  We present an analysis of electron transport through two weakly coupled
precision placed phosphorus donors in silicon. In particular, we examine the
(1,1) to (0,2) charge transition where we predict a new type of current
blockade driven entirely by the nuclear spin dynamics. Using this nuclear spin
blockade mechanism we devise a protocol to readout the state of single nuclear
spins using electron transport measurements only. We extend our model to
include realistic effects such as Stark shifted hyperfine interactions and
multi-donor clusters. In the case of multi-donor clusters we show how nuclear
spin blockade can be alleviated allowing for low magnetic field electron spin
measurements.
