thic code works for 1Dimntion slab reactor how can you make it in three dimensions for cubical reactor with the same lengths

clear clc %the slab reactor length L=30; %the number of neutrons produced per fission v=2.4; %diffusion coefficient D=0.14; %geometrical buckling B=(pi/L)^2; %macroscopicfissioncrosssection mf=0.57; %macroscopicabsorptioncrosssection ma=(v*mf-D*B); % the neutron speed V=2200; %initial condition for i=1:1:301 u1(i)=1; u2(i)=0; end %u1(1501)=0; u1(1)=0; u1(301)=0; %plot(u1) dt=0.00001; dx=0.1; stab=dt*D*V/dx^2; k=1; for n=1:1:200000 for i=2:1:301-1 if i== 75 ma=(v*mf-D*B)*1.; else ma=(v*mf-D*B); end u2(i)=u1(i)+stab*(u1(i+1)-2*u1(i)+u1(i-1))+V*dt*(v*mf-ma)*u1(i); end for i= 2:1:301-1 u1(i)=u2(i); end if n==k pause(2) plot(u2) k=k+1; else end end