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ScienceIamA Reactor Engineer that works at a nuclear power plant AMA!

Oct 28th 2016 by King_Gex • 19 Questions • 2813 Points

I graduated from Penn State with a BS degree in Nuclear Engineering in 2014. I have worked at a commercial nuclear power plant as a reactor engineer for just over two years.

Proof: http://imgur.com/W0qlLP1

Edit 1: That's all I have time to answer right now, but continue to submit questions and I will answer more tonight.

Q:

In one of those "everybody on earth disappeared/died at the same time" hypothetical situations - with no humans around, how long would it be before something automatically shut down? Like is there some "nobody's pushed a button in X amount of time, so execute auto shutdown procedure" program?

How long before something catastrophic happened?

For example, if someone did survive and was in a city powered by a nuclear power plant, how long would they have before the power shut off, and how long would it be before they were in danger from the plant?

Would "being hooked to the nationwide grid" if it still had power make any difference, like if it was powered would the plant last longer, or if it weren't would the plant try to pump 'max power' into the grid to keep up and eventually give up?

Just curious.

A:

If everyone disappeared at once, everything would continually running fine until it wasn't. Not to get into too much detail, but depending on where you were at in the cycle, power in the reactor would gradually go up or down. As that happened, or there was another significant equipment issue, you would eventually run into a set point that will automatically shut the plant down but that is only half the battle. The other half would be keeping the core cool with 0 power due to decay heat. I believe (not 100% certain) that the shutdown cooling system will automatically start after the reactor automatically shut down. So assuming I am correct, the person would not be in trouble until the cooling system failed. If I am wrong about the cooling system automatically starting he would be in danger much more quickly, on the order of hours or days. However long it would take the water to boil out of the core and cause a meltdown.

So, in your situation the person who survived would not have power after a certain amount of time because the plant would have hit a set point causing the plant to insert all of the control rods and causing it to stop producing power. If the plant had offsite power it would continue to run until it hit one of those set points, if it did not have offsite power it would insert the control rods and stop producing power.


Q:

Shutdown cooling doesn't auto start. It's one of the RHR modes that has no automatic function.

The shutdown cooling interlocks don't clear until reactor pressure drops low enough that Tsat is less than 330 degF (approximately). Usually this is set less than 100 psig. And both main EHC and the relief valves won't automatically depressurize the reactor, that takes manual operator action. Once you get below the RHR interlock, it takes manual operator action to reset the interlocks, then you need to flush the SDC loop with condensate, and perform manual warming or the loop, before finally placing RHR in service on the vessel. It's a drawn out process.

Other potentially complicating issues: unless you installed a digital feedwater control system, main feedwater has a tendency to overfeed after a scram and trip all your feed pumps off. HPCI and RCIC will cycle along with relief valves, but it's kind of a mess running those systems cycling on level 2/8, and puts a lot of fatigue on their associated injection spargers. My plant tries as hard as possible to never start RCIC after we allowed it to cycle for days following a LOOP in the 90s. We heavily fatigued the injection nozzles and our steam dryer because of the thermal shock every time it started up.

The ESBWR, if it ever gets built, will automatically go into SDC. It uses the reactor water cleanup system for SDC. The ESBWR has an oversized non regen hx with variable speed pumps to automatically control the cooldown rate. But that's the exception to the rule, and doesn't exist yet.

A:

Thanks for the added response! you were clearly an operator before. We do have digital feedwater system here for what its worth.

So, in response to the original question, it sounds like the last person alive would not have a very good day.


Q:

Do you believe that nuclear energy is a safe and sustainable source of power for the modern world?

A:

I do think nuclear power is safe and sustainable. Staying economical is the difficult part today with all of the regulations. Without carbon-free subsidies it is difficult for nuclear power to compete with other energy sources. Wind and solar get huge subsidies, and fracking has made gas very cheap as well. This is why you see smaller nuclear plants shutting down.


Q:

Thanks for the reply. I too believe that the way the system is rigged paints nuclear power in a bad light. The 'green' aspect of solar is a myth. The process of making the panels does a lot of harm to the environment; especially when considering the regulations of the country where the vast majority of panels are made (China). Nuclear power has gotten a bad name do to the fact that many of the major energy companies don't want to invest. They are just as happy refining their cheap and unsustainable petroleum.

