**7. Summary**

Safety and economics of uranium utilization for nuclear power generation were investigated and discussed. To compare the alternative candidate of plutonium breeding by FBRs, P&T technology, one of the advantages of FBRs, was also discussed.

For the safety of reactor, to remain inherent safety feature for "shutdown" function, uranium utilization in thermal reactor is necessary. The safety feature is lost in fast reactor. The core performance, breeding ability, and economy are related to a transaction in fast reactor.

The amount of conventional uranium corresponds to consumption of approximately 290 years, and it is not much enough to sustain the energy supply eternally. On the contrary, the amount of seawater uranium, which is 4.5 billion tU corresponding to 72,000 years and 4.5 trillion tU including the uranium at the surface of the seafloor corresponding to 72 million years, is almost inexhaustible.

Furthermore, by utilization plutonium in spent fuel in thermal reactor, the duration period of uranium can be increased. By once-through utilization, that can be increased to 1.6 times. By multi-recycling, which can by HMLWR, that can be increased to 2.5 times.

With seawater uranium, the electricity generation cost increases by mere 3%. With HTGR, the cost with seawater uranium is cheaper than the cost of existing LWR with conventional uranium. The cost of FBR with multi-recycling increases by 40% compared with the cost of LWR.

From the viewpoint of energy security, conventional uranium has problems, i.e., geology and concession. Therefore, seawater uranium should be recovered before exhaustion of conventional uranium from the viewpoint of energy security because the uranium mining concession, which is necessary to supply the uranium resources sustainably, is difficult to fulfill the entire requirement. Moreover, seawater uranium should be recovered by the countries facing ocean.

Plutonium utilization has problems of energy security due to the decay of <sup>241</sup>Pu. When fuel loading and/or reactor operation would significantly delay, the fuel should be refabricated and reloaded. Moreover, weapon-grade plutonium is generated in the blanket of FBR. There is a threat for the spent fuel to be seized.

For environmental burden, the safety of geologic disposal for existing LWR waste is secured by evaluating public dose with a sufficient margin. However, P&T is planned to reduce the potential toxicity, which the index should not be used for safety assessment. To reduce waste volume, P&T is effective. Only with partitioning, the repository footprint is reduced to 1/4 times. However, transmutation of MAs cannot reduce the public dose with the recovery ratio of 99.9% determined to reduce the potential toxicity. MA recycling with FBR increases the working environmental burden due to the increased dose.

As discussed above, uranium utilization in thermal reactor can achieve safe and sustainable energy supply with acceptable environmental burden.
