Re: Dispersal of Spent Fuel -Reply -Reply

[Charles Willis <CAW@nrc.gov>]

With all due respect, a response to John Sadler:

1. Yes, spent fuel assemblies are kept in pools for cooling (and Shielding).

2.  Water is an effective moderator and it becomes more effective as the temperature is
decreased because of the increased hydrogen concentration and the reduced speed of the
thermal neutrons.  

3.  A spent fuel pool could not be made critical by reducing the water temperature because
required shutdown margin is too great to be overcome in this way.

4.  Spent fuel pool criticality is highly improbable but possible if several fuel assemblies were
removed from their storage racks and braught together.

5.  Criticality would not cause a fuel element to explode, nor would it cause a steam
explosion.

6.  If fuel elements got extremely hot ( which is improbable in a pool) the water could react
with the cladding material and release hydrogen gas; the hydrogen could become explosive
when mixed with air.  Such a hydrogen explosion would have little effect on the fuel.  

7.  You are correct in believing that nuclear power plants are not risk-free work places but the
principal hazards are in construction and in commuting to and from work;  I see more cause
for concern in medical facilities.