Whereby

Hi Saffy,

"Whereby" is used to describe the way in which something is done —  essentially the "how". Where" has more than one use: it can describe the location of something or someone, or it can start a sentence as a question word, or provides further information about a situation. 

Your sentence explains how (the steps taken at an atomic or subatomic level) nuclear fusion is initiated, thus the use of "whereby". If you replace "whereby" with "where", the sentence still makes sense as you are now elaborating on a situation (a stage in nuclear fusion). 

Let's look at another sentence:

Everyone was looking around to find out ______ the blast came from. 

Here, "whereby" will not fit the bill as the focus is on the location of the blast. 

Another example:

Immigration is a thorny issue _______ politicians need to tread carefully. 

Here, "where" fits the bill because "politicians need to tread carefully" elaborates on the situation of immigration. It would sound odd to use "whereby" as we are not referring to a system of actions or answering how something is done. 

I hope this helps. 

Whereby

whereby = by which way/ method

not standard would be "in which/ with which"

But there are few misused words here (mostly just scientific stuff)
1) chemical reaction: nuclear fusion is not a chemical reaction, it is a nuclear reaction.

2) binds - in the nuclear field, binding is referred to as nuclear binding energy (minimum energy required to disassemble the nucleus of an atom into its component parts). And in the chemical field, it is referred to the force responsible for binding atoms together or binding molecules together.

3) atoms - nuclear fusion makes heavier NUCLEUS (part of an atom) from two lighter NUCLEI.

The correct definition of nuclear fusion should be: "nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons)" (from Wikipedia)

Edit: nucleus the singular form and nuclei is the plural form just for further clarification.