ARE USED
WHERE
VALVES
NUCLEAR POWER INDUSTRY
pipes that penetrate the reactor containment building (“the dome”), which are
another barrier to radioactive contamination release. Also included in this general group are special systems for the
handling and processing of radioactive
waste fluids. The ASME Code classifies
these as Section III, Class 2 or Class 3
systems.
Outside of the space that houses
ASME Section III systems is the
money-making part of the plant, typically referred to as the “
balance-of-plant systems” and the “turbine
island.” There is no significant difference between these valves and systems
and those that would be used in a large
fossil fuel-burning plant. Valves in this
part of the plant manage the steam
cycle used to spin the turbine and generate electricity. They are installed in
feedwater, main steam and condensate
systems as well as in auxiliary systems
that provide cooling water to support
these main systems. While not critical
to safety, these valves are vital to the
mission of the plant so they are no less
important.
SLOW TO ADOPT NEW
TECHNOLOGIES
For purposes of this discussion, we will
group valve technologies as exotic, special and traditional technologies. This
breakdown is not necessarily related to
nuclear safety. There are many traditional valve technologies in ASME Section III Class 1 applications. Also, the
conservative nature of the nuclear community means that tried and true technologies are often used. This
conservative nature also means the
nuclear community has been a slow
adopter of new technologies—generally
a process plant will contain the latest
valve technology. For example, for the
last 20 years, South Korea has had one
of the world’s most aggressive nuclear
plant construction programs but only
started its first plant with a full digital
control system in 2002.
EXOTIC VALVES
Exotic technologies are those highly
unusual and specialized valve designs
The conservative
nature of the
nuclear community
means that tried
and true
technologies are
often used.
unique to the new Generation III plants.
These designs require a large number of
highly specialized engineering man-hours for development despite the
prospect of relatively few installations.
However, they are a critical element in
the construction of Generation III
plants, and without these technologies,
the plant could not exist.
An example is explosively actuated
valves or squib valves. A squib valve has
no possibility of leakage because a solid
structural mass blocks valve flow. Such
valves have no disc and no seat, just a
metal mass blocking flow. The valve is
opened by firing an explosive charge,
commonly known as a squib, that generates high pressure gas. The gas propels a rod that breaks the structural
mass and allows flow to begin—similar
to breaking a dam.
The aerospace industry has used
squib valves for some time; the retrorockets on the Mars Lander were initiated by firing tiny squib valves in the
fuel system. Even some older nuclear
plants have a 2-inch design of squib
valves that isolates emergency shutdown chemicals from the reactor. The
difference between these historical
squib valves and those of the Generation III plants is that the design is
scaled up many times over. Some of the
valves required for the newer plants are
larger than 12 inches3 and must be
designed for applications that subject
them to high temperatures and pressures not previously considered for this
technology.
SPECIAL VALVES
Special valves are those unique to
nuclear power plants, but that have
been in regular use in Generation II
plants. These valves have highly special-
ized missions and would not typically be
used outside the nuclear industry,
though their technology can be based on
other industrial designs adapted for
nuclear use. Some examples include:
Main Steam Isolation Valve
(MSIV). The roots of these steam
valves lie in fossil fuel power plant
design. Given an emergency shutdown
in a fossil fuel plant, the large steam
lines feeding the main turbine are isolated by stop valves to protect the turbine. In a nuclear plant, this mission is
even more critical because these valves
are also part of the containment isolation system. For MSIVs, the challenge
is that a large amount of steam mass
flow must be stopped instantaneously.
The valves are typically large ( 20 to 36
inches) gate valves operated by pneumatic or hydraulic-pneumatic at Class
1500 high pressure and temperature
conditions. A significant engineering
challenge for this valve is management
of the momentum generated by closing
such a large valve so fast against such
high flow. This is further compounded
by the significant increase of steam flow
and pipe/valve sizes in Generation III
plants compared to Generation II.
Main Steam Safety Valves
(MSSVs). Once the previously
described MSIVs close, steam energy in
the main steam system begins to build
rapidly and must be relieved by
MSSVs. This mission is similar to the
safety valves in a fossil fuel plant. However, the precision required for operation of these valves in nuclear plants is
critical to controlling the cool-down
rate of the reactor. For example, keeping the valve open too long would cool
the fuel too quickly and actually work
to restart a nuclear reaction. The
ASME Code requires every MSSV to
be production tested at full-rated steam
flow to verify reseat characteristics.
This testing has always been done, but
nuclear design practice in the past has
been to limit flow rates to about 1 million pounds per hour steam. Unfortunately, as previously noted, the
Generation III plant designers have
increased steam flow rates significantly
without regard for the testing capabili-
