Nuclear reactors generate
energy from fission. An atom of uranium splits into two, releasing energy plus
two neutrons; and if either of those neutrons hits another uranium atom it can
cause that atom to split, which releases more energy and another pair of
neutrons, a chain reaction. Most nuclear power today is produced by large PWRs.
According to a 2005 IAEA
report, Chernobyl caused 56 direct deaths; 47 accident workers and 9 children
who died of thyroid cancer. Additionally it was estimated that as many as 4,000
people may ultimately die from long term accident-related illnesses.
Greenpeace, amongst others, dispute that study's conclusions and presume the
toll was higher.
Whatever the true toll of the
Chernobyl accident, even conceding a worst case scenario, what most
characterises the contribution of civilian nuclear power to world energy
production is its relative safety compared to all other means of energy
In terms of direct deaths per
terawatt produced since 1972, Coal killed 342, Hydro 883 and natural gas 85,
but only 8 fatalities were recorded per terawatt of nuclear power.(1) In fact,
this statistic vastly underestimates the relative hazards of fossil fuels as
the indirect deaths from pollution caused by Coal powered stations worldwide is
estimated at over 5 million per year.
A 1000 MW(e) coal plant,
depending on sulphur content, sends annually millions of tons of Carbon
dioxide, 44 000 tonnes of sulphur oxides and 22 000 tonnes of nitrous oxides
into the atmosphere causing acid rain and poor human health. Additionally,
there are 320 000 tonnes of ash containing 400 tonnes of heavy metals for which
abatement procedures themselves produce as much as 500 000 additional tonnes of
solid waste that must be disposed of.
If the potential future
climate change impact of the billions of tons of carbon emitted yearly from
conventional power plants is taken into consideration, the death toll of say,
heat waves in Europe or drought in Africa may, sooner or later, need to be
added to the already massive indirect costs of conventional power.
In a Pressurised Water Reactor
(PWR), the fuel (ceramic pellets) is packed into fuel rods. Fission heats water
to a temperature of about 320 C and via a heat exchanger this heat generates
steam that drives turbines in another loop.
The coolant water also serves
to slow the neutrons down, allowing them to be absorbed by other uranium atoms,
that is, the water acts as the moderator.
PWRs were built based on
experience gained building reactors for submarines, where a high power density
was required and in theory, if the coolant is lost the chain reaction stops. In
practice heat from short lived decay products keep the core hot. A large PWR
can produce so much power that without coolant flow the reactor can be damaged
and it is this high power density that demands a massive containment structure
and safety systems and personnel.
A Pebble Bed Modular Reactor (PBMR)
has thousands of pebbles rather than fuel rods. About the size of billiard
balls, each micro sphere has a core of enriched uranium, about half a
millimetre across, surrounded by three layers, pyrolytic carbon, silicon
carbide and graphite. Pebbles are added to the top of the reactor and taken
from the bottom. The fuel pebbles removed are inspected and replaced and
otherwise returned to the reactor. You do not need to shut the reactor down to
refuel, unlike a PWR.
Helium is used as the coolant,
entering the core at 482 C and leaving at 900 C. The high temperature of the
helium and the fact that it is directly coupled to the gas turbine make a PBMR
much more efficient than a PWR. A single reactor produces only about 110 MW.
But, if more power is required at a site, up to 10 PBMRs can be located
together and run from a common control suite; hence the name modular.
A PBMR has a number of
features that should make it much safer than a PWR. The use of pebbles means it
has a considerably lower power density in the core and with a much greater
surface area pebbles are better at dissipating heat. A loss of coolant
therefore cannot result in a meltdown that damages the reactor. The biggest
advantage of a PBMR is that as the pebbles heat, fission slows. In the event of
a catastrophic cooling-system failure, the core temperature climbs to 1,600
degrees Celsius - comfortably below the balls' 2,000-plus-degree melting point
- and then falls, making the reactors walk-away safe.
A few tons of high level waste
a year has to be disposed of carefully underground.
Vattenfall, the Swedish energy
company produces electricity from Nuclear, Hydro, Coal, Gas, Solar Cell, Peat
and Wind energy and has produced accredited Environment Product Declarations
for all these processes.
Vattenfall finds that averaged
over the entire lifecycle of their Nuclear Plant including Uranium mining,
milling, enrichment, plant construction, operating, decommissioning and waste
disposal, the total amount CO2 emitted per KW-Hr of electricity produced is 3.3
grams per KW-Hr of produced power.
Vattenfall measures its CO2
output from Natural Gas to be 400 grams per KW-Hr and from coal to be 700 grams
Thus nuclear power generated
by Vattenfall emits less than one hundredth the CO2 of Fossil-Fuel based
generation. In fact Vattenfall finds its Nuclear Plants to emit less CO2 over
the lifecycle than even green energy production mechanisms such as Hydro, Wind,
Solar and Biomass.
Of course, all these methods
emit much less carbon than fossil fuel electricity and they all have a
respected place in our energy future. Until cheap and ultra efficient large
energy storage systems become available only nuclear power can replace large
coal burning plants.
Once PBMR's are in full
production they may be able to generate energy at about 1.7 US cents per kWh,
well below the costs of new coal, gas or wind plants, and far below the cost of
other nuclear power.
In conclusion, I'll quote from
James Lovelock, who's research ultimately saved the planet when he discovered
CFCs in the atmosphere in 1973.
nuclear energy is based on irrational fear fed by Hollywood-style fiction, the
Green lobbies and the media. These fears are unjustified, and nuclear energy
from its start in 1952 has proved to be the safest of all energy sources. We
must stop fretting over the minute statistical risks of cancer from chemicals
or radiation... If we fail to concentrate our minds on the real danger, which
is global warming, we may die even sooner, as did more than 20,000 unfortunates
from overheating in Europe last summer."
The sooner construction starts
on these reactors the better.