Morbius comments. Split from Three Mile Island (survey)

[Morbius]

Originally Posted by NEOClassic

You possibly are unaware that the accident at Chernobyl was with a graphite moderating design that had once been designed in Hanford and ultimately abandoned because of fear that the graphite, being a dense form of charcoal bricks would ignite with high heat.

Jim,

The above statements are untrue.

True, Chernobyl was a graphite moderated design - but that in and of itself
was not the design defect. One CAN make a safe graphite moderated
design - like the HTGR [ High Temperature Graphite Reactor ]. For years,
the Peach Bottom Unit 1 HTGR operated safely in Pennslyvania.

[ Ironically, it's better to operate the graphite reactor at high
temperature. The higher temperature anneals the build up of
"Wigner Energy" - so that it doesn't start a graphite fire as it
did at the Windscale production reactor in Great Britain in 1957

http://www.british-energy.com/media/factfiles/mn_item57.html

]

Additionally, the Chernobyl RBMK reactor was NOT designed in Hanford.

One basic problem with the Chernobyl RBMK reactor design is that it is
a "scaled up" version of a weapons fuel production reactor like the ones
at Hanford. The Russians built their versions of the Hanford reactors.

However, in designing the Chernobyl RBMK reactor - the Russians basically
built a 2 X 2 X 2 stack of their smaller production reactor. However,
they did not redo the nuclear design of the reactor.

When you stack reactors like that - you reduce neutron leakage. The
neutrons that would have leaked out of the reactor - leak into the one
next to it, and vice-versa. Therefore, the combined stack leaks less
than the smaller reactors.

Because of the reduced leakage, the RBMK is "over moderated"- which
makes it unstable. All USA reactors have been "under moderated" and
are required by law to be such - as this makes the reactor stable.

Additionally, the Russians did some rather poor design of the control
rods - like having fissile fuel "followers".

The Chernobyl accident was triggered by an experiment the operators
were performing on the reactor. Because of a delay, this experiment
was performed in the middle of the post-shutdown "Xenon transient"
that all reactors go through. The reactor in that state was highly
unstable, and the operators could only get it to run by bypassing many
safety interlocks. The result is history.

Dr. Gregory Greenman
Physicist LLNL

 

[Morbius]

The press went overboard about the gigantic gas bubble in the water that had been neutron radiated. The Hydrogen in the bubble didn't explode but gradually neutralized the hydroxyl ions that had been left in the surrounding water.

Jim,

This part of your post is also incorrect.

The hydrogen in the bubble was produced mainly by zirconium oxidation.

The fuel elements are clad in a metal coating of zirconium. If you heat
the zirconium to a temperature that is high enough in the presence of
water, it has a greater affinity for the oxygen in the water than the
hydrogen does.

The oxygen in the water reacts with the zirconium to form zirconium oxide
and free hydrogen. Hence there are no hydroxyl ions left in the water.

[ There's no free oxygen either - only free hydrogen - which is why it
can't explode.]

A very small part of the hydrogen was formed by radiolytic decomposition.
In this case, yes - there would be free hydroxl ions. However, as you
point out, the inverse reaction is also permitted and the hydrogen
and the hydroxyl radicals recombine - thus limiting the amount of
free hydrogen and hydroxyl radicals produced this way to a very small
value.

Dr. Gregory Greenman
Physicist LLNL

 

[enigma]

I split these comments from the survey, Morbius. Please keep any discussion of the posts in idonthak's survey here.

Thanks

enigma
-Engineering Mentor

 

[Morbius]

I split these comments from the survey, Morbius. Please keep any discussion of the posts in idonthak's survey here.

Thanks

enigma
-Engineering Mentor

Enigma,

Sorry about the Off Topic posts.

Dr. Gregory Greenman
Physicist LLNL

 

[enigma]

No problem.

Ordinarily, they would have been entirely appropriate. It's just that he was posting a survey, and I didn't want a side conversation to detract from that.

Welcome to the forums, BTW!

 

[theCandyman]

The hydrogen in the bubble was produced mainly by zirconium oxidation.

