Help with while loop in sql server

Question

I have this query

 

Declare @Fnum nchar(5)

 

While (len(@Fnum)<5

Begin

 Set @Fnum += '0

End

 Print @Fnum

 

I am expecting 0000 as result

 

 But this is not happening, I only have 0 as result?

 

Can I have some help please?