The first thing needed for a clear understanding of the radioactivity is a curie to millicurie convertor:
The second thing needed is an authoritative text which describes the nature of uranium, its chemical characteristics and its radiological characteristics. Such a text is provided by the US Agency for Toxic Substances & Disease Registry. The text, “TOXICOLOGICAL PROFILE FOR URANIUM”, which is a 462 page pdf document, is available for free download at:
How radioactive is Uranium? On text page 242 (pdf page 262) we read that “Although the element uranium was discovered in 1789 by Klaproth, who named it “uranium” after the newly discovered planet Uranus,
it was not until 1896 that Becquerel discovered that uranium is radioactive.
There are 22 known isotopes of uranium, only 3 of which occur naturally (Parrington et al. 1996).
These three isotopes, 234U, 235U, and 238U, have relative mass abundances within the earth’s undisturbed
crustal rock of 0.005%, 0.72%, and 99.275%, respectively. One gram of natural uranium having this
relative isotopic abundance has an activity of 0.67 μCi. Of this 0.67 μCi, 48.9% of the activity is
attributable to 234U, 2.2% of the activity is attributable to 235U, and 48.9% of the activity is attributable to
238U (Lide 1994). This ratio is for undisturbed crystal rock only. Although the relative mass abundance of
234U is only 0.005%, it accounts for exactly one-half of the total activity. The relative isotopic abundances
given above can be altered to some extent by natural processes that are not fully understood, but which can
cause different ratios in air, water and soil as demonstrated in EPA reports (EPA 1994a).” (Source:ATSDR, Tox. Profile Uranium).
Natural uranium having the isotopic make up given above, has a rate of radioactivity of 0.67 μCuries, that is, microcuries.
1 Curie = 1,000,000 microcuries.
As we have seen in an earlier post 1 Curie is the radioactivity possessed by 1 gram of radium.
We also saw that 1 Curie equals the disintegration of 37,000,000,000 atomic nuclei per second.
Both radium and uranium emit alpha radiation. 1 curie equals the release of 37,000,000,000 alpha particles per second. 1 gram of radium produces this radioactivity. In contrast, converting millicuries to Curies, we see 1 gram of uranium achieves a radioactivity rate of 0.00000067 Curies.
Radium however is a decay product of uranium. It will be present with uranium in a uranium ore body. In an ore body the radiation rate emitted by uranium decay (progeny) products far exceeds the radiation rate of uranium itself. Although Becquerel discovered that uranium was radioactive in 1896, it took a number of years to convince many of his peers of the fact.
A clue as to the origins surrounding the disagreements of the safety of pure uranium – used for example as military shells and aircraft ballast, and still, as a form of yellow colouring in some ceramic pottery – can be traced back to this initial controversy. It can also be traced back to the poor public understanding of the distinction between the harm presented byuranium outside of the body as opposed to uranium particles lodged within the body. It further relates to the chemical toxicity of uranium – a heavy metal. It probably also relates to the form of uranium – soluable uranium or far less soluable uranium in its “heat treated” hardened state, the state depleted uranium dust takes on as a result of the high temperatures experienced by a fired DU shell.
And it further relates to the isotope of uranium being studied at the time. As we have seen above,
“Although the relative mass abundance of 234U is only 0.005%, it accounts for exactly one-half
of the total activity.” of a mass of natural uranium. 235U is the fissionable isotope of natural uranium.
Depleted Uranium is usually defined as uranium depleted of the fissile 235U. DU is the “waste” from the uranium enrichment process.
Wikipedia at http://en.wikipedia.org/wiki/Uranium-234 states:
“Depleted uranium contains much less U-234 (around 0.001% ) which makes the radioactivity of depleted uranium about one-half of that of natural uranium. Natural uranium has an “equilibrium” concentration of U-234 at the point where an equal number of decays of U-238 and U-234 will occur. Depleted uranium also contains less U-235, but in spite of its half-life that is much shorter than the one of U-238, the concentration of U-235 in natural uranium is low enough (about 0.7%) so that the U-235 depletion does not result in a significant reduction in radioactivity.” This wiki article cites the UN WHO Media Centre on line article “Depleted Uranium” available at http://www.who.int/mediacentre/factsheets/fs257/en/ This publication states: “Depleted uranium
* The uranium remaining after removal of the enriched fraction contains about 99.8% 238U, 0.2% 235U and 0.001% 234U by mass; this is referred to as depleted uranium or DU.
