**1. Introduction**

Nuclear power is an attractive energy source of clean air and carbon-free electricity that produces no greenhouse gases or air pollutants unlike power generation with fossil fuel. Moreover, it is said that fossil fuels are in danger of running out. Especially for the petroleum resources, the duration period is about 40 years. In this context, nuclear power generation (NPG), whose fuel is uranium, has been installed as alternative energy. However, fast breeder reactor (FBR), which is breeder reactor of plutonium, has been developed from the viewpoint of depletion of uranium resources [1] but not deployed as a commercial reactor yet. Many researchers and engineers believe that sustainable energy supply can be established only with FBR fuel cycle. However, we should reconsider the problem of depletion of uranium resources before coming to a decision because safety of reactor depends on reactor types. Moreover, other problems, e.g., economics and environmental burden, should be considered. To this end, safety of nuclear reactor with uranium utilization and that with plutonium breeding is

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discussed in Section 2. Sustainability of uranium resources and that with plutonium thermal use is discussed in Section 3. Economics of electricity generation with conventional uranium, sweater uranium, and plutonium multi-recycling by FBR is discussed in Section 4. Energy security for uranium and plutonium utilization is discussed in Section 5.

restricted by the requirement for the fuel block strength against thermal stress. The fuel pins are composed of coated fuel particles (CFPs). The maximum volume fraction is determined by a state of the art of fuel fabrication to restrict initial failure fraction of the CFPs. To obtain high burnup for long life cycle, the volume fraction prefers the maximum value. Moreover, the moderating power and the absorption cross section of graphite are lower than those of light water. The optimized design for criticality is not preferable from the viewpoint of the long life cycle with considering burnup. According to the result, HTGR's design condition is in the undermoderated region when the core design is reasonable and realistic from the viewpoint of the heat removal, the integrity of structure, and the long life cycle. Moreover, the solid moderator of graphite is never voided. To realize a negative power reactivity coefficient, there are two factors, the Doppler effect of fuel temperature and reactivity feedback of moderator temperature due to neutron spectrum shift of Maxwellian distribution peak [8]. As a result, thermal reactor including LWR and HTGR has the inherent safety feature due to self-regulation of power.

Safety and Economics of Uranium Utilization for Nuclear Power Generation

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On the contrary, many FBR designs allow a positive void reactivity coefficient because of the increase of threshold fission reaction of fertile material with high neutron energy over 1 MeV due to the hard spectrum. **Figure 1** shows the fission and capture cross sections and the ratio of fission cross section to absorption cross section. The ratio stands for the fission probability per neutron absorption reaction. The fission probability also rapidly increases over 1 MeV, and the probability is around unity. Then, when the coolant of sodium is voided, the neutron

Due to the positive void reactivity coefficient, the coolant is boiled, and the power burst, which melts the fuel pins, occurs upon unprotected loss of coolant flow (ULOF) accident [9]. To prevent the power burst, Integral Fast Reactor (IFR) [10] is designed with a large safety margin for heat removal to avoid coolant boiling instead of inherent safety features of neutronic characteristics for self-regulation due to negative coolant void coefficient. The concept

over 1 MeV increased, and positive reactivity is inserted.

**Figure 1.** Cross section of 238U and fission probability per neutron absorption.