The fuel elements are clad in a metal coating of zirconium. If you heat
the zirconium to a temperature that is high enough in the presence of
water, it has a greater affinity for the oxygen in the water than the
hydrogen does.

The oxygen in the water reacts with the zirconium to form zirconium oxide
and free hydrogen. Hence there are no hydroxyl ions left in the water.

[ There's no free oxygen either - only free hydrogen - which is why it
can't explode.]

Is this an intended safeguard? Very interesting!
One more thing, why isn't there any free oxygen? Does all of it combine with the zirconium?

 

[Morbius]

Is this an intended safeguard? Very interesting!
One more thing, why isn't there any free oxygen? Does all of it combine with the zirconium?

Candyman,

It's not a safeguard - it's just chemistry. In fact, the design intent is
to minimize the amount of zirconium oxidation.

You've got it correct - the only way hydrogen is made in this reaction is
that the water molecule H20 is split into H2 and O, and the O combines
with the Zirconium.

So there's no free oxygen - the hydrogen that is produced is what is left
after the hot Zirconium grabs the Oxygen atom out of the water molecule.

That's why the hydrogen bubble in the reactor can't explode. Hydrogen
gas is not explosive. A gas that is a mixture of hydrogen and oxygen
CAN explode.

Since there was no free oxygen in the reactor - there was no way that
the hydrogen could explode. [ It took a while for people to realize this ]

Since the relief valve was stuck open - allowing coolant water to gush
into the containment building - some of the hydrogen escaped the
reactor that way - and out into the containment building where there
was plenty of air with free oxygen. There the hydrogen DID EXPLODE!

However, this explosion - outside of the reactor - could not harm the
reactor itself. It also didn't harm the containment building since the
pressure from the hydrogen explosion was far less than what the
containment building was designed to withstand.

So, in the end - the hydrogen bubble was a non-issue as far as the safety
of the public. The public was needlessly scared by the hydrogen bubble
reports.

Dr. Gregory Greenman
Physicist LLNL

 

[CharlesP]

Because of the reduced leakage, the RBMK is "over moderated"- which
makes it unstable. All USA reactors have been "under moderated" and
are required by law to be such - as this makes the reactor stable.

Can you go into some technical depth here (or in a message to me) possibly listing some equations and material properties?

 

[Morbius]

Can you go into some technical depth here (or in a message to me) possibly listing some equations and material properties?

Charles,

I can give you an explanation right here.

There is an optimal amount of moderator that maximizes the reactivity
of a critical assembly. If you plot the reactivity of the system vs. the
amount of moderator - you will get a "hump" shaped curve. Check any text
in nuclear reactor physics.

All USA licensed reactors are required to be under-moderated - that is to
have an amount of moderator that is less that that which gives maximal
reactivity. Hence, they are on the uphill side of the reactivity curve to
the left of the maximum.

When a reactor loses coolant - it also loses moderator. If the reactor
is under-moderated - the decrease in moderator with the loss of coolant
shifts the point on the curve to the left - and since it's on the "uphill"
side to the left of the peak - the shift to the left means the reactivity
goes down. The reactor is self-stabilizing when it is under-moderated.

Contrast this to Chernobyl, which was over-moderated. The operating
point for Chernobyl was to the right of the peak on the "downhill" side
of the curve. When Chernobyl lost coolant, and hence moderator - the
the operating point shifted left which takes it UP in reactivity!

Chernobyl had too much moderator - so when the accident removed
some of the moderator [ the coolant water ] - the reactivity increased!

That, addition to the fact that the operators were operating the reactor
when it was Xenon poisoned following a reduction of power - meant the
RBMK reactor was in an unstable state - and the result is history.

Dr. Gregory Greenman
Physicist

 

[CharlesP]

Charles,

I can give you an explanation right here.

There is an optimal amount of moderator that maximizes the reactivity
of a critical assembly. If you plot the reactivity of the system vs. the
amount of moderator - you will get a "hump" shaped curve. Check any text
in nuclear reactor physics.

