Monday, October 12, 2009

Letters to Jesse 5: Lowering LFTR Costs While Increasing Nuclear Safety

Dear Jesse, Even if the LFTR could not be assumed to have excellent potential for lowering nuclear cost, its safety features, and handling of the nuclear waste problem would make it an excellent candidate for the role of future safe and nuclear waste free electrical producer. In addition the LFTR has excellent operational characteristics, that give it a flexibility comparable to natural gas turbine generators but with much .lower fuel price. For this reason, low cost LFTRs can replace carbon emitting natural gas turbine generators. My analysis of potential backups for renewable electrical generation facilities, pointed to the LFTR as the best backup technology. The LFTR would be priced at a competitive cost, would have lower fuel costs than natural gas generators, and would be far more flexible than batteries, pumped storage, or compressed air storage. LFTRs can be kept spinning for days with no fuel expenditure, and for indefinite periods of time with very little fuel expenditure. In fact the LFTR's performance in the back up role for renewables, would be such that the renewables being backed up would be redundant. This analysis led me to the conclusion that a single technology approach to post carbon energy could lower energy costs, while greatly increasing electrical reliability.

There are numerous sources reporting on Molten Salt Reactor/LFTR safety. (see here, and here). Since the core fluids of the LFTR are well below their boiling point, the LFTR operates at atmospheric pressure, and thus poses no danger of a steam explosion. Coolant leaks are far less likely in a LFTR are far less likely than in a water cooled reactor, and far less dangerous than in a LMFBR. Coolant leaks in a LFTR tend to be self limiting, because the coolant immediately freezes when exposed to the cooler temperature of the environment outside the reactor. Once the coolant freezes, further leaks are blocked.

Because the LFTR is safe in ways that water cooled reactors are not, safety features that are unique to water cooled reactors can be eliminated. Consider the now classic reactor dome, depicted here in a schematic for a relatively small Indian PHWR. This dome is much larger than the reactor, and one of it's safety features is that it has two separate containment walls. This design testifies to the Indian commitment to nuclear safety, and in Europe or North America would be very expensive to build. Indian labor costs are much lower, than those of more developed economies, hence the dome does not represent the sort of cost factor to Indian reactor designers that they would represent to European and American reactor designers.

The massive walls to the reactor dome not only prevent the escape of radioisotopes in the event that a steam explosion breaches the reactor pressure vessel or a pressure tube. But what if there were no possibility of an explosive release of radioisotopes? We have seen that this is exactly the case with Molten Salt Reactors including the LFTR.

During the Mid-1960's Ed Bettis, who is often credited with inventing the Molten Salt Reactor, created a number of design studies for a cluster of 4 small MSRs. Bettis's design is startling and the most startling thing about it is the way the reactors are housed. Each reactor is contained on a small cell.

Note how compact Bettis design is. The three cells Bettis drew would take far less labor and materials to build, and would require far less time to build than the Indian reactor dome. Now look at another of Bettis's schematics:
Note that no dome is depicted, only a small structure that is robust enough to contain radioisotopes released in a reactor leak. In Bettis's design. the reactor core, the heat exchanges and even the coolant plumbing could be factory built, lowered into the reactor and hooked up. The reactor housing structure itself could be built in a matter of months, or if required be prefabricated, and assembled on site. In contrast to Babcock & Wilcox small mPower reactor, the LFTR could be assembled withe much less labor, and in a matter of months rather than three years.

But would the Bettis design be safe from terrorist attack? First we should note that the Bettis design provides robust protection against the diversion of fissionable materials. The reactor housing would be around 600 degrees C, far too hot for even the most fanatic terrorists to tolerate. In addition radiation from the reactor, from the fuel cleaning process, and from fuel storage, would be far to intense for terrorists to survive more than the briefest of exposures.

But what about terrorists attacks by aircraft or truck bomb? The late Edward Teller always believed that the underground siting of reactors would create optimal conditions for nuclear safety. In his last paper, Teller and his associate Ralph Moir advocated underground sited Molten Salt Reactors as the best possible nuclear technology. Even in relatively shallow underground placements, LFTRs would be well protected from truck bombs, and aircraft attacks. Fissionable materials would be in an underground setting similar to the Bettis design and would be inaccessible to nuclear terrorists. In addition gravity, earth, the chemical nature of the hot salt fuel fluid, and the reactor housing structure would prevent radioactive materials from reaching the surface in the event of a nuclear accident.

Although Teller and Moir did not pay overt attention to the cost of their underground siting plan, it probably would not be more expensive than digging and building a small utility sub basement for an office building. Hence with the LFTR we can dramatically lower site construction costs, while improving nuclear safety.


Jesse Jenkins said...

"LFTRs can be kept spinning for days with no fuel expenditure"

Can you explain how this works/is possible? Thanks,

Jesse Jenkins said...

" In addition radiation from the reactor, from the fuel cleaning process, and from fuel storage, would be far to intense for terrorists to survive more than the briefest of exposures."

Hi Charles, sorry for another question (thanks for tolerating my ignorance!): does this imply that the interior of the reactor cell, and probably the dump and storage cell would have high levels of radiation? (and what about the steam cell?). That would certainly deter terrorists trying to steel uranium, but what does that mean for repairs to potential reactor malfunctions? Could repairs be conducted, or would the reactor have to be scrapped? Do the high levels or radiation persist after the reactor has ceased operation and if so, for how long?



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