how engineers control power station?

It's a "moderator". Soviet reactors like Chernobyl use bricks of graphite as the moderator. US reactors use water.

--Gene

So what does the moderator do? So what is the advantage of using wanter?

Peter Nachtwey

On Tue, 16 Jan 2007 02:03:26 -0500, Gene S. Berkowitz proclaimed to the world:

Thanks for the reminder. Other materials have also been used in the past too if I remember correctly.

The pistons in a car move up and down very quickly. Yet the speed of their movement is controlled by the driver pressing his foot down on a pedel. Same principle. Also, although the reaction between methane and air in a furnace is very fast, the rate of heat production is by machanical movement of a valve, often control by pneumatic signals.

The moderator makes the reaction work. Actually, a problem is that the neutrons leave the splitting atoms so fast that if they hit another atom at their original speed, they would bounce off. So to make the reaction proceed, the neutrons must be slowed down. That is the purpose of a moderator.

There are several advantages of using water:

1) As water gets hotter, the water molecules move further apart, reducing the effectiveness of water as a moderator. (Same is true with carbon, but to a lesser extent). If bubbles form in the water, the modertion effect is greatly reduced, slowing down the reaction. All of this results in a negative effect of temperature on reactivity. In other words, some negative feedback in the process. 2) Water is used to carry the heat from the reactor to the steam generators. In the SGs, hot, pressurized water flows through tubes, causing water outside the tubes at a lower pressure to boil, producing steam to power the turbine. Since they have water anyway, why use anything else? 3) Water is cheap.

In one of the nuclear engineering classes I took in the early 70's I learned that after an accident at a small research reactor in England (Windscale), no one would ever use carbon as a moderator again. Apparently someone in the USSR didn't agree.

Many years ago I worked in nuclear power and nuke plants. I still glow in the dark.

I think he was kidding. That stuff is pretty well known and documented.

The hard part about designing nuclear weapons is not determining the critical mass or geometry. It is figuring out how to reach the critical mass for an explosion faster than the material can heat up. The hotter the uranium, the less likely it is to "go critical". I think that most bombs have a hollow sphere surronded by a conventional explosive that crushes the sphere into a smaller sphere fast enough to explode.

John Shaw

The fact that individual reactions happen quickly, the overall effect of gaining enough criticality to begin raising steam will be much lower overall.

While it takes significant time to raise the rods to increase criticality, to drop them in an emergency is only a matter of releasing them to descend quickly under gravity. On a modern AGR the reactor will be cooling within 6 seconds by convection of the CO2 gas if all site supplies have failed - a highly unlikely event.

Fission of an atom of fuel can produce two or three fast neutrons. Fast neutrons are less likely to cause fission in a neighboring atom of fuel. The moderator slows down the fast neutrons, greatly increasing the probability that they will cause fission in the surrounding fuel, sustaining the chain reaction.

The hydrogen in water is a moderator, while the water can simulataneously serve as coolant. However, since water is also a very good neutron absorber, reactors that use it require enriched uranium as fuel, while a graphite-moderated reactor can use unenriched uranium.

--Gene

Moderator?

Jerry

Beryllium?

Jerry

You could say that a bomb is deliberately out of control. Designing a bomb requires that one know that amount of fissionable material that has a neutron gain of much greater than one, and can also be separated onto two parts that have very low gains. That's the easy part. The art is devising a way to quickly mate the two parts (quickly so that high but not high enough gains don;t last too long) and hold them together long enough for a good fraction (a few percent) of the material fissions. There's no room for a moderator, so the neutron gain gas to be very high, fast neutrons being not very efficient.

Jerry

In 1960? I suppose so.

The early designs used hemispheres, later a sphere with a conical insert "removed" and used artillery shells to blast and rails to guide the parts together. Ir maybe that was just disinformation.

Jerry

Yes, I remember hearing about the early designs. A solid hemisphere would be the wrong geometry and critical mass to go "prompt critical". But put two together and you have a sphere, the perfect geometry. You only have to move them together fast enough--the purpose of an explosive or artillery shell.

The important difference between a nuclear bomb and a nuclear reactor is the "prompt" part of prompt critical.

When a neutron hits a uranium atom and causes it to split, two or three neutrons are emitted. Most of these come out after a very slight delay. However, a very few neutrons come out without the delay.

If the reaction is critical based only on those few "prompt" neutrons, it is "prompt critical". The reaction will increase so fast that there is an explosion.

The level of enrichment required for a sphere of uranium to go "prompt critical" is far more than the level needed to simply go critical.