Neutron to start the chain reaction

[theCandyman]

I don't think so. I had to use this equation some in my fundamentals class. The one we used was time independent and for monoenergetic neutrons. It didn't strike me as anything difficult, but I never REALLY understood what it was I was looking at after getting the answer.

As for the boundary conditions, it depends on the geometry. If I remember correctly, sphereical geometry was the easiest with which to deal.

 

[curie]

Thanks for the enlightenment on your questions Morbius - gets the old grey matter churning!

I must say I have never had an appreciation of the concept of k, criticality, etc in the purely mathematically derived sense, although talking around the subject here I can see it. I have some brief reactor theory academic training & interact with masters students study reactor engineering & I am confident that none of these students over the last couple of years will have a clue about this level of the theory. But on the other side, the practical side, they can be equally clueless, not having done any lab work since their first undergraduate year, eg no obvious appreciation of the basic concept as to why one would take more than one reading of a varying parameter & compute an average, not being able to interpret a log scale to take a reading from a chart, etc

Onwards from student level, I don't think it is necessarily a problem - broadly speaking there are & always will be operational types & more theoretical types of person as demanded by the world beyond the teaching stage. Ultimately the most theorectical & academic mathematically-based understanding is the highest form and will always be "correct", correct meaning adhering to the most accepted advanced theory. In other different (& thus it could be said lower) levels of understanding, some concepts may be "understood in a different way" & are actually incorrect, but nevertheless they are generally adequate for the model they are applied to, eg having a full understanding of reactor physics theory as opposed to having a more operationally-based grasp. The fullest level of understanding will always work downwards, ie it can be applied to all models & situations, but if you are looking upwards from a different level of understanding, this can result in a problem. I see this often at plant nuclear/reactor safety meetings, where the committee is a mix of both types of people. Ideally it should work collaboratively as a team, getting the best out of all but all too often it ends up in academic vs operational type debates, sometimes this results in hopelessly over-complex & impracticable measures being imposed with little or no operational benefit. On no account though, should disrespect for any viewpoints be allowed to cloud the issues - this is the science & technology field, not politics! So back to increasing our understanding, where it is helpful to discuss not only the "purist" understanding but how it can be understood where it applies in the more operational setting.

 

[Morbius]

i see.

does one go about solving this equation iteratively (much like a self-consistent field equation - i assume the actually boundary conditions can be complicated) as opposed to an exact matrix inversion?

quetzalcoatl9,

Yes - the solution is solved iteratively.

It actually requires a "nested iteration" - there's an "inner iteration" and an "outer iteration".

When you solve the equation, you start out not knowing either the flux, phi; nor the
eigenvalue "k" that you are trying to solve for.

Therefore, there is an "outer iteration" that converges on the eigenvalue "k". That
iteration takes the form, typically as a "power iteration"; which you can look up in any
text on iterative methods.

Within each outer iteration, the value of "k" is fixed; but you still have an iterative problem
solving for the value of the neutron flux as a function of space and energy.

As for boundary conditions; "vacuum" boundary conditions are the norm. That is at
the edge of your problem - at the reactor boundary, or beyond the shield if that is included
[ which it should be because it can scatter neutrons back to the reactor ]; you impose
a boundary condition that says there are no incoming neutrons.

Dr. Gregory Greenman
Physicist

 

[Morbius]

Ultimately the most theorectical & academic mathematically-based understanding is the highest form and will always be "correct", correct meaning adhering to the most accepted advanced theory. In other different (& thus it could be said lower) levels of understanding, some concepts may be "understood in a different way" & are actually incorrect, but nevertheless they are generally adequate for the model they are applied to, eg having a full understanding of reactor physics theory as opposed to having a more operationally-based grasp. The fullest level of understanding will always work downwards, ie it can be applied to all models & situations, but if you are looking upwards from a different level of understanding, this can result in a problem.

curie,

I agree with your summary above. The fullest and most complete understanding is the
scientific and mathematical understanding. I also see that one can have a "working
understanding" that serves a more limited purpose. That, I believe; is where Emfuser
is.

That's why I analogized his understanding to a musician that only plays in the key of
C-Major. You can make a lot of music in the key of C-Major, and if that's what you
play in; fine, so be it.

