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.
strontium-90 reached the United States.
No significant amount of
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:
1. Radiological Agent: Strontium-90. www.cbinfo.com
2. EPA – Strontium information
www.epa.gov/radiation/radionuclides/strontium.html
3. Nuclear Fission Fragments. www.fis/frag.html
4. The tooth fairy project. www.prop1.prg.html
5. Strontium-90. www.ark21.com/strontiom90.html
6. Radiation level. www.oldbooks.net/rphp/pressrelease.html