The Axe-Man of Nuclear Reactor

The story begins with a well. It is called the well of electricity. By some means, it produces electricity from inside and it is very very hot (hundreds of degree)!! There is some kind of rods hung above the well, suspended by a rope. There is a person in charge of this well. His job is to take a fire axe, waiting for the signal. When the ringing bell informs that the well temperature is over the limit, he rapidly cuts the rope, allowing the rods to fall into the well and stop whatever happening in it. As for his good job, he is given a title as SCRAM, or Safety Control Rod Axe Man.

Who is he?

Well, the Axe man is nobody but control rods of nuclear reactor. They are made by neutron-absorbing materials such as Silver (Ag), Indium (In), Cadmium (Cd), Hafnium (Hf) or Boron (B). Their function is to increase or decrease the number of neutrons in the reactor. They are inserted or withdrawn from the core to control the rate of fission reaction (depending on the number of neutrons). In turn, it affects the thermal power of the reactor, the amount of steam produced, and hence the amount of electricity generated.

If the materials absorb all the neutrons coming to them, they are called “Black” neutron-absorbers. If only part of incident neutrons is absorbed, they are called “Grey” neutron absorbers. It is like we tend to absorb a larger amount of sun lights when wearing black clothes than white clothes (simple enough??)

Control rods can operate to serve different purposes. Therefore, they are called different names.

1. Shim rods: To make a coarse adjustment in the reactivity of a nuclear reactor

2. Regulating rods: To make a fine, small adjustment to maintain desired power

3. Safety rods: To provide a very fast shutdown in case of unsafe condition (SCRAM).

Different type of reactors and controls require different type of control rods. They are often combined together to become dual-purpose or triple – purpose rods.

Beside the control rods which control the reactivity of nuclear reactor, another important element which contributes in this job is called Poison.

Although it is called poison but it is good for the nuclear reactor. It is also made of neutron-absorbing materials like control rods. The need of Poisons is explained as following.

After a long time operation, the amount of fuel contained in the core decreases. Therefore, the fuel in excess of that needed must be added when the reactor is built. However, to maintain the normal operation of reactor, we need to decrease the amount of excess power which is generated from excess fuel loaded into the core. One method is to use movable control rods as described above. But control rods alone to balance the excess reactivity may be impractical for a particular core design as there may be insufficient room for the rods or large their mechanisms. That’s why to control a large amount of excess fuel without adding additional control rods. So, we need Poisons.

There are several types of Poisons.

1. Burnable poisons: as their names, they are burned up when absorb neutrons. So, its effect is decreasing over the core life.

2. Fixed burnable poisons: they are burnable poisons which are shaped with the core.

3. Soluble poisons: they are soluble circulated in the coolant during normal operation.