How are nuclear chain reactions controlled in nuclear power plants?

A nuclear reactor is like a device like in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate. The heated water rises up and passes through another part of the reactor, the heat exchanger.

Iam on that question know can you tell me the answer i am realy strruglingasap

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero.