The standby liquid control system is used in the event of
major contingencies as a last measure to prevent core damage. It is not
intended ever to be used, as the RPS and ECCS are designed to respond to all
contingencies, even if a quite a few of their components fail, but if a
complete ECCS failure occurs, during a limiting fault, it could be the only
thing capable of preventing core damage. The SLCS consists of a tank containing
borated water as a neutron absorber, protected by explosively-opened valves and
redundant battery-operated pumps, allowing the injection of the borated water
into the reactor against any pressure within; the borated water can and will
shut down a reactor gone out of control. The SLCS also provides an additional
layer of defense in depth against a ATWS derangement, but this is an extreme
measure that can be avoided by numerous other channels (ARI and use of
redundant hydraulics).
Versioning note: The SLCS is a system that is never meant to
be activated unless all other measures have failed. In the BWR/1 – BWR/6, its
activation could cause sufficient damage to the plant that it could make the
older BWRs inoperable without a complete overhaul. With the arrival of the ABWR
and (E)SBWR, operators do not have to be as reticent about activating the SLCS,
as these reactors have a Reactor Water Cleanup System (RWCS) – once the reactor
has stabilized, the borated water within the RPV can be filtered through this
system to promptly remove the soluble neutron absorbers that it contains and
thus avoid damage to the internals of the plant.
The ultimate safety system inside and outside of every BWR
are the numerous levels of physical shielding that both protect the reactor
from the outside world and protect the outside world from the reactor.
There are five levels of shielding:
1. The fuel rods inside the reactor
pressure vessel are coated in thick Zircaloy shielding;
2. The reactor pressure vessel itself is
manufactured out of 6-inch-thick (150 mm) steel, with extremely high
temperature, vibration, and corrosion resistant surgical stainless steel grade
grade 316L plate on both the inside and outside;
3. The primary containment structure is
made of steel 1 inch thick;
4. The secondary containment structure is
made of steel-reinforced, pre-stressed concrete 1.2–2.4 meters (4–8 ft) thick.
5. The reactor building (the shield
wall/missile shield) is also made of steel-reinforced, pre-stressed concrete
0.3 m to 1 m (1–3 feet) thick.
If every possible measure standing between safe operation and
core damage fails, the containment can be sealed indefinitely, and it will
prevent any substantial release of radiation to the environment from occurring
in nearly any circumstance.
As illustrated by the descriptions of the systems above, BWRs
are quite divergent in design from PWRs. Unlike the PWR, which has generally
followed a very predictable external containment design (the stereotypical dome
atop a cylinder), BWR containments are varied in external form but their
internal distinctiveness is extremely striking in comparison to the PWR. There
are five major varieties of BWR containments:
● The "premodern" containment
(Generation I); spherical in shape, and featuring a steam drum separator, or an
out-of-RPV steam separator, and a heat exchanger for low pressure steam, this
containment is now obsolete, and is not used by any operative reactor.
● the Mark I containment, consisting of a
rectangular steel-reinforced concrete building, along with an additional layer
of steel-reinforced concrete surrounding the steel-lined cylindrical drywell
and the steel-lined pressure suppression torus below. The Mark I was the
earliest type of containment in wide use, and many reactors with Mark Is are
still in service today. There have been numerous safety upgrades made over the
years to this type of containment, especially to provide for orderly reduction
of containment load caused by pressure in a compounded limiting fault. The
reactor building of the Mark I generally is in the form of a large rectangular
structure of reinforced concrete.
● the Mark II containment, similar to the
Mark I, but omitting a distinct pressure suppression torus in favor of a
cylindrical wetwell below the non-reactor cavity section of the drywell. Both
the wetwell and the drywell have a primary containment structure of steel as in
the Mark I, as well as the Mark I's layers of steel-reinforced concrete
composing the secondary containment between the outer primary containment
structure and the outer wall of the reactor building proper. The reactor
building of the Mark II generally is in the form of a flat-topped cylinder.
● the Mark III containment, generally
similar in external shape to the stereotypical PWR, and with some similarities
on the inside, at least on a superficial level. For example, rather than having
a slab of concrete that staff could walk upon while the reactor was not being
refueled covering the top of the primary containment and the RPV directly
underneath, the Mark III takes the BWR in a more PWRish direction by placing a
water pool over this slab. Additional changes include abstracting the wetwell
into a pressure-suppression pool with a weir wall separating it from the
drywell.
● Advanced containments; the present
models of BWR containments for the ABWR and the ESBWR are harkbacks to the
classical Mark I/II style of being quite distinct from the PWR on the outside
as well as the inside, though both reactors incorporate the Mark III-ish style
of having non-safety-related buildings surrounding or attached to the reactor
building, rather than being overtly distinct from it. These containments are
also designed to take far more than previous containments were, providing
advanced safety. In particular, GE regards these containments as being able to
withstand a direct hit by a tornado of Old Fujitsa Scale 6 with winds of 330+
miles per hour. Such a tornado has never been measured on earth. They are also
designed to withstand seismic accelerations of .2 G, or nearly 2 meters per
second2 in any direction.