The vessel failure postulated in severe accident studies results in the corium falling through to the bottom of the reactor pit. We described at the beginning of this Section various physical events related to erosion by thermal phenomena. Mode e corresponds to basemat "rupture" after its complete melt-through by the corium. This would require between one and several days, depending on the basemat characteristics (4.20 m for the standardized 900 MW(e) units and 3 m for the 1300 and 1400 MW(e) units). This period would allow the decay of short-lived radioactive products and the deposition of many others on the containment walls or in the sump.

If the corium fell through the basemat, it would soon stop in the soil beneath, but the groundwater could eventually be polluted by leaching processes*. Solutions include drilling a system of shafts round the affected unit, equipped with pumps to prevent the transfer of contaminated water to bleeding points, rivers or the sea. Any water at the bottom of the containment, injected to try and cool the corium, would be heavily laden with radioactive products and could pour out into the soil through the hole in the basemat, as could the containment gases forced out by the internal pressure. It could prove more difficult to confine such contamination. The atmospheric release would nevertheless be bounded by source term S3.

So far, we have not discussed the various holes in the foundations which could be affected by the corium and provide outlets for the pressurised gases in the containment. All light water reactor buildings comprise dynamic testing systems, designed to monitor basemat deformation with time, especially during containment pressurisation for periodic strength and tightness tests. These devices are located 1 m below the basemat surface in the 1300 MW(e) units and 1.70 m below in the 900 MW(e) units (Fig. 24). The 1300 MW(e) units are equipped in addition with a basemat drainage system, located 2 m below the surface.

FIG. 24. Rasmussen containment failure modes.

Compensatory measures were consequently defined and were the subject of procedure U4: "handling early release paths through the basemats". Since then, sealing systems, plugging beneath the reactor pit and permanent obstructions have been installed. No further action is required of the operators on this particular point, so that procedure U4 in fact no longer exists. The modifications installed are aimed at taking advantage of radioactive decay and ground filtration in the event of basemat perforation and extending the time available to make the necessary off-site provisions.

* Washing of free surfaces leading to extraction of soluble products.