Could liquid moderators prevent nuclear reactor disasters?

[eeka chu]

I was thinking, the main source of problems for reactors is cooling down the reaction in an emergency (I mean, in terms of the neutrons as opposed to heat). Chernobyl was an example of where that didn't happen - the rods overheated, bent out of shape and wouldn't fit back in as I'm aware of things.

But I seem to remember the first nuclear pile had guys standing by with pales of ?boron? loaded water that they'd pour onto the reactor if it started to overheat.

Could you not position some kind of big water tower filled with such a solution over the core of a reactor and have it empty out into it should things start going wrong? You could have a similarly sized tank underneath the core to dump the normal coolant into.

The liquid moderator would obviously benefit from being as absorptive as possible and not breaking down into an explosive hydogen & oxygen mixture - and also acting normally in the normal coolant loop if at all possible.

I can think of two reasons why this wouldn't happen. 1.) money 2.) there isn't a liquid form of moderator that will soak up enough of the neutron flux to stop the reaction (but just slowing it would be enough I would have thought, anything is better than letting it runaway).

Have I missed some point here or do they just rely on the solid moderators to do their job 100% of the time?

 

[Astronuc]

Have I missed some point here or do they just rely on the solid moderators to do their job 100% of the time?

Chernobyl (a Russian-designed RBMK) was a water-cooled, graphite-moderated reactor. The problem in that accident was that the safety systems were deactivated for an experiment, which got out of hand.

Most power reactors are Light Water Reactors (LWRs), and most are Pressurized Water Reactors (PWRs) which use soluble boron (in the form of boric acid buffured by LiOH) to control reactivity during operation. Control rods are held above the core, and drop in by gravity to 'scram' the reactor, i.e. shutdown in an emergency. If there is loss-of-coolant accident (LOCA) safety injection systems injected borated water - but they are only used during emergencies.

Boiling Water Reactors (BWRs) boil water in the core so soluble boron is not used, but rather, control rods are used during operation. To balance the reactivity and burnup in the core, the rod positions are periodically switched (between deep and shallow positions, and controlled (in-core) and uncontrolled (out of core)). BWRs have Emergency Core Cooling Systems (ECCS) and Safety injection systems, similar to those used in PWRs, are only used in an emergency.

When LWRs required shutdown, the control rods are inserted. The control rod absorb the neutrons to shutdown the reactor, and soluble boron is added to absorb even more neutrons.

 

[eeka chu]

Chernobyl (a Russian-designed RBMK) was a water-cooled, graphite-moderated reactor. The problem in that accident was that the safety systems were deactivated for an experiment, which got out of hand.

Most power reactors are Light Water Reactors (LWRs), and most are Pressurized Water Reactors (PWRs) which use soluble boron (in the form of boric acid buffured by LiOH) to control reactivity during operation. Control rods are held above the core, and drop in by gravity to 'scram' the reactor, i.e. shutdown in an emergency. If there is loss-of-coolant accident (LOCA) safety injection systems injected borated water - but they are only used during emergencies.

Boiling Water Reactors (BWRs) boil water in the core so soluble boron is not used, but rather, control rods are used during operation. To balance the reactivity and burnup in the core, the rod positions are periodically switched (between deep and shallow positions, and controlled (in-core) and uncontrolled (out of core)). BWRs have Emergency Core Cooling Systems (ECCS) and Safety injection systems, similar to those used in PWRs, are only used in an emergency.

When LWRs required shutdown, the control rods are inserted. The control rod absorb the neutrons to shutdown the reactor, and soluble boron is added to absorb even more neutrons.

Couldn't really ask for a better reply.

I watched a looooong documentary about Chernobyl and how they were planning to do that test before opening, but the politics of the situation (wanting to 'show off' the reactor to the public, but more importantly the west) forced them to just turn it on before checking that particular element.

They said the test was then rescheduled to take place during the day shift, after the reactor had been officially opened. As soon as they started the test, which involved lowering the power output of the reactor to something like 50% or less of it's maximum, they were phoned by another station saying that it needed help maintaining the grid, so they turned it back up and rescheduled the test again, but for the night shift to do.

Night shift come in, minimal staff, start the test. Start pulling out the rods to lower the reactor's output. The output drops to something like 10%, which was too low for the test to take place. They attempt to increase the output slightly by pulling back the rods and the output doesn't increase accordingly. I think they may have actually fully withdrawn the rods at this point. Still not much rise in output. They send the technicians down to manually pull some more moderator out. Still not enough.

