ISIS Press Release
17/02/06
Safe New Generation Nuclear
Power?
The Pebble Bed Modular Reactor
Peter
Saunders
A fully
referenced and illustrated version of this
article is posted on ISIS members
website. Details here
Nuclear power back on UK agenda
The UK Government issued a White Paper Our
energy future creating a low carbon
economy in 2003, saying that it was not
proposing to build any new nuclear power
stations, and that before such a decision was
taken there would be another consultation.
Now, less than three
years later, it is conducting a review of its
energy policy, and nuclear energy is very much
back on the table.
Peter Bunyard has explained in a previous
issue (SiS
27)
why nuclear power is not a solution to our energy
crisis. Apart from important safety
considerations that cannot be ignored, there is
little if any saving in greenhouse gas emissions
if we include all the processes involved, from
mining the uranium to disposing of the waste.
Nuclear energy is also very expensive. What is
more, it is at best a temporary solution, as
uranium, like oil, is a finite resource. Even at
todays rate of consumption, known reserves
that can be mined and refined economically would
be exhausted by the end of this century. If we
were to substantially increase our dependence on
nuclear energy, they would run out much sooner
than that, probably within 50 years.
However, the Blair Government still believes
that we need a substantial contribution from
nuclear energy, and we have been told that it is
looking to a new technology, the Pebble Bed
Modular Reactor (PBMR) for the next generation of
nuclear power stations. It is supposed to be both
safer and cheaper than the reactors in use today,
chiefly the pressurised water reactor (PWR) (see
Box).
The Pebble Bed Modular Reactor in South
Africa
The South African state- owned electric
utility Eskom is currently designing a PBRM in a
consortium that includes the Industrial
Development Corporation of South Africa, and
British Nuclear Fuels. The US electric utility
Exelon was a member but dropped out; its
involvement at an earlier stage is helpful,
because it meant that many Americans, including
members of the US Nuclear Regulatory Commission,
have studied the proposals carefully and raised
important issues. A detailed and well-documented
analysis has been done by Jim Harding, the
Director of Power Planning and Forecasting for
Seattle City Light.
The Eskom design is based on two German
reactors. The first, the small 15 MW AVR, began
operating in 1967 and lasted 21 years. On the
whole, it performed well. The other, the 300 MW
THTR-300, took 14 years to build, cost DM 4
billion against an original estimate of only DM
650 million, and was shut down after four years.
During its operation, there was an incident in
which radioactivity was released into the
environment. There was also a nuclear plant at
Fort St. Vrain (Colorado, USA) that used high
temperature helium as a coolant, as the PBMRs
will, but this experienced many technical
problems. After about ten years, the reactor was
decommissioned and the plant now runs on natural
gas.
On the whole, the PBMR design does appear to
be inherently safer than the PWR. Above all, it
is probably proof against meltdown. On the
other hand, there are still some important safety
issues, and it is hard to know how serious these
are.
Much the same applies to cost. The proponents
claim that the PBMR will produce electricity
economically, but this is on the basis of a new
and largely untested technology. Furthermore,
much of the expected savings arise because the
designers are so confident of the safety of the
reactor that they plan to build them without
containment buildings and close to populated
areas where the power is required, and to operate
them with much lower staffing levels than other
types of reactor. Should the licensing
authorities judge that PBMRs require the same
sort of precautions as other reactors, much of
the cost advantage will disappear.
And even if the PBMR does live up to the
claims of its proponents, it still produces waste
that has to be disposed of, it still increases
the chance of proliferation of nuclear weapons,
and it is still at best a temporary solution to
the energy problem.
The Pressurised Water Reactor vs
the Pebble Bed Modular Reactor Nuclear
reactors generate energy from nuclear
fission. When an atom of uranium splits
into two, it releases energy plus two
neutrons; and if either of the neutrons
hits another uranium atom it can cause
that atom to split, which releases more
energy and another pair of neutrons, and
so on. This is the so-called chain
reaction. If every neutron released
stimulated a uranium atom to split, the
number reacting would double at each
step, and the reaction would rapidly get
out of control, as it does in an atomic
bomb. Nuclear reactors are designed to
prevent that happening, for example by
using variable amounts of
neutron-absorbing materials such as boric
acid or carbon.
In a Pressurised Water Reactor (PWR),
the fuel - enriched uranium dioxide - is
formed into ceramic pellets and packed
into tubes called fuel rods. Water
circulates around the fuel rods and is
heated to a temperature of about 320 C;
this is possible because it is under
pressure. This water provides the heat
for a boiler that generates steam that
drives turbines and so produces
electricity.
