What factors can affect the change in the fast to thermal flux spectrum?

I’ve tried running the code for the PWR and the results of the flux spectrum of the neutrons are still fast. How can the flux of the neutron spectrum change to thermal?

The flux spectrum depends on the entire system and it’s properties (material compositions and geometric arrangement). If you truly have a PWR geometry the flux spectrum should indeed look like a typical thermal spectrum. That may mean you have an error in your problem definition. If you’re able to share your model and results, someone may be able to help diagnose what’s going on.