If a francium grenade is dropped in a sea, how much years or weeks does radiation take to kill people at close range?

Question: If a francium grenade is dropped in a sea, how much years or weeks does radiation take to kill people at close range?
- Keywords:
  francium,
  kill,
  radiation,
  range,
  sea
Asked by anon-181914 on 25 Jun 2018.
- Joanne Sharpe answered on 25 Jun 2018:
  
  Francium is way too unstable to do anything with! It is extremely radioactive though so if it was possible to make a grenade I imagine it would be very explosive. There are lots of different types of radiation, and some are very deadly but can’t penetrate very far, whereas some are only lethal at high doses but penetrate and travel further.
- Lauren Burns answered on 25 Jun 2018:
  
  The other problem with Francium is that very little of it occurs naturally. Scientists estimate there is never more than a gram of it in the Earth’s crust at any one time, so making a grenade of it would be very tricky!

Question: If a francium grenade is dropped in a sea, how much years or weeks does radiation take to kill people at close range?