The coolant water serves another
function besides transferring energy from
the fuel rods to the steam boiler.
The neutrons released during fission are
too hot to be absorbed by other uranium
atoms, which is necessary for the
reaction to continue. They have to lose
energy, and in a PWR this happens mostly
by collisions with molecules of the
water. The water acts as the moderator.
An advantage of using the coolant
water as the moderator is that if the
coolant is lost, the chain reaction
stops. There is consequently no danger of
the reactor turning into an atomic bomb.
Ordinary radioactive decay would
continue, however, and if nothing were
done, the temperature would rise to
dangerous levels, possibly leading to
meltdown. PWRs have additional safety
systems to counter this danger. They are
also surrounded by strong containment
buildings so that however badly damaged
the reactor may be, no radioactive
material should be released into the
environment, at least in principle.
A Pebble Bed Modular Reactor (PBMR) is
quite different. Instead of fuel rods, it
has 452 000 pebbles. Three quarters of
these are fuel pebbles, made of
microspheres just under a millimetre in
diameter. Each microsphere has a core of
enriched uranium, about half a millimetre
across, surrounded by three layers of
coating: first pyrolytic carbon (a form
of graphite), then silicon carbide, and
finally another layer of the pyrolytic
carbon. About 15 000 of these TRISO
(three isotropic layers) microspheres are
mixed with graphite and then pressed and
sintered (fixed under heat and pressure)
into a fuel pebble about 6 cm in
diameter. The remaining non-fuel pebbles
are pure graphite.
During operation, pebbles are
continuously added to the top of the
reactor and taken from the bottom. The
fuel pebbles removed are inspected and if
they are exhausted or damaged they are
rejected from the system; otherwise they
are returned to the reactor.
Pressurised helium, rather than water,
is used as the coolant. It flows directly
through a turbine; there is no secondary
circuit as in a PWR. The helium enters
the core at 482 C and leaves at 900 C,
and the high temperature of the helium,
the direct coupling, and the use of a gas
turbine should make a PBMR much more
efficient than a PWR.
PBMRs are relatively small. A single
reactor occupies an area smaller than a
football field and produces only about
110 MW. (A typical PWR or other light
water reactor produces about ten times as
much.) If more power is required at
a site, up to 10 PBMRs can be located
together and run from a common control
suite, which is why they are called
modular.
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How safe is a PBMR?
A PBMR has a number of features that should
make it safer than a PWR. Crucially, the use of
pebbles means it has a considerably lower power
density in the core, and as pebbles have a much
greater surface area than fuel rods (for a given
volume of fuel) it is also better at dissipating
heat. A loss of coolant should therefore not
result in a meltdown.
The moderator in a PBMR is graphite, rather
than water. That might seem to make the PBMR less
safe than a PWR because if the coolant (in this
case, helium) is lost, the moderator is still
present. In fact, this is not the case. As a
reactor heats, more neutrons are captured by
U-238 atoms, which do not split, leaving fewer
for the fissile U-235. This effect is much
greater with graphite as a moderator than with
water (because more collisions are needed to slow
the neutrons) and the chain reaction would
therefore stop before there is any danger of an
explosion.
A PWR has to be shut down, refuelled and
started up again about every eighteen months. It
is expected that a PBMR would only be shut down
for maintenance about every six years. Continuous
refuelling also means that the fuel has much the
same properties throughout, so excess reactivity
can be kept to a minimum. There are, however, a
number of outstanding questions about the safety
of a PBMR.
Fuel The core of just one PBMR
contains 5 billion microspheres. These have to be
made to high quality because it is the coatings
that prevent the release of fission products from
the fuel during normal operation. When the US
Nuclear Regulatory Commission (NRC) considered an
application to build a PBMR in the USA, they
noted that if the fuel kernel is not perfectly
centred in the microsphere it will migrate out of
the particle, so the fabrication must be accurate
enough to prevent that happening.
Another question is the packing of the pebbles
in the core. It is hard either to predict or to
control how they will arrange themselves as they
move down, and this can lead to significant local
variations from the mean operating temperature.
While the German AVR reactor had a predicted
maximum fuel temperature of 1 150 C, it turned
out to have many hot spots exceeding 1 280 C.
That is important because the coatings on the
fuel pellets begin to degrade at 1 250 C.
The current design provides for neither in-core
instrumentation nor emergency cooling systems.
