Stupid questions about nuclear energy.

[john70t]

As I understand it:
-There are rods which heat up when placed together.
-These rods have to somewhat cooled in a circulating bath of water, which also makes steam to drives turbines.
-When the rods are seperated, they no longer interact and create heat.

The stupid questions:
1). Why don't nuclear power plants have a mechanical system to seperate these rods? A hand-crank system might work in a total loss of power.

2). Why doesn't every power plant have a 30-day resevoir of gravity-fed cooling water above it?

3). Nuclear plant next to ocean. Wall?

[sc_rufctr]

No such thing as a stupid question when it comes to nuclear reactors IMO.

The stupid questions:
1). Why don't nuclear power plants have a mechanical system to seperate these rods? A hand-crank system might work in a total loss of power.

Not a bad idea but the rods are very very heavy and to be able to hand crank them would mean they would rise very slowly. (because of the reduction gearing in the mechanism)

2). Why doesn't every power plant have a 30-day resevoir of gravity-fed cooling water above it?

That would be a huge reservoir. I'm guessing around 100 Olympic sized swimming pools at a minimum.

3). Nuclear plant next to ocean. Wall?

It would have required a massive wall to protect the plants from that Tsunami.
Maybe better located a few km from the Ocean. Or on the other side of Japan away from the "Ring of Fire" fault line. (Chinese side)

[VaSteve]

Where does thee fuel come from? How do they manufacture the rods?

[IROC]

Originally Posted by john70t

The stupid questions:
1). Why don't nuclear power plants have a mechanical system to seperate these rods? A hand-crank system might work in a total loss of power.

The rods (fuel assemblies, actually) are packed so tightly together in the core that there is no room for them to move. In between each assembly (at the junction of four, actually), is a control rod that absorbs the neutrons and does essentially what you're talking about. Besides, you really don't want them to move.

The sheer size of these assemblies (and the absolute impossibility of any human to get close to that area) precludes any sort of hand-cranking of anything...

2). Why doesn't every power plant have a 30-day resevoir of gravity-fed cooling water above it?

The pressures that a reactor operate at make it impossible to gravity-feed water in. Think about an overheating radiator on your car. A reservoir containing a 30-day supply of water to cool a nuclear plant would be astronomically large. Each unit at the plant I worked at used 660,000 gallons per minute of raw cooling water. The main cooling water "pipe" was 16 feet in diameter.

3). Nuclear plant next to ocean. Wall?

They had a wall designed for a 6m tsunami. What they got was a 10m tsunami. Nuclear plants need lots of cooling water, so they have to be located near large bodies of water. For Japan, that means on the coast. You could argue that they need to build taller walls, but it is difficult and expensive to design every aspect of the plant for the largest earthquake in Japanese history.

[IROC]

Originally Posted by VaSteve

Where does thee fuel come from? How do they manufacture the rods?

Basically, uranium pellets (cylindrical, but about the size of a Lego block...) are stacked inside of sealed tubes that are maybe 15 feet tall and then a sqaure matrix of these tubes are configured into a fuel "assembly". Each assembly has a mixture of fuel and also tubes filled with gadolinium for neutronic performance, etc. The core of the reactor is made up of a number of these fuel assemblies arranged in a pattern to coincide with the control rods, etc.. The plant I worked at had 764 assemblies in the core. During a refueling outage, 1/3rd of the core is replaced and those "spent" assemblies are moved into the spent fuel pool.

[red-beard]

John,

One other thing to note, the rods produce some decay heat even when separated. This type of heat is used for powering some space-craft where solar panels cannot be used. Compared to the heat produced during operation it is small, but it is large enough to cause the problems we're seeing now.

Mike, My dad and I were discussing this yesterday. Why isn't anyone talking about that the plants actually survived. We had a massive double disaster. They had a 20 foot tsunami wall. No one could anticipate the 9.0 quake plus 30+ foot Tsunami. The entire east coast had the 20 foot wall, to stop tsunamis. And the people stayed and are fighting this.

