Scientific Facts and Figures

Basics on Uranium Isotopes

 

 

Uranium normally occurs in trace amounts in nature about 3 parts per million (ppm) or 3 micrograms of uranium per gram of dry soil, the equivalent of 1 teaspoon of natural uranium per 5 tonnes of earth.

 

An infinitesimal quantity of natural uranium is ingested in the body on a daily basis. It passes through the body with minimal effects. It is absorbed very poorly in the gastrointestinal tract (only 2% absorption). Approximately 10 nanograms (a nanogram is 1 one-billionth of a gram) is excreted daily in human urine.

 

Once mined, uranium is refined into concentrated packets of almost pure uranium. It is this highly concentrated “natural” uranium which is processed to produce enriched uranium. The enrichment process increases the isotope U235 concentration to create Enriched Uranium which is more easily fissionable for use in nuclear weapons and reactors. This process also creates depleted uranium (DU) as a byproduct.

 

All uranium whether “natural”, “depleted” or “enriched” is a chemical and radiological toxic substance emitting alpha, beta and gamma particles. All forms of uranium differ from each other by only a fraction of one percent. Both natural and depleted uranium are over 99% composed of the isotope U238. The ratio of isotope U238/U235 gives the unique signature that identifies whether the uranium is enriched, natural, or depleted.

 

 

Radioactivity (disintegrations per second) in 1 milligram of U-238 at Secular Equilibrium

 

U-238

Th-234

Pa-234

U-234

12.4 alpha particles

12.4 beta particles

12.4 beta particles

0.017 alpha particles

 

 

In the course of one year, 1 milligram of uranium emits 390 million alpha particles, 780 million beta particles and associated gamma rays. This is over one billion high-energy, ionizing, radioactive particles and rays which can produce extensive biological damage.

 

The energy of a single alpha particle exceeds the amount required to damage important macromolecules such as DNA, RNA, enzymes and proteins. It does this by breaking molecular bonds and by chemical reactions, which alter or destroy the shape, organization, and function of these molecules.

 

 

 

The difference between “DU“ and “NDU“

 

 

1. “Depleted“ Uranium (DU)

 

Depleted uranium is a byproduct of the uranium enrichment process.

Presently there is no acceptable solution for safe disposal of radioactive waste. The laws and precautions governing its use have largely been discarded since large-scale military use made them impractical. Depleted uranium is also now being made available to be recycled as an element going into manufacturing of consumer or industrial products.

 

The enrichment process also creates small quantities of the man-made isotopes U236 and plutonium (Pu239). These isotopes are included in the “depleted” uranium mass as it is too expensive to extract them.

 

For every gram of enriched uranium that is produced there are 7 grams of depleted uranium. This results in huge stockpiles of radioactive waste. It is estimated that there is over one million tons of DU stockpiled in the U.S. The quantities of plutonium in these stockpiles are a well-kept secret. It is routinely measured but not publicly reported.

 

Isotope Composition, Chemical Half-lives and Isotope Ratios in Natural and Depleted Uranium:

 

ISOTOPE

NATURAL

DEPLETED

HALF-LIFE

U-238

99.2749%

99.7947%

4.49 billion years

U-235

0.7196%

0.2015%

710 million years

U-234

0.0055%

0.0008%

248,000 years

 

 

 

 

 

 

 

 

 

2. “Non-Depleted” Uranium (NDU)

 

Non-depleted uranium is uranium with a U238/U235 isotopic ratio comparable to natural uranium but having quantities of U236 and presumably plutonium.

U236 is a man-made element not found in nature. It's presence suggests that the uranium has been through a reactor or has been mixed with reactor by-products.

While some studies have shown that U236 may be produced in nature by natural reactors, the quantity of U236 is 10,000 times less than the amount UMRC is measuring in NDU.

 

 

 

Radiation and the Human Body

 

 

In terms of pure physics, radiation is the process of transport of energy across space. Radioactivity is the process of decay of a physical element and involves the emitting of "bundles of energy", which may have a mass or not and may have an electric charge or not.

 

Relatively few natural elements undergo this process and they are called "radioactive" elements. Alpha and beta particles, gamma-rays are emitted when radioactive decay takes place.

 

When particles reach the human body they interact with its physical components. This interaction results in the deposit of part or all of the energy carried by the “intruder” particle. The particles are so tiny that their effect is not immediately sensed by the body. It is the consequences of this interaction that is felt inside the body - by disruption of the bonds that keep molecules together and by creating ions that further interact with our system.

 

Each particle emitted has a certain amount of energy. The energy multiplied by the total number of particles gives the total amount of "uninvited" energy released in the body. To illustrate this point, consider the number of alpha particles emitted by a single spherical pellet of uranium oxide (UO2) 0.0001 inch or 2.5 microns in diameter (equivalent to 1/40th the width of a human hair) and the dose rate it produces.

 

Tiny as it is, the 2.5 micron depleted uranium oxide pellet contains 210 billion atoms (2.1 x 10 to the power of 11) of U238. Each year, the pellet will emit an average 32.3 alpha particles. It also contains U234, 235, 236 which together yield an additional 5.3 alpha particles per year. Thus a single pellet of depleted UO2 will produce a total of 37.6 alpha particles per year.

 

The 37.6 alpha particles will deliver a radiation dose of 17 rads/year. With an RBE (Relative Biological Effectiveness) factor of 10, the dose rate is 170 rem/year for the surrounding body tissue. In the US, the Code of Federal Regulations regarding energy specifies an annual limit of 0.17 rem/year and a specific limit of 0.5 rem/year for an individual in the general population.

 

A quick and simple calculation shows one single pellet delivers 1,000 times the annual limit. This number is multiplied by the total number of pellets present in the body. For example, if a single or series of exposures resulted in the presence of 10 pellets then the annual limit is exceeded by 10,000.

 

Another factor to consider is "permanence". Objects or particles less than 5 micron in diameter are considered respirable, meaning that it is small enough to enter into the lungs and become permanently trapped. If the body does not manage to somehow release it then the radiation is internalized and the dosage is permanent during the individual's lifetime and even remains in their physical remains after death.

 

 

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