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Construction of Pallas foundation pit progresses

21 June 2024

The installation of 380 foundation piles – each 33 metres in length – has been completed in the construction pit for the Pallas research reactor in Petten, the Netherlands. Pallas is planned to replace the existing High Flux Reactor.

The construction pit for Pallas (Image: Pallas)

The Foundation Preparation Pallas-reactor applied in June 2022 to the Authority for Nuclear Safety and Radiation Protection (ANVS) for a permit to construct and operate the Pallas reactor. ANVS granted a construction licence in mid-February last year. Preparatory work on the foundation began in May 2023. This work is being carried out by Belgian construction firm BESIX, which was awarded a contract in November 2022.

To create the construction pit, a hole of about 50 metres by 50 metres and 21 metres deep is being dug in several phases. To do this, 30 trenches measuring one-and-a-half metres wide have been dug, into which concrete has been poured to create the so-called “diaphragm walls”. The diaphragm walls are anchored with 380 bored piles placed within them. A ring beam has been placed around the top part of the walls to connect the walls together.

Activities have started in the construction pit itself, such as reinforcing the diaphragm walls with 162 drilled steel pipes with 15 anchor cords. The anchors are between 48 and 64 meters long and are attached to the ground using a grout mixture.

The construction pit has been filled with water up to groundwater level to balance groundwater pressure.

In the next step, a layer of gravel will be applied at the bottom of the excavated pit to allow for a good quality concrete pour for the foundation of the reactor.

In a final step, 48,000 cubic metres of water will be pumped out of the construction pit.

“So far, the works have progressed smoothly thanks to the successful collaboration of NRG-Pallas, ICHOS (the main designer) and BESIX (the contractor for Pit & Foundation phase),” the Foundation Preparation Pallas-reactor said.

The work is expected to be completed by the end of 2024 and the cofferdam will be ready for the next phase, the actual construction of the reactor.

Although funding has been allocated in the coming years for the construction of the Pallas reactor, the Dutch government has yet to make a final decision on its construction. Construction will be able to go ahead if the Dutch parliament does not object to the creation of a new state-owned company and if the European Commission approves the public investment.

Former Minister of Health, Welfare and Sport Ernst Kuipers instructed the Pallas foundation not to take any irreversible steps, but to continue with the preparations for the project in the meantime to avoid unnecessary delays.

In January 2023, Pallas launched a tender procedure for the construction of the Pallas reactor and surrounding buildings under EU public procurement rules. Three pre-qualified candidates were selected to submit offers. In December, Spanish construction firm FCC Construcción was contracted to build Pallas.

The Pallas research reactor is to be built at Petten to replace the existing High Flux Reactor (HFR). The 45 MW HFR started operating in September 1960, since when its use has largely been shifted from nuclear materials testing to fundamental research and the production of medical radioisotopes. The reactor – operated by NRG on behalf of the European Union’s Joint Research Centre – has for a long time supplied about 60% of Europe’s and 30% of the world’s use of medical radioactive sources.

Pallas will be of the “tank-in-pool” type, with a thermal power of around 55 MW, and able to deploy its neutron flux more efficiently and effectively than the HFR.

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