While at 100%, CWIPs 2C and 2D tripped due to severe grass impingement on the traveling screens. The operating crew has entered the applicable procedures and rapidly reduced reactor power.
Plant status is as follows:
Rx power: 50%
CWIP 2A: Running
CWIP 2B: Not Available
CWIP 2C: TRIPPED, will not restart
CWIP 2D: TRIPPED, will not restart
Considering only the effects of Xenon and assuming no further operator action, which one of the following describes reactor power 3 hours after reaching 50% power?
A. Greater than 50% and decreasing.
B. Less than 50% and decreasing.
C. Greater than 50% and increasing.
D. Less than 50% and increasing.
B. Less than 50% and decreasing.
The reduction in power causes a reduction in Xe-135 production (due to less Xe-135 being produced directly from fission), and a very large reduction in Xe-135 burnout (due to the decreased neutron flux. A relatively high inventory of I-135 still exists, and Xe-135 production remains relatively high. This effect causes Xe-135 concentration to increase and power will start decreasing. This increase in Xe-135 concentration causes more Xe-135 decay to start occurring, and in about 4-6 hours, the I-135 concentration will have decayed sufficiently low enough to cause Xe-135 concentration to stop increasing with power now lower that the iniial 80%. In the 30 to 40 hours, Xe-135 concentration will decrease until a lower equilibrium level is reached and power will again rise and stop at a level slightly above 80% power. The total time of the Xe-135 transient is 40 to 50 hours, probably closer to 40 hours due to the small (20%) change in power. (At t=10, the peak xenon transient has passed and xenon is starting to rapidly decay and will continue to decay for the next 30 hours.) the final steady-state Xenon concentration will be less than the 100% power concentration and less than the initial Xenon level experienced at 80% power. Hence, if left long inough, final steady state reactor power would be greater than 80% at t= 40 or 45.