Sunday, December 5, 2010

Nuclear Technology Basics Part 1: Uranium Fuel Cycles

Introduction

Most reactors in the world today utilize the uranium fuel cycle to sustain fission, but there are other fuel cycles as well such as ones based on thorium and plutonium. Light water reactors (LWRs) typically have a once-through fuel cycle in which results in various degrees of spent fuel to be disposed of. Breeder reactors and various reprocessing centers can greatly reduce the quantity and half-life of material to be discarded, but nuclear reprocessing is banned in some countries because of errant political concerns rather than for any technical reason such as is seen in the US.

Uranium is a common element that is found in many locations across the world, usually in the form of Uranium oxide. Uranium oxide is a yellowish-brown powder, and is often referred to as "yellowcake". Large deposits of uranium are found in Australia, Africa, Canada, Spain, Russia, and the US where it is mined and sent to an ore processing center. Uranium mines may be either open pit mines when the uranium is close to the surface, or in underground mining tunnels for deeply-buried deposits. Most uranium in the US and Australia is mined using in-situ leeching methods where the uranium oxide is dissolved from the surrounding rock in solution using water that is acidified by carbon dioxide. A LWR reactor requires around .2 metric tonnes of uranium oxide per megawatt produced for its continual operation.

The uranium isotope, U-235 is the primary isotope of interest for power generation. In chemistry and nuclear physics, an isotope of an element is an atom that has a different number of neutrons from the typical number of an atom from that type of element. Uranium has 33 different isotopes, and all of them are radioactive with varying degrees of radioactivity and half-lives. Only .7% of the atoms in naturally occurring uranium oxide are U-235 on average, while the most abundant isotope of uranium is U-238 which accounts for 99.28% of uranium atoms found in nature. Rarer still is the naturally occurring isotope of Uranium U-234 which is slightly more than half a percent of uranium found in deposits on Earth.

In order for mined uranium oxide to be viable for usage in a LWR it must be brought to a fuel fabrication facility where uranium oxide is converted into uranium hexafluoride where the percentage of U-235 is concentrated up to three percent. This is done either through the gaseous diffusion process or the centrifuge process. In either case, "tailings" are produced as a by-product of the process. Uranium "tailings" are largely devoid of the U-235 isotope and consist mostly of U-238. This "depleted" uranium is only weakly radioactive and has many commercial uses because of Uranium's density, ranging from aircraft counter-weights, radiation shielding, boat keels, and munitions. Although uranium itself has a toxicity comparable to lead from a chemological standpoint, uranium is not easily absorbed by living organisms if ingested. The greatest danger comes from the accidental inhalation of the material if it is finely ground into a powder, because the particles can become lodged in the lungs so respiratory protection should be worn when working with powdered uranium compounds. However this is true for many fine particulate substances and is not necessarily unique to uranium. The fears of "depleted uranium" are largely unfounded and baseless.

When the uranium hexafluoride has been enriched to the desired level, it is converted into uranium dioxide which is a fine powder. The uranium dioxide is mechanically pressed into small pellets for use as fuel within a nuclear reactor fuel assembly. The pellets are stacked within tubes made from a metallic alloy of zirconium and serve as fuel rods in the nuclear reactor vessel.

Within the reactor vessel, the Uranium-235 isotope undergoes nuclear fission. Uranium-235 captures and absorbs a stray neutron to become the unstable isotope, Uranium-236. U-236 commonly decays into isotopes of barium, tellurium, krypton, and zirconium and releases energy and two or three neutrons in the process.



These stray neutrons impact other nearby atoms, causing the process to be repeated. In addition, the decay of the daughter products of uranium can create isotopes of other elements as well. Three of the more common decay chains of Uranium U-235 are represented by these equations:

U-235 + n ===> Ba-144 + Kr-90 + 2n + energy

U-235 + n ===> Ba-141 + Kr-92 + 3n + 170 MeV

U-235 + n ===> Zr-94 + Te-139 + 3n + 197 MeV

Interestingly enough, the atomic masses of the isotopes created from the decay of uranium-236 are usually around the low 90s to the mid to upper 130s because of the law of Conservation of Mass in regards to matter. The total mass of the isotopes resulting from the decay of uranium-236 and the neutrons that are released equals a mass of 236, just like the uranium-236 that they decayed from.

After a year or so, 33% of the fuel rods within a nuclear reactor are removed and the reactor is refueled with new fuel to keep the fission reaction going. The spent fuel rods are submerged in a pool of water within the power plant so that they can cool down long enough for further processing and for some of the more radioactive, shorter-lived isotopes to decay. After a few years, the assemblies containing the spent fuel are taken out to be disposed of.

