The St. Laurent plant of Electricite de France is a 500-MW(t) Magnox reactor that was first brought into operation in January 1969. The reactor is fueled on load and the machine that carries this out is called a charging machine. We shall consider the use of these machines in Chapter 7 in discussing the handling of fuel elements subsequent to their period in the reactor core. The charging machine is a very large device that is computer-controlled to move about the top of the reactor and position itself properly over each access port to unload and load the fuel. Figure 5.23 illustrates the layout of the St. Laurent reactor.

During the midnight shift on October 17, 1969, with the reactor near full power, a normal loading and unloading operation was in progress. Graphite plugs that had been placed temporarily in one of the fuel channels in the core were being replaced by fuel. The charging machine had unloaded the graphite plugs from the core into its empty storage chambers and had loaded fuel into the core from two of its full chambers, but then it stopped. Three full chambers of fuel elements are required to load one fuel channel in the core completely, and each chamber contains four elements. When the charging machine stopped, the operator overrode the automatic system, and after a series of man­ual operations, he accidentally charged a flow restriction device into the chan­nel instead of a fuel element. These flow restrictors were used to control the gas flow to individual channels. The loading of a flow restrictor into this particular channel so reduced the flow that the fuel elements were inadequately cooled.

Some of the fuel elements in the affected channel heated up beyond their melting point, and the molten fuel flowed out of the channel onto the diagrid below (Figure 5.23). This released radioactive fission products, set off alarms, and activated a reactor trip. The molten fuel (about 50 kg) was still still con­tained within the massive concrete structure; hence, little, if any, radioactivity was released outside the structure and there were no injuries. However, a year was needed to complete the cleanup operations and restart the reactor. Modifi-

cations to the machine were made, and it is no longer so easy to override the automatic system and bring the machine into manual control.

This accident and a similar one at the British Chapelcross reactor in Scotland again demonstrate the importance of carefully matching the heat removal and heat input characteristics for the system as a whole and for each component part. Again, the scope for operator error is noted, and this has necessitated steps to reduce the scope.