J.E. Matos, S.C. Mo, and W.L. Woodruff
RERTR Program
Argonne National Laboratory

September 1992

SUMMARY

This report contains the results of design and safety analyses performed by the RERTR Program at the Argonne National Laboratory (ANL) for conversion of the Georgia Tech Research Reactor (GTRR) from the use of HEU fuel to the use of LEU fuel. The objectives of this study were to: (1) maintain or improve upon the present reactor performance and margins of safety, (2) maintain as closely as possible the technical specifications and operating procedures of the present HEU core, and (3) utilize a proven fuel assembly design that is economical to manufacture. Extensive collaboration with Dr. R. Karam, Director of the Neely Nuclear Research Center at Georgia Tech, took place on all aspects of this work.

The LEU fuel assembly has the same overall design as the present HEU fuel assembly, except that it contains 18 fueled plates with LEU U3 Si2 -Al fuel instead of 16 fueled plates with HEU U-Al alloy fuel. This LEU silicide fuel has been approved by the Nuclear Regulatory Commission for use in non-power reactors.

Documents that were reviewed by ANL as bases for the design and safety evaluations were the GTRR Safety Analysis Reports, the GTRR Technical Specifications, and responses by the reactor organization to AEC questions in licensing the reactor for 5 MW operation.

The methods and codes that were utilized have been qualified using comparisons of calculations and measurements of LEU demonstration cores in the Ford Nuclear Reactor at the University of Michigan and in the Oak Ridge Research Reactor at the Oak Ridge National Laboratory. Additional qualification has been obtained via international benchmark comparisons sponsored by the IAEA for heavy water research reactors.

Only those reactor parameters and safety analyses which could change as a result of replacing the HEU fuel in the core with LEU fuel are addressed. The attached summary table provides a comparison of the key design features of the HEU and LEU fuel assemblies and a comparison of the key reactor and safety parameters that were calculated for each core. The results show that all of the objectives of this study were fully realized and that the GTRR reactor facility can be operated as safely with the new LEU fuel assemblies as with the present HEU fuel assemblies.

SUMMARY TABLE

HEU and LEU Design Data, Core Physics, and Safety Parameters
for Conversion of the Georgia Tech Research Reactor

DESIGN DATA	HEU Core	LEU Core
Minimum Number of Fuel Assemblies	14	14
Maximum Number of Fuel Assemblies	19	19
Fuel Type	U-Al Alloy	U3Si2 -Al
Enrichment, %	93	19.75
Uranium Density, g/cm3	0.65	3.5
Number of Fueled Plates per Assembly	16	18
Number of Non-Fueled Plates per Assembly	2	2
235 U per Fuel Plate, g	11.75	12.5
235 U per Fuel Assembly, g	188	225
Fuel Meat Thickness, mm	0.51	0.51
Cladding Thickness, mm	0.38	0.38
Cladding Material	1100 Al	6061 Al

			Number of
REACTOR PARAMETERS	HEU Core	LEU Core	Assemblies
Cold Clean Excess Reactivity, % Dk/k	11.7 ± 0.4	9.4 ± 0.4	17
Coolant Temperature Coefficient, % Dk/k/°C	-0.0076	-0.0067	14
Doppler Coefficient, % Dk/k/°C	~ 0.0	-0.0017	14
Whole Reactor Isothermal Temp. Coeff., % Dk/k/°C	-0.0224	-0.0232	14
Coolant Void Coefficient, % Dk/k/% Void	-0.0383	-0.0333	14
Limiting Power Peaking Factor	1.54	1.58	14
Prompt Neutron Lifetime, ms	780	745	14
Effective Delayed Neutron Fraction	0.00755	0.0075-0.0076	14
Shutdown Margin, % Dk/k	-7.1 ± 0.2	-8.8 ± 0.2	17
(Max. Worth Shim Blade and Reg. Rod Stuck Out)
Top D 2 O Reflector Worth, % Dk/k	-2.1 ± 0.3	-2.4 ±0.3	17
(For D 2 O 2" Above Fuel Meat)
Reactor Power Limits -1625 gpm Flow Rate
Based on Departure from Nucleate Boiling, MW	11.5	10.8	14
Based on Flow Instability Criterion, MW	10.6	10.6	14
Limiting Reactor Inlet Temperature, °F	172	170	14
Limiting Reactor Outlet Temperature, °F	188	187	14
Limiting Safety System Settings - Forced Convection
Reactor Power, MW	5.5	5.6	14
Coolant Flow Rate, gpm	1625	< 1625	14
Reactor Outlet Temperature, °F	139	145	14
Margin to D2O Saturation Temperature, °F	8	11	14
Max. Fuel Plate Temp. for LOCA after 8 Hours Cooling, °C	425	400	14
Maximum Positive Reactivity Insertion, % Dk/k	> 2.2	> 2.2	14

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