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Why Ceramic Cladding?

  • Economics
  • Safety
  • Long Term Spent Fuel Storage and Disposal
  • Next Generation Reactors

    Background

    Commercial nuclear fuel cladding is currently made from various alloys of zirconium metal. These alloys are used because they have reasonable strength at normal operating conditions and have fairly low neutron capture cross sections. Unfortunately, there are several drawbacks to the material:

    1. The current maximum amount of time that a fuel assembly can remain in the reactor is limited to a peak burnup of 62 GWD/MTU, generally about 5 years exposure. This is because the current cladding becomes embrittled due to increased Zirconium hydride formation within the cladding. This hydride formation decreases the strength and ductility of the metal, which increases the possibility that the cladding could fail if there is an accident.

    2. Zirconium alloy cladding will undergo a metal-water reaction above a certain temperature in which hydrogen gas is generated leading to the possibility that the there could be a hydrogen explosion . Current Federal regulations require that the maximum cladding temperature remains below 2200°F during a design basis Loss of Coolant Accident.
    3. The current generation of cladding is susceptible to fretting wear where the cladding interacts with the spacer grids. In severe cases, this wear actually leads to leaking fuel pins increasing the level of radioactivity in the primary system. This translates into increased costs for the utilities both because of the need to replace the damaged fuel rods and because of increased dose to workers when the plant is being serviced.

    4. Zircaloy loses strength rapidly as temperature increases, as shown in the graph below. It also experiences significant thermal and irradiation enhanced creep. The susceptibility to creep limits the internal pressure of the cladding to below primary loop operating pressure to prevent "ballooning." The low stiffness of zircaloy requires a large number of spacer grids to prevent flow induced vibration. These grids add to the pressure drop in the core and are the primary source of fretting wear.

    SiC Composite Cladding Advantages

    A leading commercial nuclear fuel supplier has estimated that the high-temperature tolerance of SiC cladding could allow for power upgrades in existing commercial LWRs by as much as 30%. This could substantially increase the capacity of existing plants, thereby improving their economics

    Because of the maximum irradiation limit, commercial power reactors operate on an 18 month refueling cycle. If the cladding could withstand higher irradiation, then it would be possible to increase the refueling outages to once every 24 months. This could save the utility $240 million over the lifetime of the power plant.

    SiC has a 25% lower thermal neutron cross-section than zirconium alloys resulting in greater neutron efficiency of the reactor, and therefore reduced uranium enrichment requirements.

    The greater stiffness of SiC cladding will result in fewer spacer grids which will decrease the pressure drop through the core helping with the power upgrades. The enhanced heat transfer from tailored surface roughness on the outside of the composite may eliminate the need for intermediate flow mixing grids. The SiC cladding would also not be susceptible to fretting wear which could eliminate leaking fuel rods.

    Safety

    Because of its very high-temperature tolerances the SiC cladding will reduce accident risk and consequences. An increase in the cladding operating temperature would allow for the fuel cladding to reach higher temperatures during accident or off-normal operating conditions. This increase in temperature would give a higher safety margin if a LOCA or departure from nucleate boiling (DNB) were to occur, since the cladding would not deform as quickly and would result in less potential for fuel damage and allow for a longer amount of time before safety injection would need to occur.

    CTP Tests on our Triplex SiC cladding have demonstrated a factor of 100 to 600 reduction in reaction rate in water coolant at elevated temperature (above 1200°C) as compared to zircaloy. In fact, it appears that the corrosion rate of the SiC cladding during normal operation is lower than zircaloy.

    There is no reaction with the water coolant at elevated temperatures. In fact, it appears that even the corrosion rate of the SiC cladding is orders of magnitude lower than that of zircaloy.

    Long Term Storage Reduction

    SiC Triplex cladding technology could increase the fuel burn-up of LWR fuels by as much as 70% and thereby reduce the repository burden from commercial LWRs. The increase in the length of time that fuel stays in the reactor decreases the amount of spent fuel generated.

    Next Generation Reactors

    The advanced nuclear reactor concepts selected by the Generation IV International Forum for future development have high reactor coolant outlet temperatures. For example, one of the designs of the Gas Cooled Fast Reactor (GFR) is a 600MWth helium cooled system operating with an outlet temperature of 850°C, and sodium cooled reactors have temperatures ranging from 480°C for pool type reactors to 550°C.

    These high temperatures are desired for several reasons. Liquid metal coolants, such as lead and sodium, have excellent nuclear properties as required for the fast spectrum reactors needed to burn the higher actinides and reduce the long lived toxicity of spent fuel as called for in the Advanced Fuel Cycle Initiative. In order to function effectively, these liquid metal coolants must operate at temperatures where they can flow efficiently and remove heat from the solid fuel elements.

    Most metal and refractory alloy systems are not viable at these high temperatures because of creep rupture concerns. Although the sodium cooled fast reactors already developed, such as the EBR-2 and FFTF in the US, Phoenix in France, and the BN-600 in Russia, have operated successfully with stainless steel cladding, a significant penalty in neutron economy is incurred because of the high parasitic neutron absorption of stainless steel.