Does that mean there is a hump in the cross section vs neutron energy curve?

When a reactor loses coolant - it also loses moderator. If the reactor
is under-moderated - the decrease in moderator with the loss of coolant
shifts the point on the curve to the left - and since it's on the "uphill"
side to the left of the peak - the shift to the left means the reactivity
goes down. The reactor is self-stabilizing when it is under-moderated.

That looks like a good deal.

That, addition to the fact that the operators were operating the reactor
when it was Xenon poisoned following a reduction of power - meant the
RBMK reactor was in an unstable state

What kind of curves/ equations do you have on the Xenon performance characteristic? Say reactivity vs neutron flux?

I have the Physics Today issue on Chernobyl somewhere buried in a box.

 

[Morbius]

Does that mean there is a hump in the cross section vs neutron energy curve?

NO. In general, the cross-section is a very complicated function of energy.

What kind of curves/ equations do you have on the Xenon performance characteristic? Say reactivity vs neutron flux?

Reactivity is not a strong function of neutron flux.

How criticality or sub-criticality of a nuclear assembly is NOT dependent
on the flux - but solely on the materials and the geometry of the reactor.

The assembly can be critical - even if the neutron flux is zero. Of course,
nothing happens until you get some neutrons.

Reactivity is not totally independent of neutron flux - but to first order,
it is dependent mostly on geometry and materials.

As far as Xenon poisoning is concerned. One of the most plentiful fission
products is Iodine-135. Iodine-135 decays to Xenon-135. Now Xe-135 is
THE world champion neutron absorber! It is destroyed in the reactor in
two ways - it absorbs a neutron, or it decays radioactively.

When the reactor is at power - there is an equilibrium level each of I-135
and Xe-135. When the reactor is shutdown, or the power is lowered, the
source of I-135 is stopped or reduced, and the destruction of Xe-135 by
neutron absorption is stopped or reduced.

However, immediately after shutdown - the level of I-135 is the same as
the equilibrium level - just because you shutdown the reactor doesn't
mean the I-135 goes away.

Since the I-135 level is still equal to the equilibrium level - the source of
Xe-135 doesn't change. But one of the destruction mechanisms is stopped.

Since there was an equilibrium between the source rate of Xe-135 and
the destruction rate of Xe-135; and the source rate is effectively
unchanged, while the destruction rate is reduced - the net effect is an
increase in Xe-135 levels - which poison the reactor core.

Eventually, the I-135 decays away, so the source is reduced, and without
a source - the Xe-135 decays away. However, that process takes the
better part of a day.

At Chernobyl, the operators lowered the power to run an experiment on
the reactor. The load controller in Kiev called and asked that the plant
stay online at the reduced power level. After 12 hours, the load
controller released the plant from the electrical grid.

At 12 hours, the Chernobyl RBMK reactor was in the middle of the
Xenon transient - it was experiencing maximal Xenon levels.

The operators were having a difficult time maintaining criticality - they
pulled all the control rods and bypassed safety systems that were trying
to shutdown the reactor because it was in an unstable condition!

Unfortunately, the operators triumphed over the safety systems and
proceeded to run the experiment! The RBMK is marginally stable under
normal conditions - but in the state the operators put the reactor in
while in the midst of a Xenon transient - it's hardly any wonder that they
couldn't control it.

Dr. Gregory Greenman
Physicist

 

[Astronuc]

Modern light water reactor (LWR) fuel and core configurations are designed for negative moderator temperature coefficients, i.e. they are designed such that if the coolant (moderator) temperature increases , and the moderator density decreases (beyond the normal range), the reactivity decreases, so there is a 'natural' control inherent in the reactor design.

All commercial nuclear power reactors in the US are LWRs, which use water as both moderator and coolant. In Boiling Water Reactors (BWRs) the water is boiled directly in the core, and the steam is passed directly to the turbine train to produce power. Reactivity is controlled directly with the control blades (using boron in B4C or Hf enshrouded in stainless steel) located between the fuel assemblies and use of burnable poisons (gadolinium in form of oxide) in some of the fuel rods. During the cycle, the control rods are withdrawn gradually as fission products (which also function as neutron poisons) accumulate in the UO2 fuel fuel.