For all of those who would truly like to understand most of nuclear power, raditation and health effects, dirty bombs, etc, at least a lay level, try reading "Physics for Future Presidents".

[ErVikingo]

Thanks for the info IROC.

From what I understand of these particular plants, the problem was not that the Tsunami damaged the reactor buildings but flooded the standby generator area which caused the lack of power for the pumps.

I have been to many talks and sales pitch meetings on the "new" designs and location and redundancy in backup power is a very big part of the subject.

[IROC]

Originally Posted by red-beard

Mike, My dad and I were discussing this yesterday. Why isn't anyone talking about that the plants actually survived. We had a massive double disaster. They had a 20 foot tsunami wall. No one could anticipate the 9.0 quake plus 30+ foot Tsunami. The entire east coast had the 20 foot wall, to stop tsunamis. And the people stayed and are fighting this.

Exactly. I've actually been getting a lot of my information on this crisis from the BBC news, but watched CNN for a bit last night and CNN seemed to be focusing on whether or not TEPCO was trying to cover up what was going on and whether they were being truthful, etc. That really rubbed me the wrong way. In my opinion, to even begin to suggest in this incredible event that TEPCO (or even the workers at the plant) are putting forth any less than their utmost effort to contain this problem is just irresponsible.

You're right, too. It is actually a testament to mankind's ingenuity that these plants have held up as well as they have (or that other plants all over Japan aren't in the same situation). The general public really has no idea how much time, money and engineering expertise goes into the planning and design of these plants. The safety of nuclear power is not taken lightly. There was no "inherent flaw" at the Fukushima units that led to this problem (I have actually seen that suggested in the media).

This was an incredible natural disaster and the workers at the plant should be hailed as heroes moreso than any other group of people in recent history, IMHO. I have been pretty much everywhere a human can go in that plant design and to think that these guys are in those places under those conditions is just awe inspiring.

[ErVikingo]

This is from one of the presentations I received:

The AP1000™ pressurized water reactor works on the simple concept that, in the event of a design-basis accident (such as a coolant pipe break), the plant is designed to achieve and maintain safe shutdown condition without any operator action and without the need for ac power or pumps. Instead of relying on active components such as diesel generators and pumps, the AP1000 relies on the natural forces of gravity, natural circulation and compressed gases to keep the core and containment from overheating. However, many active components are included in the AP1000, but are designated as non safety-related.

[IROC]

Originally Posted by ErVikingo

From what I understand of these particular plants, the problem was not that the Tsunami damaged the reactor buildings but flooded the standby generator area which caused the lack of power for the pumps.

Yeah, to me, this is the disappointing part. The plant is designed to safely shutdown and remain safe with zero outside power coming in and zero power being produced at the plant itself. In hindsight, the vulnerability of the diesel generators (or their fuel supply, etc) was critical.

[red-beard]

Originally Posted by IROC

Yeah, to me, this is the disappointing part. The plant is designed to safely shutdown and remain safe with zero outside power coming in and zero power being produced at the plant itself. In hindsight, the vulnerability of the diesel generators (or their fuel supply, etc) was critical.

Anyone who knows anything about Rankine cycle style plants knows you put the boiler feed pumps as low as possible to prevent cavitation. And then a giant flood occurs and the pumps are in the bottom of a sump area! Lay people (reporters) will say this is a design defect.

I don't know where the emergency equipment was positioned in the plant, but no one was expecting a wave this big, ever.

I heard one reporter ask why they haven't brought in some portable generators. I doubt he understand how many mega-Watts are needed to run the plant. Portable power is generally "kilo-Watt" in nature. A 1.3 mega-Watt Solar Saturn unit, with all accessories, and only a small amount of fuel, is still a 20 foot long trailer. And a 40 MW GT, about what is needed here, consumes about 1 gallon of #2 per second!