They check the manual. The instructions for what to do in this situation are crossed out, but not stamped with an official verification. The technicians decide to call the other reactor at Chernobyl (I think it was number 3) to ask what they should do. The guy at 3 suggests that if the crossing out isn't officially stamped, they should just do what it says. They start carrying out the crossed out writing, the output suddenly rockets well beyond 100% of the rated amount. I think they tried to lower the moderators again but by then they'd warped into a shape that wouldn't fit back into the holes, the core overpressurises and almost immediately blows the lid off.

The programme didn't mention what that crossed out writing was. I would assume it was probably something to do with checking instruments, shutting the reactor down or otherwise modifying the moderators.

I've been wondering what it is that might have happened that caused it to suddenly go from virtually zero to blowing it's self to bits in such a short space of time. Perhaps the reactor was actually running all out and the instruments were just reading that there wasn't any significant output, causing them to continue increasing the rate without realising. I think this happened with a US reactor in the late 60's or early 70's. China Syndrome, with Jane Fonda, was based on a similar idea, that the instruments in the control room weren't functioning properly - which was released at pretty much exactly the same time as the US near miss. Anyone know the particular reactor I'm talking about? There was a programme on Radio 4 about it but I've forgotten the name (edit: three mile isle?).

The only other thing I can think of is that perhaps something wasn't functioning properly in the moderator section of Chernobyl and that the mechanism in some way jammed and then slipped as they were messing around with the rods. Producing the big transient in output. Perhaps the tolerance of fit for the moderators into their slots was a bit too tight and the heat caused them to stick.

I would guess that the moderators would be designed in such a way that it'd be hard to pull them out too far beyond 100% of the reactor's rated output without going through some process that involved lots of checks to make sure you actually wanted to do that (for obvious, explosive, reasons). Fresh fuel is added to the core, but as it degrades the rods probably need some freedom to move beyond the 100% fresh fuel position to keep the output at the same point (derating). Perhaps this adjustment was the manual change the technicians made to the moderators after trying to do increase the output from the control room.

That's how it happened according to this TV documentry. They didn't really go into what they were testing, I think it was something to do with whether or not the plant could sustain it's self, what would happen if there was an outside power failure (for things like the coolant pumps).

The first people to die as a result of the accident were the technicians who were sent from the control room to see what the big boom was. Two of them walked into the reactor room, saw the glow from the exposed, lidless core and, despite leaving a few seconds later, were already irradiated. They died about an hour or two later in the car park outside.

Some people on the program mentioned hearing the explosion in their homes and being able to feel something like rain on their skin a short while after, which must have been the radiation. I suppose, potentially, ionising radiation could actually trigger neurons directly as well as indirectly.

I found a site on the net set up by a woman who lives somewhere near Chernobyl. Her dad is connected with the accident in some way and managed to get her a card to let her ride her motorbike through the place - since it now has a gigantic exclusion barrier setup around the site. She's taken a load of photos from inside the town and it's like something out of some surreal horror film - http://www.kiddofspeed.com/chapter1.html"

The magic forest, your own glow in the dark trees.

Sellafield here in the UK had a similar near accident involving some kind of fire, which caused a lot of problems since if water was used to put it out there could be hydrolysis and then explosions (they eventually used water and it didn't). These were millitary reactors, so a little part of me kind of thinks "har har" - except that I live about an hour south from it, where the big cloud of radioactive steam ended up passing by (I wasn't alive at that point in time, yay).

Then another accident in 73.

And they have a big pond outside with 450 tonnes of waste in it they still haven't got round to cleaning up. As well as the fire damaged reactor. And the retired enrichment site.

I've visited Sellafield. It wasn't very interesting. Not allowed to see much or go anywhere other than the "nuclear power is cool" visitors' centre. What with there being terrorists everywhere nowadays I doubt they're even letting people in the visitors' centre.

 

[Astronuc]

You may be thinking of the Windscale fire. Bit of a nasty mess that was.
http://en.wikipedia.org/wiki/Windscale#The_Windscale_fire

IIRC, Chernobyl 4 was going through a Xe transient. When reactor power is reduced and the neutron level decreases, the fission product Xe (particularly Xe-135) builds up. Xe-135 is a great neutron absorber. However, it decays away, and when that happened at Chernobyl, the power started to increase. I believe they could not re-insert the control rods and the power increased to the point where the water started turning into steam.

In a graphite moderated reactor, the water does absorb neutrons, but when it goes to steam (big decrease in density), there is less water to absorb neutrons, and the power goes up even faster. At some point the power surged, and there was a steam explosion that further disrupted the core - basically it blew up.

We've discussed it in the PF Nuclear Engineering forum.