New reactor sim-for your ammusmant!

Date: Oct 9, 2004

New reactor sim-for your ammusmant!

Date: Oct 9, 2004

Interesting.... the way the reactor is illustrated, the flow of the pressurised water coolant bypasses the core, going from the inlet, around the bottom, and up to the outlet.

Of course that's not how it is in reality -- the inlet flow goes down as shown, around a core support barrel, but there is no direct connection between this annular space (located just inside the reactor pressure vessel) and the coolant outlet. All the flow must go up through the reactor core and out the top, into an outlet plenum above the core (ie. below the top lid of the reactor). This again is isolated from the inlet flow by the core support barrel (and appropriate flanges connecting to the pressure vessel wall), the only openings from the barrel being to pipes which take the outlet flow out through the reactor wall, to the steam generators....

I think there is a correct illustration of a PWR on the Virtual Nuclear Tourist web site, in case anyone's interested.....

Date: Oct 10, 2004

Neet little game. It just reminds why I shouldn't be a nuclear plant engineer--the neighbors woudn't like the results! I blew the plant up three times before I averaged 1 MW. Just a little advice--always start the secondaray coolant loop pumps before withdrawing the control rods. (Duh.) Also, don't pull them out to 50% right off.

A little twitchy--just like the real thing.

Date: Oct 10, 2004

thats what I meant about being a good advert for pebble beds!

Pebble bed sim;

1) pull out controll rods!

2) Go and make coffee

3) Come back, reactors nice and hot-but not too hot!

4) Oops! better turn on circulating pumps!!

Dusty

Date: Oct 11, 2004

I managed to take it through a couple of fuel cycles before I realized that you can't refuel the reactor and got bored.

The best way to work it seems to be to prime the reactor on day 1 with 100% rods and no coolant. That should bring the temp up just a smidge so you can get things going. Next set your primary and secondary cooling pumps to 90% and remove the control rods to about 5-10%. Your reactor temp and power ratings should jump up to critical levels. Increase cooling as needed, and add control rods in ~5% increments. As your fuel drops, you'll need to drop the control rods farther and farther. You'll probably be at about 80% when you run out of fuel.

If you burn out your secondary coolant pumps, remove the rods and set the emergency coolant to 100%. This should shut down the reactor so you can perform maintence. At that point use the above procedure to bring your reactor back up to operating level.

My only other comment is, those reactors are damned touchy, aren't they? Ramp the power levels up just a little too high and you'll happily melt down your reactor. What happens if the rods get stuck, or the control mechanism accidentally causes them to drop? Someone *please* tell me that this thing is based on a 50+ year old design?

Date: Oct 12, 2004

I think LWR's are actually quite twitchy!

A combination of high power density, enriched fuels and a less than ideal coolent which means things can get out of hand very quickly.

Thats why they have to have multiple layers of automatic safty features. The Irony of TMI was that if the operators had gone off and had a cup of coffee the reactor would have SCRAMMed itself with no more damage than a blackout and a couple of days lost generation. The melt down occured because the operators overode the safty because faulty instrumentation led them to believe it was operating in error.

At Chernobyl the operators pulled the rods out too far and the reactor went to 10,000% load and, understandably, blew up. (I know that Chernobyl was not strictly an LWR but some of the charactoristics were similar)

At SL1 a technition (it is believed) pulled "one" controll rod out too far while connecting it to its drive motor result-a supercritical reactor, 3 dead technitions, and a very nasty mess

By contrast gas cooled reactors (UK style MAGNOX, AGR) have low power density, high thermal capacity and non-enriched (or very low enriched) fuel. things go wrong slowly giving operators time to think before acting. I believe the initial "emergency procedure" in a MAGNOX or AGR station was to go to the managers office and sit arround the table with some coffee and discuss it! even the worst failure would give you around several hours before it got nasty.

Dusty

Date: Oct 16, 2004

No, no, no..... sorry guys, but you've been sucked in by nuke phobia.

LWRs have a negative thermal coefficient of reactivity (required to be that way by licensing regulations), which means that any time the temp starts rising, the reactor reduces fission power naturally, without any mechanical intervention.

The TMI accident happened long AFTER the reactor was scrammed -- due to lack of removal of radioactive fission product afterheat, which CANNOT be shut off (although it does decrease fairly rapidly in the first hours and days following scram).

The Chernobyl accident didn't happen because of control rods being pulled out. It happened because of the rods being dropped in, but having a bad design (with the bottom ends made of graphite moderator material) they actually increased reactivity, instead of decreasing it (which is the equivalent of pulling the rods out....). Also, in the low power condition in which the Chernobyl reactor was in during the fateful test, was such that the thermal coefficient of reactivity was positive -- another big no-no.

The SL-1 accident apparently resulted when the technician tried to pull a rod slightly out, but due to poor design & construction, it was stuck, so he used force, until it gave, but then he ended up yanking the rod way out, causing a massive reactivity insertion and the attendant large fission power burst.....

As for Magnox or AGR reactors, I think its fair to say that natural-uranium & low-enriched-fuel reactors in general tend to have a slight positive thermal coefficient of reactivity -- at least after initial fuel loading (in reactor physics jargon, these are refered to as over-moderated reactors). One of the obvious fixes to Russian RBMK reactors following the Chernobyl disaster was to increase the fuel enrichment slightly, to achieve a core with a negative thermal coefficient of reactivity at all times and at all power levels (the other one, of course, was to replace the badly designed rods with good ones....).

The point about low power density is valid in respect to decay heat removal, as this is easier to manage/dissipate in such a core, than in a compact, high power density one. Remeber that the large natural uranium cores, moderated by graphite or by heavy water (in the case of CANDUs), in effect have two heat sinks available to them, where the LWR, with its combined moderator/primary heat transfer system, has only one.

Hope this helps.

Date: Oct 16, 2004

Thankyou Jaro.

However, as TMI goes, I am sure that I remember that a valve thet was allowing water to escape nevertheless showd "closed" on the controll panel with the result that the operators overrode something and exacerbated the problem-but it was a long time ago

OTOH I have always thought that the industry reaction to TMI was mishandeled. Nobody died! Nobody even got sick! It was not a disaster! Cock-up certainly but not a disaster.

If you never allowed for the possibility of error you wouldnt have secondry containments. the reactor (eventually) did what it was designed to do and kept everybody safe, THAT should have been the industry response!

Dusty

Date: Oct 16, 2004

Im also missing my own point which is that with conventional reactors (LWR or otherwise), it is possible for the operators, either through error, ineptitude or even malace to perform a sequence of operations from the controll panels that can cause a meltdown or even an explosion! it is also possible that an, albeit unlikly, sequence of system failures may result in the same.

if the claimes made about pebble beds prove valid then neither operator actions nor system failures will result in a excursion or a meltdown. the only way in which they can go bad is by direct physical sabotage EG demolition charges, deliberatly blocking the passive cooling ducts or dropping a tallboy on it!

Nothing can ever be 100% safe and it is impossible to make any system secure from malice, we can make it hard, but not impossible!

But (assuming the claims are valid) given the astonishing levels of inherent safty that pebble bed reactors can achieve I dont understand why anybody ever built any other type! (well I do actually )

Dusty

Date: Oct 26, 2004

I tired this game again after a few days. I did manage to average about 1.2 MW, but apparently this average was 'pathetic' (atleast it didn't melt down!)

This is a surprisingly difficult game.