Energy From Thorium Discussion Forum

It is currently Oct 16, 2018 4:59 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 24 posts ]  Go to page Previous  1, 2
Author Message
PostPosted: Mar 18, 2010 8:48 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1946
Location: Montreal
jaro wrote:
Indeed, the time constant is quite large -- even compared to LWRs (I will supply some figures shortly).

Here's the reference info :

The great majority of the neutrons produced in fission are prompt, emitted essentially instantaneously.
The prompt neutron lifetime in a CANDU 6 is 0.92 ms, a factor of 63 times longer than that in LWRs.
A very small fraction (less than 1%) of the neutrons from fission appear much later.
These delayed neutrons are produced by direct neutron decay of some of the fission products.
The delayed neutron precursors have half lives ranging from 0.2 sec to 50 sec.
In CANDU reactors there are additional delayed neutrons, produced by the photo disintegration of the deuterium in heavy water.
These photoneutrons have even longer time constants than the FP neutrons, ranging from tens to hundreds of thousands of seconds.
These delayed neutron and photoneutron time constants are such that, in spite of the small delayed fraction, the “effective” (weighted average) mean generation time will be much longer than the prompt neutron lifetime.
“For reactivity insertions at or near prompt critical, the CANDU lattice with the long neutron lifetime does not experience a drastic reduction in reactor period (time needed for an e fold increase in neutron population) and hence does not experience a sudden increase in power excursion rate, such as that experienced by reactors with much shorter neutron lifetimes.” (see attached figure)
The typical effective fuel temperature in a CANDU 6 reactor from a neutronic point of view is about 690°C for NU fuel.


Attachments:
Reactor Period vs. Reactivity for Various Prompt Neutron Lifetimes.jpg
Reactor Period vs. Reactivity for Various Prompt Neutron Lifetimes.jpg [ 67.95 KiB | Viewed 959 times ]
Top
 Profile  
 
PostPosted: Mar 18, 2010 10:19 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1056
This gentle and well behaved CANDU nature is certainly reassuring especially considering in the latter stages of its burn cycle it is consuming a large faction of its residual bred fissile; plutonium … known as a wild and frisky nuclear performer.

When this gentle and benign technology is adapted to burn only U233, one of the mellowest of nuclear fuels … the marriage will be exceedingly most tranquil and well behaved.

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


Top
 Profile  
 
PostPosted: Mar 19, 2010 4:35 am 
Offline

Joined: Apr 19, 2008 1:06 am
Posts: 2246
Indians tried out the Calandria configuration with thorium and light water coolant for the AHWR. They found that Th-U233 fuel cycle results in a positive void coefficient. Then they added some Th-Pu pins in every bundle. The net result- a net conversion of 60%, self sufficiency in U233 and additional Pu added at every cycle.
It does not seem likely that we can use all the U238 or thorium without a fast spectrum. Indians are busy optimising the fast U-Pu cycle by using metallic fuel before taking up thorium fuel on an industrial scale.
A better conversion ratio can be obtained in a thermal cycle only by changing to the thorium fuel in the PHWR. Shppingport Light Water Breeder experiment is too complecated for replication on an industrial scale.


Top
 Profile  
 
PostPosted: Mar 23, 2010 8:56 pm 
Offline
User avatar

Joined: Nov 30, 2006 9:18 pm
Posts: 1946
Location: Montreal
Lars wrote:
There are some areas of concern that I would pursue if it were my favorite design. Namely, a better understanding of ......the thermal coefficient of reactivity .....if there are no neutrons captured in the resonance region

A colleague has noted an interesting trend in the development of CANDU reactors, that has a bearing on this issue.
In the early days, small prototype CANDU-style reactors had fuel bundles with just seven fat fuel pins, instead of the current 37 pin standard with thinner pins (both having the same 4" overall OD).
The thinner pins have more D2O coolant, which results in more neutron moderation and resonance absorbtion in the fuel.
Consequently, when the fuel channel experiences coolant voiding (as in overheating), the reactivity goes up slightly because of reduced resonance absorbtion.
By the same token, the fat fuel pin version has little resonance absorbtion to begin with, so coolant voiding has a neutral or even slightly negative reactivity effect (depending also on lattice spacing).
Of course with solid fuel, fat pins are impractical, due to heat transfer limitations.
No such limitation with molten salt fuel.


