Nuclear Poison

A nuclear poison is a substance with a large neutron absorption cross-section. Simply put, it can absorb neutron very effectively.

Where do these poisons come from? For starters, in the nuclear reactor, there will be material that is produce by the fission reaction that can absorb a lot of neutrons. Because we need neutrons to keep the reactor running, having less neutrons will result in less power being generated. In fact, if the poison level is unchecked, it may cause the nuclear chain reaction in the reactor to stop.

The radioactive materials in the reactor will decay. From time to time, there will be materials in the reactor that its decay process will result in a new substance that have a high neutron absorption cross section. This too will lead to an increase in the amount of nuclear poison.

Let's say that u have a jug of water. Then, a sponge is put inside the jug. After a while, you will noticed that the level of water inside the jug has decreased. Imagine that the water is the neutron and the sponge is the poison. Neutron level will be reduced by the existence of the poisons.

But sometimes, you will need to keep some amount of poison in the reactor. Why you ask? It is because the poisons can be used to regulate the reactivity of the reactor. For example, you want to run the reactor with a 200 grams of U-235. Since the radioactive material will decay, the amount of fuel will decrease over time. In order to solve the problem, you will need to put more than 200 grams in the reactor. The more fuel you use, the reaction rate will be higher. So to effectively control the neutron level, poison is put inside the reactor.