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Q & A: Nuclear fission and conservation of mass and energy

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Most recent answer: 01/12/2017
Q:
I recently watched with great interest a PBS program which described in layman's terms how Uranium 238 transforms into the different chained elements, to include U235. It also explained the basics of the chain reaction caused by splitting U235 using E=MC^2 as the basis for energy release. This is where I was a little unclear. The split was described as one U325 nucleus splitting into two separate nuclei with some individual particles released (can't remember if they are protons or neutrons) Those particles then collide with other U235 atoms in proximity triggering subsequent splits and particle releases as part of a chain reaction. My question is that the mass doesn't appear to be transforming into energy (E=MC^2). Rather it appears that it is simply splitting and being cast off, so what causes the energy release? This assumes that the number of particles in the remaining two nuclei + the particles independently released still equal 235. There was a general reference in the program to how the Strong Force reduces the size of the resultant smaller nuclei, but it didn't say if matter within each was converted to energy or if the number of particles are additionally reduced through such a conversion. Thanks for any clarification you can provide.
- David Benson (age 50)
Virginia Beach, VA, USA
A:

The usual reaction of nuclear fission is  neutron + U235  -->  other nuclei plus more neutrons plus some kinetic  energy as depicted in the following Wikipedia diagram. 

If you put the original U235 nucleus and the added neutron on a sensitive scale and weigh the combination, then put all the resultant nuclei and neutrons on the scale and weigh them, you will find that there is some mass lost.   That mass loss times c2 will equal the kinetic energy of the final state particles.  

The total number of neutrons plus protons as well as the total charge is unchanged.

LeeH


(published on 01/12/2017)

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