Energy From Thorium Discussion Forum

It is currently Dec 09, 2018 10:05 pm

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 34 posts ]  Go to page Previous  1, 2, 3
Author Message
PostPosted: Apr 27, 2010 6:15 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1938
Location: Montreal
Axil wrote:
I think where delayed fission really matters is in the precise timing that is to be found in the design of a nuclear weapon.

No.
There is no such thing as "delayed fission" -- only delayed neutrons from fission products that decay by neutron emission.
Anyway, neither has ANY role in nuke weapons.
What *is* important in nuke weapons is that some nuclides don't fission by thermal neutrons, only fast ones: in that case, you don't have to worry about pre-detonation criticality (or high sub-critical multiplication creating large neutron background).
One good example is Np237.
Typically, thermal-fissile nuclide critical mass is far lower than fast-fission critical mass -- hence the problem !
In the case of reactors, this is also a problem, since delayed neutrons are NOT fast, so any core that has a significant amount of that type of fissile nuclide will be less responsive to delayed neutrons -- in the extreme, not responsive AT ALL ! (i.e. uncontrollable)


Top
 Profile  
 
PostPosted: Apr 30, 2010 2:15 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1055
jaro wrote:
Axil wrote:
I think where delayed fission really matters is in the precise timing that is to be found in the design of a nuclear weapon.

No.
There is no such thing as "delayed fission" -- only delayed neutrons from fission products that decay by neutron emission.
Anyway, neither has ANY role in nuke weapons.
What *is* important in nuke weapons is that some nuclides don't fission by thermal neutrons, only fast ones: in that case, you don't have to worry about pre-detonation criticality (or high sub-critical multiplication creating large neutron background).
One good example is Np237.
Typically, thermal-fissile nuclide critical mass is far lower than fast-fission critical mass -- hence the problem !
In the case of reactors, this is also a problem, since delayed neutrons are NOT fast, so any core that has a significant amount of that type of fissile nuclide will be less responsive to delayed neutrons -- in the extreme, not responsive AT ALL ! (i.e. uncontrollable)


Thanks, jaro you are very generous with your time and patience with us beginners. I do like to get into the esoteric details. IMHO, to fully appreciate the Lftr, one needs to understand its fuel, U233 in some detail.

For one thing, when U233 absorbs a neutron, it briefly becomes U234 before U234 fissions. Most of the time, (92%) this excited U234 nucleus will fission, but sometimes it won’t (8 %).

Traditionally, neutrons in reactors have been categorized by the reaction that produces them. Thus, all neutrons from neutron induced fission are called "fission" neutrons, with two subsets, prompt fission neutrons and delayed fission neutrons.

When U234 fissions in the prompt fission process, it produces two or three prompt neutrons. I still think that there is some variable delayed time period described by some Gaussian time distribution that U234 takes to break apart in the fission process. Why would U234 be any different in this regard from other delayed neutron precursors?

This means the mean delta time that U234 takes to fission is a major time component in the mean prompt neutron lifetime that defines the rate of multiplication of prompt neutrons.

I looked at this study "Benchmark Critical Experiments of Uranium-233 Spheres Surrounded by Uranium-235”. The multiplication of U233 prompt neutrons is very low at only 6.31. When you add some U235 to it, it goes way up to the mid 80’s.

There might be some sort of U233 anti-proliferation argument to be made from the teapot experience as follows:

If you need U235 or Plutonium to make U233 critical, why not eliminate U233 from the mix altogether as a redundant and a needlessly complicating factor that can cause unpredictable performance?

In conclusion, U233 is confusing to me. The more I look into U233, the less I understand it, and the more questions that I have about it. But at the end of the day, it is still very fascinating.

_________________
The old Zenith slogan: The quality goes in before the name goes on.


Top
 Profile  
 
PostPosted: Apr 30, 2010 6:04 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1938
Location: Montreal
It won't be all that confusing once you've spent a bit more time studying the stuff...

Some of the confusion comes from a poor choice of words in certain reference material.

This is a good example:
Axil wrote:
...all neutrons from neutron induced fission are called "fission" neutrons, with two subsets, prompt fission neutrons and delayed fission neutrons.

....the "delayed fission neutrons" should NOT be read as "neutrons from dalayed fission", but rather as "fission neutrons that are delayed".
Again, the reason for the latter is that some neutrons from fission only come out from fission products that decay by neutron emission.


Top
 Profile  
 
PostPosted: Apr 30, 2010 10:55 pm 
Offline

Joined: Dec 03, 2008 5:23 pm
Posts: 137
Location: Oak Ridge, TN
Axil wrote:
When U234 fissions in the prompt fission process, it produces two or three prompt neutrons. I still think that there is some variable delayed time period described by some Gaussian time distribution that U234 takes to break apart in the fission process. Why would U234 be any different in this regard from other delayed neutron precursors?

This means the mean delta time that U234 takes to fission is a major time component in the mean prompt neutron lifetime that defines the rate of multiplication of prompt neutrons.

Ok, this seems to be very off topic for "skipping the intermediate loop".

No, No, No! fission events (time between neutron absorption and fission) happen very quickly (generally less than 10^-12 seconds) and can be assumed to be instantaneous. The physics behind fission and delayed neutron production are totally different. The delayed neutrons are the result of the neutron emission during radioactive decay of fission products, typically following a beta decay. Hence these fission products are delayed neutron precursors and they have a half-life leading to the delayed neutron emission.

The time it takes a neutron to fission is nil compared to the time it takes the neutrons to travel (their speed) and for thermal reactors, the time that it takes to moderate them. For fast reactors, the average neutron speed is fast and there is no slowing down time, hence a short prompt neutron lifetime (~10^-5 seconds), while in thermal reactors the moderation time is significant leading to a prompt neutron lifetime of ~10^-3 seconds.

Axil wrote:
I looked at this study "Benchmark Critical Experiments of Uranium-233 Spheres Surrounded by Uranium-235”. The multiplication of U233 prompt neutrons is very low at only 6.31. When you add some U235 to it, it goes way up to the mid 80’s.


The multiplication in this report is a measure of the subcriticality (M = 1/(1-k)). The multiplication factor should go way up when you add more fissile mass to the system such that keff is increasing. This experiment was specifically design for obtaining criticality with a combination of U-233 and U-235.

Axil wrote:
If you need U235 or Plutonium to make U233 critical, why not eliminate U233 from the mix altogether as a redundant and a needlessly complicating factor that can cause unpredictable performance?

Of course you do not need U-235 or plutonium to make U233 critical. It has a bare-sphere critical mass of about 15 kg.

I suggest that this discussion be terminated or moved to another thread.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 34 posts ]  Go to page Previous  1, 2, 3

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group