Strontium-90
By Richard David
What is strontium?
Strontium is found naturally as a non-radioactive element. Strontium (chemical symbol Sr.) is a silvery metal that rapidly turns yellowish in air, it occurs in nature as four stable [isotopes]. Strontium has 16 known isotopes, but only strontium-90 has a half-life sufficiently long (twenty-nine years) to warrant any concern for Department of Energy environmental management sites such as Hanford. Naturally occurring strontium is found as four stable isotopes Sr-84, -Sr.-86, Sr.-87, and Sr.-88. Twelve other isotopes are radioactive with Sr.-90 as the most important radioactive isotope in the environment.
Isotopes are different forms of an element that have the same
Number of protons in the nucleus but a different number of neutrons.
Description.
Strontium-90 is solid at 298 K
Color: silvery white
Classification: Metallic.
Strontium does not occur as the free element. Strontium is softer than calcium and decomposes in water more vigorously. Freshly cut strontium has a silvery appearance, but rapidly turns a yellowish color with the formation of the oxide. The finely divided metal ignites spontaneously in air. Volatile strontium salts impart an excellent crimson color to flames and these salts are used in pyrotechnics. See picture below:
Figure 3
The picture above shows the result of adding different metal salts to a burning reaction mixture of potassium chlorate and sucrose. The red color originates from strontium sulphate. The orange/yellow color originates from sodium chloride. The green color originates from barium chlorate. The blue color originates from copper (1) chloride. The lilac color that should be evident from the potassium chlorate is washed out by the other colors all of which are more intense. With the production of these colors, scientist developed multiple uses.
Who discovered strontium:
In Scotland during the 1790s, Adair Crawford and William Cruickshank first detected non-radioactive strontium in the mineral strontiante, and witherite (SaCO-3). When they mixed witherite with Hydrochloric acid they did not get the results they expected. They assumed that their sample of witherite was contaminated with an unknown mineral, a mineral which they named strontianite (SrSO-3). Sir Humphrey Davy, a British chemist, first isolated strontium in 1808 through the electrolysis of a mixture of strontium chloride (SrCl-2) and mercuric oxide (HgO). Today, strontium is obtained from two of its most common ores, celestite (SrSO-4) and strontianite (SrCO-3), by treating them with HCL, forming strontium chloride. The strontium chloride, usually mixed with potassium chloride (KCl), is then melted and electrolyzed, forming strontium and chlorine gas (Cl-2).
What are the properties of Strontium-90?
Strontium-90 is produced by nuclear fission, while the other three stable isotopes (Strontium-84, 6%), (Strontium-86, 9.9%), and (Strontium-87, 7%) occur naturally.
Figure 1.
Whenever a nuclear weapon is tested Strontium-90 is produced as a by-product: See figure #1.
When Uranium-235 undergoes fission the average of the fragment mass is about 118, but very few fragments near the average are found. It is more frequently Uranium breaks into unequal fragments, and the most probable fragment masses are around mass 95 and 137. Most of these fission fragments are highly unstable (radioactive), and some of them such as celsium-137 and strontium-90 are extremely dangerous when released to the environment.
Figure 2
Fragment are produced whenever a nuclear explosion occurs, one of the common are Xenon and strontium. See figure 2
Highly radioactive, the xenon decays with a half-life of 14 seconds and produces the stable isotope cerium-140. Strontium-94 decays with a half-life of 75 seconds, finally producing the stable isotope zirconium-94. These fragments are not so dangerous as intermediate half-life fragments such as cesium-137. This particular set of fragments from uranium-235 fission undergoes a series of beta decays to form stable end products. See figure 3.
Figure 3.
What is strontium-90 used for?
Two strontium compounds, strontium carbonate (SrCO3) and strontium nitrate (Sr (NO3) 2, burn with a bright, red flame and are used in fireworks and signal flares. Strontium carbonate is also used to make certain kinds of glass and is the base material for making most other strontium compounds.
Strontium-90 is used as a radioactive tracer in medical and agricultural studies. The heat generated by strontium-90’s radioactive decay can be converted to electricity for long-lived, lightweight power supplies. These are often used in remote locations, such as in navigational beacons, weather stations and space vehicles.
Strontium-90 is also used in electron tubes, as a radiation source in industrial thickness gauges, and for the treatment of eye diseases. Controlled amounts of strontium-90 have been used as a treatment for bone cancer.
How does strontium get into the environment?
Strontium-90 was widely dispersed in the 1950s and 1960s in fall out from atmospheric testing of nuclear weapons. It has been slowly decaying since then so that current levels from these tests are very low. Strontium-90 is also found in waste from nuclear reactors. It is considered one of the more hazardous constituents of nuclear wastes. The accident at the Chernobyl nuclear power plant also introduced a large amount of Sr-90 into the environment. A large part of the Sr-90 was deposited in the Soviet Republics. The rest was dispersed as fallout over Northern Europe and worldwide. No significant amount of strontium-90 reached the United States.
What are the health effects of Strontium-90?
Strontium-90 is chemically similar to calcium, and tends to deposit in bone and blood-forming tissue (bone marrow). Thus, strontium is referred to as a “bone seeker.” Internal exposure to Sr-90 is linked to bone cancer, cancer of the soft tissue near the bone and leukemia. Strontium mimics calcium in the body, becoming part of the bone marrow tissue and damaging blood-producing cells.
In 1958 Barry Commoner and others showed that the teeth of every baby in the US had some level of strontium-90 accumulation as a direct result of fall-out from nuclear testing in the 194s and 50s.
Radiation dose and damage:
Indication of a substance’s activity alone does not say anything about its biological radiation effect. The relation, between the activity of a radioactive substance, and the radiation dose, which is caused by radiation. Which is expressed by the so-called dose factor, which has to be determined individually for each radioactive substance. Today, the absorbed radiation dose is computed and indicated mostly as effective dose equivalent. The unit used is the “Sievert” (Sv). In the past, the unit “rem” (roentgen esquivalent man) was used, with 100 rem = `1 Sv.
The lethal dose is about 10 Sv (1000 rem), i.e. death occurs within a few days after radiation exposure. At doses of 4-6 Sv, the survival rate probability is 50% within th first four weeks after exposure. From a dose of abo;ut 500 mSv directly changes the blood cell count. Up to 100 mSv no direct radiation damage occurs but there may by random stochastic effects.
Reference:
- Radiological Agent: Strontium-90.
- EPA – Strontium information
- Nuclear Fission Fragments.
- The tooth fairy project.
- Strontium-90.
- Radiation level.