If nuclear power was more prevalent, the world would be more inclined to find a use for the spent nuclear material (other than enriched uranium) that is produced during the process of creating energy.

Stay non glowy my friend.

A:

Yeah, you got that right. I think France actually reprocesses their spent fuel and then reuses it. We have the technology but do not do it in the US anymore because of nuclear proliferation. Another note on solar that you never hear about, is throwing away solar panels is hazardous wast because they are made with some nasty heavy metals.


Q:

CANDU FTW.

A:

We don't have any heavy water reactors in the US, but maybe one day.


Q:

None? Wow.

Considering the exchange rate, a CANDU system would be a bargain.

A:

Yeah, I'm not really sure why the US never picked up on it. It seems like we do not have any interest in new or foreign designs. The thermodynamic efficiency is also slightly less in a candu. Their efficiency is between 28-30% where as light water reactors are around 33%.


Q:

The NRC in the 90s did a comparison of the CANDU and Canadian nuclear standards to their own. The CANDU didn't meet the same standards for a number of things including positive void coefficient, some differences in station blackout response, on site tritium inventory limits, among others. I'll need to find it (have it on my drive at work). The CANDU plants also don't have full reinforced containments like we use. They typically use a sub atmospheric containment system.

CANDU plants also have very low hot excess reactivity, to the point that doing a 10% load drop can cause a xenon transient that pulls you down to 60% or more. The low HER helps with the positive void response but their units can be xenon precluded from restart.

Don't get me wrong, it's a good design, but we just don't have any mostly for regulatory reasons.

A:

TIL ^


Q:

Are the heat exchangers between the cooling water from rivers and the closed loop radioactive water "shell in tube" or "plate in frame"?

How are the heat exchangers safely cleaned?

A:

It is shell in tube. The river water is in the tube side and radioactive steam is in the shell side (for the condensers). They are cleaned by either reversing the flow of the water through them or physically going in and cleaning them at reduced power. At a reduced reactor power the dose rates are low enough for workers to clean them. The most important part to clean is the waterbox where build up form "dirty" river water deposits.


Q:

Are you a safety inspector who works in Sector 7G?

A:

Me and Homer have a lot in common, but no I am not Homer Simpson.


Q:

Any thoughts on the Fort Calhoun Station closing?

A:

Any nuclear power plant closing is a shame. Fort Calhoun was a small (compared to other commercial plants) reactor but still employed 700 people. Small plants still have to abide by the same regulations and security measures that other plants do. That makes them more expensive to run per-megawatt hour than it would cost larger plants. There are several other small plants planned to close due to economic pressure. Closing nuclear plants means that energy will be made up by other means, which today, means it will be replaced with a CO2 emitting energy source.


Q:

Do you work with a lot of nukes (Navy sailors with nuclear power training)?

A:

At my facility many of the operators are nukes and they often transition to supervisory roles. They are very common here.


Q:

Do you ever wish that ADM.Rickover was still alive and overseeing the reactors?

A:

He was in charge of the naval nuclear reactors and promoted pressurized water reactors. He did a very good job overseeing the naval reactors and for that reason I wish he could have lived forever. I also think if he were still alive he would be an excellent pro-nuclear advocate.


Q:

Do you have hot chicks with automatic weapons at your workplace?

A:

They carry semi-automatic AR-15's and a hand gun.


Q:

But are they hot.

A:

I am sure some are :D its difficult to judge with all of the gear they have on.


Q:

I'm guessing your on the east coast, so, do you believe that a tragedy such as the one that occurred in Fukushima with the cooling systems failing due to the earthquake may occur in the United States due to the hurricanes that tend to hit the east coast? If not, what measures do we have in place that prevent this, if we do, what are they?

What type of reactor do you work at? No worries, you don't need to mention the name of where you work, I just know that there's a couple of types here in the United States.

Would nuclear energy ever be made available to consumers in terms of powering small vehicles or commercial planes? The United States already powers submarines and carriers this way, would it ever become available to the everyday consumer.

