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The nuclear powered car part II

Now I recently commented on one aspect of the folly of the ”nuclear powered car”, one of my worries was that some unqualified and reckless person might attempt to service a reactor thus causing a radiological accident.

Now I will consider some of the other aspects of the nuclear powered car. We need to consider what type of reactor fuel should be used.

We could opt for a light water reactor; we could create a very small light water reactor using solid fuel. This reactor could generate steam or very hot water which could drive the engine. The problem with this is the fuel. I think that to make the reactor very small we would need to use a fuel with a high level of enrichment. We would need to use a burnable poison in the fuel to keep the reactivity level roughly constant during the life of the core.

I imagine that the fuel and the core would be very like that which is used in a nuclear submarine, in a nuclear submarine the core is used for a very long time. Also the core of both the car and the submarine needs to be able to respond rapidly to changes in power output. In contrast a typical nuclear power plant will run the fuel at a roughly constant power output for about one year rather than yo-yoing the power output.

For example about 300 grams of 239Pu in a breakfast mug with some water will go critical and thus form a primitive nuclear reactor. The fuel would be a security problem I will go through the possible fuels which I can think of right now.

A high plutonium MOX containing 239Pu in a 238U matrix. This fuel would breed some more plutonium during use which would prolong the lifetime of the core. The great problem I see is that such a fuel would have a high potential for misuse. While a 50 : 50 mixture of 239Pu and 238U might not be suitable for direct use in a bomb, it would be possible by a simple chemical separation to obtain bomb quality plutonium so this fuel is a non starter.

If a plutonium containing a large amount of 240Pu was to be used then this fuel would be bad in quality for building a bomb, even after the chemical separation. 

If we consider low energy neutrons (0.0253 eV) then for 239Pu we have the following data

 

Total cross section 1026 barns

Elastic scattering cross section 8 barns

Fission cross section 747 barns

Radiative capture cross section 270 barns

 

While for 240Pu we have the following data

 

Total cross section 291 barns

Elastic scattering cross section 1.6 barns

Fission cross section 0.059 barns

Radiative capture cross section 289 barns

It is clear to me that the 240Pu will capture neutrons and form 241Pu, this might sounds like a harmless effect but it is far from nice. The problem is that 241Pu decays by beta decay to form 241Am which is an intense emitter of alpha emitters. This americium would increase the amount of radiotoxic (alpha emitting) medium lived waste. If the used cores from the cars were reprocessed than the lanthanide rich fraction would be very alpha active. In conventional PUREX reprocessing the lanthanides and the post-plutonium actinides end up in a mixture with all the other fission products.

I know that if you were to run a light water reactor on pure 239Pu some 241Pu would be generated but with a power reactor grade plutonium which is 240Pu rich this problem would far greater.

To avoid the generation of plutonium, americium, curium and the other transuranium elements we could opt for a highly enriched 235U, this would be a bad choice for security reasons. Such a fuel (when fresh) would have even greater misuse potential than the bomb grade 239Pu. So as a result I think that this fuel is totally out of the question.

Another alternative fuel would be a blend of natural thorium with some fissile matter. While pure 233U or 235U in 232ThO2 (thorium dioxide) would be a security problem, because after a simple chemical separation you could obtain bomb grade uranium, we might be able to imagine a fuel where a 239Pu / 240Pu is in the thorium dioxide matrix. This fuel would become more and more plutonium lean with use, while the level of uranium-233 would grow during the lifetime of the core. The 233U level would grow and would hopefully become steady thus allowing the reactor to function for a long time.

The great disadvantage I see of this fuel is that the used fuel would have considerable misuse potential. If the uranium was separated form the fuel then the isotope signature would be very favourable for bomb making and very unfavourable for world peace / goodwill to all men. So for these reasons I think that I can not think of a perfect fuel to run a small reactor on for a long time.

I hope to get onto some of the other problems which I can imagine would plague a nuclear reactor powered car in another post. While you might think that the “nuclear powered car” is a stupid idea which should not be considered, it is not a totally stupid idea even when it might be a stupid product. The idea allows us to consider some of the issues associated with a small mobile power reactor.

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2 Responses

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