However, it is limiting. There's a whole body of music and music theory that goes
beyond the key of C-Major. So Emfuser tellinig me that a device can't be super-critical
unless it has neutrons in it; is like a "musician" telling a composer that there's no note
between "F" and "G". The composer counters with, of course there is; "F#".

Then the "musician" comes back and says that "in the real world" nobody uses "F#".
That's just something to write on the chalkboard. Nobody out making music and selling
records uses "F#".

Obviously, there's a very good use for "F#", and all the other notes that are not in the
key of C-Major.

If this were a forum about Music; one wouldn't want the contention to stand unchallenged
that there was no need for "F#" and the other notes beyond the key of C-Major.

I think one's understanding should be grounded in a complete understanding of the
subject. I hear too many engineers that just "plug and chug"; they plug values into some
formula they learned in school, without an understanding of what the formula really means.

There are many cases where someone used the wrong formula; or forget that the formula
is an approximation and forget under what conditions the approximation is valid.

I recall NASA had some trouble a while back where they lost [ crashed ] a mission
because someone used an approximation that wasn't valid. I would sure hate to have
something like that happen in the nuclear field.

Dr. Gregory Greenman
Physicist

 

[theCandyman]

It actually requires a "nested iteration" - there's an "inner iteration" and an "outer iteration".

When you solve the equation, you start out not knowing either the flux, phi; nor the
eigenvalue "k" that you are trying to solve for.

Isn't flux a given measurement?

 

[Astronuc]

Isn't flux a given measurement?

What is the definition of flux? The number of neutrons passing through a unit area per unit time (n/cm²s), and it can vary between 0 (never negative) and whatever value can be achieved in a transient before the fuel and reactor geometry become disrupted.

At zero power, the neutron flux is due to whatever spontaeous fissions of certain transuranics and the few fissions they induce, which could be on the order of 10,000 - 100,000 n/cm²s, as compared to full power where the flux is on the order of 10¹³ n/cm²s.

The objective in having a startup source is to provide the neutron detectors with a strong enough signal to monitor the criticality of the core. In subcritical configuration, the neutron activity is a function of the source strength. When a reactor has gone through some cycles, and some of the fuel has achieved some reasonable level of burnup to where there is sufficient transuranics, a sourceless startup might be possible. If a reactor has incore detectors (IIRC all commercial plants were built initally with ex-core detectors), it is more likely that one could do a sourceless startup.

As for targeting particular eigenvalues, one can only expect the same eigenvalue in a subsequent startup if all initial conditions are the same, i.e. the core configuration is more or less in equilibrium, which means the same core average burnup (efpd), same power history, same burnup distribution, same feed batch size and enrichment, same burnable absorber content and distribution, same temperature spatial distribution. In reality, this never never happens.

As for reactor design, in a sense doing a core design is much like reactor design, but with many parameters already determined, e.g. the number of assemblies and their geometry (lattice), coolant temperature, control element configuration. Some of the key variables in a core design include energy, batch size, enrichment and distribution, and burnable absorber (either discrete or integral) and distribution. A core designer however does not go back and re-write the physics of the lattice code (CASMO, Phoenix, WIMS, TGBLA) or core simulator (SIMULATE, ANC, Panther, POLCA, MIRCOBURN, PANACEA).

Even today, the nuclear physics codes are still tweaked, particularly with respect to handling the presence of burnable absorber adjacent to guide tubes and water rods where the moderation is more significant.

Presently, ANL is conducting a program - National Numerical Reactor - which uses multigroup transport theory to do a core simulation. This has been coupled with a sophisticated CFD code which calculates the coolant properties, which then provides the input for the moderation of the core.

 

[quetzalcoatl9]

so let me see if i got this right..(bear with me, as I'm just a lowly chemist :) )

1) Without the ability to detect the neutron current density, operators would essentially have no idea what is going on in the reactor. Other detection means, such as coolant temperature, gamma radiation, etc. are all indirect methods and are insufficient to completely guarantee safe startup of the core.

2) Neutron detection, like any detection means, may have significant error in detecting weak signals (I assume that He-3 is used in detection, like in INS studies?)

