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  • MICHAEL SHORT: We've actually got a special guest today.

  • It's Jake Hecla, one of the seniors at NSE

  • who's gone on to Chernobyl for the second time, just returned

  • from there two weeks ago.

  • So if you remember on Tuesday, we

  • went through all of the physics and intuition

  • about why Chernobyl happened.

  • And we left off on what does it look like today.

  • So Jake is going to tell you what does it look like today.

  • JAKE HECLA: All right, so first off I'm

  • actually going to go over a bit of the reactor physics involved

  • with the Chernobyl accident.

  • I realize you guys have already covered this to some extent.

  • But I didn't plan for that.

  • So it's in my presentation.

  • MICHAEL SHORT: It'll be a good review.

  • JAKE HECLA: Yes, also I am a little sick.

  • So I'm probably going to start coughing, apologies.

  • I'm not dying.

  • It's just a cold.

  • AUDIENCE: Radiation poisoning.

  • JAKE HECLA: I have heard that joke about eight times

  • in the last two days.

  • And I'm so done with it.

  • But yes, it's not radiation poisoning.

  • AUDIENCE: [INAUDIBLE].

  • JAKE HECLA: Yeah, all right, where is Chernobyl?

  • Ah, dang it.

  • Come on, no, go the other way, the other way, yep.

  • OK, there.

  • OK, so one of the first questions

  • I got when I said I'm going to go and visit Chernobyl is wait,

  • isn't that a war zone?

  • Not quite.

  • So the Ukrainian, the war in Ukraine

  • is mostly in this portion over here.

  • It's not entirely under Rebel control in that area.

  • And I say "rebel" in quotation marks

  • because rebel means Russian.

  • However, if you notice those arrows,

  • Russian forces are built up all along that border.

  • So while it's not an active war zone,

  • it's certainly not a place to be spending

  • a large amount of time.

  • That said, Chernobyl is north of Kiev by about,

  • I don't know, let's see, 200, 250 kilometers.

  • So it's not completely out in the sticks, right.

  • Hopefully this gives you good sense of roughly where it is.

  • All right, so what is the Chernobyl nuclear power plant

  • look like?

  • It consists of four finished reactors.

  • There are two unfinished reactors, unit 5 and 6, that

  • are not shown in this image.

  • Units 1 and 2 are located at the right.

  • Those were constructed in the 1970s and early 1980s.

  • All of these reactors or the RBMK type.

  • Units 1 and 2 operated with some success--

  • I'll go into that later--

  • for a number of years before the accident that happened in 1986.

  • We also had some call outs up here that

  • show the, some of the incidents that I'll

  • talk about here a little bit later in the presentation.

  • But this just gives you a general idea of the layout.

  • So it's two separate buildings for units 1 and 2.

  • And then units 3 and 4 are in one building, all connected

  • by this turbo generator hall.

  • So this is where the generators that

  • turn the steam from the RBMK into power, r.

  • This is one giant--

  • well, before the accident, this was one giant, not separated

  • hallway, basically.

  • So you could walk from one end to the other, theoretically.

  • All right, so what is an RBMK?

  • An RBMK is a light water-cooled, graphite-moderated,

  • channel-type reactor.

  • This means that it does not have a giant pressure vessel

  • like you would see in a VVER or an equivalent American

  • light water reactor.

  • Why does that mean anything?

  • Well, building giant pressure vessels is very difficult.

  • If any of you've done research on manufacturing

  • of nuclear reactors, you'll find out

  • that the equipment necessary to construct a reactor

  • pressure vessel is not actually something

  • we even have in the US anymore.

  • Is it Korea that does it for us now?

  • MICHAEL SHORT: Japan Steel Works.

  • JAKE HECLA: Japan that does it now.

  • In the Soviet times, it was very, very difficult

  • for the Soviet Union to produce such pressure vessels

  • at any kind of reasonable rate.

  • So the RBMK got around this by using

  • individual channels that were their own pressure vessel,

  • so to speak.

  • So the way this works is, let's just start on the cold side.

  • You take in cold water, goes here through these things.

  • These are main circulating pumps--

  • MCPs, as you'll see them referred

  • to later in the presentation--

  • goes up through the bottom up the core.

  • These are the hot fuel rods.

  • The water goes from liquid to steam phase as it's

  • flowing through the channels, comes out the top,

  • goes to the steam water separators.

  • Steam goes to the turbines, turns the turbines,

  • makes electricity.

  • The important thing to remember here

  • is that we've got a giant graphite core.

  • The graphite is what is doing the moderating

  • in this circumstance.

  • It is not the water.

  • This allows you to run very low-enriched uranium.

  • So you could theoretically run an RBMK

  • on I believe it was 1.2 percent was as low as they could go.

  • But regardless, extremely low-enriched uranium,

  • which is convenient if you don't want to waste a lot of time

  • enriching uranium.

  • The problem with this is that you have a giant core.

  • If you recall the scattering cross-section for graphite,

  • it's pretty small.

