Xe-135, a secondary fission product, has a voracious appeitite for neutrons, and a civilian power reactor, once shut down, cannot be restarted until the xenon is sufficiently decayed. Such a delay would be unpleasant for any ship, and unacceptable for a warship. What methods are used to increase reactivity in naval nuclear propulsion to avoid this problem?
2006-11-06 00:59:13 · 5 answers · asked by Anonymous in Science & Mathematics ➔ Physics
Xenon is a gas that is vacuumed off
2006-11-06 01:01:22 · answer #1 · answered by Anarchy99 7 · 2⤊ 5⤋
Very good question. The answer is all in the design of the reactor, and what is important.
In nuclear engineering, we talk about reactivity, which is a measure of how far from critical you are. If your reactivity is 0, then you are critical and the power in you reactor is constant. If your reactivity is >0 then you power will increase, and the larger you reactivity the faster the power will increase. And if your reactivity is negativity, then your power will go down. Doing things like removing control rods can be explained by adding a positive reactivity. Poisons like Xe-135 add a negative reactivity.
In nuclear power plants we limit the maximum amount of positive reactivity you can physically add to the reactor. This limits how fast the power level in the reactor can increase, and allows us to always be able to react to a change in core conditions. These limits are good, and they are one of the reasons why nuclear reactors (in the US) are safe.
For navy reactors these limits are much higher. They are much higher to allow the positive reactivity to overcome the Xe-135 poisoning. There are many ways to increase the max reactivity. You can use higher enrichments, larger reactor sizes, etc.
Also note that the Xenon is still contained in the fuel rods, so you can not 'wash it out.'
2006-11-06 06:45:09 · answer #2 · answered by sparrowhawk 4 · 2⤊ 2⤋
This is speculation, but cold water injection?
This would reduce the doppler broadening and thus reduce the neutron capture cross section for some of the poisons allowing the reactor to start up.
Other thoughts:
Provide sufficient fuel density to allow delayed criticality even in the presence of Xe135, provide a measured mass of material that undergoes delayed single neutron generating fission, such that these extra neutrons following shutdown will make up the losses (Thorium?)? I am thinking that it may be possible to design so that the delayed neutrons from the other decay products make up for the losses to Xe135.
Can the military stuff be taken prompt critical? Bloody dangerous, but it would hint at sufficient fuel load to allow start up even with heavy poisoning.
Guessing here, and I don't have the rubber bible to hand so I cannot find a suitable 'salt' (if this is even the approach taken).
Regards, Dan.
2006-11-06 04:23:09 · answer #3 · answered by Dan M 3 · 0⤊ 2⤋
Well, nuclear technology in general is not classified.
To increase the reactivity in the reactor you need to withdraw the control rods further than you would expect for a certain power level. Then re-insert the rods slowly as the Xenon decays.
2006-11-06 02:45:04 · answer #4 · answered by Jeff M 3 · 1⤊ 1⤋
classified...and wont tell you
2006-11-06 01:06:38 · answer #5 · answered by Jack Kerouac 6 · 1⤊ 1⤋