Turning MSRs into high-performance items

Turning MSRs into high-performance items
The role of moisture separator reheaters as passive components has changed radically.
They are no longer attended to only when they become a maintenance concern,
an operational problem, or poor performers that lose potential megawatts.
T
he moisture separator reheater (MSR)
is employed in a nuclear steam cycle
in conjunction with its saturated
steam turbines. Since the expansion of the
steam takes place largely within the wet
steam range, MSRs are used to dry the
exhaust steam leaving thehigh pressure turbine, and then to reheat the dry steam
before it enters the low pressure turbine. I11
most cases, MSR vessels comprising both
the moisture separation system and the
reheater are used. They can contain one or
two stages of reheat - using maul steam as
heating stearn for the former, and
main steam as well as extraction steam for
the latter.
The rise of power uprating and plant life
extension have eiven
MSRs a more vromi,.,
nent roleiri generation and uperatiory'tl flexibilitv. MSl<s,lre ,bow callr~lilv,,n toacLivt.l\,
contkbute to the power uprate proces8.
They must also contribute to generation and
operational flexibility.
Nuclear power plant uprating emerged
as a viable considerationin the early 1990%
when steam generators began to fail and the
idea to redesign them to produce increased
steam output was prompted by design
advances and the advent of competition in
power generation. On the other hand, MSR
redesign and reconstruction have benefited
from over 25 years of experience and technological advances.
MSRs were not originally given much
attention in older nuclear plant designs;
they were considered a relatively basic
adjunct, and hence MSR problems arose
The aufhar is senior vice preliden~,Thermal Engineering
International. Lor Angeler, California, US
components within the existing internal
MSR structure. While this is probably the
early in plant life.Recently MSR technology least expensive approach, it produces
has improved to make it a component less than full utilisation of modern MSR
uniquely able to contribute to plant uprat- technology.
The recent Hatch 8% power uprate proing and life extension.
Levels of design confidence, gained gramme was an interesting case in point.
through experience, have overcome some of Here, the cost of extensive HP turbine modthe excessive conservatism that previously ification was reduced by routing added
inhibited the full utilisation of MSR benefits. uprate cycle steam flow directly to the
For example, older designs provided for MSRs' HP reheaters. nus, of course, greatly
actual terminal temperature differences increased LP turbine superheat, increasing
(?TDs) from as low as 22°C to as high as MWe output and minimising LP turbine
45"C, which could still produce acceptable moisture-induced blade erosion.
superheat levels at LP turbine inlets. These Full MSR intemals
TTDs have now been reduced to as low as redesign a n d reconstruction
5"C, producing a much higher LP inlet
steam superheat. The beneficialeffects here Where the reconstruction approach is indion ~ W e a i and
n LP turbine blade, lifc Jrr c'lted, theexisting MSRshellsarc n,nipletr
clrac Another eu.~mple is that modern ly gulled. ,\dvanced MSI< conip,>n?nt
reheaters can be placed in service earlier, technology and unproved structural design
during load ramping. Their ability to can be used to produce maximum MWe
endure transients and large thermal gradi- gain through near-100% moisture separaents cnmw frnm their flexible structural tion, minimum 7TD (resulting in maximum
superheat to the LP turbine) and lowest
desigi
attainable pressure drop. The recent project
at Korea ElectricPower's I<ori3 and 4 is an
UPGRADE OPTIONS
example. Here, TTDs have been reduced
In considering the MSRs' role in a power from 31.8"C to 4.5"C and 8MWe gains were
uprate programme, there are several achieved in each unit.
approaches to evaluate. Each approach is Complete MSR vessel replacement
viable, but the choice depends on ownership circumstances and the particular As well as using modern MSR technology,
nuclear power plant's type, age, specific entirely replacing the MSR vessel allows the
MSR shell size to be optimised. This
needs, and uprate targets. They are:
approach is particularly attractive where
Existing MSR refurbishment
excessive internal steam velocities have proHere, the functioni~lginternals - reheaters duced flow-assisted corrosion (FAC) damand moisture separationsystems- are selec- age. This was the case at the recent MSR
tively replaced by advanced-technology replacement project for Virginia Power's
North Anna 2. In such a case while crossaround cvcle-steam viuinr centrelines can
be maintiined, as they were at North Amla
2, other, smaller-bore piping connections
have to be relocated.
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UPGRADING TO UPRATE
Upgrading MSRs to meet plant uprate
needs breaks downinto two basic classifications: rectifying existing MSR performance
and reliability problems; atid enhancing
MSR thermohydraulicperformance.
Rectification
includes
eliminating
mechanical and/or structural failures due
to thermally-induced aacks and distortions
in key internal support elements and
reheater tubing. These past failurescannow
be overcome through the use of a flexible
tube-support system that allows for controlled, intermittent relief of mismatched
thermal expansions between hotter reheater
tubes and progressively cooler shrouding
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