5.3.3 The EBR-1 Meltdown Accident

The U. S. Experimental Breeder Reactor I (EBR-1) had the distinction of being the first reactor to generate electricity. Construction of the reactor began in 1948, and electric power production started in December 1951. The reactor was designed for a thermal output of 1 MW(t) and an electrical power output of 200 kW(e). Of course, the power production was more for demonstration than for economic viability.

The core of the reactor is illustrated schematically in Figure 5.25a. During its lifetime, the reactor was operated with four different core configurations, all with fuel in metallic form. The first three cores were of highly enriched ura­nium, consisting mainly of U-235. The second core had a uranium-zirconium alloy fuel containing 2% zirconium. The fuel pins were 1.25 cm in diameter, and 217 pins in a triangular array were mounted in a central hexagon 19 cm across, forming the core of the reactor. The small size of this core illustrates the great compactness of liquid metal-cooled fast reactors. Around the central U-235 re­gion there was a blanket region containing natural uranium rods, as shown in Figure 5.25a. The coolant for the reactor was a sodium-potassium mixture (NaK) that is liquid at room temperature (see Chapter 3).

With the second core, power oscillations were observed at very low core flows. In an experiment to examine this effect beginning on November 29, 1955, with the core flow totally stopped and certain safety interlocks cut out, power was rapidly raised in order to determine the magnitude of a previously observed increase in reactivity with temperature. It had been intended to termi­nate the experiment with the fuel temperature at 500°C, but through the com­bination of this temperature effect and an operator error, the temperature rose to more than 720°C. At this temperature the uranium metal fuel and the stain­less steel can begin to interact, leading to the melting of about 40% of the core, but without explosion, plant damage, or radiation hazard.

As explained in Chapter 4, bringing the pins closer together in a fast reactor causes an increase in reactivity or neutron population. The mechanism by which the EBR-1 core meltdown occurred was related to this. It was possible for the rods to bow as illustrated in Figure 5.25b, and this gave an increase in reactivity that was self-propagating as the increased temperatures increased the amount of bowing. This accounted for the temperature effect that was being investigated at the time and that was subsequently explained theoretically. The core of EBR-1 was later removed and replaced by another core designed to eliminate the bow-

(4) External Air cooled control rods annulus

Graphite

7j across inside of flats

Upper shields and seal plales

(b)

Figure 5.25: The EBR-1 meltdown incident.

ing effect by the use of spacer ribs. The expansion of the ribbing with increasing temperature causes the core to expand, giving a negative rather than the previ­ously observed positive temperature coefficient of reactivity.

The EBR-1 reactor, which was finally shut down in December 1963, gave in­formation of great value related to the design of fast reactors. Now all fast reac­tor cores are designed with significant amounts of restraint so that they always have a negative temperature coefficient of reactivity. In fact, it may be possible in the future to design fast reactor cores that are inherently safe in that they ex­pand to switch off the nuclear reaction even if the control rods fail to actuate. This is one of the features of fast reactors that make them in some respects even safer than thermal reactors.