Re: What would happen if a necular meltdown got to the core of the earth?

The concern at Chornobyl was that molten debris from the reactor core 
(uranium fuel, zirconium pressure tubes, graphite, structural steel and 
other materials) would react with and penetrate the relatively thin 
concrete foundation at that plant. At the Chornobyl site, the water table 
is fairly high -- only a few meters below the surface. Because of the high 
water table and relatively sandy soils there, any contaminants, including 
radioactive material could move quickly and contaminate the large Kiev 
Reservoir on the Dnieper River. This provides drinking and irrigation water 
for several million people downstream of the Chornobyl site (Kiev 
population ~3 million).

This is much less a concern at nuclear sites in the US and other western 
countries since the concrete foundation is much thicker (2 to 4 m thick) 
and usually founded on bedrock. If a core melt accident did breach the 
bottom of the reactor vessel, it would probably first interact with the 
containment sump -- a tank in the bottom of the containment that is filled 
with water. Depending on how fast this occurred, the process might stop 
there. If not, it the molten mass would then begin to interact with the 
concrete. As it did so, it would continue to lose heat and eventually cool 
to the point where it was no longer molten. The process would stop before 
it reached groundwater. The main point is that there simply is not enough 
thermal energy in a molten reactor core to significantly penetrate the 
plant's designed safety features (such as the thick foundation) and earth's 
surface

20 years ago at Three Mile Island Unit 2 the plant suffered an accident in 
which the core partially melted. Some of the molten core did react the 
bottom of the reactor vessel, but it rapidly cooled and actually formed a 
protective barrier against further attack. 


George J. Vargo, Ph.D., CHP
Pacific Northwest National Laboratory
International Nuclear Safety Program
509-375-6836; -2019 (fax)
vargo@pnl.gov
http://insp.pnl.gov:2080