3) New reactors need startup sources to initiate fission. Older reactor components, having been irradiated and are currently in some decay process, can become natural neutron emitters and hence it may be possible to startup the fission cycle without a startup source.

4) HOWEVER...these natural emitters in the core emit low levels of neutrons and so the neutron detectors may be partial "blind" as to the true neutron flux in the reactor - and so a critical (or worse, supercritical) configuration can be achieved, at least momentarily (until the neutron flux starts to rapidly shoot up, i presume). The operator is going completely on "faith" at this point.

In analogy, if I have a gas oven i may a) start up a pilot flame (neutrons) and b) turn on the gas (fission fuel) in order to get more flame (neutrons). That would be safe operation. Alternatively, I could just turn on the gas and then light the flame. But since I really have no way of detecting the gas, it could be possible that the entire room has filled with gas prior to ignition, at which point the room+gas non-equilibrium state will proceed to follow the potential energy surface down to a new energy minimum

5) Therefore, startup of an older reactor without an ignition source puts the general public at risk needlessly.

Since it seems operators are willing to startup with such risk, I assume changing out the source must be a hassle?

 

[Astronuc]

Remember, the initial question was reactor startup, and startup of the first core (all fresh fuel) definitely requires a source of neutrons other than those from fission. That source is placed in the reactor in order to induce a 'detectable' signal so that the state of the core IS known.

See slide 32 of
http://www.energy.kth.se/courses/4A1627/Material2005/PhysicsPart/05%20Reactor%20Kinetics%20and%20Operation%20Rev%200.pdf

See also -

When a reactor is started up with unirradiated fuel, or on those occasions when the reactor is restarted following a long shutdown period, the source neutron population will be very low. In some reactors, the neutron population is frequently low enough that it cannot be detected by the nuclear instrumentation during the approach to criticality. Installed neutron sources, such as those discussed in Module 2, are frequently used to provide a safe, easily monitored reactor startup. The neutron source, together with the subcritical multiplication process, provides a sufficiently large neutron population to allow monitoring by the nuclear instruments throughout the startup procedure. Without the installed source, it may be possible to withdraw the control rods to the point of criticality, and then continue withdrawal without detecting criticality because the reactor goes critical below the indicating range. Continued withdrawal of control rods at this point could cause reactor power to rise at an uncontrollable rate before neutron level first becomes visible on the nuclear instruments.

http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v2.pdf - page 116 of 128
See also - http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v1.pdf

12.2.1.2.9.1 Reactor Startup Source
The reactor startup source is shipped to the site in a special cask designed with shielding. The source is transferred under water while in the cask and loaded into beryllium containers. This is then loaded into the reactor while remaining under water. The source remains within the reactor for its lifetime. Thus, no unique shielding requirements are required after reactor operation.

Lungmen PSAR, Chapter 12

Neutron detectors work on the basis of activation (not with He-3, but something like Rh or Co) or fission (e.g. fission detectors). See page 76 of -

http://www.hss.energy.gov/NuclearSafety/techstds/standard/hdbk1013/h1013v2.pdf

Startup sources usually stay in the reactor, unless the operating utility applies for a licensing amendment to remove them and do a sourceless startup.

3) . . . Older reactor components, having been irradiated and are currently in some decay process, can become natural neutron emitters . . .

'Older reactor components' should be changed to 'older fuel' or 'irradiated fuel', or 'high burnup fuel', which is typically loaded toward the periphery of the core. In a PWR, the startup sources are placed in assemblies toward the core periphery and near the ex-core detectors.

Some (many or most ?) plants employ in-core detectors in addition to ex-core detectors. I would imagine that this would facilitate sourceless startups.

4) HOWEVER...these natural emitters in the core emit low levels of neutrons and so the neutron detectors may be partial "blind" as to the true neutron flux in the reactor - and so a critical (or worse, supercritical) configuration can be achieved, at least momentarily (until the neutron flux starts to rapidly shoot up, i presume). The operator is going completely on "faith" at this point.