Fire The PBMR core contains a
large amount of graphite, and this is an obvious
hazard because graphite can oxidise at 400 C and
the reaction becomes self-sustaining at 550 C,
both well below the operating temperature. There
could also be dangerous reactions if water vapour
were allowed to enter. Using helium at high
pressure clearly reduces the chance that either
air or water can reach the graphite, but it
remains to be seen by how much. For example, it
has been shown that if a pipe were to break and
the helium system lose pressure, air inflow could
occur [9]. A fire in a PBMR would be
especially serious, because there is to be no
containment building and the reactors are meant
to be built near the towns they serve.
Reliability of key components
The PBMR has a number of components that are
the first of their kind, and how well they
perform in practice has obvious implications for
both safety and cost.
External threats However
well they are designed and built, nuclear
reactors are subject to threats such as
earthquakes, plane crashes, floods and terrorism.
Articles on PWRs often point out that their
strong containment structures offer resistance to
such threats. Similar articles on PBMRs, which do
not have the same protection, do not mention
external threats. Instead they are so blandly
reassuring that they become misleading (see Fig.
1).
Fig. 1 How Exelon explains
away the problem of nuclear waste to the public
Waste Management:
Whether we use PWRs, PBMRs or any other form
of reactor, we still have to dispose of highly
dangerous radioactive waste. There seems broad
agreement in principle that it should be possible
to store the waste safely in geologic
repositories, but identifying actual sites is
turning out to be very difficult. The USA has
been concentrating on one particular location,
Yucca Mountain in Nevada, but this has still not
been commissioned after 15 years. Moreover, it is
estimated that if there were a thousand 1 GW
light water reactors in the world, a new
repository equivalent to Yucca Mountain would be
required every three or four years.
Supporters of PBMRs claim that the waste
should be easier to deal with because the coating
will ensure that the spent radionuclides are
contained for extremely long periods of time.
Even if the pebbles can be relied upon to retain
integrity for thousands of years without
additional processing, the total volume of waste
is still very much greater than with a PWR. Eskom
has argued that the repository requirements would
be much the same for both types because spent PWR
fuel requires overpacking and spent pebbles do
not, but this remains to be seen.
Proliferation of weapons grade uranium
An important selling point for the PBMR is
that individual modules are small and (so it is
claimed) relatively easy to build and operate.
They could therefore be sold to countries that
could not afford to buy and run conventional
reactors. Harding points out that this would mean
such countries would receive shipments of fuel
which had been enriched to 9.6 per cent U-235.
Thats about twice the enrichment required
for light water reactors; and about 90 per cent
of the separative work required for weapons grade
uranium, the rest could be done using a small gas
centrifuge plant.
The Interdisciplinary MIT Study recommends
that the International Atomic Energy Agency
should do more to prevent proliferation and that
nuclear power should not expand unless the risk
of proliferation from operation of the commercial
nuclear fuel cycle is made acceptably small. The
widespread use of PBMRs would increase this risk
rather than decreasing it.
Cost cutting involves compromising safety
standards
The Eskom Board estimated in 2004 that a
demonstration plant could be built for about 10
billion Rand. Subsequent units, they claimed,
would cost about one fifteenth of that, and would
make the total costs (recovery of capital and
operating) 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.
Harding rightly argues that the estimates both
for the demonstration plant, and even more so,
for the follow-on plants are based on a
large number of extremely optimistic safety,
reliability and efficiency assumptions.
Above all, a lot of the claimed savings come from
weakening the usual safety standards.
Conclusion
In theory, the PBMR has a number of advantages
over most current reactors. If all goes well, it
could turn out to be both safer and more
economical. But thats a very big if,
and past experience with nuclear reactors and
indeed new technologies in general tells us that
things are unlikely to go as anticipated.
Even if the problems that are bound to arise
during development and the operation of the first
prototypes can be solved without substantially
increasing the costs, there remains the problem
of safety. The design may make incidents such as
fires and accidental release of radiation less
likely than with, say, a PWR, but are they so
unlikely that PBMRs should be built near towns
and without containment structures? If licensing
authorities decide they are not, a considerable
part of the economy disappears.
The intractable problems of waste,
proliferation, and the finiteness of the uranium
supply remain. For those committed to nuclear
energy, the PBMR may well turn out to be an
improvement on existing reactors, though it will
be at least five years before we can know. But it
makes no difference whatsoever to the wider
debate. The arguments against nuclear energy
remain overwhelming.
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