Question: Initial boundary conditions of a PDE

Hello, I'm currently trying to approximate the solution of a PDE with Maple, but I'm having troubles with the boundarie conditions .
For instance, when I set : (dT/dr)(r=3,t) = 0 , everything works fine .

With (dT/dr)(r=3,t) = -T(r=3,t), it's still okay .
But if I set (dT/dr)(r=3) = -T(r=3,t)^4, Maple just ignores the second member and works as if it was 0 . There is no error notification, but I don't get a satisfying solution .. 

I have no idea where this problem comes from, if could someone help me .. 

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