No doubt that a two way trip with 1.5 years on the surface will give astronauts a major dose, probably enough to make it their one and only trip in a career. (They will take on something like the lifetime limit of a nuclear power employee, albeit over the course of only a few years.) However, this is clearly not a show-stopper at all. Especially if some basic shielding is provided by spacecraft fuel, water, and some low-z materials as needed. Note that several astronauts have already spent about a year in LEO, meaning that they got perhaps a third as much radiation exposure already.