Thanks for your response Tim. You answers are reassuring.

1) Reguarding reactor SCRAM. Has it not happened in the past where the control rods would not go in due to core damage?

2) Reguarding ceramic not melting. Have you seen the pictures of the concrete "lava" flow at Chernobyl? If you get something/anything hot enough it will melt or burn. There is no way a hot spot could develop? Is all this melted core stuff a thing of the past?

3) You seem as optimistic about fusion as I am about renewables.



Kraken:

I will concede that we are going to need a LOT of energy. Electric cars would only make this problem worse for the electric power grid.

 

Okay, I've started this research, and I'll post what I have so far. This is a FAR FROM COMPLETE list of some, but not all, of the benefits we have from nuclear reactors and nuclear power.

1. Medical and biological uses.
a. Radiation treatment for cancer patients. High energy radiation can be used to treat malignant tumors. I know of a fantastic lab at Fermilab, in the Chicago suburbs, which you can read much more about here: http://www-bd.fnal.gov/ntf/ntf_home.html.
b. Other types of radiation therapy. Total body irradiation a radiotherapy technique used to prepare the body to receive a bone marrow transplant, for instance, and other types include the treatment of trigeminal neuralgia, severe thyroid eye disease, pterygium, prevention of keloid scar growth, and prevention of heterotopic bone formation.
c. Radiation tracers used in nuclear medicine to find holes in arteries, trace the flow of blood etc. through the body, etc.
d. Diagnostic x-rays. I think that needs no explanation.

2. Basic scientific research. Nuclear reactors produce neutrinos in sufficient number and with sufficiently predictable properties that they can be studied. This has led to a number of scientific experiments and projects involving the research of neutrinos coming from reactors at nuclear power plants. One such experiment was able to show that the atmospheric neutrino deficit was not due to nu-e to nu-m oscillations, which has added to the search for neutrino mass. A similar experiment provided scientists with information that limits the mixing angle (called theta one-three) with the best precision of any other scientific experiment that currently exists. Without going into all the technical details of what these things mean, let’s just say that the potential benefits to science and the world from these kinds of experiments are enormous, if unknown right now. No one could have predicted the age of electronics when scientists first discovered the electron, and it’s impossible to say what a better understanding of neutrinos may bring someday.

3. Industrial uses.
a. Radiography by means of gamma or x rays: This is a method used in industrial production. The piece to be radiographed is placed between the source and a photographic film in a cassette. After a certain exposition time, the film is developed and it shows internal defects of the material if there are any.
b. Gauges. Gamma ray penetration can be used as level indicators and to measure thickness.
c. Smoke detectors. Two ionization chambers are placed next to each other. Both contain a small source of 241Am that gives rise to a small constant current. One is closed and serves for comparison, the other is open to ambient air; it has a gridded electrode. When smoke enters the open chamber, the current is disrupted as the smoke particles attach to the charged ions and restore them to a neutral electrical state. This reduces the current in the open chamber. When the current drops below a certain threshold, the alarm is triggered.
d. Avoiding static. To avoid the build-up of static electricity in production of paper, plastics, synthetic textiles, etc., a ribbon-shaped source of the alpha emitter 241Am can be placed close to the material at the end of the production line. The source ionizes the air to remove electric charges on the material.
e. Radioactive tracers. This works in much the same way as in medicine. Tracers can flow with a fluid or gas in a system and allow you to find a leak in a closed system, measure wear on a surface, etc.
f. Sterilization. All kinds of things are sterilized with ionizing radiation, from medical instruments to food. The advantage of using ionizing radiation for this instead of non-ionizing radiation is that the object can be sealed in plastic before sterilization, and the radiation can act through the plastic, so that the object is sterile when opened. For food, there are strict regulations to avoid induced radioactivity.

4. Energy. This one is obvious and I know that other posters have already covered it to some extent. Our energy needs are growing, and the energy production capability of nuclear power is the basis for this whole conversation.

 

I want to congratulate everyone who has been involved with this tread. Nuclear power is an emotional issue (certainly for me anyways) and yet we have made it 9 pages without getting so heated that the thread had to be closed. This is a major accomplishment around here! Lets keep up the good work!

 

I think it is due to the moderators (in whom we live in fear) culling out abusers and bringing civility back to public discourse. We can agree to disagree without being disrespectful. Wiser engineers taught me this lesson a long, long time ago.

