Two presentations on fuel design for conversion were given by Panel 2.1 speakers: Daniel Wachs (Idaho National Laboratory) reported on efforts to develop LEU fuels for converting U. S.-origin reactors (Wachs, 2011),

and Yu. S. Cherepnin (Dollezhal Scientific Research and Design Institute of Energy Technologies [NIKIET]) described progress and prospects for reduction of fuel enrichment in Russian-origin reactors (Cherepnin, 2011).

Fuel Design for U. S.-Origin Reactors

Daniel Wachs

Highly enriched uranium (HEU) fuel elements in U. S.-origin research and test reactors consist of aluminum-clad plates (see Chapter 1) that contain a UAlx or U3O8-aluminum dispersion fuel meat clad in aluminum or a uranium-zirconium hydride (UZrHx) fuel meat clad in stainless steel (TRIGA fuel). Work carried out by Argonne National Laboratory and the Idaho National Laboratory, in cooperation with other American, European, and Korean organizations, has resulted in the development of three LEU dispersion fuel systems1 for conversion of plate-type reactors:

• UAlx (density = 2.3 grams of uranium per cubic centimeter [gU/cm[26]])

• U3O8 (3.2 gU/cm3)

• U3Si2 (4.8 gU/cm3)

These fuel systems are adequate for converting all but “high per­formance” research and test reactors.[27] [28] There are six HEU-fueled high — performance research reactors in the United States3 as well as four HEU-fueled high-performance research reactors in Europe that cannot be converted with these existing LEU fuel systems. The U. S. reactors are shown in Table 1-1 in Chapter 1; the European reactors are the following:

• Belgian Reactor 2 (BR2) at the Belgian Nuclear Research Centre in Mol, Belgium

• Forschungsreaktor Munchen II (FRM-II) at the Technical Univer­sity of Munich, Germany

• Jules Horowitz Reactor (JHR), under construction at the CEA Cadarache Research Centre in Cadarache, France (discussed in Chapter 4)

• Reacteur a Haut Flux (RHF) at the Institut Max von Laue-Paul Langevin (ILL) in Grenoble, France

Higher-density LEU fuel systems based on uranium-molybdenum (UMo) alloys are now under development for use in converting these U. S. and European reactors. Test irradiations have been carried out on several UMo alloys to assess their suitability for use as fuel for these reactors. Testing revealed that alloy phases with high U/Mo ratios (e. g., U-10Mo[29]) were most stable under irradiation because they suppressed the formation of fission gas bubbles.[30]

Two LEU fuel systems based on this alloy are now under development by Idaho National Laboratory and partners:

• UMo dispersion fuel: A UMo alloy dispersed in an aluminum ma­trix with uranium densities up to 8.5 gU/cm3. An LEU fuel system based on this material is being developed for conversion of BR2, RHF, and JHR.[31]

• Monolithic UMo fuel: Metallic UMo foils with a uranium density of 15.5 gU/cm3. An LEU fuel system based on this material is being devel­oped for conversion of ATR, HFIR, NBSR, MITR, and MURR (Figure 2-1).

Test irradiations of fuel elements containing both of these materials are now being carried out to develop and qualify these fuel systems.