Do Radioactive Wastes Produce Heat?

A background piece by Gordon Edwards, July 12, 2011

Once radioactive materials have been created, nobody

knows how to shut off the radioactivity.  Radioactivity is

caused by the spontaneous disintegration of unstable

atoms; in fact the unit of radioactivity is the "becquerel"

which indicates that one disintegration is taking place

every second.  If we knew how to shut off radioactivity,

there would be no nuclear waste problem.

In a nuclear reactor, under normal operation, more than 

90 percent of the heat  is produced by splitting uranium 

atoms -- this process is called nuclear fission, and it CAN 

be stopped. That's why every reactor has control rods 

(for slow shutdown) and shut-off rods (for fast shutdown).

But the broken pieces of uranium atoms -- the fragments that
are left when uranium atoms have been split -- are extremely
radioactive, and that radioactivity cannot be shut off. So even
after the fission process has been totally stopped (no more
splitting of atoms), a lot of heat is still being produced
by the disintegrating atoms of the radioactive waste
materials in the core of the reactor.

This "unstoppable" heat, due to radioactivity alone, 

is called "decay heat".

For example, in a 1000 megawatt reactor, immediately

after shutdown the decay heat would be about 200

megawatts.  And if the pumps do not keep pumping

water through the core to remove that decay heat,

the fuel will get hotter and hotter as more and more

heat is added, so the temperature will rise higher and 

higher until the fuel itself begins to melt at about 2800 

degrees celsius.

The radioactive wastes inside a nuclear reactor are

not just hot, but they actually GENERATE heat -- like

a furnace that just never stops burning.  So there is

really no limit as to how hot the surroundings can get

if that heat is allowed to keep building up.  The wastes 

from the core of a nuclear reactor that has recently 

been shut down generate so much heat that they will 

eventually melt through any container they are in, unless 

there is a method for removing the heat as it is being 

produced.  That's why nuclear reactors need to keep

pumping coolant through the core of the reactor for

days and weeks after shutdown.

Because of the decay heat, the irradiated fuel that is

removed from the core of the reactor after it is "used

up" or "spent" has to be stored in a deep pool of 

circulating water which is used to cool the fuel for at 

least 7 to 10 years.  If the fuel is uncovered during the

first few years it will slowly overheat and eventually

damage its metallic container at hundreds of degrees 

celsius and release a lot of radioactive gasses and 

vapours into the air.

The problem is that even though the fuel is "spent"

(meaning the fission process no longer works well) 

it still contains an enormous quantity of radioactive junk

(mostly the unstable broken pieces of atoms that were

created when the atoms were split -- these are called

the "fission products") and that's where the heat comes

from -- from the unremitting and unstoppable disintegration

of trillions and trillions of unstable atoms.

AECL has produced graphs showing how the decay 

heat from buried spent fuel will heat up the underground

rock formations, which will finally return to their "ambient 

level" of temperature after about 50,000 years.

   This graphic is taken from one of the technical appendices of AECL's 

   EIS for the Geologic Disposal Concept. The dark horizontal layers

   represent underground rock strata, whereas the brightly coloured

   portion in the first graphic represents the buried spent fuel.  Colours

   are used to indicate temperatures.  The decay heat from the spent fuel

   raises the temperature of the surrounding rock strata for thousands of 

   years, because the heat cannot escape.  However the heat output does

   decrease over time, so the temperatures will eventually return to more

   normal levels, close to the "ambient temperature" they started with.

Gordon Edwards, Ph.D., President,

Canadian Coalition for Nuclear Responsibility.