C-14 production in reactors

[daleboyce@charter.net]

Hi all,



The reason we have trouble finding references to C-14 released from

Chernobyl is that there probably wasn't much produced.



>From Kessler "Nuclear Fission Reactors" there is about 3 curies per gigawatt

year produced in PWR reactors due to N14(n,p)C14 and O17(n,alpha)C14.

Unfortunately, he doesn't say much about graphite reactors, so I had to

grind a little to try to set some bounds on production in graphite. What

follows is a handwaving explanation of what I did.



To get 1 gigawatt year electric you need about 3 gigawatt years thermal

which requires about 2.8e27 fissions. Each fission produces 2.5 neutrons,

one of which goes on to produce another fission. Let's assume that roughly

one neutron also gets captured in the fuel assembly, and or control rods.



That leaves 1.4 e27 neutrons that have a remote possibility of making C-14.

The mean range before neutron capture by C12 to make C13 is about 25 meters

in graphite.  If you plug in all the numbers you could hypothetically make

400 curies of C-14 from C-13 capture.



However, half a neutron per fission escaping the core sounds pretty high, so

I looked a little further and found also in Kessler that the mean life of a

neutron in a PWR reactor is 2.5E-5 seconds. A 2200 meter/sec neutron travels

about 0.055 meters in this time, so as an estimate of a correction to the

fraction of neutrons lost to the graphite I multiply 400 Ci by .055 meter

and then divide by the 25 meter range in graphite.  I then get around 1

Ci/year.



Graphite reactors have a lower power density than PWR. Without much else to

go on to account for the different power density (which effects neutron

lifetime), I opted to multiply by the ratio of number of collisions needed

to on average to thermalize neutrons with graphite vs. light water (6).  So

my back of the envelope guestimate for C-14 produced in Chernobyl is 6 Ci/

gigawatt year(e) of operation.



I'd be interested to see if anyone has any real data.



Regards, Dale

daleboyce@charter.com





----- Original Message ----- 

From: "Nathan Russell" <windrunner@gmail.com>

To: "Michael McNaughton" <mcnaught@LANL.GOV>; <radsafe@list.Vanderbilt.Edu>

Sent: Thursday, August 26, 2004 9:02 AM

Subject: Re: request for help with Wikipedia article





> On Thu, 26 Aug 2004 07:33:07 -0600, Michael McNaughton

> <mcnaught@lanl.gov> wrote:

> > At 08:04 PM 08/25/2004, Nathan Russell wrote:

> > >I'm an editor (and sysop) of the free Wikipedia encyclopedia -

> > >http://en.wikipedia.org/ -

> >

> > I find Wikipedia to be one of the most valuable resources on the

internet

> > for non-scientific subjects. On science, it is still growing. On health

> > physics, it has barely begun. The subscribers to radsafe could make a

very

> > important contribution to Wikipedia. Please take a look at Wikipedia and

> > learn more about it. The most amazing fact is how well the Wiki concept

> > works; it restores my faith in humanity.

>

> Thanks for the kind words.  We already have some content that I feel

> is accurate (e.g. that LNT is not a good model), but on the Chernobyl

> accident in particular - why does it seem like I can't find any

> figures for the amount of activity released that are even close to

> each other?

>

> I've seen figures for the total from 2 to 12 EBq, and around 2-3 for

> Cs137 alone.  Is it safe to assume that Cs137 is the majority of the

> remaining activity?  I assume some C14 was generated in the moderator,

> and a great deal of that made it into the atmosphere, but is that not

> a significant source of contamination?

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