The SPC strengthens Tokamak security to intensify fusion research

Swiss Plasma Center © Nadia Barth / EPFL

Swiss Plasma Center © Nadia Barth / EPFL

It's a world first. After seven months of work, more than 220 tons of high-density polyethylene have transformed the shielding around the Tokamak à Configuration Variable (TCV). Never before has such a quantity of this material been used on such a large scale. The goal: to protect the public from ionizing radiation following the recent improvement of TCV's performance. This complex endeavor, a result of collaboration with the Operational Vice-Presidency of EPFL, was successfully completed in August 2023. It promises to propel fusion research to new heights.

Since TCV start of operation in 1992, the reactor was surrounded by 50cm thick baryte concrete walls made of blocks to stop neutron and photons. Its very elongated vacuum chamber, highly flexible magnetic coils configuration and unique microwave heating system allowed TCV to provide significant breakthroughs in plasma research towards a fusion reactor. Recently, to further study fast ion physic, the TCV tokamak has been equipped with two 1MW Neutral Beam Heating (NBH) systems that directly heat the plasma ions. Heating the ions enhances the Deuterium-Deuterium fusion reaction rate and hence increases the neutron production inside the reactor. This modification required additional shielding around the torus hall to ensure the safety of areas open to the public against ionizing radiation, following the standards of the Federal Office of Public Health.

High-density polyethylene, lightweight and efficient
A collaboration with the Jozef Stefan Institute in Ljubljana allowed for the modeling of neutron and gamma ray transport, leading to a new design that includes increased thickness of existing walls, the addition of a roof, and the installation of doors. Usually, concrete is used for neutron shielding, but the TCV building foundations could not support the three additions if they were made of concrete. The solution? The use of high-density polyethylene. Although different materials were evaluated, it became clear that using polyethylene for additional shielding was the preferred solution due to its high hydrogen atom content and low density of about 1 kg/l. In fact, a 20 cm thick polyethylene plate has a similar moderating power to 50 cm of concrete but with 8.7 times less weight. Therefore, polyethylene was chosen as the main neutron moderator. In total, 220 tons of this material were installed in just 7 months of work. This is the first time such a quantity of polyethylene has been used on such a large scale!

Extremely fruitful collaboration
The complexity of the work, launched in collaboration with the EPFL Operational Vice Presidency (VPO), required meticulous planning that began in 2020. The VPO took charge of project management while the SPC focused on the scientific aspect. This partnership proved essential for the success of the project. Technically, the shielding consists in recycled concrete blocs from the upper layers over the first 3m of the torus hall existing walls, 20cm thick PE slabs on all remaining 50cm thick concrete walls, 35cm thick PE dismountable roof, a concrete belt surrounding the roof and 35cm thick borated PE for doors. After the completion of the work in August 2023, a dose measurement campaign at ten reference points was conducted the following month. It demonstrated that this new shielding is a complete success. Experimental measurements align with model predictions within a factor of 2, and neutron attenuation exceeding 1000 was measured in the TCV control room.

MCNP simulations of neutron doses for a source of 4e13n/s on a vertical cut of the TCV building without and with neutron screening © Henri Weisen
© Henri Weisen

Overall safety
Finally, in addition to the shielding, the opportunity was seized to modernize the building housing the TCV: the entire lighting system now operates with LEDs, three monoblocks and other ventilation systems have been replaced, fire safety has been reviewed, and emergency procedures adapted to ensure a faster response in critical situations.

With this technological leap, the Swiss Plasma Center paves the way for safer and more promising fusion research, giving a new boost to the quest for a sustainable energy source for our future.