Does the military monitor you?

A:

Wow quite a question. Yes, I am on the east coast.

Fukushima was some pretty substantial circumstances. It was the combination of the earthquake and the tsunami. The earthquake took out their offsite power which plants in the US and the Japanese plants are designed to handle. The Fukishima plants all shutdown properly once they lost power and were using their diesel generators to cool the reactors. Then, the tsunami hit which happened to be larger than their tsunami wall. Due to poor plant design, the Fukushima plants had their diesel fuel tanks outside on the ground which the water swept away leaving them with no power source to run the cooling pumps.

Here in the US, our plants are analyzed for earthquakes, tornadoes, floods, and any other naturally occurring event. Being on the east coast a tsunami isn't much of concern, but even if it was our diesel tanks and emergency equipment is required to be at a certain height above sea level in a water tight room.

We have also implemented measures after fukushima to prevent something like that happening in the US. It is implemented in 3 phases. Phase 1 is being able to rely on currently installed equipment to maintain key safety functions with an extended loss of AC power. Phase 2 involves being able to use portable equipment that can maintain key safety functions i.e. portable generators, portable pumps, portable fans, communication equipment, etc. Phase 3 involves the ability to ship equipment to any plant in the us by truck or air in 24 hours. There are two offsite facilities in the US ready to ship this equipment if a similar event occurs.

I work at a boiling water reactor. In the US there are boiling water reactors as well as pressurized water reactors.

I do not think that nuclear energy is a viable source for cars or planes. Space probes are technically nuclear powered but not in the way you are thinking. Space probes use a cool technique using plutonium and thermo-couples. Submarines use nuclear power to generate steam which uses a turbine and generator to create electricity, then they use an electric motor to power a sub. So imo that would be too complex of a system to power a car or plane.

And finally, no the military does not monitor us. We are monitored but the US nuclear regulatory system.

Sorry for such a long answer :D


Q:

No need to apologize, a very thorough answer is appreciated when a thoughtful reply is given. In other words, I appreciate you taking time out of your schedule to answer it.

In other words, the "what if" is always thought of and plans are made to prevent it. My biggest surprise is that there are 2 off site facilities which would play into helping to prevent and off set such a catastrophe.

So, basically, we might never see it in hands of everyday consumer, probably for the best.

Now, your reactor must definitely be connected to a grid providing energy to a lot of homes and businesses. Do you think that the current way which our grid system is set up in which many states share the same grid should be changed? I look at Texas and see that they have their own, if we do change it and move towards states controlling their own grids, how would nuclear energy be the best answer for states to power their own grids?

A:

At my site we provide power for 3,000,000 homes.

I like how the grid is currently set up because it allows us to be very versatile. If a certain region needed more energy for some reason it is very easy to pay slightly more and obtain the additional needed energy. This could still be a backup option if the grid is still connected and another state needed power from a neighboring state.

It would be difficult for each state to provide their own power because not all states have the means to do so. For example, coal, oil, natural gas, and nuclear all require a water source to cool the plants which not all states have. The aforementioned plants make up 90% of electricity in the US, the other 10% come from hydro electric (need a dam) or renewable energy.

Nuclear would be a good option to power a state's own grid because it is a reliable energy source and does not take up much space. To put it in perspective, there are a total of 4 nuclear plants in New Jersey which power 51% of New Jersey residents. With global warming in mind, the operation of the plants without CO2 production is also a plus.


Q:

Have you ever watched "The China Syndrome" ?

A:

I have never seen the movie but I know a bit about it. I believe if the plant in question melted through the entire Earth it would've ended up in the Indian Ocean and not China though.


Q:

What's a nap in the reactor core like?

A:

A nap? like someone sleeping lol? Since the core is under quite a bit of water, imagine someone on a pool float with a margarita in hand relaxing.


Q:

Funny thing is that a few meters of water between you and the reactor core already exposes you to less radiation than a person standing outside.

https://what-if.xkcd.com/29/

A:

Yeah, that's exactly right. If anything the people swimming in the water would dirty up the water.