NO! A fresh core has no transuranics, unless it is loaded with MOX (with Pu isotopes). After one cycle of operation, part of the core is replaced with fresh fuel, with the one-cycle (once-burned) fuel returned. After the second cycle, another batch of fresh fuel is added, with the remainder being a mix of once- and twice burned (one and two cycles of operation) being coresident. The burnup is still too low to be effective for use of ex-core detection.

Older reactors might have some high burnup assemblies with sufficient spontaneous fissions to permit a sourceless startup. In that case, the spontaneous fisson rate must provide comparable strength to that of the startup source.

5) Therefore, startup of an older reactor without an ignition source puts the general public at risk needlessly.

Operators follow procedures designed to prevent putting the public at risk! Failure to follow procedure can lead to discharge, and if deliberate, will lead to arrest.

As for whether or not sourceless startups have been performed, perhaps Emfuser knows, or I could check with reactor engineers whom I know.

 

[Emfuser]

WRONG! I have worked on design and design methods for REAL devices; real
nuclear reactors, and other real nuclear systems.

These reactors and systems have been BUILT and TESTED!

NO - you can't paint me as someone who is in an ivory tower somewhere.

My experience is AT LEAST as "real" as yours!

For somebody claiming not to be in an ivory tower somewhere, you most certainly talk like you are in one. The fact still remains that I work in nuclear power, you work in research. I work on reactor engineering and core design at a nuclear power plant, you work at a lab. I have tried to make it abundantly obvious that, while we share some similar nuclear background, that we operate in different working worlds

WRONG! My thesis supervisor was the late Prof. Allan F. Henry - who used to be
the head of codes and methods development at Westinghouse. Prof. Henry was a
stickler for the proper use of these terms. In his textbook, which is one of the seminal
texts in nuclear reactor theory; Prof Henry made a point of the fact that "critical" is a
property of the geometry and materials, and does NOT mean neutrons!

And you are WRONG! It is those "in the industry" that have BASTARDIZED the term;
and use it incorrectly.

Do you have a key on your keyboard, or perhaps a keyboard shortcut that generates the string "WRONG!"? You seem to enjoy condescending just a wee bit.

It must drive you absolutely bonkers that the industry, and even some nuke engineering textbooks use "critical geometry" and "critical mass". No matter how much you want to stonewall and beat on a strawman, a rose, by any name, is still a rose. Like it or not, we're talking about the same thing, just from different perspectives. Just because we use different language does not make it "WRONG!", as you so put, just different. The entirity of the rest of your post continues to beat on the "Emfuser doesn't understand, because he doesn't use "criticality" the same way that I do!" strawman.

No matter how much you talk down my different colloquial language about criticality, it doesn't mean it's "WRONG!". I'm sorry that it's so bothersome to you. We're actually talking about the same thing.

I note that you sidestepped my challenge to answer the questions I posed in my last
post. It would have been illustrative for you to have attempted them.

Sorry, but I did not have the time last Thursday. Your questions are nuke engineering 101 style questions. I can answer them if you want, but you might blow a gasket when I use "critical mass" & "critical geometry", followed by usage of "critical" in terms of neutron population change.

Now suppose I introduce a single neutron heading into the assembly. [ This is the type
of problem nuclear weapons designers have to consider.] The interactions of neutrons
with the material is stochastic - that is it is probabilistic - NOT deterministic. There is
a finite probability that the neutron thus introduced will be absorbed without starting a
chain reaction. That's because the radiative capture cross-section of the Uranium is
finite.

All we can say is that there is a certain probability that the neutron so introduced will
lead to a chain reaction - it is not guaranteed. So how do we determine that probability?

That probability is a VERY REAL physical property of the assembled nuclear system.

It turns out - that the probability is a function of the "k"; or "criticality" of the assembled
system - something Emfuser claims doesn't exist because there are no neutrons.

There's a whole inter-related body of physics and knowledge here that requires that
"k" and "criticality" exist as a function of the geometry and materials independent of
whether there are neutrons present or not.

Emfuser, you are analogous to the child that is playing a toy piano wherein all the black
keys are just painted on. The only notes you have at your disposal are those in the key
of C-Major. The problem is you don't see any use for the other notes.

Well - there is a WHOLE WORLD of music and music theory beyond the key of C-Major.

