It will depend on which models you are using. I will have answered your question better if the question is more clear. There are several reactivity parameters, and reactivity coefficients due to the coolants' temperature and density are the important ones. The March-Leuba model is one of the standard lumped parameter models for a boiling water reactor. One of the articles are available online (Article Nonlinear Dynamics and Stability of Boiling Water Reactors: ...

Dear Vikas Pandey ,

Thank you so much for your valuable comment. I am trying to modelling the pressurized water nuclear reactor in Matlab/simulink. Here I have attached some papers.

Dear Mohammad Al Mahfuz, the answer depends on the level of spatial and temporal detail you are aiming at. Vikas Pandey already answered for static calculations. For time-dependent problems, relatively simple point-kinetic equations for reactivity could suffice for some purposes. However, for others one might need a full 3D dynamic simulation. (E.g. Thesis Analysis of the Rod Insertion Method for Control Rod Worth M...

There exist quite a selection of different programs for such purposes (see NEA Data Bank, Computer Program Services at www.oecd-nea.org/dbcps/ ). Are you sure you want to start programming from scratch? It is true, on the other hand, that fuzzy logic is relatively new approach and I am unaware of any programs using it to calculate reactivity of a nuclear reactor.