It is easier to design and build compact nuclear reactors with uranium that has a higher fraction of U-235. The higher the U-235 content, the easier it is to overcome the effects of impurities in the coolant and cladding and the easier it is to overcome the inevitable effects of fission products that absorb neutrons. There is nothing secret about this fact; it is obvious from studying the history of nuclear reactor technology development. There are readily available examples related to university research reactors, the compact reactors built by the US Army, the Soviet era space reactors, and even the general knowledge that Rickover based his submarine reactors on highly enriched uranium – otherwise known as HEU.
Aside: This is a serious topic, so I want to lighten it up by sharing a photo that I took at a Veteran’s Administration hospital while I was going through my exit physicals from the US Navy. As an atomic advocate who knows how sensitive people can be about discussing HEU, and as a man who works hard to avoid acronyms, I was amused. End Aside.
One of the reasons that I decided to include the above aside with a humorous HEU sign is that I want to challenge you to think about the political and economic implications associated with the arbitrary definition of highly enriched uranium. According to the internationally accepted political definition, any uranium that contains 20% or more fissile uranium (U-235) is considered to be HEU. At that level, all kinds of material restrictions kick in. The world has spent enormous sums of money developing sophisticated new research reactor fuels in order to replace the simple aluminium clad fuels that were the basis for research reactors for the first three decades or so of the Atoms for Peace era.
The ostensible reason for the line at 20% is that anything greater makes it too easy for a dedicated weapons developer to build a functional explosive device. However, that limitation has also hampered or outright prevented a number of valuable technology innovations. In addition, it has successfully raised the barrier to entry for any nation that would like to develop its own profitable nuclear fuel manufacturing capability.
While it is obviously possible to build large functional reactors using lower enrichments, small reactor designers have to overcome a number of challenges to build a reactor that can reliably produce heat to serve a variable load. If a reactor designer wants to produce a system to serve isolated loads, they have to design something that can power its way through xenon transients during a reasonably chosen fuel lifetime. It is a less costly challenge to overcome with a higher concentration of fissile material that does not lose a large portion of the produced neutrons in absorption in fertile material.
Doing that with lower enrichments requires some design and material sophistication. I do not know the technical details as well as some people, but I understand that lower enriched fuels require cladding that has a tiny neutron cross section. One of the more popular choices has been extremely pure zirconium alloys; zirconium has a low affinity for absorbing valuable neutrons, but it is naturally contaminated with hafnium, a strong neutron absorber. Separating hafnium from zirconium is a well established technology, but the people who own the intellectual property chose not to share it. A nation that wants an independent capability has to develop it.
In the past few days, Iran has publicly admitted that it is interested in building smaller reactors that can power tankers and perhaps even reactors that are small enough to squeeze inside submarines. Though submarines are rightfully considered to be sophisticated systems and though they have been a technical challenge even for nations like Canada and Australia, Iran appears to have all of the right ingredients to be reasonably successful.
The country has been operating and maintaining submarines for at least 2 decades, it has a firm base of engineering talent (in the days of the Shah, the population of Iranians in US engineering programs was quite high), and it has a supportive government. Politicians need to remember that the United States, the Soviet Union, France and China all successfully developed nuclear powered submarines using 1950s vintage technology. (The UK did not independently develop its nuclear submarine technology.)
As an internationally isolated pariah, Iran also has a strong motive for developing what could be a powerful game changer – the ability to build smaller, load following nuclear power plants. From an economic power point of view, it is very difficult to embargo a country that has plenty of indigenous energy. It is very difficult to stop a nation from finding someone who is willing to engage in trade if it has ships that almost never need to be refueled or resupplied.
From a military point of view, compact nuclear reactors are valuable, especially considering Iran’s geographic location astride a key international trading route. However, nuclear powered submarines would allow an angered Iran to threaten to sink ships in almost any location around the world.
Continued demonization and isolation is a path that is likely to yield results that almost no one (other than arms suppliers) wants. Engagement and recognition of real world technical facts will be a more productive path towards a peaceful and prosperous world. I suspect that I have just opened myself to accusations of following a Neville Chamberlain path. Let me assure you that I am not an advocate of appeasement, but proving that point would take a lot more time than I have available this morning.