1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Nuclear Energy

  1. May 6, 2006 #1
    I'm seriously lost!

    Here's the problem:
    It has been estimated that the Earth contains 1.0 x10^9 tons of natural uranium that can be mined economically. If all the world's energy needs (7.0 x10^12 J/s) were supplied by 235(U) fission, how long would this supply last? Assume that the average energy released in a fission event is 208 MeV. (Hint: See Appendix B for the percent abundance of _92^235(U).)

    My (pathetic attempt at a) solution

    OK, I've converted to 343000 tons of usable 235U and that this is equal to 3.11x10^12g which, using avogadro's number equals 7.97x10^33 nuclei available for a reaction. this converts to 1.65x10^36MeV which converts to 5.33x10^32 ev/kg

    Now I need to get from here to how many years this would last. I tried multiplying the number of kg on hand (3111643658) by 5.33x10^32 ev/kg which gives me 1.66x10^42ev and since there are 6.24x10^18 J in an ev this gives me 2.66x10^23J

    BUT I tried to go from there to 1s/7x10^12J=3.80x10^10s which converted to 1204 years... but it's wrong.

    Does anyone see where I went wrong?

    Much appreciation!
    (Who is hugely grateful that this is the LAST physics problem I will ever have to work!)
  2. jcsd
  3. May 7, 2006 #2
    You intersperse grams with kilograms...plus you may have transposed some digits...so you're off by factors of 10.

    Consider, you get 5.33x10^32 ev/kg ... but it is really 5.33x10^23 ev/g, which is 5.33x10^26 ev/kg.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook