Plutonium

Historical eventMay 211946

Plutonium

While working with a mass of plutonium known as the demon core, Manhattan Project physicist Louis Slotin accidentally exposed himself to a lethal dose of hard radiation, making him the second victim of a criticality accident in history.

80th anniversary in 2026

Everything on May 21Original source

In context

When
Tuesday, May 21 194680 years ago
Category
Historical eventA dated happening — a battle, treaty, launch, discovery, disaster or decision — recorded on this calendar day.
Era
20th century · 1940s
Anniversary
80th in 2026
Recorded from
Wikipedia “On this day” feed

At a glance

Dates: From 1946.

What happened

Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric.

May 21 through the years

Other entries on the same calendar day, in order.

  1. 1946Plutonium
  2. 2006Republic of Montenegro (1992–2006)
  3. 2010JAXA
  4. 2011Harold Camping
  5. 2012Qafa e Vishës bus accident
  6. 20122012 Unity Day parade rehearsal bombing
  7. 20142014 Taipei Metro attack
  8. 2015Islamic State
  9. 2017Ringling Bros. and Barnum & Bailey Circus
  10. 2024Greenfield tornado
  11. 20242024 Taichung MRT attack

Also in 1946

  • October 22Operation OsoaviakhimOver twenty-two hundred engineers and technicians from eastern Germany are forced to relocate to the Soviet Union, along with their famil…
  • July 5Bikini AtollNamed after Bikini Atoll, the site of the nuclear-weapons test Operation Crossroads in the Marshall Islands, the modern bikini was introd…
  • October 271946 Bihar riotsInter-religious riots in which Hindu mobs targeted Muslim families began in the Indian state of Bihar, resulting in 2,000 to 30,000 deaths.
  • May 1Pilbara strikeStart of three-year Pilbara strike of Indigenous Australians.
  • July 22King David Hotel bombingKing David Hotel bombing: A Zionist underground organisation, the Irgun, bombs the King David Hotel in Jerusalem, site of the civil admin…
  • December 26Bugsy SiegelAmerican gangster Bugsy Siegel opened The Flamingo Hotel & Casino in Las Vegas, the oldest casino still in operation on the Las Vegas Strip.
  • June 21946 Italian institutional referendumBirth of the Italian Republic: In a referendum, Italians vote to turn Italy from a monarchy into a Republic. After the referendum, King U…
  • August 7Turkish StraitsThe Soviet Union informed Turkey that the way in which the latter was handling the Turkish Straits no longer represented the security int…

Everything we record for 1946 →

Sources

Cite this page

“Plutonium”, What's On This Date, https://whatsonthisdate.com/app/event/plutonium-1946-may-21, accessed October 24, 2026.

Cite the underlying source as well — the source links above go to the record this entry is adapted from.

Questions about this entry

When did Plutonium happen?

May 21 1946 — 80 years ago, on a Tuesday.

Where does this text come from?

Wikipedia “On this day” feed. Summaries are short adaptations that link back to the source, not reprints.

How do I cite this page?

Use the ready-made citation near the sources at the bottom of the page, and follow the source link to cite the underlying record as well.

Text adapted from Wikipedia under CC BY-SA 4.0 and from Wikidata (CC0). Images come from Wikimedia Commons; credit and licence are shown with each image. Editorial entries are © What's On This Date. Sources & licensing · How this site works.