Pulse

Today we were supposed to pulse the nuclear science center's reactor -- that is, to blow $1.50 worth of a control rod (called the "transient rod") out of the reactor, making about a 15 ms pulse of 1000 MW of energy at peak. Well, in order to do this according to our guidelines, we need to have the reactor critical at 300W with the transient rod all the way in. Unfortunately, the reactor operators had turned the reactor on that morning at exactly at the right time to put us at the peak of the Xenon curve. This added enough negative reactivity to prevent our reactor from going critical even with all four shim safety rods and the regulating rod out. So, we turned the reactor up to 1MW in an effort to burn off some of the Xe-135, but that also increased our fuel temperature which also lowered the reactivity of the reactor.

In sum:

• To get the reactor critical at 300 W, we had to remove all rods but the TR completely, and remove the TR to about 5%, which is about $0.20 of reactivity removal.
• When we took the reactor up to 1MW, where the fuel temperature is about 600 deg F instead of 100 deg F at 300 W, we managed to burn off at least $.02 of Xenon, but of course the thermal reactivity coefficient made our reactor a lot less reactive.
• We inserted the SS rods back to about 92% and the TR back to 0% to bring the reactor power back down.
• At about 300 W, we took the shim safeties out again, in the hopes that our reactor period would be positive (a slightly supercritical configuration instead of a subcritical one). Alas, our period was still the -80.6 second one characteristic of the longest-lived delayed neutron group.
• Therefore, our reactor is lame because it doesn't have enough fresh fuel in it.

This is the first time I've been really sad over missing out on a lab like this. Pulsing the reactor is awesome to watch, and I've been looking forward to this lab for months, actually.