RE: Response to Norman Cohen -- cooled fuel (numbers)

["Franta, Jaroslav" <frantaj@AECL.CA>]

Title:

Ruth   Weiner wrote:

Besides, even their scenario would not duplicate the Chernobyl accident because the latter involves fuel in the reactor, and cooled fuel that has been out of the reactor for at least several years is what is transported

....which I think is an important point that is not emphasized enough and often enough.

My favourite article on this topic is Bernard Cohen's "Physics of the Reactor Meltdown Accident," in a 1982 issue of the journal Nuclear Science and Engineering (Vol. 80, pp 47-60).

The radioactivity in spent nuclear fuel (SNF) is proportional to the amount of heat produced.

In figure 1 and table II of that article, the following data is shown for a typical 3000 MW (thermal) reactor (I added the last two, long-term figures) :

time after shutdown    heat output (MWth)

0                                    152

30 seconds                ~127

1 minute                      ~110

15 minutes                  ~58

1 hour                          ~40

4 hours                        ~25

12 hours                      ~20

1 day                           ~16

10 days                         7.82

1 month                         4.87

3 months                       2.82

1,000 days (2.7y)      ~0.23

10,000 days (27y)     ~0.046

 

.....which shows that there is roughly a 200-fold difference in radioactive content of fuel fresh out of a reactor (with several hours of fallout time included for Chernobyl) and SNF that's being transported following several years storage at on-site pools.

More importantly - as others have pointed out already - the most dangerous, volatile radionuclides like I-131 (half-life 8.07 days) are long gone. Also, the SNF shipment only carries a small fraction of the total core load for which the above figures are given.

Also interesting in that article is table I, in which we see that most of the radioactivity (and heat) at shutdown is due to fission products with a half-life of a few seconds to two hours - only 7.4% is due to those with a half-life of about a day and longer. Ninety days later, most of the radioactivity is still due to fission products with half-lives ranging from hours to a couple of months (none of that 10,000-year stuff -- which are mostly the trans-uranic elements like plutonium, but which contribute next to nothing to the total radioactivity, until at least several hundred years later...).

 

Jaro