meltdown question

[ivymike]

ok, so a neighbor of mine used to work at the Japanese nuclear plant that's having the trouble currently. He's a bit of a blow-hard, and this evening he decided to bend my ear about the reactor. He said that he can't see why they're having so much trouble with it, they must just be trying to save money by not draining the water. I said that it sounds like they're having a heck of a time avoiding melting of the fuel and eventual loss of containment. He responded "all they have to do is drain the water and the fission will stop." I was like "um, no... they need the water in there to slow the reaction and help remove heat" He said "no, it's a fast neutron reactor, and the water is where the fission happens. if you drain the water, the fission will stop, but it'll get hot inside." Anyway, he started doing his "you have no business questioning me" routine, so I left. A quick look on wikipedia convinced me that I was more right about it than he was (boiling water reactor, and water helps to moderate), but it left me wondering whether there were any grains of truth in what he was saying, or things that he could have been told and misinterpreted during a safety course or similar. Are there any reactor designs where draining the water inherently results in stopping the fission?

 

[VEBill]

I'm not an expert, but it sounds like he was mixing up light water reactors and CANDU.

 

[zdas04]

Sounds like he was mixing Scotch and BS.

I'm not sure what technology the reactor is, but I do know that if you drain the water, while fission may slow to near zero (in either a fast neutron or slow neutron reactor) the heat that the decay products generate is still significant and can melt stuff. The reactors I worked on (which are nuclear waste now), draining the reactor was in the same category of catastrophe as pumping in cold water.

David

 

[IRstuff]

Well, if he's right, then are about a few thousand engineers and physicists that are completely wrong, including anyone else who worked at those facilities. So, one guy right, thousands wrong? What are the odds?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[waross]

I under that the early Candu reactors could drain the heavy water as one of the fail safe mechanisms. Then they realized that the residual heat would destroy the reactor. More recent designs do not use an intentional heavy water dump as an emergency shutdown measure. There are moderating control rod are suspended from electro-magnets and drop by gravity into the core and there is a poison injection system that will stop the reaction.
Still, in the event that everything goes bad and the heavy water loop ruptures, the reaction will stop. The reactor will probably be damaged beyond repair but there will not be a melt-down.
As I remember one Candu reactor used a heavy water circulation pump of about 10,000 HP and it was on an inverter/battery UPS that kept it running until the diesel gensets started and came online.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prex]

1) It is not a fast reactor
2) There is no heavy water in this plant
3) The chain fission reaction was shut down by the insertion of the control rods immediately after (or during) the earthquake, and it is not possible it will start again
4) All nuclear reactors continue to generate heat after the shut down because of the decay heat (this by no means can be referred to as 'fission' heat). If this heat is not removed, the fuel elements will eventually melt and the whole reactor core will be out of reach for thousands of years.
5) If water is removed from the core, the melt down is a matter of minutes: the problems they have at present come from not being able to correctly cool the core, because the emergency diesel generators, that should supply the emergency circulating pumps, were flooded by the tsunami.
ivymike, your neighbor is just an irresponsible and silly guy shouting in the crowd of others like him.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[Latexman]

When someone puts their back to the wall and challenges the world, run, don't walk, away.

Good luck,
Latexman

 

[TheTick]

IF it is a slow reactor, loss of water will halt the reaction.

BUT, there's still plenty of heat to dissipate. More than enough to melt the core. Plus, there are other radioactive byproducts still decaying and generating heat.

I know I'm not the only ex-navy nuc around here. Your neighbor needs to realize he is not the only one in the world capable of understanding what he "knows". He worked there? What did he actually do? REM sponge?

 

[ivymike]

so just be sure I understand
- there are some reactors where the water itself is critical to sustaining the chain fission reaction, and some where it is not
- this design happens to be the latter
- in the former, all the fission happens in the water, and the "fuel" simply decays to release neutrons?
- in this design, the chain reaction is stopped via insertion of the control rods, which should have (did?) happen early on in the recent chain of events

 

[prex]

Water is critical to sustaining the chain fission reactors in all water cooled reactors: however not because 'the fission happens in the water' (a totally false statement), but because the neutrons are slowed down by the water, this slowing down being necessary to this type of 'slow' (non 'fast') reactors.
If the water goes away, then the chain reaction can't continue, but the situation becomes even worse because of the lack of cooling.
And yes, the control rods were inserted immediately, I can tell this for sure, though I was not there to watch: otherwise all the story would have turned into a muuuch bigger disaster.

prex

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes and launchers for fun rides

http://www.levitans.com

: Air bearing pads

 

[IRstuff]

As was stated in a Japanese newscast, even with the control rods in position, there is sufficient internal heat that cooling water is mandatory.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Latexman]

What's the worst case scenario? The nuclear fuel melts, runs down into the reactor, and accumulates in a quatity greater than critical mass? Then? BOOM!?

Good luck,
Latexman

 

[IRstuff]

WC is that the rods melt through the containment and into the ground, making both the reactor and area around unusable and uninhabitable for millenia to come. Ground water containment might be mitigated because the plant is so close to the ocean, but local marine ecology is likely to be foobar as well.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[TheTick]

No "boom".

 

[IRstuff]

Smaller ones only.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[IRstuff]

Actually, I don't think the rod material is capable of reaching explosive criticality, since the density of U-235 is relatively low; my recollection is that it's on the order of a few percent, while weapons-grade uranium is on the order of 90%.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Latexman]

OK

Good luck,
Latexman

 

[TheBlacksmith]

Agreed, the light water (normal but very pure water) is a moderator, making fission possible. Without the water, no fission. However, secondary decay does not require water and generates on the order of 10% of the thermal output of the nuclear fission and tapers off after a shutdown. Still, this is an enormous amount of heat to dissipate, hence the need for water and circulation through a heat exchanger after the nuclear reaction is stopped. Another ex Navy NUC.

The Tick - REM sponge; good one.

 

[ivymike]

I asked... didn't ask if he was a REM sponge. He said he was "basically a Level 3 over inspections." In his current job (after several changes over the years) he supervises technicians doing ultrasonic inspections of machined parts (for misc equipment, non-nuclear).

 

[IRstuff]

"Without the water, no fission" should be "no EXTRA fission."

U235 happily splits on its own, water or not. That's how metal hunters find uranium deposits in the first place.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize