nuclear-news

Future MSR designs and problems

a) The MSR design is much like a PWR design: each has a reactor, steam generator, and turbine/generator for the three primary sections.  However, as shown in the Idaho National Lab drawing above (INL), there are four loops in this design.  PWR has three circulating fluid loops: cooling water, boiler feedwater/steam, and the primary heating loop,  Yet, the MRS has a fourth loop, for radioactive molten salt for MSR.    Any MSR design that hopes to be economic will also be huge, likely in the 1000 MWe output size, to employ economy of scale.  This requires scaleup of approximately 500-to-1 compared to the ORNL project.   With a cycle efficiency of approximately 30 to 33 percent, the thermal output will be approximately 3500 MWth.   Scaleup from ORNL size by 500 times is an enormous challenge.   Note that scaleup with a factor of 7 to 1 is a stretch, yet such a factor (using 6) requires four steps (40, 250, 1500, and 3500) to use round numbers.   Each larger plant requires years to design, construct, and test before moving to the next size, and that is if the larger design actually works the first time.    It is also instructive (and very, very expensive) that the MSR design has a dual-compressor and heat removal fluid instead of the conventional steam condenser system.  Costs and operating problems for this design are much, much greater than for a PWR.

b) The materials of construction for a very hot molten Fluoride salt mixture will likely be extremely expensive, if made of Hastelloy N to prevent the widespread cracking found at ORNL.   It remains to be seen if even Hastelloy N will have a sufficient strength and thickness after 40 years of service.

c) Pumping the very hot, corrosive, molten salt mixture will require expensive alloy materials, and due to the salt’s density, high horsepower for pumping.   Also, pumping a hot molten radioactive salt requires sophisticated pump seals to ensure safety and prevent leaks.   As described above, the thorium MSR design will have four main circulating loops, while a PWR system has only three.   However, the cost for MSR hot molten salt circulation pump will be more expensive than the PWR pressurized water circulation pump due to the high-cost alloy required, and the almost double horsepower motor to drive the pump.

d) If a molten salt pump is not used, circulation can be achieved by a thermal density difference loop.  However, this also presents serious design and control problems.

e) The steam generator design presents a complex and likely insurmountable problem. Even if a successful design is somehow created, leaks of high-pressure water into the low-pressure molten salt are inevitable and will create all manner of hell. Havoc is too mild for the mess that will happen.   Water that contacts the hot molten salt will explode into steam, possibly rupturing the piping or equipment and flinging radioactive molten salt in all directions.   In addition, the steam generator’s material of construction also must resist the hot, corrosive molten salt.  The steam generator will also likely be made of Hastelloy N, which adds to the already high cost of the plant.   It is also notable that the INL MSR design has two heat exchangers for the steam generator loop, which decreases overall cycle thermal efficiency.   It does not increase safety, as water will leak into the molten salt.

f) Controlling the plant output, adding more fuel, and removing unwanted reaction byproducts, all are obstacles.

g) With the low thermal efficiency, MSR plants will require approximately the same quantity of cooling water as uranium fission plants.   That, as discussed previously in TANP, is a serious disadvantage in areas that are already short of water.

Conclusion

It can be seen then, that thorium MSR has few advantages, if any, over PWR.  They each have three or four circulating loops and pumps, however MSR will have much more expensive materials for the reactor, steam generator, molten salt pumps, and associated piping and valves.   There will be no cost savings, but likely a cost increase.  That alone puts MSR out of the running for future power production.

The safety issue is also not resolved, as stated above: pressurized water leaking from the steam generator into the hot, radioactive molten salt will explosively turn to steam and cause incredible damage.  The chances are great that the radioactive molten salt would be discharged out of the reactor system and create more than havoc.  Finally, controlling the reaction and power output, finding materials that last safely for 3 or 4 decades, and consuming vast quantities of cooling water are all serious problems.

The greatest problem, though, is likely the scale-up by a factor of 500 to 1, from the tiny project at ORNL to a full-scale commercial plant with 3500 MWth output.   Perhaps these technical problems can be overcome, but why would anyone bother to try, knowing in advance that the MSR plant will be uneconomic due to huge construction costs and operating costs, plus will explode and rain radioactive molten salt when (not if) the steam generator tubes leak.    There are serious reasons the US has not pursued development of the thorium MSR process.  Reports are, though, that China has started a development program for thorium MSR, using technical information and assistance from ORNL.   One hopes that stout umbrellas can be issued to the Chinese population that will withstand the raining down of molten, radioactive fluoride salt when one of the reactors explodes.