Originally posted by BigJon@Work:
Can you explain this terminology? [/b]
Originally posted by analog:
[b] OT---Chernobyl's moderator void worth was four dollars - -- those fellows let it get away from them
Bigjon - i didnt think anybody but old nukes would be interested. It'll take defining a couple other terms, but here's my best try for you----
A "Dollar" is a unit of reactivity. It's the amount of reactivity that will put a reactor "Prompt Critical".
To grasp those terms, first:
Think of a reactor as a multiplying engine. Instant by instant it multiplies its present power level by a multiplying factor K. K is kept very close to 1.00000 with the control rods. Therefore power stays constant, or at most changes slowly, since 1.000 X 1.000 is close to 1.000, instant after instant after instant. To raise power you adjust K slightly above 1.000, and to decrease power you set it slightly below.
"Reactivity" is defined as (K-1)/K, which is very nearly (K-1). This is zero for a reactor at steady state (K=1.000), and slightly positive (or negative) when raising (or lowering) power. Adding reactivity raises power, removing reactivity reduces power.
Okay with reactivity?
Next, think of the concept of "doubling time". It's the amount of time for reactor power to change by a factor of two. That's a more intuitive unit than reactivity, because you can feel it. Most reactors have a meter that reads it out directly, although more often in units of "period" (time to change by a factor of e, 2.71) or "decade" (factor of ten). A doubling time on the order of a minute would be quite a comfortable start up rate for a commercial reactor. Ten seconds would be way too fast for me, though maybe not for OT. A reactor that's at steady power will never double, so has doubling time (and period) of infinity. The shorter the doubling time , the faster the reactor is increasing its power.
"Prompt Critical": If somehow enough reactivity gets added to push doubling time down into the millisecond range, the reactor power will go through the roof before anyone (or anything) could react. With your calculator, figure 2 to the 100th power - you're looking at that kind of power change in a second... The extreme case is called "Prompt Critical". A "Dollar" of reactivity is the amount it takes. Reactor designers are extremely careful to limit the rate at which one can intentionally add reactivity. A few cents is quite a bit of it.
"Void Worth" is reactivity change due to boiling of water in the reactor core.
That Russian reactor had the unfortunate characteristic that boiling in its core added reactivity and lots of it. The full amount it could add was greater than a dollar, in fact four dollars, according to a report that circulated around the US power industry a while after the accident. Those poor guys bypassed some of their built in safeguards, then disconnected the reactor from its heatsink which started in-core boiling and let it go prompt critical. Of course a huge steam explosion resulted, followed by a graphite fire...
That's why when I lived in South Florida I prayed Castro would never finish that big Russian reactor in South Cuba. And I'm pro-nuke!
I hope this helps you. Please feel free to email me if not.
<small>[ 08-26-2005, 02:24 PM: Message edited by: analog ]</small>
Cogito ergo doleo.