Purdue Engineers Create Safer, More Efficient Nuclear Fuel, Model Its Performance

New findings regardingthe research will be detailed in a peer-reviewed paper to be presentedon Oct. 6 during the 11th International Topical Meeting on NuclearReactor Thermal Hydraulics in Avignon, France. The paper was written byShripad Revankar, an associate professor of nuclear engineering;graduate student Ryan Latta; and Alvin A. Solomon, a professor ofnuclear engineering.

The research is funded by the U.S.Department of Energy and focuses on developing nuclear fuels that arebetter at conducting heat than conventional fuels. Current nuclear fuelis made of a material called uranium dioxide with a small percentage ofa uranium isotope, called uranium-235, which is essential to induce thenuclear fission reactions inside current reactors.

"Althoughtoday's oxide fuels are very stable and safe, a major problem is thatthey do not conduct heat well, limiting the power and causing fuelpellets to crack and degrade prematurely, necessitating replacementbefore the fuel has been entirely used," Solomon said.

Purdueresearchers, led by Solomon, have developed a process to mix theuranium oxide with a material called beryllium oxide. Pellets ofuranium oxide are processed to be interlaced with beryllium oxide, orBeO, which conducts heat far more readily than the uranium dioxide.

This "skeleton" of beryllium oxide enables the nuclear fuel to conduct heat at least 50 percent better than conventional fuels.

"Theberyllium oxide is like a heat pipe that sucks the heat out and helpsto more efficiently cool the fuel pellet," Solomon said.

Amathematical model developed by Revankar and Latta has been shown toaccurately predict the performance of the experimental fuel and will beused in future work to further develop the fuel, Revankar said.

Pelletsof nuclear fuel are contained within the fuel rods of nuclear fissionreactors. The pellets are surrounded by a metal tube, or "cladding,"which prevents the escape of radioactive material.

Becauseuranium oxide does not conduct heat well, during a reactor's operationthere is a large temperature difference between the center of thepellets and their surface, causing the center of the fuel pellets tobecome very hot. The heat must be constantly removed by a reactorcooling system because overheating could cause the fuel rods to melt,which could lead to a catastrophic nuclear accident and release ofradiation – the proverbial "meltdown."

"If you add thishigh-conductivity phase beryllium oxide, the thermal conductivity isincreased by about 50 percent, so the difference in temperature fromthe center to the surface of these pellets turns out to be remarkablylower," Solomon said.

Revankar said the experimental fuelpromises to be safer than conventional fuels, while lasting longer andpotentially saving millions of dollars annually.

"We can actuallyenhance the performance of the fuel, especially during an accident,because this fuel heats up less than current fuel, which decreases thepossibility of a catastrophic accident due to melting," Revankar said."The experimental fuel also would not have to be replaced as often asthe current fuel pellets.

"Currently, the nuclear fuel has to bereplaced every three years or so because of the temperature-relateddegradation of the fuel, as well as consumption of the U-235. If thefuel can be left longer, there is more power produced and less wastegenerated. If you can operate at a lower temperature, you can use thefuel pellets for a longer time, burning up more of the fuel, which isvery important from an economic point of view. Lower temperatures alsomeans safer, more flexible reactor operation."

Solomon said a 50percent increase in thermal conductivity represents a significantincrease in performance for the 103 commercial nuclear reactorscurrently operating in the United States.

"Just a 5 to 10 percentincrease would be pretty significant, so a 50 percent increase would bequite an improvement," Solomon said.

The next step in theresearch is to test the new fuel inside a nuclear reactor to make sureit stands up to the extreme conditions inside reactors over its entirelifetime.

"We know it holds up well to very high temperatures,and now we are at the point where we want to irradiate this materialand see what it does," Solomon said.

The researchers also hadcreated fuel pellets containing fingers of another high thermalconductivity material called silicon carbide, but the silicon carbidereacted with the uranium oxide at elevated temperatures. New fueldesigns made of compatible uranium compounds are presently beingstudied. The research paper being discussed in October concentrates onthe model's accuracy in predicting the results of experiments withsilicon carbide and beryllium oxide, Revankar said.