Moderator temperature coefficient(MTC) uncertainty

[jinggang]

HI, Guys:
    I have one question about MTC uncertainty on PWR. In most current industry design code, the calculation MTC uncertainty limit is 2 pcm/F, and 2 pcm/F is used mostly in safety analysis. However, during the startup physics test, the measured MTC uncertainty criterion is 3 pcm/F compared to the calculated value.  Why the (M-C) uncertainty is not 2 pcm/F in the startup physics test?
    Your kind answer is highly appreciated.

[rlbinc]

You guys can back me up on this - but I'll take a stab at it.

MTC is more negative during startup because the Boric Acid Concentration will be higher during startup than during power operation at rated conditions. As power is increased towards rated thermal power, Power Defect will add negative reactivity, which will be countered by dilution.

I'm a BWR guy, so that's as good as I can do over there in PWR land.

[HydroDave63]

This may help...

http://www.nukeworker.com/forum/index.php/topic,6848.msg72659.html#msg72659

[Fermi2]

You guys can back me up on this - but I'll take a stab at it.

MTC is more negative during startup because the Boric Acid Concentration will be higher during startup than during power operation at rated conditions. As power is increased towards rated thermal power, Power Defect will add negative reactivity, which will be countered by dilution.

I'm a BWR guy, so that's as good as I can do over there in PWR land.

What do you mean higher? MTC is more negative at power than at Hotstandby because Boron Concentration is lower. At certain concentrations, ie refueling MTC is slightly positive.

During startup we use a combination of Control Rods and Dilution to offset Power Defect and Xenon.

As for the uncertainty I'm not sure. Perhaps you read the "code" wrong. Usually a number like that means you have to meet a given value INCLUDING the uncertainty in which case 3PCM would be far more conservative.

Mike

[tr]

Getting back to the original question (uncertainty in design versus uncertainty in testing), this may be a case where the base values differ.  It may be that the values calculated for physics testing are different than the values calculated for the safety analysis.  At the plant I work at, the safety analysis physics data is biased to be conservative, while the physics data used for startup test verification is biased to be "real."

[Fermi2]

Which is exactly what I posted.

Mike