There's the key of D, E,...as well as the existence of "minor keys", and "diminished keys"
... ALL of which you see no use for because you are only familar with the notes in the
key of C-Major.

Then you would have the audacity to tell Mozart that he doesn't need any notes beyond
those of C-Major; when Mozart has actually composed symphonies, that you HAVEN'T!

*yawn*
How's that ivory tower?

This is much more analagous to me speaking Italian while you speak Latin. Sure, I learned to speak Latin. I understand it and recognize that it's still spoken by some, and has its uses, but I'm speaking Italian.

I doubt that you have done say a reactor noise analysis? Have you ACTUALLY done
reactor noise analysis?

We usually don't do them unless we have reason to. It's a collaborative between nuclear design and analysis (my group) and the reactor internals system engineeer.

OK - you're a startup engineer. Did you DESIGN the reactor?

A friend of mine was a startup engineer. I'm familiar with the job. You've essentially
been given the reactor - you DIDN'T design it. You didn't deal with all the physics that
require the mathematics and concepts that I'm talking about.

No, I'm not just a startup engineer. That's just a portion of my job. Yes, I participate in the core design we do every cycle.

If you do go study at a nuclear engineering grad program; you would find out that
"criticality" is a property of the geometry and materials.

Well... critical mass & critical geometry sure are fun, aren't they? I'm quite amused that the commercial operating usage of "critical" bothers you so much.

You still sure that I don't know what I'm talking about over here? Like I said before, that reactor is good and hot, sitting at a nice effective critical state. That turbine is still spinning, those electrons are still moving. I guess I'm not as "WRONG!" as you say I am, regardless of how much furious howling you want to do over what is little more than different perspectives.

Oh and no, we still don't require a source on hand at all times for subcritical operation.

 

[Emfuser]

Remember, the initial question was reactor startup, and startup of the first core (all fresh fuel) definitely requires a source of neutrons other than those from fission. That source is placed in the reactor in order to induce a 'detectable' signal so that the state of the core IS known.

Correct

Only on that first core, of completely fresh Uranium, do we need a source for startup. After all, we're not going critical without any neutrons. (waits for Morbius to come in fuming over my language )

Startup sources usually stay in the reactor, unless the operating utility applies for a licensing amendment to remove them and do a sourceless startup.

We can yank them after the first cycle, but they usually sit there until the utility wants to analyze to remove them. They are an insert for the fuel assemblies, and quite easily removed. The burnt fuel from the cycle provides ample neutrons for subsequent startups.

Some (many or most ?) plants employ in-core detectors in addition to ex-core detectors. I would imagine that this would facilitate sourceless startups.

We all have incores. Whether or not they are fixed or movable will vary by NSSS design. They have no use as sources during startups. They are used to determine core behavior during power ascension and periodically at power.

Older reactors might have some high burnup assemblies with sufficient spontaneous fissions to permit a sourceless startup. In that case, the spontaneous fisson rate must provide comparable strength to that of the startup source.

One, two cycles at the most before sources can be pulled.

Operators follow procedures designed to prevent putting the public at risk! Failure to follow procedure can lead to discharge, and if deliberate, will lead to arrest.

As for whether or not sourceless startups have been performed, perhaps Emfuser knows, or I could check with reactor engineers whom I know.

Failure to follow a procedure carries a penalty range anywhere from reprimand to termination or even legal action by the NRC. That's right.

Startups without secondary sources are quite the norm.

 

[Emfuser]

In analogy, if I have a gas oven i may a) start up a pilot flame (neutrons) and b) turn on the gas (fission fuel) in order to get more flame (neutrons). That would be safe operation. Alternatively, I could just turn on the gas and then light the flame. But since I really have no way of detecting the gas, it could be possible that the entire room has filled with gas prior to ignition, at which point the room+gas non-equilibrium state will proceed to follow the potential energy surface down to a new energy minimum

Be careful with your analogies. They might be declared as "WRONG!", and you'll be sent to nuclear jail! No lay person or passerby should be able to understand this stuff!

 

[Morbius]

For somebody claiming not to be in an ivory tower somewhere, you most certainly talk like you are in one. The fact still remains that I work in nuclear power, you work in research. I work on reactor engineering and core design at a nuclear power plant, you work at a lab.

