# Thorium vs Uranium MJ/kg

by DLHill
Tags: energy, power, reactor, thorium, uranium
 P: 32 In a reactor using Th232 as a fertile material to breed fissile U233, what is the amount of energy released per kg Th232 compared to a reactor using about 20% enriched uranium?
P: 21,827
 Quote by DLHill In a reactor using Th232 as a fertile material to breed fissile U233, what is the amount of energy released per kg Th232 compared to a reactor using about 20% enriched uranium?
The rate of energy release depends on two things: 1) energy per fission, and 2) fission rate.

The fission rate is carefully controlled such that the fuel state (e.g., temperature) remains within certain specified design limits, and would be constrained to certain limits if the system is upset.

20% enrichment would be rather high for a commercial (LWR) reactor, for which fuel is limited to 5%. Some small research reactors use highly enriched fuels since the core are small, and the fuel is not so optimized for the core size. Fast reactors, do use higher enrichments of 20% or more, and usually Pu-239 in the form of (U,Pu) ceramics (MO2, MN or MC, where M = U, Pu).

The energy per fission is:

Isotope                         U-233   U-235   Pu-239
Energy per fission   (MeV)      197.9   202.5    207.1
Energy in anti-neutrinos (MeV)    6.9     8.8      7.1
Recoverable energy    (MeV)     191.0   193.7    200.0
Another ~ 3 - 12 MeV can be recovered from capture gammas from the neutron capture (n, γ) reaction. The neutrons may be captured by fuel, but not cause fission, or they are captured by the coolant and structural (non-fuel) material, e.g., cladding.

Ref: http://www.kayelaby.npl.co.uk/atomic...4_7/4_7_1.html

In a thermal Th-breeder system, there would be some fissile species, e.g., U-233 or U-235, usually U-235 in the beginning and later U-233 as it becomes available.

In LWRs, about 8-10% of fissions are fast-neutron induced fissions in U-238, with the remaining fissions mostly due to thermal neutrons fissioning U-235 and Pu-239, as the Pu is produced in the reactor.

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