confined divertor area would be damaged.

Hassanein & Sizyuk, whose paper in Nature on the problems with the existing ITER design may well have influenced the French nuclear regulatory agency in its decision to halt construction of the reactor until the problems have been rectified, suggest that completely different tokamak designs - such as the snowflake or super-X magnetic containment configurations - might offer better protection by distributing the core-plasma particles over a wider area on the surface of the divertor, decreasing the heat loads on the divertor plate.

In the divertor of a Super-X tokamak, the plasma exhaust is spread over a wider area than in the conventional divertor hitherto adopted by ITER. Or the divertor plates and strike points could be moved further from the core plasma, and a special divertor chamber could limit penetration of the secondary plasma into the operating chamber, contaminating the primary plasma. 

https://wattsupwiththat.com/2023/07/27/nuclear-confusion-why-the-gee-whizz-factor-costs-so-much/