APR: your source for nuclear news and analysis since April 16, 2010

Wednesday, March 16, 2011

FACTS: Some terms

Many terms are being used concerning nuclear energy that people aren't familiar with, and one of the terms that unfortunately has overlap into non-nuclear use is the term "Critical."

-In nuclear energy, an assembly of fissionable material is "subcritical" if the rate of fissions per unit of time is decreasing
-An assembly of fissionable material is "critical" if the rate of fissions is constant, in other words a self-sustaining chain reaction
-An assembly of fissionable material is "supercritical" if the rate of fissions per unit time is increasing, no matter how slowly

When the media uses phrases like this: "Heat values in spent fuel pools may reach critical level" they are talking about a temperature limit above which the fuel may begin to melt. They normally do not have any idea what nuclear criticality really is or what the terms mean

RADIOACTIVE CONTAMINATION is the presence of a radioactive substance where it should not be

RADIATION is particles or rays (alpha particles, beta particles, gamma rays) given off by a substance that is radioactive

If some uranium is where it should not be, the surface on which it is sitting is now radioactively contaminated. The contamination emits radiation. Usually radiation cannot make another substance radioactive, or if it can, it is not for very long. Whatever the case, radiation impacting the human body CANNOT make it radioactive.

FUEL MELTING is when reactor fuel structures overheat and begin to stretch, bulge, and lose structural strength

FUEL BURNING can happen if the heat is so high that combustion -- actual chemical reaction -- can take place which is very rare and practically impossible in realistic scenarios

SPENT FUEL has already been "used up" in a reactor, and while it still contains some uranium, is radioactive, and gets hot, it doesn't have enough unfissioned fuel left in it to be useful in a reactor any more and in fact if inserted may not be able to reach criticality

Hopefully this will help clear up a few of the misstatements I've seen in general press reporting in the last six hours or so.

ATOMIC POWER REVIEW 5:50 PM Eastern Wednesday 3/16


  1. Thank you very much for your very informativ blog - it gives very interesting insights. Do you have any sources, that can say how high the radiation levels are, split between Alpha, Beta and Gamma rays / cores? This would significantly help to understand the situation of the operators in the plant.

  2. I do not have that, but really most of the exposure will be high energy gamma both in general ranges or zones, and in high intensity streams if there are cracks in containment. There would also be contamination around emitting all three, but remember that a sheet of paper can stop an alpha particle. The neutron flux would be expected to be generally low, except inside the reactor buildings.

  3. Poor guys - I was hoping to hear, that most would be Alpha or Beta and that they could be 'safe' in their protection suits.