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Figure 7.12. 5 MeV neutron and gamma-ray relaxation length for some materials.

Figure 7.12. 5 MeV neutron and gamma-ray relaxation length for some materials.

NASA by Dr Raj Kaul and Nasser Barghouty [NASA, 2005b] on a polyethylene-based plastic called RFX-1 has raised expectations that lighter atom structures may be effective shields; references and calculations my be found in NASA [2005c].

### Residual radioactivity

After a fission reactor has been switched off, it keeps emitting secondary radiation from decaying nuclei (see Figure 7.9). The intensity decreases with time t, measured typically in days, and is a function of the length of time t0 the reactor has been operated. A simplified relationship for the residual power emitted and valid for 235U fuel is [Glasstone, 1955, p. 119]

where P7+^ is the residual power of the combined gamma and beta particles, and P is that of the fission reactor. Equation (7.18) shows the decay is not exponential but follows a power law, therefore is somewhat slow. For instance, after 30 days of operation (representative of long interplanetary missions) it takes about 30 days of ''cooling off'' to have the residual radiation down to 0.01% of the reactor power. A 1-MW reactor would still release 100 W after 30 days from shut-down.

The activity, measured in curies (see Appendix A at the back of the book) is directly proportional to power, so activity follows the same power law, with the constant in front of equation (7.18) replaced by 1.4 (instead of 5.9 x 10-3).

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