 The ultimate success of the pressurized water reactor , he wrote , was due less to any superior characteristics of water , but rather to the decision to power the prototype of the Mark I submarine thermal reactor with a pressurized version of the Materials Testing Reactor at Oak Ridge . Once pressurized water was established , other possibilities became too expensive to pursue , but Weinberg remained interested in other possibilities . According to Freeman Dyson , he was the only nuclear pioneer who supported the wide universe of reactor designs . 
