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By Charles Rhodes, P.Eng., Ph.D.

The main reasons for adoption of sodium cooled U-238 fueled Fast Neutron Reactor (FNR) technology and Th-232 fueled Molten Salt Reactor (MSR) technology are:
a) Fuel sustainability;
b) Minimum production of long lived nuclear waste;
c) Low pressure primary coolant makes reactor safe for urban installation.

Both FNRs and MSRs share the aforementioned major features. However, at this time I have a strong bias toward U-238 fuelled FNRs due to more favorable temperature and chemistry. We can build FNRs today using off the shelf regulation approved materials that will reliably operate in the temperature range 400 C to 500 C. The peripheral FNR piping can use conventional flanged joints for ease of both assembly and service. Most of the FNR service work can be done by lowering the primary sodium temperature to 120 degrees C, at which temperature electronic robotic systems can easily function. About a week after FNR shutdown the primary sodium is only slightly radioactive. Another benefit of pool type FNRs is that they provide considerable allowance for differential expansion in peripheral piping and for minor pipe connection mis-alignment.

The practical realities with MSRs are that: they need to operate in the temperature range 550 C to 700 C, they require piping and piping fixtures to be fabricated from materials that do not exist on the market and are not approved for use in fired pressure vessels, they require a moderator change every four years, and they are very difficult to assemble and service because they require almost all welded peripheral piping. The highly radioactive fuel/coolant freezes at a high temperature, meaning that the coolant needs to be drained before any service work can be done on the system. There then must be provision for raising the highly radioactive fuel/coolant back to at least 600 degrees C before reinserting it in the reactor. On top of that, each MSR requires ongoing attention by persons with considerable radio-chemical expertise.

We can build power FNRs today. The Russians are already doing so. Even if we can obtain access to 100% of the practical MSR operational knowledge gained by the Chinese, we will not be in a position to design a fuel sustainable power MSR for at least another 30 years.

In my view the reason for the present large Russian and Chinese lead in FNR and MSR technologies has been sustained long term corruption of both the US and Canadian governments by the fossil fuel industry. This corruption still persists today.

One of the MSR developmental challenges is piping and mechanical pipe joints containing gaskets that are suitably chemical resistant, pressure resistant and work at over 450 degrees C. Other important MSR issues are the moderator and provision for differential thermal expansion (TCE). Related to the moderator issue is the issue of the effect of fuel residence time in the moderator on reactor stability, especially in accident conditions.

The longest MSR operating time that I am aware of is 17,000 hours. We need to be talking 10X to 30X that time for a MSR to make financial sense for electricity generation.

Almost all the present MSR proponents are contemplating a reactor plus moderator plus intermediate heat exchanger replacement every 4 to 5 years. That equipment replacement requires hot radioactive fuel transfers and perfect mechanical pipe joints. Just physically aligning those pipe joints with hot radioactive equipment is no small feat. It is difficult enough for real personnel to accurately align a single turbine shaft to a generator shaft at room temperature in a non-radioactive environment. The advocates of frequent MSR reactor changes require a whole series of near perfect pipe alignments that are complicated by radioactivity and TCE problems.

I believe that some years ago there was a reactor project in the UK that required comparable pipe alignments, but that technology was abandoned due to its practical implementation costs.

I am no longer young. I am not in a position to significantly influence future MSR development. However, I can influence FNR development and deployment, as I am doing today at www.xylenepower.com.

Fuel sustainability is important due to the adverse distribution of natural uranium ore. In the ocean the concentration of uranium is about 3 parts per billion. On land the average uranium concentration is a few parts per million, but there are relatively few deposits of uranium of sufficient concentration for economic mining.

In an August 12,2021 email Peter Ottensmeyer set out the fuel sustainability case for FNRs as follows:

I am concerned about there being enough nuclear fuel for my kids and grandchildren. From my vantage point, Canada has a uranium reserve of about 500,000 tonnes, which should be plenty for us, since we need about 2000 tonnes per year for our present level of nuclear energy generation using CANDU reactors. However, we are presently mining and exporting it at the rate of 16,000 tonnes per year (e.g. 2016/17/18). At that rate the Canadian natural uranium reserve will be exhausted in about 30 years, around 2050. That means that after 2050 there won’t be any economic Canadian fissile material for either internal use or export.

In reality the problem is significantly worse than this simple calculation depicts, because in order for Canada to meet its 2050 climate change mitigation objective, Canada must quadruple its present rate of nuclear power production.

The economically mineable world uranium reserves will last perhaps a few decades longer, before they and their fissile contents are gone as well.

So I look for more fuel-efficient alternatives. FNRs (Fast Neutron Reactors) not only give a factor of 100 improvement in fuel efficiency, they simultaneously make all of the stored previously mined 3 million tonnes of U-238 into usable fuel. That is fertile material, not fissile. It cannot be used in thermal reactors and is not of any help in thorium-based reactors either. However, the FNRs can convert the U-238 into fissile Pu-239 that let’s thorium reactors run.

Being surrounded by CANDU reactors for most of my life I am certainly well aware of how little fissile material it takes to start such a reactor and to make it run: only the 0.72% U-235 in natural uranium. The CANDU is one of the most fuel–efficient thermal reactors around, about 50% better than the LWRs. Nevertheless a CANDU reactor requires 100 tonnes of new natural uranium every year in order to replenish the fissile component that is used up. A CANDU reactor has a lifespan of about 30 years before its pressure tubes must be replaced and then runs for another 30 years. So in 60 years it has used 6000 tons of mined uranium. For easy comparative calculations, let’s say the CANDU has a power of 600 MWe.

Compare that to two GE-Hitachi 300 MWe PRISM reactors, which are FNRs operating with core fuel rods having 15% fissile material and each reactor requiring 20 tonnes of core fuel. It requires 1200 tonnes of natural uranium to furnish the first enriched starting charge of core fuel for the two reactors and at most another 1200 tonnes of natural uranium to establish fuel recycling, for a total of 2400 tonnes. Since the fissile complement is maintained (or even augmented) the total fuel requirement after startup is only about 1 tonne of U-238 containing fertile material per year which may be depleted uranium, used thermal reactor fuel or natural uranium. So in rough numbers, after 60 years the two 300 MWe reactors together would require at most a total of 2460 tonnes of mined natural uranium.

After 60 years, if that is their lifetime, the FNRs would pass on their fissile content, undiminished, to the next FNRs of equal power, which then require only the 1 tonne per year of a source of fertile U-238. So the second set of FNRs can run for 60 years using only 60 tonnes of mined uranium. It is perfectly clear that the requirement for an “enormous quantity of fertile material” is a one-time start-up event.

A new CANDU reactor (or other equally efficient new thermal reactor) would still need its charge of 100 tonnes of mined uranium every year, or 6000 tonnes over its lifetime. At his point the FNRs with their requirement of 1 tonne per your, or 60 tonnes lifetime total, are 100 times more fuel efficient than the CANDUs.

No matter what reactor you have, you get about 200 MeV total for each fission. If you get that energy primarily from U-235 you still need to mine the 99% U-238. If you get it via transmuted U-238, you eventually only need to mine 1/100 as much uranium.

In the greater scheme of things, the amount of fissile material needed to start a reactor (bring it to criticality) does not matter at all. How that fissile material is used over the reactor life to convert the fertile fuel into fissile fuel is what matters when it comes to efficacy of using uranium.

You wouldn’t throw the bulk of a chocolate bar away after you’ve had only one lick of it when you know you can bite off chunks to enjoy the rest. Why would you insist on throwing the bulk of uranium away when you know how to extract all of its energy?

This web page last updated October 3, 2021

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