What is the GI project? Who is the customer?

The G1 nuclear reactor is the^1st military nuclear reactor built in 1955 by the Commissariat à l'énergie atomique (CEA) at Marcoule, and commissioned in just one year. It was also the first reactor to produce "nuclear" electricity. The UNGG (Uranium Naturel Graphite Gaz) reactor is made of 1,200 tonnes of graphite, from which it takes its name G1 (G for graphite), and has been shut down since 1968, awaiting nuclear dismantling.

 The G1 sampling and coring project involves investigations and sampling. The aim is to define the radiological and physical state of the reactor in order to initiate dismantling scenarios and identify waste streams (particularly for graphite).

CEA awarded the contract to the Onet Technologies-Eiffage Génie Civil partnership in 2010; following an initial phase of on-site investigations and topographical surveys, the intervention scenarios were used to define specific tooling, which was qualified on a test mock-up in 2013. Following a^1st phase of sampling in 2016, higher-than-expected tritium releases forced CEA to suspend the contract and install a dedicated outfall. The contract was resumed in January 2021 following the negotiation of an amendment.

What does Onet Technologies offer and what is its scope?

Onet Technologies is managing the project in partnership with Eiffage Génie Civil. Each company has its own speciality: Eiffage EGCN is in charge of through-coring in civil engineering (operations already carried out on EDF's UNGGs between 2008 and 2009, with Onet Technologies at the time); and the reactor's graphite, with the challenges of controlling the trajectory (reaching precise zones and avoiding singularities in the reactor) and ensuring that the graphite does not heat up above 60°C.

Onet Technologies is involved in all other sampling work (investigations and small-diameter coring in the fuel channels, sampling of reactor equipment and instrumentation, sampling of the steel belt holding the graphite in place), as well as fittings, ventilation connections and adjustments, and radiation protection.

What professions are involved?

Most of Onet Technologies' professional skills have been mobilized since the start of the project: radiation protection engineering, safety, mechanics, general installation, the decommissioning and waste remediation process, and ventilation. For the resumption of the project, the mechanical engineering department carried out equipment adaptations, preventive maintenance and tool training for new operators, and the ventilation department for ventilation assembly, adjustment and testing.

What are the project stages?

The project is organized in several stages: firstly, the development phases, with SAS/ventilation carried out prior to each phase of work. The ^1st phase of work will last 6 months. It consists in carrying out investigations and taking samples from the fuel channels and sealing plugs. The work will be carried out in a demonstration hall with no ambient ventilation: all operations will be carried out using the reactor's negative pressure and ventilated containment enclosures.

Then comes the extraction and sampling of control rods, which will take around 4 months, depending on the CEA's exercise of its option, before the coring is carried out by Eiffage Génie Civil. Finally, in the last half of 2022, a steel sample will be taken from the lower part of the reactor (access to which is highly restricted).

The coring phase requires a high degree of precision: of the order of 10 cm at a depth of 13 m, after traversing 7 m of reinforced concrete, 1 m to 1.5 m of void and 5 m of graphite, sometimes at an incline. The qualification stages on a mock-up in 2013 enabled these configurations to be simulated, but the exact configurations of the reactor (concrete quality, positions of internal structures) remain uncertain data; regular measurements of deviation, coupled with ITV and location on a topographical model, will enable trajectories to be corrected or core holes to be repositioned.

For all graphite samples, the CEA wishes to verify the presence or absence of residual Wigner energy still trapped in the graphite; energy which can be released at around 100°C and which may present a fire hazard. A large number of samples are to be submitted to the CEA, which will carry out analyses using nuclear measurements in situ or in the laboratory to define the activities and chemical compositions.

Onet Technologies and Eiffage Génie Civil are pressing ahead with the project to ensure optimum safety conditions and meet deadlines.