CZ-SK-MO 2006 Problem 1

Define a sequence of positive integers $\{a_n\}$ through the recursive formula: $a_{n+1}=a_n+b_n(n\ge 1)$ ,where $b_n$ is obtained by rearranging the digits of $a_n$ (in decimal representation) in reverse order (for example,if $a_1=250$ ,then $b_1=52,a_2=302$ ,and so on). Can $a_7$ be a prime?