Thread by @EnergyJvd

When people (like me) confirm that nuclear energy is "safe", what do we mean by that?

I'll tell you what I mean by it, by defining five levels of nuclear safety as it pertains to historical accidents, existing technology and modern nuclear designs. (thread)

The first level of safety is "no safety", meaning that the reactor will support runaway chain reaction as soon as it's turned on, destroying itself within seconds. It won't explode like a nuclear bomb but it will certainly look like some sort of explosion.

The second level of safety is where measures are taken to ensure a destructive runaway chain reaction can't occur during normal operation, through engineering negative feedback into the design.

In other words: as the reactor heats up, the reaction stops.

This "safety" feature is engineered into all nuclear reactors great or small, for the simple reason that operating reactors that lack this feature would be impractical, if only because you probably wouldn't be able to find rad workers crazy enough to operate them.

However, it's true all reactors (except a specialised subset) can theoretically be rigged (sabotaged) to disable or weaken negative feedback, making them potentially vulnerable to destructive chain reactions.

Chernobyl is the best known example of that.

Now, please rest assured that the severe lack of negative feedback security that led to Chernobyl is not today tolerated in any reactor, not even the remaining RBMK reactors. They were all upgraded. As such, the second level of safety is assured today in all reactors.

Apart from destructive runaway, there's a second source of safety concern: cooling the reactor.

While it's easy enough to shut down a chain reaction, ensuring sufficient cooling to prevent reactors melting after shutdown due to residual heat production is not guaranteed.

Large commercial reactors generate so much residual heat that they require significant mechanical cooling in order to prevent them from melting down after shutdown. Such a meltdown would destroy the reactor and cause major radioactive contamination, possible also off-site.

The best known example of this risk manifesting is the Fukushima Daiichi plant in Japan after it was hit by a monster tsunami.

Due to failure to re-energize the mechanical cooling of the reactors quickly enough, the reactors overheated and melted down.

So the third level of safety is to ensure there's always sufficient mechanical cooling available to prevent meltdown.

This consists of multiple independent cooling systems and sources of power to energize them.

Sadly, at Fukushima, this level of safety proved insufficient.

While some fairly simple and rather cheap alterations or upgrades to the systems as the plants in Fukushima could have prevented the meltdowns there, it is clear that relying on mechanical cooling to prevent meltdowns is not a very attractive option.

Enter the concept of "passive cooling" or the ability to cool the reactor without help from mechanical systems.

This is an easy and reliable solution to be sure, but the drawback is that it only works on small, low-power reactors cores that tend to be a lot less economical.

My personal opinion is that passive cooling is a nice idea, but not worth the cost, so I prefer the standard practice of large cores cooled with mechanical cooling.

And yes, I know that this brings a risk of meltdown, and all the problems that come with it.

To prevent such problems, however, a fourth level of safety is available, namely engineering the containment so that it allows heat to escape, but not radioactivity, in the (unlikely) case that a meltdown does occur.

This is achieved using a core catcher.
sarkaritel.com/core-catcher-s…

I couldn't find a nice animation of how a core catcher works, but what it does it is prevents a molten core from causing too much non-condensable gas to be generated within the containment. This prevents the endless build up of pressure, and hence prevents containment rupture.

So if this system had been in place at the Fukushima plants, it would not have prevented the meltdowns, but it would have prevented the containment breech and hence release of radioactivity.

All of today's modern nuclear plant designs have core catchers, which means that - while they are not 100% free of meltdown risk - the chance of a significant release of radioactivity in case a meltdown does occur is even smaller than the already small risk of meltdown itself.

So my point is this: when antinuclear commentator tell us there's a "small chance of grave consequences" they are talking of Fukushima-type accidents.

Modern plants with core catchers rather have a "small chance of becoming total loss but without causing off-site contamination".

The fifth level of safety is achieved when all sources of risk are zero, by creating a reactor that cannot go runaway critical and does not contain significant amounts of radioactivity.

This is known as the "ultimately safe reactor" and it is what "Thorium!" fans dream about.

You can read about it here, and while I am also a fan, I would say that todays moderns plants are good enough, far better than fossil fuels or renewable energy, so we don't need to "wait" for even better technology to come along.
nucleargreen.blogspot.com/2012/06/uri-ga…