- 273 - 3.3.3. Other facilities in the fuel cycle (Mélox, FBFC) Mélox Design of the facility To define the means of flood protection when designing the plant, the licensee used RFS I.2.e to determine which data to consider. These data were then provided by Compagnie Nationale du Rhône (CNR). They concern the following levels: the maximum 1000-year flood (CMM), represented by the 1000-year return flood flow increased by 15%; the 100-year flood combined with failure of the Vouglans dam. The flow of the thousand-year flood increased by 15% is retained as the most penalising with respect to the rise of the river Rhone. In event of the CMM, the Rhone can in fact only exceed the 37.5 m NGF level (elevation of the left bank embankment) by a small amount, but without exceeding the level of the right bank. The height difference between the embankments of left and right bank of the Rhone favours overflowing on the left-bank side. The 100-year flood combined with failure of the Vouglans dam is more penalising with respect to the rise in the water of the secondary canal situated behind the right-bank embankment. In the event of a 100-year flood concomitantly with Vouglans dam failure, the level would reach 36.65 m NGF at the Marcoule site (KP 209.5). This value has been adopted as the flood safety margin level (CMS) since the design data were re-evaluated. The Mélox plant is installed on a platform at elevation 40 m NGF. This gives a margin of more than 2 m with respect to the CMM and more than 3 m with respect to the 100-year flood combined with failure of the Vouglans dam. The pits of building 500, which are dug below the 40 m NGF level, are designed to withstand the hydrostatic thrust corresponding to the CMM. These pits are designed for a rise in the water table. All the plant structures are watertight up to the basemat, to which is added the height of the door sills. The margin is 9.73 m with respect to the average height of the water (30.57 m NGF), and 6.93 m with respect to the maximum level recorded over 10 years (33.37 m NGF). As the quantity of water inside the process premises is very limited, the risk of internal flooding has not been considered. The plant has been designed (with the installation of a network of pipes and the dimensioning of the openings, connecting galleries, passageways, cooling units and flat roofs) from the reference rainfall levels indicated in the table below and taken from the graph of heights/durations/frequencies of the national meteorological service of Nîmes-Courbessac. Duration Return period (years) 6 min 15 min 30 min 1 h 10 16 mm 28 mm 45 mm 68 mm 100 24 mm 41 mm 68 mm 105 mm Nîmes-Courbessac rain-gauge station - Maximum adjusted precipitation over the 1964-1992 period Measures to protect the facilities against the flood risk The target key SSCs in the event of flooding are the cooling units (situated externally), the emergency diesel generator sets (situated in external containers) and the backup diesel generator sets (situated in different rooms within building504) that enable the safeguarded functions to be maintained. The plant has been designed to protect these key SSCs against a reference flood .
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