Nuclear Operation: Understanding PT Limiting Curve & Radiation Effects

[matt222]

Hi all, I would like to understand the PT limiting curve during the operation, the effect of ebrittlment radiation on Reactor Pressure Vessel, what I don't understand first is what the results problem of changing PT diagram during the operation compared with the beginning of the operatoin,

I understand that during the heat-up/cool down of the reactor there is problem with sudden increase in the pressure so that they set apoint which is call low temperature overpressure protection to prevent this problem. but its not clear to me what I just said at the first pragraph really mixed up

attached the PT curve

 

[Astronuc]

Early in life, one can pressurize the reactor up to the nominal operating temperature at lower temperature when the vessel is not brittle. As the irradiation embrittles the reactor vessel, one has to go to higher temperature before pressure can be increased above the LTOP.

Basically, one wants to avoid pressurizing a brittle, or non-ductile, vessel.

The pump curve has to do with cavitation. As the temperature increases, the pressure must increase to prevent cavitation.

PWRs heat up on pumps. The four loop plants heat up on three pumps, and the fourth one isn't kicked in until water is hot enough, and the viscosity low enough such that fuel is not lifted off the core support plate.

 

[gmax137]

... what I don't understand first is what the results problem of changing PT diagram during the operation compared with the beginning of the operatoin,

The neutron flux during power operation makes the steel reactor vessel more brittle - the neutrons actually change the steel. It happens slowly, so typically the PT curves get revised every 10 years or so.