Leakage in a nuclear condenser

[Luxdot]

TL;DR Summary

What happens if there is a leakage in a condenser?

Hi!

I'm a bit curious, what would happen if there would be a leakage in the condenser in a nuclear plant (PWR or BWR). As I understood it the stean in the turbine is expanded to about 4 kPa and I guess the pressure outside in the freshwater that is pumped into the condenser is at atmospheric pressure. So what would happen if there were a leak? Would there be a rupture in the pipes? Would steam be released to the surrounding? Would this be prevented by the difference in pressure?

 

[anorlunda]

Summary:: What happens if there is a leakage in a condenser?

As I understood it the stean in the turbine is expanded to about 4 kPa and I guess the pressure outside in the freshwater that is pumped into the condenser is at atmospheric pressure. So what would happen if there were a leak? Would there be a rupture in the pipes?

That is 4 kPa absolute, not guage. It is a vacuum relative to atmospheric pressure. So if there is a leak, the cooing water will leak in, not steam leak out.

Reading between the lines, are you thinking about radiation? The steam in the turbine is not significantly radioactive anyhow. So a condenser leak in a nuclear plant is no more dangerous than a condenser leak in a fossil fuel plant, or a whistling cap on your teapot.

 

[Luxdot]

Thanks! So due to the vacuum, the seawater will leak into the condensor but the steam will not be leaked out? Will there be any other consequences? I'm aware that there is low radiation in the turbine so it won't be dangerous in that way. But is there a risk of some kind of explosion of some kind or is the consequence that seawater will penetrate the condensor?

 

[anorlunda]

Seawater is corrosive. So a condenser leak will probably cause a plant trip and shutdown to prevent corrosion of the equipment. But no special hazard to the public.

Edit: I forgot to mention, zero explosion hazard.

 

[dRic2]

he steam in the turbine is not significantly radioactive anyhow.

In a PWR yes, but in BWR the steam should be radioactive, right? Because in a BWR the steam is produced by the water-moderator boiling in the core.

 

[anorlunda]

In a PWR yes, but in BWR the steam should be radioactive, right? Because in a BWR the steam is produced by the water-moderator boiling in the core.

It is slightly radioactive. Water molecules can't become radioactive (except see Nitrogen 16 below). It is impurities in the water that can carry radioactive isotopes. So in a BWR, they take special precautions to make sure that the water going through the reactor is a pure as possible.

https://www.nrc.gov/docs/ML0037/ML003722491.pdf
That is a document discussing the special precautions for BWR water chemistry.

Wikipedia says:
https://en.wikipedia.org/wiki/Boiling_water_reactor

• Contamination of the turbine by short-lived activation products. This means that shielding and access control around the steam turbine are required during normal operations due to the radiation levels arising from the steam entering directly from the reactor core. This is a moderately minor concern, as most of the radiation flux is due to Nitrogen-16 (activation of oxygen in the water), which has a half-life of 7.1 seconds, allowing the turbine chamber to be entered within minutes of shutdown. Extensive experience demonstrates that shutdown maintenance on the turbine, condensate, and feedwater components of a BWR can be performed essentially as a fossil-fuel plant.

Note what they say about shutdown maintenance.

 

[Astronuc]

In a PWR yes, but in BWR the steam should be radioactive, right? Because in a BWR the steam is produced by the water-moderator boiling in the core.

Water molecules can't become radioactive (except see Nitrogen 16 below). It is impurities in the water that can carry radioactive isotopes. So in a BWR, they take special precautions to make sure that the water going through the reactor is a pure as possible.

Some radioactivity can be carried over to the turbine, besides N-16, which has a very short half-life, but can be an issue depending the location of the main steamlines with respect to the areas where personnel are locate. With nuclear fuel, there is always some tramp UO₂, which is kept at a very low level. Fresh fuel is much cleaner these days, but is fresh fuel can pickup tramp UO₂ when loaded into the core. Most tramp these days comes from failed nuclear fuel rods when there is degradation of cladding and oxidation of the fuel pellets.

In BWRs with failed fuel, one has to be concerned the noble gases, Kr-85m, Kr-87, Kr-88, Xe-133, Xe-135 and X-138, which can be carried over to the turbine, as well as isotopes of Br and I, which decay into isotopes of Kr and Xe, respectively. Radioisotopes of Xe decay to Cs, which is a concern along with I. There is an off-gas treatment system, as well as a reactor water filtering system, that endeavors to filter the various nuclides out of the water.

As was mentioned, condenser tube failures are at a lower pressure than the outside cooling water, so leakage is into the condenser, and undesirable contamination can and does occur. In the past, some BWRs have had damage to condenser tubes with salt water intrusion.

When the plant is shutdown, the pressure in the condenser is about the same as outside, so leakage would then be a concern.

 

[artis]

@Luxdot You should also take into account the difference between PWR and BWR , as already said here PWR differs in that it has two loops, one is the loop that goes through the reactor core (a high pressure loop where boiling is prevented) , the other loop takes the heat from the first one through a large heat exchanger which also serves as a steam generator.
In a BWR (notice the boiling water in abbreviation) on the other hand the steam generator is the reactor vessel itself, where in the lower part the core sits and as the water flows by the core it heats up and becomes steamy.
So the steam separated within the reactor core vessel itself is then directed to the turbine so this causes it to be more radioactive.
It's a single loop system and this is why @Astronuc mentioned the need for a "cleaner" water because any impurities will get "activated" as they go through the core.
The now famous Soviet RBMK also has a single loop system, only there the steam is separated in a separate steam separator (yes, pun intended) or steam generator.