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In 1987, in Brazil, a simple metal capsule left behind in an abandoned hospital triggered one of the worst civilian nuclear accidents in history.

that's pretty much the first sentence of the linked article, no?

When you make a post, I think HN asks you to summarize the article in the comments (or something like that, I haven't submitted anything in a while)

No. There is a textbox for the URL and a textbox for questions, but the idea is not to use it as a summary.