Top
 Profile  
 
PostPosted: May 03, 2010 5:47 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1056
Delayed neutrons make it possible to leave the reactor in a subcritical state at least as far as only prompt neutrons are concerned: the delayed neutrons come a shat time later, just in time to sustain the chain reaction when it is going to die out. In the timeframe of delayed neutron production, neutron production overall is amplified greatly by the small fraction of the delayed neutrons produced, either increased or lessened by this negative feedback loop mechanism. Thus, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to inherent reactor safety and even in reactors requiring active control. The bigger that this delayed neutron fraction is, the more controllable and docile the reactor becomes.

Doing the operation of the first MSRE, with all other factors held constant except the fuel used; U233 or U235, the delayed neutron fraction characterized by U233 was found to be surprisingly low in both absolute terms and in relation to U235.

To quantify specifically .0019 (reference ORNL 4812) for U233 as opposed to .0064 for U235 (reference wikipedia). That is just under 4 times different.

This is the great mystery for me. How can two isotopes that differ by a scant 2 neutrons produce delayed neutron precursor isotopes upon fission that bare so different in their delayed neutron production profile.

With the disclaimer that there is some sort of comparison error that I don’t yet see, my uninformed opinion is that there must be some inherent characteristic between the two isotopes themselves that leads to this wide difference in delayed neutron production capacity and not solely in the inventory of associated delayed neutron precursors.

The people that wrote ORNL 4812 were also greatly surprised and mystified. They did not offer an explanation; they just accepted it as a fact of nature and simply added this behavior to their mathematical model.

ORNL 4812 page 40: “…Delayed neutron fraction much lower than with the 235 fuel”

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


Top
 Profile  
 
PostPosted: May 03, 2010 8:37 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
Axil wrote:
Doing the operation of the first MSRE, with all other factors held constant except the fuel used; U233 or U235, the delayed neutron fraction characterized by U233 was found to be surprisingly low in both absolute terms and in relation to U235.

To quantify specifically .0019 (reference ORNL 4812) for U233 as opposed to .0064 for U235 (reference wikipedia). That is just under 4 times different.

This is the great mystery for me. How can two isotopes that differ by a scant 2 neutrons produce delayed neutron precursor isotopes upon fission that bare so different in their delayed neutron production profile.

With the disclaimer that there is some sort of comparison error that I don’t yet see, my uninformed opinion is that there must be some inherent characteristic between the two isotopes themselves that leads to this wide difference in delayed neutron production capacity and not solely in the inventory of associated delayed neutron precursors.

The people that wrote ORNL 4812 were also greatly surprised and mystified. They did not offer an explanation; they just accepted it as a fact of nature and simply added this behavior to their mathematical model.

ORNL 4812 page 40: “…Delayed neutron fraction much lower than with the 235 fuel”


I see on page 40 that the "233U zero-power experiments and dynamics tests confirmed the
predicted neutronic characteristics." Then in the footnote that
"The 233U critical concentration was lower, the reactivity coefficients larger, and the delayed neutron fraction much lower than with the 235U fuel."

There are a very few fission products that produce delayed neutrons. The quantity of delayed neutrons depends on the fission yield of these fission products. Nothing is mysterious about this. Where do you get the impression that ORNL was greatly surprised and mystified?


Control of any feedback system depends on the control path being faster and having a larger range than the item it is controlling. Certainly, the delayed neutrons are slower and allow for slower control mechanisms. With a epi spectrum and thorium in the fuel the doppler feedback mechanism is very fast (pico seconds) and has sufficient range for most disturbances. The French analysis shows they can lose up to half of the delayed neutrons and remain stable even in the face of extreme disturbances.


Top
 Profile  
 
PostPosted: May 03, 2010 11:48 pm 
Offline
User avatar

Joined: Aug 21, 2008 12:57 pm
Posts: 1056
Quote:
Where do you get the impression that ORNL was greatly surprised and mystified?