Emfuser,

I may work at a lab - but it is a lab that does "real world" design and engineering - not
the "ivory tower" type of analysis that you ineptly tried to paint my work as being.

It must drive you absolutely bonkers that the industry, and even some nuke engineering textbooks use "critical geometry" and "critical mass".

Again you are confusing terms. One can have a "critical mass" in a system, and not
have the system be "critical". You are being sloppy in your terminology.

"Critical Mass" and "Criticality" are NOT synonomous!

For example, does a nuclear weapon contain therein a "critical mass" of special nuclear material? YES.
Does it contain a "super-critical mass" of special nuclear material? YES.
Is the warhead, as it is sitting on the missile inside the submarine "critical", or "super-critical"? NO - it is sub-critical.
It has a"super-critical mass" of material; however, it is "sub-critical".

The terms "critical mass" and "critical" are NOT synonomous!

No matter how much you want to stonewall and beat on a strawman, a rose, by any name, is still a rose. Like it or not, we're talking about the same thing, just from different perspectives. Just because we use different language does not make it "WRONG!", as you so put, just different. The entirity of the rest of your post continues to beat on the "Emfuser doesn't understand, because he doesn't use "criticality" the same way that I do!" strawman.

No - it isn't a strawman. For the designs we do at the lab - we operate in the super-critical
realm. As I stated in my previous posts; there are VERY REAL physical properties of a
super-critical assembly that doesn't contain neutrons - for example the probability distribution
that a neutron injected at a give position, with a given energy, and in a given direction;
will produce a chain reaction. That property is a function of the criticality of the
super-critical system.

However, YOU claim that super-criticality doesn't exist for such a system unless it has
neutrons in it. If that were the case, then the above property would be undefined. But
it IS defined - because super-criticality and "k" are properties of the system - NOT
properties of the neutrons.

The best analogy is the underdamped / overdamped / critically damped car suspension.
That is a property of the SUSPENSION and its spring constants, viscous damping
coefficients, and masses - NOT the motion of the car. The property exists even if the
suspension isn't moving.

No matter how much you talk down my different colloquial language about criticality, it doesn't mean it's "WRONG!". I'm sorry that it's so bothersome to you. We're actually talking about the same thing.

No - there's a fundamental difference. You think that the criticality and the eigenvalue
are properties of the neutron distribution - and not the geometry and materials and that
is just plain "WRONG". If you were to do graduate study in nuclear engineering - and
you said the things you said on this board, your professors would tell you that you are
"WRONG".

Perhaps you could address the concept of "neutron importance". Do you need to have
neutrons in the reactor for the neutron importance distribution to exist?

Sorry, but I did not have the time last Thursday. Your questions are nuke engineering 101 style questions.

Yes - they were that type of questions - and you would have FLUNKED based on your
previous and current statements.

We usually don't do them unless we have reason to. It's a collaborative between nuclear design and analysis (my group) and the reactor internals system engineeer.

So how do you do such an analysis with a flawed concept of criticality.

No, I'm not just a startup engineer. That's just a portion of my job. Yes, I participate in the core design we do every cycle.

Just as long as someone with a better understanding of the job checks your work; I'll
have to be OK with that.

Well... critical mass & critical geometry sure are fun, aren't they? I'm quite amused that the commercial operating usage of "critical" bothers you so much.

I just surprised that you would haven't been better trained.

You still sure that I don't know what I'm talking about over here? Like I said before, that reactor is good and hot, sitting at a nice effective critical state. That turbine is still spinning, those electrons are still moving. I guess I'm not as "WRONG!" as you say I am, regardless of how much furious howling you want to do over what is little more than different perspectives.

In the limit of the reactor being critical and full of neutrons; then your definition of criticality
works.

However, if you consider a super-critical system without neutrons - like an assembled
nuclear weapon - then you are WRONG - because your definitions require neutrons, and
the concepts and properties exist independent of the presence of neutrons.

How would you characterize an assembly - like the assembled core of "Little Boy" that
I used as an example before - without neutrons. Is that core "super-critical" even though
it doesn't have neutrons in it? If not "super-critical", then is it "sub-critical" or "critical".

