Avoiding nested ifelse without using "dplyr" library

Actually, I want to assign to each credit_category a specific Risk-weight.

I've been using nested ifelse as shown below, but I am trying to find something else in order to avoid the nesting, without using "dplyr" library. Any ideas ?

tab_nonsec$RW = ifelse(tab_nonsec$credit_category=="AAA", 0.005,
                       ifelse(tab_nonsec$credit_category=="AA", 0.02,
                       ifelse(tab_nonsec$credit_category=="A", 0.03,
                       ifelse(tab_nonsec$credit_category=="BBB", 0.06,
                       ifelse(tab_nonsec$credit_category=="BB", 0.15,
                       ifelse(tab_nonsec$credit_category=="B", 0.3,
                       ifelse(tab_nonsec$credit_category=="CCC", 0.5,
                       ifelse(tab_nonsec$credit_category=="Unrated", 0.75,
                       ifelse(tab_nonsec$credit_category=="Defaulted",1,0
                       )))))))))