It was shown in Section 1.3.3 of this chapter, that the fission of U-236 into two equal fission products of palladium 118 releases 236 MeV of energy. Section

3.3.1 and Fig 1.8 show that this symmetrical fission is rare; asymmetrical fission with the release of two or three neutrons is much more likely. Calculation similar to that in Section 1 of this chapter, but for the more typical asymmetric fissions, gives an energy release per fission of 200 MeV, most of which appears in the form of kinetic energy of the products of the fission process and ultimately as heat energy. An ap­proximate distribution of the fission energy is given in Table 1.5 from which several points of interest can be obtained:

Table 1.5

Liberation of energy due to fission Instantaneous

Kinetic energy of fission fragments 168 MeV

Kinetic energy of fission neutrons 5 MeV

Prompt gamma rays 7 MeV

180 MeV

Delayed

Beta particles from fission product decay 7 MeV

Gamma rays from fission product decay 6 MeV

Antineutrinos 10 MeV

23 MeV [4] [5]

3.3.5 Decay heat

As previously mentioned, some of the energy released in fission is delayed; it is not released at the time of fission. This energy mainly comes from the sub­sequent decay of the fission products and is released as an exponential decline over a long time. Thus a reactor will continue to be a source of heat — decay heat — even though the fission process has ceased.

Immediately after shutdown a reactor will be pro­ducing about 6,5% of full power output; this falls to about 1.5% after an hour but is still about 0.5% three days later. Considering that full power may be of the order of 1500 MW (thermal), each 1% represents 15 MW of heat that is being produced. This has obvious implications on reactor design and operation in that there must be provision to remove a substantial amount of heat from the reactor core even after it is shutdown.

3.3.6 Summary of the fission process

Asymmetric binary fission is the most likely, with one fragment between 90 and 101 and the other between 132 and 143. The average number of neutrons produced by thermal fission (y) of uranium 235 is 2.43 fast neutrons of average energy 2 MeV. A small fraction (0.68% for U-235) of the fission neutrons are delayed (mean neutron lifetime for U-235 fission :

12.9 s). The energy released by the fission is 200 MeV, most of which — but not all — appears in the fuel in the kinetic energy of the fission products. A sig­nificant amount of energy is due to the decay of the radioactive products; the resulting handling and shielding problems and the decay heat must be catered for in the reactor design.