Astronuc said:
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 During the cycle, the control rods are withdrawn gradually as fission products (which also function as neutron poisons) accumulate in the UO2 fuel fuel.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
