ADVANCED METHODOLOGY AND MODELING OF NUCLEAR CORE RESTRAINT DEFORMATIONS IN A LIQUID METAL, FAST REACTOR ENVIRONMENT

Julie Ann Dewberry

A fast reactor core is composed of closely packed fuel assemblies that are hexagonal ducts. The ducts determine the location of the fuel. They also provide a coolant path and the ability to insert or withdrawal the assemblies. When the assemblies are brought radially closer together the reactivity increases. The methods and core design can control the core reactivity. Therefore, fuel motion plays an important role in the fast reactor safety. Typical fast reactor core environment includes thermal gradients across the fuel assembly causes the assemblies to bow. There are axial thermal gradients that causes the fuel assemblies to grow or lengthen. The fast neutron flux gradients cause the assembly ducts to swell and contribute to bowing over time due to irradiation creep and swelling. Bowing ducts affects the reactivity, present handling problems and the duct’s ability to maintain its integrity. Handling problems occur if bowing and swelling are greater that the inter-duct gap clearance. The effects of creep and swelling can be manipulated to reduce the core reactivity insertion. Thus, the permanent core bowing deformations caused by creep and swelling can be used along with the choice of steel materials to assure net negative reactivity. One of the core design is to have load pads on the fuel assemblies located at the core and at the top of the assemblies. The bowing can be so severe that it causes contact at the load pads. The contact on the load pads causes a bowing and swelling shape that can be used to reduce or ensure there is a negative reactivity. The most serious adverse effect would be compaction of the fuel in a fast reactor leads to large positive reactivity effects. Thus, the core reactor design is to have the fuel motion predominantly away from the middle of the core instead of the reverse. The load pads assist in minimizing core compaction. The Fast Flux Test Facility tested various materials used to construct fuel assemblies. Creep and swelling laws for design purposes were developed from the extensive testing. Two materials are selected for further investigation. These are 316 20% CW stainless steel and HT-9 materials. Custom sub-routines included the creep and swelling correlations to be used with ANSYS, a commercially off-the-shelf software. A detailed ANSYS model of a fuel assembly was made using shell elements. This evaluation shows an expected behavior for both materials evaluated. Additionally, it addresses whether the intra-assembly gap remains closed or opened if there was a row of fuel assemblies evaluated within the core environment.

ADVANCED METHODOLOGY AND MODELING OF NUCLEAR CORE RESTRAINT DEFORMATIONS IN A LIQUID METAL, FAST REACTOR ENVIRONMENT

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