# HELP! I'm not sure how to do this problem.

1. Apr 22, 2007

### holldoll508

Alright... this question is on my homework for my accelerated physics class. I have a test tomorrow and I'm lost on how to do it so
Here is the question:

A very small sample of fuel for a nuclear fission reactor contains .012 kg of Uranium-235. Suppose that each fission event from this fuel produces Barium-141 and Krypton-92, along with 3 additional neutrons. The nuclear reactor is then able to provide power to homes, which uses an average of 7.2x10^9 J of energy every month.
a) Determine the number of fission events possible from this fuel sample.
b)How long could the Uranium-235 fuel sample provide power for the average home?

Uranium-235 = 235.0439 u
Barium-141 = 140.9144 u
Krypton-92 = 91.92515 u

Here are some conversion factors:
1 u = 1.66x10^-27 kg

I'm not exactly sure what equation to use and how to use it. Here is the equation
E = mc^2

This will sound weird but I already know the answers.. Our teacher gives us the answers but she only takes points for the work and plus I need to know how to do it for the test. Here are the answers:
a)3.08x10^22
b)119 months

I would really appreciate it if you could help me out and give me pretty good detail so I can figure it out. Thanks!

2. Apr 23, 2007

### Kurdt

Staff Emeritus
Part a) is fairly simple in that you know the mass of a single uranium-235 atom and you know the mass of U-235 in the fuel rod. If each atom is one fisiion event then all you have to do is find how many atoms constitute a mass of 0.012Kg.

For part b) you have to find the mass defect between the parent nucleus and the products of fission. You can then find out how much energy a single fission event releases and multiply it by how many events there are. Then you will compare it by the energy consumed per month by the households.