In Pressurized Water Reactor, the water is pressurized to limit the boiling to some amount of so-called nucleate boiling (heat is passed from primary (reactor cooling) system through a steam generator and steam is produced in a physically separate circuit). The control rods are withdrawn above the core during operation, so reactivity is controlled by soluble boron (in the form of boric acid, with an appropriate buffering agent like LiOH) in the coolant as well as burnable poisons in the fuel (oxides of gadolinium or erbium, or coating of ZrB2 on the surface of the fuel pellets).

In either case, as the coolant density decreases below normal range, the moderation decreases and the reactors go subcritical. If there is some significant perturbation in the reactor operation, the control rods are inserted automatically and the reactor is shutdown.

RBMK's are graphite-moderated reactors using water as coolant. It turns out that the water in such a system behaves as a neutron absorber (light water - with H and O has a reasonably good neutron capture cross-section) and as the water heats and density decreases the reactivity increases, which is the opposite of the way the water moderator in an LWR behaves.

Not remembering the details, I presume the Xe in the Chernobyl 4 reactor was decreasing and the reactivity began to increase. Also as the neutron flux increased, it would start to 'burnout' or consume the Xe-135. As the heat increased, the liquid water turned to steam, which further increased the reactivity - the power continued to increase. Since the control rods (safety systems) had been over-ridden, the staff could not respond fast enough to reinsert them - they probably had a matter of seconds once they realized (if they did) what was happening and the rest is history.

For some background on RBMK, see - http://www.world-nuclear.org/info/inf31.htm - which discusses somewhat the concept of "Positive void coefficient" in the coolant/moderator.

As for TMI-2, it was in its third month of commercial operation when the problem occurred. Lack of experience contributed to the problem. As a result, all reactor operators receive constant training, testing and assessment. Training includes very detailed simulators that simulate normal and abnormal reactor operating conditions. If the operators fail the simulator or training - they are not allowed to operate the reactor.

 

[CharlesP]

The operators were having a difficult time maintaining criticality - they pulled all the control rods and bypassed safety systems that were trying
to shutdown the reactor because it was in an unstable condition!

Somehow I am not getting to the relationship that I want. How does the excess Xe-135 cause instability? Instability means that as N goes up then K goes up. Pulling rods raises K. Stability means that as N goes up then K goes down. How fast did the power rise at Chernobyl?

 

[Morbius]

Modern light water reactor (LWR) fuel and core configurations are designed for negative moderator temperature coefficients, i.e. they are designed such that if the coolant (moderator) temperature increases , and the moderator density decreases (beyond the normal range), the reactivity decreases, so there is a 'natural' control inherent in the reactor design.

All commercial nuclear power reactors in the US are LWRs, which use water as both moderator and coolant. In Boiling Water Reactors (BWRs) the water is boiled directly in the core, and the steam is passed directly to the turbine train to produce power. Reactivity is controlled directly with the control blades (using boron in B4C or Hf enshrouded in stainless steel) located between the fuel assemblies and use of burnable poisons (gadolinium in form of oxide) in some of the fuel rods. During the cycle, the control rods are withdrawn gradually as fission products (which also function as neutron poisons) accumulate in the UO2 fuel fuel.

In Pressurized Water Reactor, the water is pressurized to limit the boiling to some amount of so-called nuclear boiling (heat is passed from primary (reactor cooling) system through a steam generator and steam is produced in a physically separate circuit).

Astronuc,

Not quite - the limit is not "nuclear boiling" [ there's no such thing].
I believe the term you are searching for is "nucleate boiling".

Specifically, the limit is controlled by concern for "departure from
nucleate boiling". The type of boiling that you see in a saucepan on the
kitchen stove - where little bubbles form on the bottom of the pan is
called "nucleate boiling". There's a very limited amount of that type
of boiling going on inside a PWR.