The fission products that were created during the nuclear fission process can be divided into short, intermediate, and long-lived half-life categories.The half-life of an element is the average amount of time for the atoms within a sample of material to have undergone radioactive decay into another element. Most of the fission products have short half-lives that are less than a year. Although many of these isotopes are highly radioactive, they undergo decay during their period in the spent fuel pool and do not present a problem from waste disposal standpoint. Isotopes with an intermediate half-life can be somewhat problematic as they can range anywhere from a year to a century or two and can emit moderately high levels of radiation such as with the case of strontium-90 and cesium-137. These elements can be transmuted into less dangerous isotopes through further neutron bombardment but it is much more cost effective to simply dilute them with inert compounds to the point to where their radioactivity no longer poses a problem. Isotopes with half-lives lasting longer than three centuries can make up to 20% of the spent fuel to be disposed of, but one must keep in mind the inverse relationship between half-life and radioactivity.

Although the half-life of some of these fission by-products can be up to several billion years, they are only weakly radioactive to the point of being barely above the background levels of radiation that all of us are exposed to in our daily lives. As a case in point, potassium-40 has a half-life of 1.3 billion years, and it can set off alarms from radiation detection equipment. However, it is quite abundant in foods with large amounts of potassium in them, such as bananas and it is also found in our bones. However, it is very weakly radioactive as a person only gets an exposure of a few picocuries per year. Eating one banana a day for each day in a year would increase your exposure to radiation by 3.6 milirems per year and the average person receives several hundred milirems per year from naturally occurring background sources with no ill-effects.

Exposure to Radon Per Year By County (Red means high levels of radon)



New Cases of Cancer Diagnosed Per Year By County (High rates are purple)



In countries such as France that use nuclear reprocessing the useful isotopes are separated from the spent fuel assemblies. Since over 90% isotopes within a spent fuel assembly consist of un-fissioned uranium-235, and fissionable plutonium-239 this greatly reduces the volume of the material to be disposed of. The material from a spent fuel assembly can be reduced through reprocessing to a piece of material the size of a cigarette lighter with a half-life of three centuries. The fuel created from this process is known as mixed-oxide fuel, or "MOX" fuel. There are many different types of fuel reprocessing. The most common type is the PUREX method, although research is being conducted into "pyroprocessing" techniques.

Unfortunately, it is often politics that drive policy not common sense and the US is no exception. Although the once-through spent fuel disposal method is wasteful from the standpoint of throwing away a source of useful nuclear fuel, there is not much of it at all. There are three categories of "nuclear waste"; low-level waste, intermediate waste, and high-level waste.

Low-level waste consists of anything from pens and pencils from the offices within a nuclear power plant to the gloves and protective gear worn by personnel. Low-level waste from a nuclear power plant is often very weakly radioactive if it is radioactive at all, and is typically burned or buried close to the surface of a special landfill. Intermediate waste includes things like the actual components of the reactor itself in addition to the materials used in the construction of a nuclear reactor. There is usually not much in the way of intermediate waste to be disposed of and it is often buried in a shallow repository. High-level waste consists of the spent fuel that is marked for disposal.

This material is a metallic solid that has been encased in glass, lined with concrete, and sealed into an extremely durable cask. Tests have demonstrated the ability of these casks to withstand impacts with freight trains. Doomsday scenarios featuring terrorists stealing spent fuel material in order to construct bombs leave out the fact that the concentration of Uranium-235 needs to be enriched up to at least 90% for it to be weapons-grade material. Spent fuel does contain plutonium-239 which can be used for a plutonium bomb, but it is also contaminated with plutonium-240 which is a poison for a nuclear bomb as it absorbs neutrons without fissioning, effectively stealing the neutrons that would be able to strike plutonium-239 that would cause the rapid fission reaction. Fission would occur, but not in the rapid fashion that you would need it to for a nuclear bomb. To make things worse for a terrorist, it would be very difficult to separate the plutonium-239 from the plutonium-240 and it would require highly specialized equipment. It would simply be cheaper and easier to build a special reactor dedicated to producing weapons-grade material like most nations do.

Finally, the amount of high level waste to be disposed of is quite small. All of the high-level waste ever produced in the US as a by-product of nuclear energy could easily fit into a room the size of a high school gymnasium, just two stories high. Compare this to the mountains of coal ash and carbon dioxide generated by the burning of fossil fuels which will be just as toxic millions of years from now as it was the day that it was created.

In part two, we will be taking a look at thorium-based fuel cycles and how nuclear reprocessing works in detail. I hope that this post was easy to read and understand and that it was not too long or boring. Stay tuned!

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