Steels are corroded more by water than by liquid metals. As a result the coolant circuits of water-cooled reactors become considerably more contaminated with 60Co and 56Mn than those of sodium — or lead-cooled reactors. This has a marked effect on the radiation doses experienced by the plant operating personnel, particularly the maintenance staff. The principal radiation source in the primary circuit of a sodium- cooled reactor is 24Na, but this decays quickly (T1/2 = 15 hours) so that a few days after shutdown, provided there have been no fuel failures, radiation levels are low and do not impede access for maintenance. If failed fuel is present the primary circuit cover gas may be contaminated with radioactive Kr and Xe.

Collective doses in the range 10-100 man-mSv per year are repor­ted for sodium-cooled power reactor plants, in contrast to doses in the range 1000-10000 man-mSv per year for BWRs and PWRs.

5.3.2 Sodium Fires

Sodium burns readily in air to produce dense white fumes of sodium peroxide. The reaction is

2 Na + O2 ^ Na2O2 + 11 MJ per kg of sodium.

The peroxide combines with atmospheric water vapour to form sodium hydroxide which is corrosive. The ignition temperature in air is ~250 °C for a quiescent pool but lower, ~125 °C, for a spray such as might emerge from a leaking pipe. Burning sodium can generate temperatures greater than 1000 °C and can cause severe damage to steel structures.

Because the primary sodium is radioactive it is necessary to enclose all the sodium-carrying vessels and pipes in a continuous leak jacket. The space inside the leak jacket is filled with inert gas (nitrogen or argon) and provided with sodium presence detectors. Major secondary circuit components have also to be provided with leak jackets, but they are not necessary for minor secondary components and pipework, which can be located in air-filled spaces with fire-suppression pans on the floors. These pans work by collecting any leaking sodium and directing it to a drain tank while restricting access of air to it so as to minimise the fire, as shown in Figure 5.4.

Sodium pool fires can be extinguished by smothering with common salt (NaCl) or, better, graphite powder which expands by exfoliation to form a blanket over the surface of the sodium.