Kilocurie to Curie

In 1987, scrap metal scavengers in Goiânia, Brazil broke open an abandoned caesium-137 teletherapy source containing about 1,375 Ci (50.9 TBq). The glowing blue Cs-137 powder fascinated locals — they rubbed it on skin, gave it to children, and spread it across multiple homes. Four people died, 249 were contaminated, and the cleanup produced 3,500 m³ of radioactive waste. The incident became the textbook case for why sealed sources must be tracked and securely stored throughout their entire lifecycle, and why the IAEA created its Code of Conduct on the Safety and Security of Radioactive Sources.

Yes, multiple times. In Ciudad Juárez, Mexico (1983), a stolen Co-60 teletherapy source was sold as scrap and melted into rebar, contaminating 4,000 tonnes of steel and exposing thousands. In Samut Prakan, Thailand (2000), a junked Co-60 source killed three scrap workers who pried it open. In Yanango, Peru (1999), a welder pocketed an Ir-192 industrial radiography source and carried it in his pocket for hours — his leg was amputated. The IAEA documents over 30 serious radiation accidents involving orphaned or stolen sources since the 1960s, collectively killing dozens and injuring hundreds.

Cobalt-60 has a 5.27-year half-life, so a 500 kCi source drops to 250 kCi after five years and becomes too weak for industrial throughput after about 15–20 years. The spent source pencils are returned to the manufacturer (typically in Canada or Russia) for reprocessing or secure storage. Transport uses heavily shielded Type B casks certified to survive a 9-meter drop and 30-minute fire. The manufacturer often offers a swap program: deliver fresh sources and take back decayed ones in the same shipment, minimising the number of high-activity transports.

The Fukushima Daiichi disaster released an estimated 10–30 PBq (10,000–30,000 TBq) of caesium-137 directly into the Pacific Ocean between March and July 2011 — the largest single marine radioactive release in history. For comparison, the Sellafield reprocessing plant in the UK discharged about 40 PBq of Cs-137 into the Irish Sea over decades of operation (1952–2000). Soviet dumping of entire reactor compartments from nuclear submarines in the Arctic added further inventory. Despite these numbers, ocean dilution is vast: Pacific Cs-137 levels from Fukushima peaked at about 50 Bq/m³ near the plant and dropped below 2 Bq/m³ within a few hundred kilometers.

This is exactly why the IAEA, NRC, and national agencies track high-activity sources so aggressively. A kilocurie Cs-137 or Co-60 source dispersed by conventional explosives would contaminate a few city blocks — not causing acute radiation casualties (the blast itself is deadlier) but creating a costly, panic-inducing cleanup lasting months. The actual health risk to the public would be low, but the economic and psychological damage would be enormous. Post-9/11 programs like the US GTRI (now NNSA OSRP) have recovered or secured thousands of orphaned high-activity sources worldwide.

When Marie and Pierre Curie isolated radium in the early 1900s, it became the reference standard for radioactivity because it was the most intensely radioactive substance known and could be weighed on a balance. The Radiology Congress of 1910 defined the curie as the activity of one gram of Ra-226 — roughly 3.7 × 10¹⁰ disintegrations per second. That number was not chosen for mathematical elegance; it simply fell out of radium's half-life and atomic mass. It is one of the few scientific units defined by a specific lump of material rather than an abstract principle.

One curie is enormous by everyday standards. A human body contains about 0.1 microcuries of K-40 — one ten-millionth of a curie. A smoke detector holds about 1 microcurie. To reach one full curie of K-40, you would need roughly 140 kilograms of pure potassium. Conversely, a single spent nuclear fuel rod can contain millions of curies. The curie was designed for the world of radium laboratories and nuclear reactors; for anything you encounter in daily life, the microcurie or picocurie is the appropriate scale.

Yes. The NRC, DOE, DOT, and EPA all accept curie-based units in filings, license applications, and transport documents. While 10 CFR Part 20 lists dose limits in both rem and sievert, the curie remains the default activity unit in most US regulatory practice. License conditions specify possession limits in millicuries or curies; transport labels use the Type A₂ values in curies; and waste manifests record activity in curie-based units. The US is unlikely to mandate a switch to becquerels without a broader metrication push that no one in Washington is championing.

Marie Curie personally processed tonnes of pitchblende ore to isolate fractions of a gram of radium salts — which she stored in her desk drawer and carried in her coat pocket. Her notebooks from the 1890s are still so contaminated with Ra-226 that they are kept in lead-lined boxes at the Bibliothèque nationale de France, and researchers must sign a liability waiver and wear protective clothing to view them. She died in 1934 of aplastic anaemia, almost certainly caused by decades of unshielded exposure to alpha, beta, and gamma radiation from radium, polonium, and radon gas in her poorly ventilated laboratory.

It is not oddly specific — it is just 3.7 × 10¹⁰ Bq, the measured disintegration rate of one gram of Ra-226 rounded to two significant figures. When the curie was standardized in 1910, they measured radium's activity as precisely as they could and pinned the unit to that number. Later, more precise measurements showed the actual activity of one gram of Ra-226 is closer to 3.66 × 10¹⁰ dps, but the curie was redefined as exactly 3.7 × 10¹⁰ dps to keep the number clean. So the curie no longer exactly matches one gram of radium — it is off by about 1%.