[IROC]

Originally Posted by red-beard

Anyone who knows anything about Rankine cycle style plants knows you put the boiler feed pumps as low as possible to prevent cavitation. And then a giant flood occurs and the pumps are in the bottom of a sump area! Lay people (reporters) will say this is a design defect.

Very good point. NPSH is critical. The plant I worked at had all of the floor levels noted in feet above sea level. The lake level was nominally at about 550 ft. The main floor of the reactor building was 565. The emergency cooling pumps (RHR, Core Spray, HPCI, RCIC, etc) were at the 519 level. Over 30 feet below lake level...

[GH85Carrera]

Now this is ironic:

Greg Palast » Tokyo Electric to Build US Nuclear PlantsThe no-BS info on Japan’s disastrous nuclear operators

[IROC]

Originally Posted by GH85Carrera

Now this is ironic:

Greg Palast » Tokyo Electric to Build US Nuclear PlantsThe no-BS info on Japan’s disastrous nuclear operators

Well, there's two sides to every story, but I'm not buying his side. Lying on test reports and certification documents is one thing (and completely unacceptable), but to essentially label the nuclear industry as a bunch of bungling idiots is just too much. Quotes like:

What dim bulbs designed this system?

TEPCO and Toshiba don't know what my son learned in 8th grade science class: tsunamis follow Pacific Rim earthquakes.

[krystar]

some of these web blogs are targeting the joe the plumber population.
"why didn't they just design the system to be earthquake and tsunami proof?" yea...try designing something to withstand 9.0 earthquake, 30ft tidal wave, meteors from space that weren't blown up by bruce willis, godzilla....oh and still come in below the budget.

reporters are good for investigating and reporting. don't expect them to come up with engineering specifications of a solution.

[javadog]

Originally Posted by krystar

reporters are not worth a damn at investigating and reporting... don't expect them to come up with engineering specifications more detailed than how to flush a toilet.

I'm sure this is what you meant to say...

JR

[944Larry]

Looks like we have some good knowledge here on this board! My question is; Why do nuclear plants put out so much radiation when they go berserk but nuclear weapons which apparently have a much more potent fuel percentage wise not need all this shielding. I have been within a few hundred yards of a nuke, people were working all around the thing and I never saw any of those little dose tags like x-ray techs wear on anybody.

[A930Rocket]

His kid probably learned in 8th grade right after the 2004 quake/ tsunamis.

I sure didn't know about them until 2004, but what do I know.

TEPCO and Toshiba don't know what my son learned in 8th grade science class: tsunamis follow Pacific Rim earthquakes.

[MikeSid]

Originally Posted by 944Larry

Looks like we have some good knowledge here on this board! My question is; Why do nuclear plants put out so much radiation when they go berserk but nuclear weapons which apparently have a much more potent fuel percentage wise not need all this shielding. I have been within a few hundred yards of a nuke, people were working all around the thing and I never saw any of those little dose tags like x-ray techs wear on anybody.

My completely uneducated guess is that radioactive material in the weapon is contained safely by a lining like lead which surrounds the radioactive material until detonation. I would assume assembly of the warhead is done robotically or with heavy personal shielding.

[IROC]

Originally Posted by 944Larry

Looks like we have some good knowledge here on this board! My question is; Why do nuclear plants put out so much radiation when they go berserk but nuclear weapons which apparently have a much more potent fuel percentage wise not need all this shielding. I have been within a few hundred yards of a nuke, people were working all around the thing and I never saw any of those little dose tags like x-ray techs wear on anybody.

Until the uranium fuel is irradiated, it is perfectly safe. You can walk up and lick a fuel assembly. In other words, until the nuclear reactions start, the fuel (or bomb material) is essentially harmless. Once irradiated, it becomes highly activated.

Edit: Another important point is that not only does the fuel become highly activated (radioactive), but the components of the reactor do too. The reactor vessel istelf, piping, pumps and everything are now radioactive through a process called ionization.