DiffEQ's and Bessel Functions and their kin "are" how we determine the neutron flow through the core, where they are, how their concentration changes with time and position (height and radius and efficiency and movement up/down and speed and cross-section) of the control rods.
DiffEQ's are pretty fundamental to FEA analysis (boundary conditions of whatever you want to measure, or plot, or control at the edges and boundaries of your FEA structure.
So, yes, you need to understand them IF if you are going to be doing something that requires them later on. But, really, unless graduate school is your real goal in life, or unless you are going to be using them later on a constant basis that you cannot pick up from your FEA training and practice (if you use FEA at all), you've passed. Don't bother taking the class again, unless you really, really want that grad school for a PhD or for your self-image of success. You certainly can't erase a C.
Get better grades in something important.
Get hired.
Start paying taxes. 8<)