A small container of radioactive Caesium-137, which went missing on January 16 from a truck ferrying machinery and tools from an ONGC exploration site near Machilipatnam to Rajahmundry 120 km away, was retrieved from a scrap shop at Kalindindi village in Krishna district of Andhra Pradesh late Wednesday evening. The mysterious disappearance of the 1 ft × 1 ft box had triggered concern and some panic, as both police and ONGC officials searched frantically for it. However, the container was found intact, and the isotope safe.
Caesium-137 (atomic mass 137) is a heavier, radioactive isotope of Caesium (Cs) whose most stable form is Cs-133. Silvery white, soft, and malleable, Cs-137 is one of the very few metals that exist in liquid form at room temperature.
(A more common example is mercury.) Cs-137 is most commonly produced as a byproduct in fission reactions of uranium and plutonium in nuclear plants or nuclear explosions. It is, thus, part of the spent fuel. One of the biggest contaminations of Cs-137 happened during the Chernobyl accident of 1986, when about 27 kg of the metal entered the atmosphere. Cs-137 has a half-life of about 30 years.
It decays through the emission of beta particles (a high-energy electron or positron, or positive electron) and gamma rays (a form of electromagnetic radiation like X-rays). Any danger from Cs-137 emanates from these natural emissions of beta particles and gamma rays. Exposure to very small amounts of Cs-137 is not harmful.
Such exposure happens through presence of the metal in very small amounts in the atmosphere and in the soil. However, accidents like Chernobyl or Fukushima can contaminate water bodies with Cs-137, and the metal can then be ingested along with food and water. Very high exposure — an extremely rare occurrence, according to the US Environment Protection Agency — can result in an increased risk of cancer.
Cs-137 is used in a variety of measuring instruments in the construction and other industry. It is also used, as in ONGC’s case, in well-logging devices in the drilling industry for the characterisation of rocks.
The substance that went missing in Andhra has a radioactivity of 80 GBq (giga or billion becquerel), according to the Atomic Energy Regulatory Board (AERB). This means that about 80 billion atoms in the substance was decaying every second. ONGC said the substance was encased in a box that had a lead shield to absorb all the gamma rays that were emitted. Unless the box was opened — which was not easy — there was no harm of exposure, the AERB said. “AERB would like to inform the general public, (that) the radioactive source is in a heavily shielded rigid container and well secured, and hence not easily retrievable. Moreover, the source is in non-dispersible form,” it said in a statement.