  • And the amount of energy lost per collision

  • is likewise also fairly small.

  • So the core on this thing is, let's see, 11, yeah,

  • 11.5 meters across.

  • The core for an equivalent American reactor--

  • so well, there is no real equivalent to this--

  • but for, let's say, an AP 1000 reactor

  • of equivalent electrical output, is about four meters across.

  • So the core is huge.

  • As I already discussed, this is what the individual pressure

  • channels look like.

  • So cool water comes in the bottom, goes by the fuel rods,

  • pops out the top.

  • The RBMK had some serious design flaws.

  • So as I said, the core is huge.

  • This allows local power anomalies

  • to form really, really easily.

  • If you look at the core, one portion

  • can be kind of neutronically separated from the others

  • because neutrons just don't make it all that far when

  • diffusing across the core.

  • So you can have very, very high power in one corner

  • and very low power in the other, which is not something that

  • can develop in a physically smaller core, which

  • has a characteristic scale equivalent to that

  • of the neutron being free path.

  • Further, the encore flux monitoring on the RBMK

  • is seriously deficient.

  • So there are a variety of neutron detectors

  • that exist around the periphery of the core.

  • But they're wholly insufficient to catch these local power

  • anomalies.

  • Chernobyl actually found out the hard way on this one.

  • In 1982, unit 1 suffered a quote "localized core melt,"

  • not really something that can happen in LWR, really

  • any other type of reactor.

  • But a couple of the fuel channels

  • actually experienced one of these local power

  • anomalies and ended up melting.

  • So if you go into the control room of unit 1,

  • you can see that on the fuel channel cartogram on the wall,

  • there are two of them that are just Sharpied out.

  • And those are the ones that melted.

  • Further, it has a positive void reactivity coefficient.

  • What does that mean?

  • Well, when the water boils in the core,

  • the density of the water there goes down.

  • And the power of the reactor ends up

  • going up because the water is primarily

  • acting not as moderator but as a neutron absorber.

  • This is bad for a whole variety of reasons.

  • And they found out quite catastrophically

  • in 1986 exactly why.

  • Further, the system is extremely unstable at low power.

  • So how did the 1986 accident happen?

  • It was part of this thing called a turbo generator rundown test.

  • The general idea is that if you have an off-site power failure,

  • and your main circulating pumps are no longer

  • have off-site power, you somehow need

  • to keep water flowing through the core, such

  • that the fuel does not melt.

  • The problem is that the backup, large diesel generators,

  • are just that.

  • They're large.

  • They're diesel.

  • And therefore they're very, very slow

  • to come online and come up to full power.

  • The way that you can bridge this gap

  • is by using the energy that you've stored in the turbines

  • to effectively power the main circulating pumps

  • until the diesel generators can come up online.

  • When unit 4 was fully constructed in 1983

  • and turned on for the first time,

  • they had never actually done this test

  • where they did a turbo generator rundown,

  • despite the fact that it was required by law in the Soviet

  • Union that all new power stations should

  • have this test performed.

  • It was delayed until 1986.

  • And yeah, it was delayed until 1986 is the long story short.

  • The test procedure-- sorry for all the text on this slide--

  • is basically as follows.

  • So you would ramp the reactor down.

  • So you would bring it from a normal thermal output

  • of up to 2,400 megawatts thermal,

  • down to 600 or 700 megawatts.

  • You'd bring the turbo generators up to full speed.

  • So you'd store as much energy in them

  • as you possibly could, then cut off the steam supply such

  • that now you are just extracting energy from the spinning turbo

  • generator.

  • This would then be used to power the main circulating

  • pumps, each of which took about 40 megawatts.

  • There are eight of them total.

  • I believe six could be used for normal operation.

  • The rundown would take somewhere in the range

  • of 60 to 70 seconds.

  • And hopefully by this time your diesel generators

  • would be turned on, pumping water,

  • and everything would be fine.

  • What happened in the test was decidedly quite different

  • from that.

  • So on April 26, 1986, they attempted

  • to begin this test about six hours behind schedule

  • because there was an incident in another part of Ukraine,

  • in which a coal power plant went offline.

  • So what happened was the authority

  • for the grid in the area ordered that Chernobyl

  • should stay online at full power for an additional six hours.

  • They began the test by bringing power down.

  • But as a result of running for an extra six hours,

  • they'd built up a significant amount of xenon precursors

  • in the core.

  • So when they started turning the power down,

  • the power started going down, and down, and down.

  • And they were unable to arrest its drop.

  • What ended up happening was that the power dropped all the way

  • down to 30 megawatts thermal.

  • And the reactor operators kind of panicked.

  • Their response to this, instead of canceling the test,

  • was to pull out as many control rods

  • as they could get their hands on.

  • They did so.

  • And this managed to rescue the thermal output of the reactor.

  • And it bumped up to around 200 megawatts thermal.

  • At this point, the reactor was in an extremely unstable state.

  • Mind you, almost all of the rods that they could get their hands

  • on were out of the reactor.