Tuesday, March 10, 2009

Q&A on LFTR Load Following, Peak Load Potential.

Question:
One of the more attractive potential applications for LFTRs is as Load Follower components for new Renewable Energy Systems to provide energy into the grid during the times the sun does not shine or the wind does not blow.

LFTRs have been described as excellent candidates for this application (the first molten salt reactor built at ORNL was for an aircraft reactor that required throttled operation).

How does the throttle function work on LFTR (what changes to produce a change in power output)?

Can LFTRs safely change power output multiple times an hour without risk and successfully back-up renewable energy systems?
First the LFTR's negative coefficient of reactivity means that a LFTR's chain reaction can be stopped by raising its core temperature. Coolant and fuel salt expands as core temperature increases. As core salts expand, they begin to overflow the core. As fuel salts begin to leave the core with the overflow, reactivity drops until when enough coolant salt leaves the core, reactivity stops completely. Heat is withdrawn from the core through the heat transfer system. As power demand increases more helium flows through the heat transfer system and picks up heat from the heat exchange. The helium carries the heat to the power generating turbines. Since the LFTR stops at maximum heat, the turbine can go from rest to full power as fast as it can spin up. As heat is drawn from core salts, their temperature drops and they begin to contract. As the core salts contract, coolant salt along with fuel salt is drawn back into the core. As fuel salts are drawn back into the core, reactivity increases, producing more heat.

Since reactivity is regulated by core salt heat, and core salt heat is regulated by power demand, there will always be a power reserve within the heated reactor salts that can be drawn on in response to power demand from the grid. Power production can be diminished simply by decreasing the flow of helium through the heat transfer system. Decreasing heat transfer from the core automatically lowers reactivity.

Operating as a peak power source or as a load follower actually benefits core life. Lower reactivity and less time spent at peak reactivity levels means less neutron damage to graphite, core metals and other core materials. If a load following LFTR averages producing 50% of its rated power, its core life is doubled. If a peak load LFTR produces power 25% of the time, its core life is quadrupled. Since utilities pay a premium for load following and peak load power, as opposed to base load power, load following and peak power generation are potentially a real money maker for LFTR owners.

The only drawback to LFTR backup for renewables would be that with its superior flexibility and reliability, low cost and potential to produce constant power 24 hours a day, the LFTR back ups would tend to make renewable base sources redundant.

4 comments:

David Walters said...

Charles, it is not that a LFTR can spin a turbine from zero to 3600 RPMs quickly.It is that it can provide the torque necessary to go from minimum load (as determined by the safe operation of the turbine...all of which have minimum load set points, usually about 20% of full load) to full load...but the turbine ALWAYS spins at either 1800 or 3600 RPMs because that is "the speed of the system".

David

Charles Barton said...

David, You are undoubtedly correct, A more accurate account then would be that 100% of potential torque is immediately available although most likely it would be applied gradually to avoid over stressing the system. The turbines would be kept spinning at times of no power demand by decay heat.

David Walters said...

That's correct. Speed never changes. If the LFTR can handle the rise and lowering of demand, the limiting factor is only the turbine's ability to handle that torque. Most modern turbines of the Rankine cycle can handle 10 to 20MW-per-minute load changes easily.

Newer yet-to-be-designed closed-cycle Brayton cycle turbines I can see having this capability built in.

I think you are actually raising and very important point. The ability to both load follow, change load, and, especially go to *minimum* load is a "product" that has immense value for any system operator. The LFTR having this ability is going to be a great 'selling point' for the technology. I think it needs to be emphasized more. Thanks for doing this.

David

Charles Barton said...

Thank you David.

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