Module B1 Study Book - the Graduate School of the Environment

a) Mining and extraction
75,000 tonnes of ‘yellowcake’ [the result of crushing the ore and dissolving
it in sulphuric acid to produce the basic uranium] are demanded annually
by the nuclear power stations of the world. The environmental hazards of
mining have included [less so today, after much campaigning] exposure of
the workers to radiation also from radon gas, a ‘daughter product’ of uranium
by natural decay, and the fact that each tonne of uranium extracted generates
1000 tones or so of ‘tailings’. These can be 20 times as radioactive as the
uranium, and contain other undesirable material, so safe storage is an
issue.
b) Enrichment
The U3O8 is converted into uranium hexafluoride [UF6 ‘hex’]. The original
method [first developed for weapons-grade fuel] uses gaseous diffusion
through a porous membrane, from which U-235 emerges at a faster rate
than the U-238 and can thus be recovered.
In the more recently developed, cheaper method, a centrifuge spins the gas
at high velocity, with the heavier U-238 going to the outside, leaving the
central layer richer in U-235.
One tonne of enriched uranium leaves 6 tonnes of depleted uranium, which
is mostly treated as waste, with some going into weapons and some going
to breeder reactors.
c) Spent fuel
After three years in the reactor, the spent fuel [ie no longer producing enough
fission to be economic] contains a range of proportions of ingredients,
according to type of plant, mode of operation and degree of original enrichment
of the fuel.
For a PWR with 3.5% U-235, typical values per kg would be :
Item * = kg x 10-3 [ie thousandths of a kilogramme]
U-235 7* At about the same concentration as natural uranium
U-238 940* Reduced by neutron absorption to plutonium and other
actinides
Plutonium 9* Less than half of the total produced from the U-238 via fission,
which contributed to the power output
Fission products 38* Lighter radioactive isotopes, with widely varying halflives
[fractions of a second to several years]. Almost all the initial radioactivity
of the spent fuel.
Actinides 6* That is heavy radioactive isotopes, producing most of the
radioactivity after a few hundred years.
d) Disposal of spent fuel
Normally, it is submerged in water, in a vessel surrounded by a radiation
shield, and with a heat extraction system, for at least a year. Then it is subject
to either:- i) Direct disposal: left in tanks for several decades, by which time
the radioactivity and heat production will have fallen to levels allowing easier,
more compact, storage. Adopted by, eg, USA and Sweden. Or:-
ii) Reprocessing ie separating it into uranium, plutonium and other wastes
[last two items on ‘spent fuel’ table]. World capacity for reprocessing is about
5,000 tonnes/year, mostly in UK [47%], France [40%] and Russia [8%] with
Lecture: Nuclear Power 49