2.1. Thorium Based Fuels

If introduced into the LWR fuel cycle, thorium as a primary fertile material in the core has a good potential for reducing the TRU generation simply due to its lower than uranium position in the periodic table. Longer neutron capture and decay chains have to take place in thorium to result in Pu or MA isotopes. However, significant quantities of U233 and long lived radioactive Pa231 generated in a thorium fuel mitigate this advantage. U233 is a valuable fissile isotope that, if recycled, can improve fuel utilization and due to its continuous generation from the thorium can increase fuel burnup and reduce core reactivity swing during a fuel cycle. At the same time, U233 is a weapons usable material with relatively low spontaneous neutron source. Thus, its accumulation reduces attractiveness of the thorium based fuel cycle from the proliferation resistance standpoint. Addition of natural or depleted uranium to the fuel for the sake of limiting U233 concentration in uranium below the weapons usability limit will result in increased TRU generation.

Although Th232 has a lower resonance integral than U238 its response to the fuel temperature increase (Doppler broadening) is greater, which results in a more negative fuel temperature reactivity coefficient than that of the uranium fuel – a desirable feature in case of transients with rapid reactivity increase. At the same time, the smaller delayed neutron fraction of U233 than of U235 has a negative effect on reactivity initiated accidents. This is particularly important for the Th-TRU mixtures where βeff is considerably smaller than that of the uranium fuel all through the in-core residence time.

In the fuel performance area, thoria is found slightly more advantageous than UO2. ThO2 thermal conductivity is slightly higher than that of UO2 at comparable temperatures in addition to about 500 °C higher melting point [Oggianu S.M. et al.,] (Table 2.1.I).

In ThO2 compound, thorium has its highest possible and the only oxidation state +4, which makes it very stable and almost chemically inert [Belle J. et al., 1984]. Therefore, thoria is expected to be a very durable waste form for the spent fuel. If the fuel recycling strategies are considered, however, reprocessing of the thoria fuel is somewhat complicated for the same reason and therefore it is expected to be more expensive than reprocessing of UO2 fuel. The presence of U232 isotope inevitably generated in Th fuel primarily as a result of (n,2n) reaction in U233 may further complicate reprocessing and add to its cost. U232 decay chain products emit highly

penetrating hard γ radiation which will require additional shielding and remote handling of the fuel. This complication is less critical if TRU recycling in conjunction with use of ThO2 matrix is desired because TRUs other than U232 are likely to dominate the γ and neutron radiation doses and will require remote handling as well.

The heavy metal density in thorium oxide is slightly lower than in uranium oxide which is additional disadvantage (Table 2.1.I).

Table 2.1.I. Physical properties of some nuclear fuel materials, [CRC Press, 2000]

U UO2 Pu PuO2 Th ThO2 Melting point, °C 1135 2827 639 2390 1750 3390 Phase change, °C 660 1400 Theoretical density, g/cm3 18.9 10.96 19.8 11.50 11.7 10.00 Thermal Conductivity at 600 °C, W/cm-°C 0.42 0.045 0.45 0.057

Finally, the interest in breeder reactor technologies including Th cycle in 1960’s and 1970’s resulted in accumulation of significant experience and produced a vast knowledge database covering different aspects of thorium fuel cycle [Lung M. et al., 1997]. This knowledge and experience can be effectively put in use should the interest in thorium cycle in connection with TRU burning be renewed.

The focus of this section is on establishing the practical limits for Pu and MA burning efficiency and on the feasibility of thorium based fuel in PWRs. The main parameters of interest are the rate of total Pu and MA destruction and residual fraction of trans-uranic nuclides (TRU) in discharged fuel. The former parameter is, effectively, the number of kilograms of TRU that are burnt per unit energy produced by the fuel. The latter parameter indicates the amount of TRU that will have to be recycled or disposed of in the nuclear waste repository.

The fuel composition (relative amounts of Th, Pu, MA and U in the fuel) and lattice geometry will affect both of these indices: the burning efficiency and rate of TRU destruction. Therefore, the study reported here consists of several parts. First, homogenous reactor grade PuO2-ThO2 mixtures are studied covering a wide range of possible compositions and geometries. Then, the effect of the addition of a small amount of natural uranium to the fuel was investigated. This option is important for the once-through TRU burning scenario where the discharged fuel will be sent directly to the repository. In this case, U233 generated from Th232 has to be

isotopically diluted (denatured) in order to eliminate potential nuclear proliferation threats. Next, MAs were also considered as part of the fuel and the efficiency and destruction rates of Pu, MA and total TRU were investigated.

The PWR fuel lattice allows a certain degree of freedom in optimizing the fuel to moderator ratio. This ratio defines the degree of neutron moderation and, therefore, absorption and fission reaction rates in different HM nuclides in the fuel. For that reason, a scoping study was carried out to evaluate the effect of the fuel lattice geometry on Pu and MA destruction performance for each fuel composition considered.

Heterogeneous core geometries may be beneficial if Pu and MA can be concentrated in separate fuel assemblies in the core allowing more flexibility in fuel lattice optimization and fuel management schemes. Therefore, the potential of heterogeneous core configuration to burn Pu or MA more efficiently was also explored on the basis of two dimensional 2x2 assemblies colorset segment. Figure 2.1.1 schematically depicts the topology of all investigated cases.

Pu Denatured with NU

H/HM variation

Homogeneous assembly

Undenatured Heterogeneous colorset

Denatured with NU

Pu + MA Pu Pu + MA

Figure 2.1.1. Topology of investigated cases.

Finally, feasibility of utilization of TRU-loaded thorium based fuels in the current generation of PWRs was studied by a comparative analysis of the reactivity coefficients and soluble boron worths for a number of realistic TRU-Th cases, typical MOX and conventional all-U fuel. In current PWRs, only moderate changes in the fuel assembly configuration are possible in order to