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The nuclear powered car part III

Dear Reader,

I have already considered the problem of long lived alpha waste, the theft of the nuclear powered car fuel by a bomb maker and the horror of a back street repair job which goes wrong. Now I will consider one of the other aspects of the nuclear powered car, the problem of what happens when you park your car for a few minutes and then start to drive again.

If we were to use solid fuel which is form of pellets, rods or plates then the car reactor would suffer from the problem of xenon poisoning. If the power output is kept at a constant level for a long time then the xenon-135 level will settle down to a constant level. But if the power level of the reactor is increased or lowered then the continued formation of xenon-135 and its consumption by the neutrons in the core may cause a transient. In a core where the reactivity is suddenly lowered then the xenon-135 transient will tend to push down further the power output of the pile, but in a recently used core where the reactivity is increased by withdrawing control rods the xenon-135 transiant will tend to cause a surge in nuclear reactivity.

Also if the core has been operating at high power recently, then if it is shut off then it is very hard to restart the reactor safely due to the xenon-135. It tends to poison and stop the reaction, if the core is provoked into going critical then the free neutrons will start to burn up the xenon-135. Suddenly when the xenon-135 has been consumed then the poisoning effect is suddenly lost, this can lead to a power surge. The xenon-135 transient is something which I think greatly contributed to the power surge at Chernobyl. If the operators had taken a core when had laid idle for weeks, and run it up to full power and then SCRAMed it then they would have been more likely to have done the experiment in a safe and effective way.

One solution to the xenon problem would be to choose a fuel in which the noble gases can not accumulate. If we were to use a homogenous aqueous solution of uranyl sulphate or a molten salt reactor then the fission gases could be constantly purged out of the fuel. These volatile fission products would be trapped in either an activated carbon, a zeolite or some fancy metal organic framework such as the zinc salt of benzene-1,4-dicarboxylic acid. This would make the management of the waste from the car reactor more easy. Much of the medium lived heat producing fission product waste (Cs-137 and Sr-90) would be separated from the fuel, Also this arrangement will lower the amount of Cs-134 formed in the fuel.

The homogenous aqueous reactor has the advantage that if it overheats the moderation will become less effective so it will tend to slow down, the radiolysis bubbles and the boiling off of water will tend to shut down the reactor. This reactor design is very resistant against reactivity accidents. But it has the disadvantage of having liquid fuel. If a serious mechanical accident occurred then it might be possible to have a spill of used fuel. I think that the idea of a potential highly radioactive liquid spill in the event of a dire road accident would stop the nuclear licensing body from issuing a license for the car. So we now have another reason why the car can not be made and sold.

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