Here is a physically realistic system - not some "chalk board" hypothetical system; but
a REAL system that existed. You tell me, in your vernacular; is the case of the assembled
Little Boy before neutrons are present - "sub-critical", "critical", or "super-critical"?

Dr. Gregory Greenman
Physicist

 

[Morbius]

Only on that first core, of completely fresh Uranium, do we need a source for startup. After all, we're not going critical without any neutrons. (waits for Morbius to come in fuming over my language

Emfuser,

The WHOLE REASON for having that source is that YOU CAN go critical without any
neutrons.

It is possible to take the reactor into the critical or even super-critical regime without
neutrons. However, if you don't have any neutrons in the system; then your instrumentation
doesn't indicate that you are in the critical or super-critical regime.

Without an indication by the instuments - you could keep pulling rods when the system
was already super-critical. Then some cosmic rays generate a burst of neutrons for you,
and you have suddenly gone from a super-critical reactor without neutrons to a
super-critical reactor with neutrons that are rapidly proliferating.

It is exactly THAT scenario that the presence of the source is meant to forestall.

It is this very scenario that we are trying to prevent - a super-critical reactor without
neutrons - because the instrumentation relies on neutrons, you don't know it.

Dr Gregory Greenman
Physicist

 

[quetzalcoatl9]

It is this very scenario that we are trying to prevent - a super-critical reactor without
neutrons - because the instrumentation relies on neutrons, you don't know it.

wait...so was my understanding that - startup without a source is dangerous - actually correct or not?

Morbius, is sourceless startup in an older reactor ok?

 

[Morbius]

wait...so was my understanding that - startup without a source is dangerous - actually correct or not?

Morbius, is sourceless startup in an older reactor ok?

quetzalcoatl9,

Yes - in an older reactor - or more precisely a reactor with "old" fuel; then you don't need
a source because of the presence of fission products that provide a background source
of neutrons.

What you are trying to prevent is the case where there are no fission products present to
provide a source of neutrons; and there is no neutron source to provide the neutrons.

In other words, you have a reactor; but no neutrons running around in it. The problem is
that all the instrumentation for the reactor runs off neutron detectors. If there are no
neutrons to detect; then you don't get any readings on your instruments; regardless of
what the state of the reactor is.

In this case, your instruments would just read ZERO. However, if you are pulling control
rods at the time; you can take the reactor into a critical or super-critical state while the
instruments still read ZERO.

So with the instruments reading ZERO all the time; you don't know when you crossed the
threshhold into having a critical or super-critical reactor. You don't know when to stop
pulling rods.

You can essentially get the reactor into a state that is very super-critical; but without the
neutrons. However, should a "stray" neutron - like those produced by cosmic ray events
or whatever - comes across this very super-critical reactor, you've now suddenly got a
super-critical reactor with neutrons in it that are multiplying rapidly and producing energy.

This sudden unexpected production of energy could be a problem; and be unsafe.

The reactor became "unsafe" when it transitioned into a state where it could support a
self-sustaining, or self-multiplying chain reaction - i.e. became "critical" or "super-critical"
and not when the neutrons showed up.

That's the essence of my disagreement with Emfuser; the reactor became "critical" or
"super-critical" when the conditions for a self-sustaining or self-multiplying chain reaction
were created - NOT when the neutrons "took advantage" of the configuration.

Nuclear engineers define "criticality" and "super-criticality" to be a property of the
materials and geometry of the reactor, bomb, or whatever - NOT a function of the
neutrons. "Criticality" or "super-criticality" tells you what the neutrons would do;
whether or not they are actually present.

So in the example above; the reactor became "unsafe" when it became "critical" or
"super-critical". The consequences of that "unsafe" condition are only realized when
the neutrons show up.

Dr. Gregory Greenman
Physicist

 

[quetzalcoatl9]

thank you, i think i get it.

the definition of criticality seems logical to me.

 

[Emfuser]

Emfuser,

The WHOLE REASON for having that source is that YOU CAN go critical without any
neutrons.

It is possible to take the reactor into the critical or even super-critical regime without
neutrons. However, if you don't have any neutrons in the system; then your instrumentation
doesn't indicate that you are in the critical or super-critical regime.

