Hacker News Reader

Comment by jhayward | Hacker News Reader

> so it would never be at risk of a melt-down

This is a misunderstanding of what a melt-down is. In every case we know of involving commercial reactors, fuel melt-down happened when the reactor was non-critical, completely devoid of fission.

Meltdown happens when the heat generated by decay of products produced when the reactor was critical cannot be removed fast enough, such as when a cooling system fails or coolant does not cover the fuel modules.

Any reactor with solid fuel and a high enough energy density will have a meltdown possibility, unless it is somehow using a reaction chain that precludes the production of high-activity fission products.

View on HN

I wanted to make almost the same comment, but this part is not 100% accurate:

> In every case we know of involving commercial reactors, fuel melt-down happened when the reactor was non-critical, completely devoid of fission.

This is indeed true for Three Mile Island, Fukushima and Saint-Laurent-des-Eaux in 1969, but IIRC not for Chernobyl, and this was clearly not the case in Saint-Laurent-des-Eaux in 1980. (Yes, Saint-Laurent-des-Eaux faced two indendant meltdown events…).

More generally, as long as you're using light water as a coolant and moderator (PWR and BWR), you're likely to have negative void coefficient, which prevents fission-induced meltdown, but for other technologies, the risk exists.

I'll accept the correction regarding Saint-Laurent-des-Eaux, thank you.

However the steam explosion(s) that blew apart the reactor assembly at Chernobyl did not melt the fuel rods there although of course there may have been some localized areas of melting. The melt that created the tons of corium was due to decay heat. It occurred many hours after the power excursion and explosion(s).

Regardless of precisely what a “melt down” is or whether this was the appropriate term here, the Chernobyl explosion was caused by an excessive amount of fission output in a super-critical reactor. As I understand it, in a fission- or accelerator-driven reactor, if you turn it off, fission stops rapidly. Decay heat will remain, but that’s a smaller amount of output power and is presumably a good deal less dangerous.

That's way enough to cause an accident though, see Three Mile Island and Fukushima.

Fukushima successfully SCRAMed their reactor. Previously produced fission byproducts continued to decay after criticality was lost. While the thermal power of this decay process was an order of magnitude below peak, it greatly exceeded passive cooling capability. Without backup power to actively cool the core, meltdown is unavoidable: this decay heat is far more than enough to destroy the core.

This is a real risk for current reactor designs. Loss of backup power means a loss of any safe shutdown option.

Reply on news.ycombinator.com