  We consider random walks in a random environment that is given by i.i.d.
Dirichlet distributions at each vertex of Z^d or, equivalently, oriented edge
reinforced random walks on Z^d. The parameters of the distribution are a
2d-uplet of positive real numbers indexed by the unit vectors of Z^d. We prove
that, as soon as these weights are nonsymmetric, the random walk in this random
environment is transient in a direction with positive probability. In dimension
2, this result can be strenghened to an almost sure directional transience
thanks to the 0-1 law from [ZM01]. Our proof relies on the property of
stability of Dirichlet environment by time reversal proved in [Sa09]. In a
first part of this paper, we also give a probabilistic proof of this property
as an alternative to the change of variable computation used in that article.
