Thursday, January 9, 2020

From Lithium to Potassium

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Solution Shows Promise to Avoid Unexpected Shutdowns Due to a Vulnerable Supply of a Key Chemical

EPRI Gears Up to Demonstrate a New Chemical for pH Control

A chemical used in Western nuclear plants for pH control is vulnerable to shortages, and EPRI is investigating its replacement with a more abundant chemical. Results to date are promising. “Good pH control is a key part of how we operate our plant,” said Jim Connolly, chief nuclear officer at South Texas Project. “EPRI is the perfect organization to help industry through this issue.”

One critical task for nuclear plant operators is to make sure that the water used to cool the reactor core doesn’t become acidic to the point of damaging plant materials and fuel. Early in the development of pressurized water reactors, engineers selected chemicals that could be added to the water to maintain the proper pH. For the Russian-made WWER type pressurized water reactors, they chose potassium hydroxide. For Western pressurized water reactors, they opted for lithium hydroxide enriched with the lithium-7 isotope.

The rationale for lithium-7 hydroxide was reasonable: Lithium-7 is produced during reactor operations when boron in the cooling water captures neutrons, so its use for pH maintenance simplified maintaining the optimal mix of chemicals in the water. In addition, the less common lithium-6 isotope in naturally occurring lithium produces radioactive tritium when hit by neutrons. The excess tritium could unacceptably increase radiation dose risk for staff in Western pressurized water reactors.

Fast forward many decades, and China and Russia are the only producers of enriched lithium-7. A 2013 U.S. General Accountability Office report flagged concerns about reliance on just two suppliers. In 2014, a mechanical malfunction at a Chinese production plant resulted in a global shortage of lithium-7 hydroxide.

Fortunately, no U.S. nuclear plants ran out of lithium-7 hydroxide, but the shortage raised concerns about the long-term viability of lithium-7 hydroxide use. Operators add lithium-7 hydroxide when they restart reactors after refueling, which usually occurs every 18 months. If lithium-7 hydroxide were not available, reactors could not be restarted. Operating a reactor without the use of lithium-7 hydroxide for pH control would result in significant corrosion of pipes and infrastructure.

“The shortage was a wake-up call for the industry,” said Scot Greenlee, senior vice president of engineering and technical services at Exelon Nuclear.

EPRI had already started investigating how to avoid shortages by replacing lithium-7 hydroxide with the far more abundant potassium hydroxide. Today, EPRI is nearly halfway through a 10-year research program to inform the switch, applying WWER experience with potassium hydroxide to Western plant designs.

“EPRI is responding to an industry need,” said Jim Connolly, chief nuclear officer at South Texas Project, a nuclear plant in Texas. “EPRI brings broad technical know-how to this issue.”

Led by EPRI Senior Technical Executive Keith Fruzzetti, researchers have conducted modeling and laboratory experiments to address technical gaps—and to determine whether potassium hydroxide replacement is safe enough to demonstrate in an operating nuclear reactor.

Results to date are positive, and EPRI and South Texas Project are planning a demonstration at the plant’s Unit 1 for fall 2021. Pending approval by South Texas Project, the demonstration would span three refueling cycles.

“So far, there is no showstopper, so to speak,” said Greenlee. “Everything is pointing to potassium hydroxide being able to be used in place of lithium-7 hydroxide.”

Along with its abundance, potassium hydroxide offers other advantages. Because it’s less expensive than lithium-7 hydroxide, its use could save $100,000 per year per unit. Relative to lithium-7 hydroxide, potassium hydroxide requires less work to maintain a constant pH because its concentration in the coolant water is less variable. The radioactive byproducts of naturally occurring potassium isotopes are more readily managed than the radioactive byproducts of naturally occurring lithium isotopes.

Use of potassium hydroxide may also reduce the risk of unplanned distributions of power in the reactor core. Corrosion product deposits (known as crud) accumulate on fuel surfaces, and boron and lithium can precipitate within the crud to form lithium borate. Unplanned power distributions caused by these precipitates raise safety and operational concerns that may require dropping power output by as much as 25%.

EPRI modeling and laboratory studies demonstrated that because potassium hydroxide is more water-soluble than lithium hydroxide, its use can reduce lithium borate precipitation enough to cut fuel costs by as much as 2–3%. Based on preliminary EPRI calculations, this could translate to savings of $1–2.5 million per unit per refueling cycle.

Crud can lead to corrosion in fuel cladding, which may require repairs or even fuel replacement at a cost of more than $1 million. EPRI found that potassium hydroxide could reduce corrosion.

“The original motivator for potassium hydroxide was to eliminate the vulnerability to lithium-7 hydroxide shortages,” said EPRI Senior Program Manager Lisa Edwards. “It’s been a pleasant surprise to identify additional benefits that have nothing to do with the chemistry of pH control.”

Prior to the demonstration, EPRI is addressing remaining technical gaps about potassium hydroxide use. For example, Western pressurized water reactors use nickel-based alloys that are not used in WWERs, and researchers are examining the performance of these alloys in the presence of potassium hydroxide. Another set of lab experiments is testing the effects of potassium hydroxide on fuel rods exposed to pressures, temperatures, and other conditions typical in operating reactors. The rods used in the experiments are the same as those used in nuclear plants except that they are heated electrically rather than with uranium fuel pellets.

As part of the plan for the South Texas Project demonstration, researchers would examine the effects on the fuel.

“If plants replace lithium-7 hydroxide with potassium hydroxide, they will need to make various operational changes,” said EPRI’s Fruzzetti. “Based on our experiments, we are developing a strategy to inform how operators could implement and manage these changes.”

“Good pH control is a key part of how we operate our plant,” said South Texas Project’s Connolly. “EPRI is the perfect organization to help industry through this issue.”

Key EPRI Technical Experts:

Keith Fruzzetti, Dennis Hussey