Thermal aspects of alternative fuels for use in nuclear reactors

SuperCritical Water- cooled nuclear reactors are a 4^th generation reactors that use light water coolant at temperature higher than its critical level. Uniform and non-uniform axial heat flux profiles applied with a range of alternative fuels such as thorium dioxide, uranium dicarbide, uranium nitride etc.

There are two reactor basic arrangements for SuperCritical Water cooled Nuclear Reactors(SCWRs)-A fuel was deemed fit it the fuel centerline temperature remained lower than 1850oC. 

Uranium dioxide was the main choice for its wide and traditional use as a nuclear fuel. Although several studies have concluded that fuel centerline temperature can significantly exceed the industry acceptable limit. Unconventional fuels like mixed oxide, thoria, uranium dicarbide, uranium nitride and uranium carbide. Uniform and non-uniform axial heat flux profiles were applied, non-uniform profiles included- upstream cosine, cosine and downstream cosine. The sheath materials chosen were Inconel 600, Inconel 718 and stainless steel 304. A shielding material is acceptable if the outer sheath temperature is below 850oC.

SCWR is using SuperCritical Water as a reactor coolant. It is a light water and above its critical point. Various fossil fuel based plants are using this water as a working fluid for the turbine. Major thermodynamic cycle options for direct cycles SCWR are with no-reheat and single reheat. Direct cycles are allowed with increased coolant factors such as high temperature and pressure. The no-reheat cycle SCW exits the channel and enters into the turbine. The single reheat cycle is obtained by using Steam-Reheat channels.

Significance of Sheath materials

Sheath material is suitable for SCWR use if the temperature remains below the design limit specified at 850oC. The main choice for sheath material is zirconium alloy for its high mechanical strength and outstanding neutron transparency. Although when the temperature increases to 500oC, it is steeply corroded. Therefore zirconium alloy is unfit as a sheath material because the coolant temperature vary from 350oC to 625oC.

Other sheath materials are- alloy Inconel 600, Inconel 71 and stainless steel 304. Inconel alloys are nickel based high temperature materials offering high mechanical strength, hot and cold workability and supreme corrosion resistance. With increase in temperature above 750oC, alloy 718 attains significant reduction in its yield stress and tensile strength. Stainless steel 304 is selected for offering good corrosion resistance but its structural strength is very low and wall thickness needs considerable increase. It is not fit as the thermal efficiency and neutron economy would be significantly reduced. Following the review, Inconel 600 is the main sheath material.

For fuel supply, MOX is the perfect option as it is developed from irradiated UO2 and Thoria is special because it is a non-uranium based fuel. The fuel centerline temperature industry limit goes beyond only by MOX fuel. To make this fuel suitable for advanced reactors, fuel bundle design changes are required or channel power should be reduced. Thoria, uranium nitride and uranium carbide are possible SCWR nuclear fuels because their fuel centerline temperatures remain lower than 850oC. Thoria offers an additional benefit due to reduced dependence on uranium reserves.