Super Critical Light Water Reactors

From: Matthew Beasley (dontspam_beasleys_at_teleport.com)
Date: 07/22/04 

Date: Thu, 22 Jul 2004 06:45:26 GMT

I had noticed one of the types of advanced reactors added to the list of
research areas is the super critical light water reactor. At first I
was real confused since I though all light water reactors were super
critical at power ;-) Then I realized that they were talking about the
thermodynamics of the working fluid rather than the neutron
multiplication ratio.

I have found a couple of papers that described potential designs. The
first one I found was:
http://www.hut.fi/Units/AES/courses/crspages/Tfy-56.181_03/Danielyan.pdf

The other I found was:
http://wwwsoc.nii.ac.jp/aesj/publication/JNST2001/No.12/38_1081-1089.pdf

If anyone knows of better references online, I am most definitely
interested in them.

Both papers are fairly close in their descriptions of possible overall
plant makeup.
The Danielyan paper describes a reference design where the typical
current power level of 3,300MWt is maintained, resulting in a much
larger 1,600MWe output due to the higher thermodynamic efficiency.
The Oka paper describes a reference design where the more typical
1,000MWe is maintained but the thermal power is reduced via the higher
thermodynamic efficiency.

I note that neither paper describes a cross compound turbine in the
reference desing. One describes a 1800-RPM turbine and the other a
3600-RPM turbine. (Cross compounds are where there are two separate
turbo-alternators: a 3600-RPM high-pressure unit and a1800-RPM
low-pressure unit. 3600-RPM provides for a smaller HP turbine, and
1800-RPM allows longer LP turbine blades.) I believe most coal plants
of this power range use cross compound turbines.

One of the first questions I had was how to get the plant started up and
shut down. Only the Oka paper touches on startup. They describe a
system similar to coal powered plants were a part capacity water
separator is inserted inline for startup. Once the reactor output is
high enough to get superheated exit conditions, the startup separator is
taken offline. The part that diverges from the coal plant designs that
I am familiar with is that the SCLWR design rejects the separated water
to the condenser through a regenerative heat exchanger. If a
recirculating pump were used to re-inject the separated flow into the
high pressure feed instead to provide the startup recirculation, the
pumping power would be much lower. If a recirculating system were used,
however, there would be no demineralization of the recirculated flow. I
would think this could be dealt with by bleeding a much smaller flow
back to the condenser and provide diluting demineralized feed water if
recirculated flow becomes to contaminated / activated. An additional
advantage would come in at shutdown where the startup process could be
reversed to provide an orderly shutdown. This would eliminate the need
to reject a mixed flow to the condenser or suppression pool once the
reactor dropped below supercritical pressure.

Both papers described how the SCLWR would be more sensitive to loss of
feedwater. This is due to the fact that the SCLWR does not have a
recirculation flow like a BWR, or natural circulation cooling like a
PWR. Forced flow MUST be maintained to keep the fuel cool. The
Danielyan paper just left it with the statement that auxiliary feed
water would just need to be better than that applied to current BWR and
PWR plants. The Oka paper touches briefly on using a auxiliary
feedwater turbine powered by the reactor exit, but does not mention what
happens once the pressure vessel reduces below supercritical and the
exit conditions become a mixed water / steam flow. I would assume a
water separator would be needed. I would propose that a larger
capacity separator could be used allowing the entire flow under isolated
cooling conditions to be run through the separator. Steam could go both
the turbine and to the steam dump (either to the condenser if available
or to the suppression pool). The water could be recirculated to the
reactor to provide cooling by a second pump located on the auxiliary
feedwater turbine. This change would make the pumping power and
behavior much more like that of a BWR under shutdown conditions. Once
pressure dropped far enough, regular shutdown cooling could take over
just like in a BWR. To save capital costs, the startup separator could
also be used for isolated cooling. This would mean that the separator
would need to be well shielded to handle potential high activity under
accident conditions. Just as in the papers, the backup system could
still be de-pressurization and flood using the lower pressure injection
system like that of a BWR.

Another difference between existing plants and a SCLWR is what will
happen to contaminants in the water. In a PWR, obviously most
contaminants never make it to the secondary system and are removed by
the reactor water cleanup system. In a BWR, the steam separator manages
to remove much of the dissolved and suspended contaminants and return it
with the recirculated water to eventually be removed by the reactor
water cleanup system. In the SCLWR, any contaminants will leave with
the supercritical water either in solution or suspension. I would think
the impact would be that water condensing out in the turbine would be
able to collect the contaminants, resulting in the moisture separator
drain water and feedwater heater drain water becoming much more active.
In addition, the condensate demineralizers will collect all contaminants
instead of them being divided between the condensate demineralizer and
the cleanup system. Another possibility is that the contaminants will
plate out on the turbine and make it even hotter that a BWR turbine.

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