What one wants to prevent is "departure from nucleate boiling" which is
when the temperature of the heated surface exceeds the "Leidenfrost"
temperature. If the temperature of the surface exceeds the
Leidenfrost temperature - the water won't "wet" the surface.

Next time you fix pancakes - try a little experiment with the griddle
before you cook the pancakes. As the griddle heats up - drop a couple
drops of water on the griddle. You will see the drop spread out on the
surface of the griddle and very quickly evaporate.

However, if you get the griddle hot enough - the drop will "dance around"
on the griddle surface for several seconds - much longer than the life
of the drop that spread out on the griddle surface.

When the drop dances around like that - the griddle has exceeded the
Leidenfrost temperature. The griddle is so hot that where the drop
touches the griddle it flashes a small part of the drop into steam. This
steam forms a layer that insulates the water drop from futher heating,
and allows it to dance around. As the steam layer diffuses away, and
the drop again can momentarily contact the griddle surface - there will
be more steam made - and again the drop will be insulated. Eventually,
all the drop will have been turned to steam - but it takes longer than if
the water wet the surface - as is the case at lower temperatures.

One wants to prevent this "vapor blanketing" and insulating effect of
steam production. When there's an insulating steam layer - the heat
transfer from the heated surface goes down - and the temperature of
the heated surface goes up - because the coolant is being less effective
at doing its job.

There is a limit called the DNBR - Departure from Nucleate Boiling
Ratio. It is the ratio of the heat flux at which one would get DNB -
Departure from Nucleate Boiling - i.e. exceeding the Leidenfrost temp.
to the heat flux in the reactor. This value is specified in the reactor's
"Tech Specs" - the limits of which are conditions of the operating license.

If the DNBR, was given as 0.85 - then the power of the reactor is limited
so that the heat flux stays more than 15% away from DNB conditions.

The control rods are withdrawn above the core during operation, so reactivity is controlled by soluble boron (in the form of boric acid, with an appropriate buffering agent like LiOH) in the coolant as well as burnable poisons in the fuel (oxides of gadolinium or erbium, or coating of ZrB2 on the surface of the fuel pellets).

In either case, as the coolant density decreases below normal range, the moderation decreases and the reactors go subcritical. If there is some significant perturbation in the reactor operation, the control rods are inserted automatically and the reactor is shutdown.

RBMK's are graphite-moderated reactors using water as coolant. It turns out that the water in such a system behaves as a neutron absorber (light water - with H and O has a reasonably good neutron capture cross-section) and as the water heats and density decreases the reactivity increases, which is the opposite of the way the water moderator in an LWR behaves.

This is only the case if the reactor is "over-moderated" - like the RBMKs
are. One can build a graphite moderated reactor in which there is not
enough graphite to do all the moderation required. That means that
some of the moderation has to be done by the coolant water. If the
coolant water is providing needed moderation - then you get a
negative coolant temperature coefficient - just like in an LWR!

Not remembering the details, I presume the Xe in the Chernobyl 4 reactor was decreasing and the reactivity began to increase. Also as the neutron flux increased, it would start to 'burnout' or consume the Xe-135. As the heat increased, the liquid water turned to steam, which further increased the reactivity - the power continued to increase. Since the control rods (safety systems) had been over-ridden, the staff could not respond fast enough to reinsert them - they probably had a matter of seconds once they realized (if they did) what was happening and the rest is history.

Not quite. At about 12 hours since the power was lowered the Xenon
levels were not yet decreasing. The problem was that the core was so
poisoned with Xenon - the operators fully withdrew the control rods.

The RBMK also had a bad control rod design - one in which the rod had
an active fuel "follower". So during the first part of a rod insertion -
the rod was actually adding more fuel to the core - before it added
the neutron absorber.

For some background on RBMK, see - http://www.world-nuclear.org/info/inf31.htm - which discusses somewhat the concept of "Positive void coefficient" in the coolant/moderator.

