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A term expressing the departure of a reactor system from criticality. A positive reactivity addition indicates a move toward supercriticality (power increase). A negative reactivity addition indicates a move toward subcriticality (power decrease).

Rad Sponge:
I understand the -alpha (t)(mod) is negative for most of the core cycle and becomes less negative as fuel burns out and core density changes effectively causing less probability for thermalizing neutrons.

What I don't understand is that there is a chance around 200F at EOC that alpha (t)(mod) can go positive. What makes it go positive?

You're talking in a BWR core right? In a commercial PWR Core it gets EXTREMELY negative at EOL and in fact we have a TS Limit on it.

In a BWR as the core ages it physically gets bigger with less fuel. This is of course due to withdrawal of the Control Rods combined with being an EOL core having less U235/Pu239. The core becomes overmoderated, there is less competition for Neutrons. Remember water acts as a poison too. In an Overmoderated Core at low temperatures the primary affect of heating up is having less water molecules to absorb the Trons so the primary affect of a heat up is to reduce poisoning by H2O. This occurs at about 200F to maybe just under 400F. After this point the core is hot enough and for a given concentration of water there are less water molecules that water actually starts acting like a real moderator again.

Think of it this way. At 200 Degrees assume a substance has a 1000 PPM Concentration.
Now at 500 Degrees it may have the SAME concentration but for a given area there will be less of that substance because it expanded. While your concentration remained the same at 1000PPM there are less millions to be a part of. Therefore less actual pieces of your substance.

At low temperatures the predominant affect is as you heat up you're creating "holes" in the water. Trons make it through these "holes" to the fuel. As you heat up the water, even though you have the same volume there is less water present do to expansion so your holes are bigger, You're not creating new holes and there is a lower Moderator to fuel ratio so water starts acting like a moderator again.

Yes at EOL in a BWR it can be positive.

It's the same in a PWR Fuel Pool with a high boron concentration. At lower temperatures the primary affect is less particles of boron present so in spent fuel pools and during refueling a commercial PWR will have a positive Alpha T.


Jason, here's a picture of what BZ is talking about.  I'm a picture kind of guy.

The picture is crappy.  Here is a better link.

Something else about the origional definition...

A positive addition of reactivity doesn't necessarily mean a move toward supercriticality.  Prompt or stepped changes in addition rate can change power regardless of reactivity. 

The algabraic sign of either term isn't as important as the relative magnitude.  Power will follow the bigger term regardless of the sign of the other.  Only in a steady state reactor (which doesn't really exist) will reactivity alone affect the direction of power.  Semantics, I know.

BWRs may have a positive coefficient due to the combined effects of Low Temperatures and Core Age (depletion). So this would be a problem during a late cycle cold startup.
Once heated up, alpha T is supplanted by big brother alpha V and the issue becomes insignificant.

At its worst condition, it's a small positive. I have seen the range of BWR alpha T being between -2.5 x 10-4 to +0.5 x 10-4

We don't use fast startup rates. An 80 to 120 second period ( + 1/3 to + 1/5 decade per minute) is about as fast as we push it. No extra points for startup rate, like the Navy.


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