Tell the corp to sell higher and then contract production to your own country.
I think then, in the contract itself, it will show market price but sell at the price it was assigned to sell at in accordance to its trade strategy.
Same goes for plutonium and other raw materials - just tell it to sell its product to enough local nuclear corps to meet quota (or lower quota by lowering hiring and/or salaries). Of course, this also drives up the price of the nuclear corp even higher and raises costs for your country.
I could be wrong about all this.
But the alternative - to actually make money selling nuclear stuff, is to contract it to other players. Doing it with slaves probably results in the greater over all cost impact on the empire.