Bob Wilson

 

Good question, and I don't know if it has happened. I will say that it's theoretically possible if the path of the control rods were somehow blocked or severely bent. However if we're talking about an emergecy shutdown scenario I have a hard time imagining the core being damaged before the rods drop. Again, it comes down to the common misconception that fuel failure in a Three Mile Island-type accident is the result of a nuclear reaction gone horribly out of control. Not the case. Instead it's that decay heat that I was talking about earlier, where the reaction byproducts decay naturally after shutdown and could fail the assembly if not sufficiently cooled. In that case the damage occurred after reactor SCRAM.

It's worth pointing out that reactors "trip" (automatic SCRAM) all the time. It sounds bad, but it serves to demonstrate how sensitive the automatic safety systems are to off-normal conditions. Even something like a power interruption to the grid will do it, as in the case of that big northeastern blackout a few years go. The systems sensed that the load suddenly dropped, the power had no place to go, so the plants shut themselves down.

The point I'm trying to make is that the odds are extraordinarily low that inserting control rods would be the problem. They would be dropped well before the core got into any kind of condition that their path would be blocked.

Ahh, Chernobyl. What you say is generally true, that you can get pretty much anything hot enough to burn or melt. The reason that it wouldn't happen in a pebble bed reactor is that we have the ability to calculate the worst case scenario and design those pebbles to withstand it. For example, we know how much uranium we can put in a core at any given time, we can calculate the rate of reaction, and from that we can figure out how much heat the core would generate and for how long. We can also make conservative assumptions about cooling (say, reduce it to nothing more than conduction through the ractor vessel) and from that we can calculate the maximum temperature that could be expected in our worst case scenario. Then you multiply by a safety factor (something greater than 1 in order to give you a margin for error) and you come up with a design criterion for the ceramic casing on those pebbles. I can pretty much guarantee that concrete would not be the material of choice. In fact, the designs I've seen consists of several layers of several different materials to give them the strength and heat resistance needed. I figure if we can build ceramic tiles that can withstand the heat of the Space Shuttle re-enterig the earth's atmosphere, we can do this reliably. Later I'll try to find an internet reference regarding how they are put together.

I'm optimistic that we have the capability to find the answers that we desperately need. All it takes is sound research and engineering, and the money to back it up. The scientists and engineers I don't worry about. It's the people in charge of the purse strings that make me edgy.

 

Let's take out the normal reason why a reactor can trip, which causes the rods to go into a fail safe mode. What could prevent the rods from going into a fail safe mode. Let's say a terrorist where to get a device in there that would prevent the rods from going in. Is it impossible for this to occur?

 

Well, the only time anything gets in or comes out is when you're replacing spent fuel assemblies with fresh fuel. The rest of the time the system is sealed by some of the biggest bolts you'll ever see (or not see). But the way you install a control rod, at least in the PWRs that I am familiar with, is to drop it all the way into its assembly while you assemble the core. Re-starting the plant involves pulling those rods back out. Long story short, you couldn't put the reactor back together if there were something in the way of the control rods. A terrorist absolutely could not do it, even if he were one of the guys in containment putting it back together. Keep in mind that I'm not familiar with every single plant design. My knowledge is specific primarily to pressurized water reactors (PWRs). BWRs (boiling water reactors) tend to use a blade system and I'm not really sure how they go together. But from the standpoint of physics I have to believe that you start at zero power with the control elements inserted, same as a PWR. Otherwise how would fission be controlled while they button up the reactor vessel?

 

Ok. So the rods are not exposed in the sense that I imagined. EDIT: ^^ and I meant to say: take OUT the normal reasons...

On a separate note, I could have sworn I saw an Iranian official holding a bar of enriched uranium on the news. How is that safe? Is it not radio active in that state?

 

Thanks for taking the time to post that information Tim. It helps allot to blow past the hype.

 

Yeah, the rods are inside the pressure boundary. That's part of the reason that we use electromagnets to control them: it's the easiest way to manipulate them without actually having to put some sort of moving part through the vessel head.

As far as the uranium is concerned, that doesn't surprise me. I wouldn't necessarily recommend sleeping with it under your pillow for several years straight, mind you, but the level of radioactivity due to natural decay is pretty low before you expose it to a neutron flux and start splitting atoms on purpose. You can work around it safely without ill effects. Just don't try it with a fuel rod that's been in an operating reactor!