Without an indication by the instuments - you could keep pulling rods when the system
was already super-critical. Then some cosmic rays generate a burst of neutrons for you,
and you have suddenly gone from a super-critical reactor without neutrons to a
super-critical reactor with neutrons that are rapidly proliferating.

It is exactly THAT scenario that the presence of the source is meant to forestall.

It is this very scenario that we are trying to prevent - a super-critical reactor without
neutrons - because the instrumentation relies on neutrons, you don't know it.

Dr Gregory Greenman
Physicist

(sorry for my long absence from the thread)

We are most definitely in agreement there. For that fresh new core, you absolutely need a source to go critical. I look forward to doing startups on new cores in the future, as new plants go online.

 

[Homer Simpson]

That was awesome! I've got to visit back here more often.

I'm definitely glad the pilot doesn't design the jets we fly on, and I'm sure as hell glad the Phd that designed them doesn't pilot them.

I'm betting the pilots have a lot of "who's the friggin newfie idiot that designed this piece of crap" comments and I'm sure that the designers scoff when (insert Morbius TMI operator slam here) pilot error brings their planes down.

Keep up the entertaining threads!

 

[Morbius]

(insert Morbius TMI operator slam here)

Homer,

My slam / opinion of the operators of TMI comes from attending a seminar by
Prof. Kemeny who led the commission to investigate TMI.

Prof Kemeny stated that the operators of TMI didn't take into consideration the
equation of state of the water coolant. Prof Kemeny stated that when he visited the
TMI control room, he asked for a "steam table" - a book that gives the equation of state
of water - i.e. at what pressures/temperatures is the water a liquid, and when is it a gas;
and the dividing line between them - the saturation line.

It took the TMI operators about a half-hour to find a steam table. This is something they
should have had available right at hand. What good is it for the operator to know the
pressure and temperature of the water coolant if the operator doesn't know where those
conditions are on the water phase diagram? THAT'S what's important - how far is the
PWR in this case to boiling conditions that you want to prevent?

The accident at TMI was reversible, and could have been forestalled; up to the time the
operators made an irreversible error. The operators noted that the main coolant pumps
were vibrating. The reason the pumps were vibrating is that the coolant conditions in the
reactor were at saturation - the coolant was boiling. The pumps were vibrating because
they were pumping a two-phase mixture of liquid water and steam.

The operators were unaware of that - because they had not consulted a steam table to
ascertain the thermodynamic condition of the water coolant. They decided to shutdown
the pumps to avoid damaging them. THAT was when they lost the core!

Perhaps if the operators had asked themselves under what conditions the pumps would
vibrate like that; perhaps they would not have made an irreversible error. It would be
better to risk damage to the pumps; than to let the core melt.

The operators at TMI basically just reacted to circumstances; and never "got out ahead"
of the accident progression. The philosophy of the NRC in the past was that all one had
to do was to present the information to the operators, and the operators would take care
of the situation.

That's no longer the case. The operators are now more limited in what they can / can not
do. The accident at TMI is analogous to what in the aerospace industry is called CFT -
Controlled Flight into Terrain; which means the plane is always under the pilot's control
as he flies it into the ground.

Dr. Gregory Greenman
Physicist

 

[Astronuc]

In fairness to the operators, there were major issues with the utility and vendor management. As a results of the TMI incident, staff was enhanced with several folk, including the shift technical advisor (STA) who understood core neutronic and thermal hydraulics. In addition, operators were give much more extensive training and with better simulators, especially in the areas of dealing with fault modes and accidents.

Interface between vendor (supplier of the NSSS) and utility engineering and operations was improved, and the industry was forced to be more 'proactive'.

Complacency is still an issue, not only with nuclear, but any technology based industry.

 

[curie]

Just adding an operational-based aside here, that will be obvious to most, but may not be for readers that are novices to reactors. Another important reason for having a source is to help determine the neutron detectors are functioning correctly. If they are reading reading zero all the time, there is more chance there could be a problem with the signal than if they are displaying a positvie signal, which would be expected to show fluctuation & which can be tested by moving the source too in some reactors.