On page 83, “the Circulating-fuel reactors especially if operated on the thorium fuel cycle have an unusually small delayed neutron fraction.”

Something that is “unusual” connotes …surprising, unexpected, astonishing, weird, bizarre, or uncommon.

ORNL 4812 page 85: With regards to reactor dynamics…”We have not found it very useful to try to formulate simple generalized statements about the dynamic behavior of these underlying characteristics. Instead, we have tried to develop reliable computational models for predicting behavior…”

ORNL was long on observation and short on theory as far as predicting behavior is concerned, a wise approach.

Quote:
The French analysis shows they can lose up to half of the delayed neutrons and remain stable even in the face of extreme disturbances.
.

I prefer the experimentalist to the theoretician as the ultimate agents of truth.

At the end of the day, analysis is just an opinion. You can never be sure if theory is right until it is tested against reality.

Many analysts look at experimental results with surprise and wonder saying “that’s unusual!” ; …like unexpected.

Quote:
There are a very few fission products that produce delayed neutrons. The quantity of delayed neutrons depends on the fission yield of these fission products. Nothing is mysterious about this.


Image

These fission products look very similar to me in both type and magnitude…almost identical. I can’t yet see how such similar delayed neutron precursor profiles produce a factor of four difference in product. I guess the devil is somewhere in the details. I don’t say it wrong; I just say I can’t see it. Let’s get into the details.

After all, there is only a two neutron difference between U233 and U235.

Quote:
I see on page 40 that the "233U zero-power experiments and dynamics tests confirmed the
predicted neutronic characteristics."


As far as delayed neutrons go, the ORNL 4812 number for the delay neutron fraction (.0019) disagrees with all the modern values for the U233 modern delayed neutron fractions (.0026 give or take) that I have seen. Could it be that ORNL was wrong in both their predictive analysis as well as their observations? Or maybe isotopic purity makes a big difference here?

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


Top
 Profile  
 
PostPosted: May 04, 2010 1:33 am 
Offline

Joined: Oct 29, 2007 6:27 pm
Posts: 277
Pu-239 has a smaller delayed neutron fraction in comparison with U-235. Although the fission product data looks the same (for which you kinda use the wrong graph, but let's not go too much in detail - since I've also got other things to do). The same is true for U-233....It's all related to unstable fission products close to the neutron emission lines of stability on the nuclide chart...It's that simple! Otherwise those data would be completely different for solid fuels. Of course for reactor theory, this isn't relevant...You want to capture and model the effect, which is probably what they did...Furthermore, it's unlikely they knew all the characteristics of U-233 as we do today.

_________________
Liking All Nuclear Systems, But Looking At Them Through Dark And Critical Glasses.


Top
 Profile  
 
PostPosted: May 04, 2010 2:56 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3063
Axil wrote:
Quote:
Where do you get the impression that ORNL was greatly surprised and mystified?

On page 83, “the Circulating-fuel reactors especially if operated on the thorium fuel cycle have an unusually small delayed neutron fraction.”

Something that is “unusual” connotes …surprising, unexpected, astonishing, weird, bizarre, or uncommon.

...

As far as delayed neutrons go, the ORNL 4812 number for the delay neutron fraction (.0019) disagrees with all the modern values for the U233 modern delayed neutron fractions (.0026 give or take) that I have seen. Could it be that ORNL was wrong in both their predictive analysis as well as their observations? Or maybe isotopic purity makes a big difference here?


I read unusual in this context as different than the norm that the reader is expecting (namely delay neutron fractions from 235u).

In ORNL4191 written before they had started with 233U they predicted a delayed neutron fraction of 0.0026 and an effective delayed neutron fraction of 0.0017 (page 59). They did analyze and predict pretty well just how the reactor would behave (especially given the lack of computing resources they had). The measured effective delayed neutron fraction was 0.0019 (ORNL 5018 pg 12-9). The difference is that approximately 1/3 of the delayed neutrons are emitted while the fuel salt is outside the core - hence an effective value that is approximately 2/3rds of the full value. I see no major surprise or discrepancy here.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 24 posts ]  Go to page Previous  1, 2

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