Abstract
As insurance against reactor runaway or other unplanned excursions, gas-cooled, graphite-moderated reactors are provided with a secondary shut-down mechanism which serves as a back-up to the primary control rod system. This back-up includes a hopper located above fuel channels in the core, equipped with a quick discharge mechanism, which is filled with boron-containing spheres. In an emergency, this hopper discharges the spheres which then cascade down the channels and ''poison'' the uranium fission reaction by absorbing thermal neutrons - the propagators of the chain reaction. Within six months time, a process was successfully developed based on silicon carbide reaction-bonding, which yielded a strong, hard, oxidation-resistant, boron-containing shut-down ball. Test materials were exposed to water saturated argon for three hours at each of several temperatures. While normal boron carbide-graphite balls were completely vaporized, the Cerashield balls remained basically unaffected. Had the reactor at Chernobyl been outfitted with Cerashield shut-down balls, it might never have become famous.
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