[en] The concrete vessel surrounding a PWR is partly imbedded in the ground, its foundation extending below the natural groundwater level. Moreover, the concrete vessel is covered by a mound (mound-type design). To prevent radioactive substances from entering the groundwater after having escaped from the plant, the concrete vessel is surrounded by two concentric ring walls made of bentonite, a natural mineral, which walls extend down to a layer in the ground which is impermeable to water and up to the surface of the mound. Groundwater can be pumped into the concrete vessel from the annular space between the ring walls through a filter so as to reduce the groundwater level in the annular space. Suitable choice of the earth used for the mound and the partition walls made of foils can also ensure that leakage water or rain water from the mound can only flow into the annular space. (DG)