siemens - Public Service Commission

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A higher structural integrity by stronger construction of the internal
partitions and the 4th pass box, to endure sudden pressurization and/or
decompression. This eliminates deformation and rupture failures.
An easier access for inspection and maintenance, which is possible by a large
manway and by the ability to remove internal covers, rather than entire
components.
A service-worthy full circumference flat surface seal, on the back of the 4-
pass box. The box is welded to the tubesheet. The flat surface seal is
subject to considerably lower differential pressure than in your current
design due to tube pass sequence.
Tube passes in a “downhill” top-to-bottom sequence of 1,3,4,2 which
provides a logical thermal head arrangement and a true co-current two-phase
flow, thus promoting condensate drainage and preserving stable hydraulics in
the U-bend sections. This arrangement also results in very low pressure drop
across the internally welded 4-pass box, thus preserving the sealing gasket,
on top of the box.
Boffom discharge holes from the 4th pass box. The box is designed also as a
“sump” to avoid condensate buildup and submergence of lower tubes.
Since the end pass (4th pass) is nested above the 2nd pass, the maximum
potential for subcooling, under unlikely worse case conditions, (insufficient
excess steam during low load, transients and/or full heating steam pressure) is
considerably reduced.
The rate of excess steam is regulated by a valve to establish optimal rate.
This is the highly recommended and best design for the CR-3 MSRs. It provides a
larger effective surface, but by utilizing the side segments of the circular tubesheet,
which are not used in the original design. Such tube arrangement reduces the cross
flow velocity and, as a result, caused a lesser pressure drop across the tube bank. It
also reduces the susceptibility to local vibration. The lower pressure drop, coupled
with a comparable heavier bundle, reduces the “lift-off’ tendency of the bundle.
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