The Russians were also dumping quantities of lead and boron.
Granted, the first order of business is to keep the fuel from melting into one large critical mass blob. Seawater will work for that but once/if the fuel rods melt, only high boiling point materials (sand, lead, boron) that will dilute and moderate molten uranium have any kind of chance of slowing down the reaction. That is if there is not a critical reaction that becomes explosive or the fuel and contaminated control assemble vaporizes.
Yes, they were dumping in boron and lead. It didn’t work out as well as I think the Japanese situation will work. The #1 failure of the Chernobyl reactor was a cheap-assed design without a containment vessel. Follow that with using a graphic moderation.
Then again, the Soviets never were one for redundant systems. Build ‘em big, cheap and fast was their motto.
The Japanese are supposedly injecting boric acid solution in with the seawater as well.