A defect in a steam tube or a weld in a steam generator of a sodium-cooled reactor may give rise to a leak in the form of a jet of

500

0.25 0.5 1 2 4

Moles of sodium per mole of steam

Figure 5.5 Sodium-water reaction temperatures.

high-pressure water or steam into the secondary sodium. The water and sodium react chemically to produce sodium hydroxide, which is strongly alkaline, and hydrogen. The reaction between sodium and liquid water is

2 Na + 2 H2O ^ 2 NaOH + H2 + 8 MJ per kg of water.

Figure 5.5 indicates the temperature of the reaction products as a function of the ratio of the reactants, assuming the reaction is adiabatic and the reactants start at 350 °C with the water as liquid. If the reactants are in the stoichiometric ratio the products reach a temperature of about 1450 °C in steady flow.

Formation of sodium hydroxide is not the whole story because in practice some sodium oxide is formed by the reaction

2 Na + H2O ^ Na2O + H2 + 7 MJ per kg of water,

Self wastage

Original leak

Figure 5.6 Propagation of a small steam generator leak.

but this makes little difference to the temperature of the reaction products.

Small Steam-Generator Leaks. A small hole, such as a crack in a weld or a defect in a steam tube, might be equivalent to a circular hole of diameter 0.1-0.2 mm and give rise to a leak of steam or water into sodium at a rate of 10-100 mg s-1. This would form a hot under­sodium “flame” that could be very damaging. If the “flame” were to play on an adjacent tube it would heat and soften it, the caustic reaction products would corrode it, and the supersonic steam flow would erode the corroded steel and “blow it away”, until the tube was penetrated. The water or steam flowing in the tube would act to cool it and delay penetration for tens of seconds or longer depending on the size of the original leak, but eventually the tube would fail and the leak would propagate. Figure 5.6 illustrates the processes taking place as a small leak propagates in this way.

The steam generator has to be equipped with a protective system to minimise the damage caused by a small leak; the system has to prevent the leak from propagating and also prevent the spread of caustic reaction products that might damage the secondary sodium
circuit and in particular the intermediate heat exchangers. The system acts to detect the leak, and then to take protective action.

A small leak can be detected by monitoring pressure, the presence of hydrogen, or acoustic noise. Excess pressure in the expansion tank is normally used to actuate a trip system. It is reliable as a means of detecting a leak but not particularly sensitive in a large steam gen­erator. A hydrogen-in-sodium signal (as described earlier in section 4.2.7) can be used to actuate a trip but it may be a poor indicator of a leak because there is always some hydrogen present by diffusion through the steam generator tubes as oxidation takes place on the steam side. If the trip level is set too low spurious trips are unaccept­ably frequent. Hydrogen leak detection has the additional disadvant­age that it is difficult to design a system to respond quickly, in less than 10 s or so. Acoustic leak detection is attractive in principle but in practice it is difficult to discriminate against the noise of the coolant flow and the mechanical rattling of steam tubes against their support grids. Hans and Dumm (1977) survey in considerable detail the various methods of detecting leaks.

On detection of a small leak a trip is initiated to isolate the steam generator on both the steam and sodium sides. The steam is dumped through the safety valves and the intermediate heat exchanger is isol­ated to prevent contamination with sodium-water reaction products. Operation of the dump system trips both the reactor and the turbine. The isolation and dump system is shown diagrammatically in Figure 5.7. (The bursting discs would not operate in the event of a small leak.)

Once a leak is detected it has to be located and repaired. Acoustic methods may be useful for location, because if one side of the unit is pressurised it may be possible to hear the gas issuing from the leak. An alternative method is to seek sodium hydroxide on the water side of the tubes by chemical means, because it is found that sodium migrates through small leaks against the pressure difference. When the leak has been located the usual method of repair is to plug the affected tube or tubes.

Bursting disc

Effluent separator

Steam dump valve

Steam isolating valve

SDV Sodium dump valve

SDT Sodium dump tank

SIV

Figure 5.7 Steam generator isolation and dump systems.

In a steam generator made of austenitic steel there is a danger of more extensive damage because of the susceptibility to caustic stress- corrosion cracking. If after manufacture parts of the unit such as the tubeplate welds are left in a state of stress, and there is a leak nearby, the sodium hydroxide formed may cause cracking of the stressed region (Broomfield and Smedley, 1979). This is an important dis­advantage of austenitic steels in steam generators and is one of the reasons for the use of ferritic materials.

Large Steam-Generator Leaks. If there is a large leak, such as might be caused by a steam tube breaking in two (an event often known as a “double-ended guillotine failure” or DEGF), water or steam would be ejected into the sodium at a rate of the order of 1 kgs-1. Unless protective action is taken such an event might propagate to failure of other tubes. As in the case of a small leak a reaction “flame” would be formed, but it would be large enough to embrace several tubes. Somewhere in the flame region the reacting mixture would be in the

Figure 5.8 Propagation of a large steam generator leak.

stoichiometric ratio above 1300 °C, hot enough to cause the steel of pressurised steam tubes to become soft and to burst. Figure 5.8 illus­trates the possible situation.

One kilogram of steam reacting with sodium generates about 1.5 m3 of hydrogen at 1 atmosphere and 350 °C, so in the event of a large leak (1 kg/s or more) large volumes of reaction products are formed very quickly. The secondary sodium circuit is exposed to the steam pressure of 16-20 MPa, possibly higher as the sodium and water react. The protective system has to relieve the pressure quickly. This is usually done by means of bursting discs that release the reaction products to an effluent system that traps the sodium and caustic material in a dump tank and vents the hydrogen and steam to atmosphere. Figure 5.9 shows such a system in outline.

When the bursting discs have opened the secondary sodium circuit is open to the atmosphere, so the intermediate heat exchangers are the only barrier between the radioactive primary sodium and the environ­ment. It is therefore essential that the integrity of the heat exchangers

should not be compromised. The capacity of the effluent system has to be sufficient to protect them from damaging pressures, and the isol­ation valves have to act quickly enough to prevent contamination by corrosive reaction products. Because of the possibility of propagation it may be necessary to size the bursting discs and effluent system to cope with the simultaneous severance of several steam tubes.