As for TMI-2, it was in its third month of commercial operation when the problem occurred. Lack of experience contributed to the problem. As a result, all reactor operators receive constant training, testing and assessment. Training includes very detailed simulators that simulate normal and abnormal reactor operating conditions. If the operators fail the simulator or training - they are not allowed to operate the reactor.

During my graduate studies at M.I.T., I also attended a seminar by
Professor Kemeny - who led the commission that investigated the TMI
accident. At one point, while visiting the TMI control room after the
accident - Kemeny asked the operators for a "steam table" - that's a
book that gives the Equation of State of Water - that is for each value
of temperature and pressure - it tells you whether water is liquid, vapor,
or boiling. Kemeny said it took the TMI operators about 15 minutes to
find a steam table!

The operators at TMI didn't know how far away from boiling conditions
they were. Kemeny said anyone who read the accident chronology in the
newspaper understood the problem.

I knew EXACTLY what he meant. I remember reading the accident
chronology in the newspaper. At one point, the reactor operators
reported that they had "stabilized" the reactor at such and such a
temperature and such and such a pressure.

I wondered how far they were from boiling. So I reached up to get my
copy of Keenan and Keyes steam tables which was sitting on my filing
cabinet - and looked up the Equation of State for Water at the
conditions specified in the newspaper.

Those conditions were right ON the "saturation line" or "boiling curve".
I said to myself - "They didn't "stabilize" the reactor - they're BOILING!"

The reason the temperature and pressure in the TMI reactor stabilized
was that the water was boiling - and the operators thought everything
was fine - that they had solved the problem - when the worst possible
thing was currently happening!

Dr. Gregory Greenman
Physicist

 

[Astronuc]

Morbius, Thanks for the correction - I did mean 'nucleate' boiling as you described.

With respect to Departure from Nucleate Boiling Ratio (DNBR) -

$$DNBR = \frac{{\large{q}\prime\prime}_{DNB}}{{\large{q}\prime\prime}_w}$$

where $${\large{q}\prime\prime}_{DNB}$$ is the heat flux that causes DNB under local temperature and pressure conditions, or perhaps more accurately is the heat flux predicted to cause DNB under the local conditions according to some correlation (usually based on ex-core experiments)

and $${\large{q}\prime\prime}_w$$ is the local wall heat flux under the operating conditions.

Of course, there are more practical considerations, such as corrosion of the Zr-alloy cladding. The heat flux is limited in order to limit the local cladding temperature in order to prevent excessive cladding corrosion and crud (metal oxide corrosion products from the primary cooling circuit surfaces, primarily steam generator tubing). Some heavy crud deposits have been associated with reactivity anomalies in some reactors - but this is now controlled (avoided) by limiting coolant temperature and better controls on water chemistry.

In operating PWRs, typical DNBR's have been on the order of 1.26 or 1.3, a margin of 0.26-0.3 to DNB. The relatively high margin was due to uncertainties in the DNB (of CHF) correlation and the uncertainties in the core monitoring systems. With improved models and monitoring systems, some plants can reduce the DNBR to about 1.17 (IIRC), but it is always greater than one by definition.

There are some European PWRs that operate with considerable levels of nucleate boiling, but plants in the US operate with much less.

 

[Morbius]

Morbius, Thanks for the correction - I did mean 'nucleate' boiling as you described.

With respect to Departure from Nucleate Boiling Ratio (DNBR) -

$$DNBR = \frac{{\large{q}\prime\prime}_{DNB}}{{\large{q}\prime\prime}_w}$$

where $${\large{q}\prime\prime}_{DNB}$$ is the heat flux that causes DNB under local temperature and pressure conditions, or perhaps more accurately is the heat flux predicted to cause DNB under the local conditions according to some correlation (usually based on ex-core experiments)

and $${\large{q}\prime\prime}_w$$ is the local wall heat flux under the operating conditions.

Astronuc,

Exactly - the limit is for the most limiting local value of DNBR.

Thank you for refreshing my memory on typical values of DNBR.
[ I may have also inverted the ratio ].

It's been a long time since I've had to contend with those considerations.

Dr. Gregory Greenman
Physicist