Didcot
Fuel Ash Disposal
The
Potential Hazards of PFA
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| Everything in this world
is potentially harmful if present in sufficiently
large quantities and if handled, stored or disposed
of in an inappropriate manner. PFA is no exception.
In "reasonable" quantities, it is believed
not to represent a major hazard to the environment.
However there are risks if a large amount of it
is dumped in one place, while the actual infilling
operations are certainly damaging to the natural
environment. In Radley's case, some of the infilling
in the past has visually improved the environment
(Lakes A-D: compare
with photos); The situation with lakes
G-P is questionable. Regarding future infilling,
it is certainly a case of enough is enough. |
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Radiological
All
coal contains low concentrations of radionuclides,
principally those of the decay series of U238,
U235, Th232 and also K40. Its combustion releases
volatile radionuclides (which include Polonium210
and Radon226, which are alpha-emitters) and, more
importantly, concentrates non-volatile heavy radionuclides
in the resultant ash. From the
figures given, the concentration factor is
somewhere between 3 and 10. Moreover, because
coal is formed from plant residues, and plants
have the ability to sequester and thus concentrate
heavy metals in their environment (Such a process,
where a biological entity absorbs and retains,
and thus concentrates, a substance, is called
biomagnification.) there is no reason to suppose
that the composition of ash residues should reflect
the originally prevailing radionuclide and heavy
metal concentrations in the original environment.
Indeed radioactive discharges from
coal fired powered stations greatly exceed those
from nuclear power stations of equivalent power.
COMARE, the Committee on
Medical Aspects of Radiation in the Environment,
considered this in 1988 and commissioned a study
by the the NRPB to investigate. As a result,
COMARE, in their 3rd report in 1989, concluded
that [atmospheric] discharges from coal-fired
plants [in Oxfordshire and Berkshire] make a very
small contribution to the total radiation dose
received by people living around the sites. In
their 10th report in 2005,
COMARE revisit this issue and note that Didcot
power station is now the major radiological polluter
in the area with emissions exceeding those of
Aldermaston and Burghfield combined, by a factor
of two. A further report relating to Berkshire
and South Oxfordshire is promised in the near
future.
The
radioactive content of the retained ash is likely
to be somewhere in the region of 1000 times greater
than that released into the atmosphere. This is
not to say that the ash is likely to be dangerously
radioactive. The activity is below the threshold
for it to be considered radioactive waste and
alpha emitters are only hazardous if ingested.
In modest quantities, the ash represents a negligible
hazard (though some concerns have been raised
over its use of a building material in certain
circumstances). One million tonnes of ash all
in one place does however represent quite a lot
of radioactivity (1 TBq).
However, for the hazard to be realised, the ash
would have to be subject to wholesale rapid dispersal,
so burying it is a whole lot better than discharging
it directly into the atmosphere.
It is
worth noting that UK coal ash has lower than average
content of U238 and Th232. Coal from international
sources generally has 2-3 times higher concentrations
of U238 and Th232 than UK coal, but lower concentrations
of K40. The total radioactivity of fly ash is
typically in the region of 1 Bq/g
(irrespective of source) of which 65-90% is K40.
This is about 10 times the average natural level.
Also, 0.35 Bq/g of
natural uranium in decay equilibrium represents
a uranium content of about 2ppm by mass, or, put
another way, one million tonnes of ash may contain
up to about 2 tonnes of uranium.
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Chemical
A related
concern is likely contamination by neurotoxic
heavy metals such as lead (Pb), cadmium (Cd),
mercury (Hg), arsenic (As), chromium (Cr) and
vanadium (V) which may be present in the ash in
small but highly variable amounts. Also one would
expect these to be concentrated by the same factor
as the radioactivity (~ x10). However the potential
for leaching of such contamination from fly ash
is often considered to be low because of their
low solubility and the alkaline nature of the
leachate. However this may understate the long-term
problem. Natural acidification over time combined
with plant sequestration and biomagnification
processes provide possible mechanisms whereby
these contaminants, if present, may eventually
enter the environment.
Chromium,
vanadium and arsenic are water soluble and occur
in leachate and supernatant outflow in small but
significant amounts (typically combining to around
0.2 mg/litre).
PFA
contains significant amounts of alkali (group
I and II) metal oxides (Na2O, K2O, CaO, MgO) which
become “slaked” into alkaline hydroxides on reaction
with water. This is what accounts for the alkalinity
of the PFA. Again, this may be highly variable
even between coal batches from the same source
and one can speculate whether this was responsible
for the pollution
incident in 2003. Also, it means that PFA
that has been slaked or “conditioned” is less
useful for use as a building material (see below).
The PFA stockpiled in Radley's lakes is therefore
degraded for this purpose, if it were ever to
be recovered (though it could be restored by recooking).
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| Regulatory
and Epidemiological
PFA
is a “Controlled Waste” in the UK and has no special
requirements for its disposal at appropriately
licensed facilities. It is included in the European
Waste Catalogue but is not hazardous material
as defined by the EC Hazardous Waste List.
It is
also a “Green List” material for transfrontier
shipment and for transportation it is classed
as Non-Hazardous under the Classification, Packages
and Labelling Regulations 1994.
Epidemiological evidence
from power workers in the UK exposed to airborne
fuel ash suggests, somewhat surprisingly, that
it has no adverse effects whatsoever. However
this may be partly attributable to particle shape
and size which tend to be spherical in the 10
micron range. Smaller more irregularly shaped
silicaceous particles, such as those that might
arise from crushed cenospheres, should still be
considered as potential carcinogens. |
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Other
Links and Sources of Information
Other
Information Relating to Potential Biological Hazards
of PFA.
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Units
of Radioactivity
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Radioactivity
is measured in one of two units, the Becquerel
(Bq) which is the preferred (SI) unit in Europe.
and the Curie (Ci) which is commonly used in the
USA.
- 1 Bq = 1 Becquerel =27 pCi = 27 picoCuries
= 1 disintegration per second
- 1 GBq = 1 GigaBecquerel = 1,000,000,000Bq
=27 mCi
- 1 TBq = 1 TeraBecquerel =1,000,000,000,000
Bq =27 Ci
- 1 Ci = 1 Curie = 37 GBq
- 1 Bq/g = 1 MBq/tonne
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