* The main difference between DU and natural uranium is that the former contains at least three times less 235U than the latter.
* DU, consequently, is weakly radioactive and a radiation dose from it would be about 60% of that from purified natural uranium with the same mass.
* The behaviour of DU in the body is identical to that of natural uranium.
* Spent uranium fuel from nuclear reactors is sometimes reprocessed in plants for natural uranium enrichment. Some reactor-created radioisotopes can consequently contaminate the reprocessing equipment and the DU. Under these conditions another uranium isotope, 236U, may be present in the DU together with very small amounts of the transuranic elements plutonium, americium and neptunium and the fission product technetium-99. However, the additional radiation dose following intake of DU into the human body from these isotopes would be less than 1%.”
Fissile vs fissionable
Note that the uranium isotope 235U is fissile not because of its radioactivity rate but because of the neutrons it possesses. It is not as radioactive as 234U. Depleted uranium is generally composed of 234U and 238U. 238U is fissionable but, unlike the fissile 235U it cannot sustain a chain reaction.
(When hit by a neutron possessing a specific energy, 235U will break apart or fission. When 235U breaks apart, it releases more than 2 neutrons, and thus is able a sustain a chain reaction in a suitable pile of 235U. Each atom which breaks apart releases 200 million electron volts of energy.)
WISE, a voluntary organisation, provides a simple camparison chart of the natural Uranium iosotopes this at this website:
This chart gives:
U234 = 231.3MBq/gram (MegaBequerel) or 231,300,000 Bq/gram or 0.006251351 curies/gram
U235 = 80,011Bq/g or 0.000002162 curies/gram
U238 = 12,445Bq/g or 0.000000336 curies/gram
(using the Becquerel to Curies conversion table at
The relative radioactivity rates of uranium compared to the decay products may have led to the
idea that the radioactivity of uranium is harmless. As we shall see this is a false position.
Radium is a decay product of uranium. It is plain to see that uranium mining removes uranium ore from the
ground, on site processing removes most the uranium and leaves behind in concentrated form uranium progeny
(decay products) such as radium in tailings dumps and dams on the surface. What was once shielded in an ore body
is now available to the biosphere. Radon gas, a decay product of radium, vents directly into the air from the tailings heaps
and dams left by uranium mining.
The reader may have seen photographs and video of uranium workers dressed in dust proof suites and wearing respirator face masks, busily handling drums of uranium yellow cake. (http://en.wikipedia.org/wiki/Yellowcake).
It is therefore self evident that industry applies occupational health and safety regimes in such workplaces. (The requirement to wear protection varies from task to task). Uranium is not harmless. This begs the question as to why it is that uranium tailings dumps and dams are left open to the environment. Uranium tailings have a complex nature. It consistes of the highly radioactive uranium decay (progeny) substances. These all have rates of radioactivity hundreds and thousands of times greater than that of uranium. Why don’t workers in protective gear have to process this dangerous waste into safe storage containers?
Why is it left in the open to blow in the wind and vent into the air?
If uranium mine waste was stored in containers, where would the containers be stored? How long would the containers
before leaking? How much would safe storage cost to the nuclear military/industrial complex?
Tailings Dams at the Olympic Dam uranium mine
Olympic Dam mine in South Australia contains the world’s largest uranium deposit. 10 million tonnes of radioactive tailings are brought to the surface every year. The mine operations consumes 35 million litres of water a day., taken free of charge from the Great Artesian Basin.
(Source: Still frame from “A Hard Rain”, David Bradbury, Frontlilne Films
Next: Radiological and Toxicological Hazards of internalised Uranium.