POWER November 2011 - 86

NUCLEAR SAFETY
encompassed a large percentage of the plant
primary system components and was not possible
considering the tight project schedule.
Instead, SCrP was used on carefully selected
valves, piping, pump casings, impellers, valve
bodies, and process radiation monitor hous2.
RWCU pump SCrP application.
Courtesy: Radiological Solutions Inc.
ings (see EPRI 1003373, Stabilized Chromium
Process Final Report, August 2002).
After performing a chemical decontamination
to reduce worker exposure, the majority
of the major piping systems in Unit 1 were
removed for replacement, and at one point it
was even possible to walk into the drywell in
street clothes.
Source Term Elimination
The EPRI SCrP consists of three discrete steps.
First, the component is electropolished.
Electropolishing wetted surfaces reduces the
true surface area of the component by creating a
micro-smooth finish, as observed at a magnification
of greater than 100x. Mechanical polishing
of metal surfaces does improve the finish,
4. SCrP applied to valve internals.
Courtesy: Radiological Solutions Inc.
3. RWCU valve internals. Shown are
the as-received (top), electropolished (center),
and post-application states of the SCrP (bottom).
Courtesy: Radiological Solutions Inc.
but it leaves rough edges and asperities. These
rough edges or pinnacles then have a greater
corrosion potential than the base materials
would normally exhibit. Electropolishing preferentially
removes the pinnacles and asperities
while creating a microscopically smooth surface.
Also, fewer activated corrosion products
are deposited on the smooth surface layer than
would be found in the micro-crevices created
from mechanical polishing alone.
The second step is chromium application.
To ensure that the surface is clean for the
chromium application, it immediately follows
the electropolishing step. A thin, 3,000to
13,000-angstrom layer of chromium is
applied to the wetted surface. This layer of
chromium provides a chromium-rich corrosion
inhibition layer.
The final step is passivation or pre-oxidation.
Pre-oxidation of electropolished and/or
chromium-enriched surfaces allows for the
initial corrosion film to be formed in a controlled
environment. The corrosion film at
the component surface will be free of radionuclides.
With the reduced corrosion potential,
subsequent activated corrosion products
will build up at a much lower rate.
Stabilized chromium surfaces are most
5. SCrP applied to the inside of a
valve body. Courtesy: Radiological Solutions
Inc.
passive in reducing chemistry environments,
which are typical of pressurized
water reactor primary systems and BWR
under hydrogen water chemistry (HWC)
and noble metals application.
The following BFNP Unit 1 components
had SCrP applied prior to reinstallation.
RWCU Pumps. The SCrP-treated reactor
6. SCrP applied to the outside of
a valve body. Courtesy: Radiological
Solutions Inc.
water cleanup (RWCU) pumps that were intended
for Unit 1 service were installed in Units
2 and 3 when those pumps needed replacement,
as there was time in the schedule to acquire new
pumps for Unit 1. While installed in Units 2 and
3, the dose rate rise was very low, and, interestingly,
higher flow rates at normal operating amperages
were observed. Increased RWCU flow
is desirable for BWR plants because it has the
potential to reduce steady state operating impurity
levels in the reactor coolant. It is believed
that the smoother surfaces and minimization
of high and low spots in the flow path result
in lower pressure drop across the pump and,
therefore, greater flow.
Figure 2 illustrates the SCrP application
on a pump impeller and casing. Photographs
of the SCrP application steps on a RWCU
valve are shown in Figure 3.
RHR Testable Check Valve. The residual
heat removal (RHR) system testable check
valves were identified by the ALARA staff to
be high dose contributors on Units 2 and 3,
as these valves historically have required frequent
maintenance. SCrP was applied to the
valve internals (Figure 4) and to the inside and
outside of the valve bodies (Figures 5 and 6).
86
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POWER November 2011

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