It takes money folks. Developing an indigenous nuclear reactor design would alone cost more than 6 billions PKR in R&D. This is the major hurdle. Another one might be the design of class 1 level Pumps. For the rest of the components and stages of nuclear power plant construction, expertise has been achieved.
And, I dont see there is any need to go for indigenous design currently. After completion of K2 and K3, and subsequently C5 (probable), we wont be needing too much of nuclear electricity sources. Also, there is no foreseeable potential of export in this field as hardly anyone would buy our nascent designs. So, there is no considerable practical motive to spend this much of resources just to look scientific. Our plutonium demands are also being met with ease in Khushab.
I am not saying that this is an entirely useless field. But, we should go for indigenous nuclear power plant after we are done with other more important jobs.