What safety measures prevent a LOCA at nuclear power plants?

[georgeh]

What safety systems are there to prevent a LOCA??
(Loss Of coolant Accident)
I've googled but nothing has surfaced that relates to LOCA.

 

[jdsneeder]

What safety systems are there to prevent a LOCA??
(Loss Of coolant Accident)
I've googled but nothing has surfaced that relates to LOCA.

Most safety systems are designed to mitigate the *effects* of the LOCA, not prevent it. These items generally consist of pressure suppression or pressure capability of the containment, pressure relief of the reactor vessel, core reflood, containment of radioactive material, ventilation, etc.

Plant design and construction are such to preclude a LOCA, i.e. pipe schedule, snubbers for earthquakes, fail safe valves, etc. These items are not normally considered "safety systems"

Try searching at nrc.gov for more detailed information. Some may have been removed after 9/11 though.

 

[Astronuc]

Well, as you mentioned, LOCA refers to a Loss of Coolant Accident.

The only way to 'prevent' a LOCA is to prevent conditions of the reactor and reactor cooling system (RCS), which if they occur could lead to a LOCA.

The reactor protection system - i.e. the detectors which monitor the core power level, the instrumentation which causes the reactor to trip if the power or power ascension rate exceeds tech specs, and the control rods which are inserted into the core during a trip - constitutes the principal preventative measure to avoid a LOCA.

The structural integrity of the RCS pressure boundary, which is verified by periodic testing and inspection, is another method for 'preventing' LOCA. The RCS boundary consists of the Reactor Pressure Vessel (RPV) and the piping/tubing of the RCS.

In BWR, the primary system includes the main steam lines which carry steam from the RPV to the turbines, the recirculation lines, the feedwater (FW) lines and all other piping/tubing including condensers and FW heaters. In a PWR, the RCS includes the hot legs (RPV to Steam Generators (SGs)), the SG tubing, the cross-over lines (SGs) to Reactor Coolant Pumps (RCPs), the cold legs RCPs to RPV, the Pressurizer (attached to one of the hot legs), and all piping attached the main RCS piping and components, including the Emergency Core Cooling Systems (ECCS). Both BWRs and PWRs have pressure relief valves to prevent the coolant pressure from causing failure of the RCS piping.

However, there are certain 'hypothetical' power transients, which could 'theoretically' cause pressure pulses in the reactor which could cause failure of the piping or pressure vessel. The goal is to prevent such 'hypothetical' accidents from ever occurring!

 

[Morbius]

What safety systems are there to prevent a LOCA??
(Loss Of coolant Accident)
I've googled but nothing has surfaced that relates to LOCA.

George,

One of the design philosophies for nuclear power plants is that one has to have
systems to mitigate events - even when one can't identify a mechanism for it.

The "design basis accident" for a LOCA is that a main coolant pipes breaks clean
through - and the ends of the pipe are displaced a few feet so as to allow coolant to
escape from both broken pipe ends.

I don't know of anyone that could give you a scenario of HOW that would happen -
let alone how one would prevent it from happening. There's no mechanism or
physics in the plant that would tend to drive one toward breaking a pipe like that.

So there's no causality mechanism that one tries to prevent.

However, the philosphy of designing a nuclear power plant is that, even in the absence
of a mechanism to cause the accident - the plant has to be designed to mitigate the
accident.

The philosophy is to mitigate accidents - even when you can't envision how they
would happen. If you can imagine some way for the accident to happen - then you
design the system so that it won't.

Now the one major accident the USA has had - Three Mile Island Unit 2 - was
a small-scale LOCA. Instead of the coolant pipe splitting - a valve stuck open.
this small scale LOCA could easily have been handled by the emergency cooling
systems designed to mitigate a large break accident.

The one thing the plant's designers hadn't counted on was that the operators would
TURN OFF the emergency cooling systems before they had investigated and found
out what problem these automatically triggered systems were responding to.

Dr. Gregory Greenman
Physicist

 

[Homer Simpson]

As said, one of the reactor trips is on high heat transport pressure, but other things could cause a LOCA. For instance... Weak inspection/maintenance. Look up some stuff on the Davis-Besse BWR close call... that could have been a bad one.