Thursday, January 12, 2006

The Stones that Boil - Nuclear Waste Management.

Nuclear energy is based on nuclear fission, a process in which a heavy nucleus such as that of uranium splits in two, releasing radioactive "fission products" of lighter nuclei. In most of the nuclear power plants in operation today, the heat generating unit is a pressurized water reactor (PWR). In this arrangement, the heat generated by nuclear fission in the reactor nuclear fuel [See Footnote] is removed by the "primary" coolant which is circulated in a closed primary circuit. Here, the pressure is sufficiently high to maintain the primary coolant (water) in liquid form at the operating temperatures employed. Heat is transferred from the primary circuit to the secondary circuit (also known as the "water-steam circuit") where steam is generated and is then directed at the steam turbine to generate electricity.
Originally, the PWR was designed for nuclear ship propulsion by the U.S. Navy. From its principle, in the 1950's Westinghouse started to develop civilian electricity generating units, the first of which was the Shipping-port 1 plant, a 90 MW PWR that started producing electricity in 1957. Modern PWR's produce anywhere up to 1400 MW. While different designs of PWR's exist, in respect to the production and properties of radionuclides ("fission products") the various designs are effectively similar, according to a common aim to confine the radionuclides safely and to keep both the occupational radiation exposures and radioactive emissions into the environment at a minimum very low, acceptable level.
The containment is not perfect, since minute cracks develop over time in the some 50 miles of piping used in the water circuits, and it is well known in the nuclear industry that all waters, whether used for cooling (heat transfer), cleaning, or for other applications will become radioactive to the level of constituting Intermediate Level Nuclear Waste (ILW). To simply dump this water into the environment - e.g. into the sea - is highly undesirable, and potentially dangerous. Therefore, some means for intercepting the radioactive ions it contains must be sought, and this is where a class of remarkable materials called "zeolites" makes their entrance.
The first identified zeolite was recorded in 1756 by the Swedish mineralogist, Cronstedt, who discovered that on heating the mineral the stones erupted into a froth of hot liquid and steam, lending the appearance that the stones themselves were boiling. Thus, he termed them "zeolite" which from Greek derivation means "stones that boil". The observation provides a hefty clue as to the unusual properties of zeolites, which are highly porous and are able to absorb large quantities of water and indeed other substances within an extensive microporous network. The micropores are of molecular dimensions, typically 13 A (1.3 nm), or less, and absorb substrates on a molecular scale. There are about 40 naturally occurring zeolites known and about another 150 or so that have been artificially synthesised. In current parlance, zeolites may appropriately be described as "nanomaterials". Thus, zeolites find a wide range of important applications e.g. as catalysts for use in the petrochemical industry; in the separation of air into oxygen and nitrogen; as molecular sieves, for use as drying agents and for the separation of hydrocarbons (e.g. para-xylene for polyester manufacture); and in ion-exchange.
The ability of a zeolite to perform ion-exchange rests upon its structure which consists of a negatively charged aluminosilicate framework, which is balanced by the presence of a number of positively charged metal cations (e.g. Na+, Ca2+, Mg2+, H+) equal to the total number of negative charges. It is the molecular structure of the framework that defines the nature and size of the micropores. If a vessel is packed with zeolite, and water containing different cations from those present in the zeolite is passed through it, an exchange can occur in which cations are absorbed from the water and simultaneously released from the zeolite. This process finds large scale application in industry, where Ca2+ and Mg2+ cations in "hard" water which block pipes in the form of "lime scale" can be absorbed into an initially Na+ containing zeolite, simultaneously releasing Na+ cations into e.g. cooling water where they can do no harm, so that it continues to flow unimpeded.
The principle is exactly the same for removing radioactive cations from effluent waters arising in the operation of a nuclear power plant. Major fission products are radioactive caesium (caesium 134 and caesium 137) and strontium 90 (which all of us have in our bones to some extent from testing nuclear weapons in the past, since it feeds into the biological calcium cycle). One naturally occurring zeolite called clinoptilolite has a specifically high affinity for caesium and strontium, and is present in a volcanic mineral known as "tuff". Tuff occurs in rich beds in various regions of the U.S. and in northern Armenia. Indeed, zeolites are generally associated with volcanism. Tuff may be processed and the resulting zeolite rich material packed into tanks for the purpose of filtering waste waters, such that the water which emerges from it is sufficiently low in radionuclides for sea disposal or even for agricultural purposes - an important feature in parts of the world such as Armenia, where water is at a premium, and otherwise the millions of gallons per year required to "cool" a nuclear power station would be wasted, as well as being a potential source of radioactive environmental contamination.
Once the zeolite is sufficiently "saturated" with Cs+ and Sr2+, it is removed from the tanks which hold it, for disposal and fresh zeolite is introduced. The radioactive zeolite may then be sintered (heated at 1200 - 1400 degrees C) to partly vitrify it (turn it into a glass) especially at the surfaces, which tends to lock-in the radioactive ions against any future radioactive exchange into the environment. The zeolite may also be dissipated into a boron containing glass or imbedded in concrete. In this way, the material is converted into "bricks" which can be removed for storage in a suitable repository.

PWR's fall into the category of Light Water Reactors (LWR), which use ordinary water both as a coolant and a moderator. Cooling and heat-transfer I have already described, but the action of a moderator is to slow down energetic neutrons. This is necessary since the fissile form of uranium, uranium 235 which occurs only to the extent of 0.7% in natural uranium, is most efficiently fissioned by absorbing a "slow" neutron. Since ordinary (light) water is only moderately effective in slowing neutrons, a fuel enriched (to about 3.5%) in uranium 235 is used in order that the operational flux of slow neutrons is sufficient to sustain a chain reaction. Naturally occurring uranium (without enrichment) can be used as a nuclear fuel in reactors that use "heavy water", which is more efficient as a moderator, but the former design using light water is generally preferred.

1 comment:

James Aach said...

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