[en] The deformation of a subassembly in response to the core environment is frequently the life limiting factor for that component in an LMFBR. Deformation can occur as diametral and axial growth or bowing of the subassembly. Such deformation has caused several handling problems in both the core and the storage basket of EBR-II and may also have contributed to reactivity anomalies during reactor operation. These problems generally affect plant availability but the reactivity anomalies could lead to a potential safety hazard. Because of these effects the deformation mechanisms must be understood and modeled. Diametral and axial growth of subassembly ducts in EBR-II is due to swelling and creep and is a function of temperature, neutron fluence and stress. The source of stress in a duct is the hydraulic pressure difference across the wall. By coupling the calculated subassembly growth rate to the available clearance in the core or storage basket a limiting neutron fluence, or exposure, can be established

[en] One of the current design objectives for a liquid metal reactor (LMR) is the inherent shutdown-cooling capability of the reactor, such that the reactor itself can safely reduce power following a total loss of pump power without activating the reactor shutdown system (RSS). Following a loss-of-flow (LOF) accident and a failure of RSS, in EBR-II, reactor core damage and plant restartability is of considerable interest. In the LOF event, high temperature in the reactor causes negative reactivity feedback that reduces reactor power. After an accident, reactor fuel performance is one of the factors used to assess the restartability of the plant. A thermal-hydraulic-neutronic analysis was performed to determine the response of the plant and the temperature of individual subassemblies. These temperatures were then used to assess the damage to driver fuel elements caused by the station blackout accident. The maximum depth of cladding wastage from molten eutectic at temperatures >715⁰C was found to be 0.0053 mm for the hottest subassembly; this value is considerably less than the 0.28 mm cladding thickness. 12 refs

[en] In-reactor bowing of subassemblies has caused some handling difficulties at EBR-II, the extent of the problem increasing with the subassembly stiffness. Analysis of four particular bowed subassemblies with respect to their core environment and structural design has indicated that residual bow is primarily the result of radiation-enhanced creep during irradiation

No abstract available

[en] The experimental test procedure employed the use of a high-temperature furnace which heated pre-irradiated elements to temperature and maintained the environment until element-cladding breach occurred. Pre-irradiated elements of the Mark-II design were first encapsulated in a close-fitting sealed tube that was instrumented with a pressure transducer at the top of the tube and a thermocouple at the element's top-of-fuel axial location. The volume of the capsule was evacuated in order to better identify the pressure pulse which would occur on breach and to minimize contaminants. Next, a three-zone fast-recovery furnace was heated and an axial temperature profile, similar to that experienced in the EBR-II core, was established. The encapsulated element was then quickly inserted into the furnace and remained there until clad breach occurred. The element was then removed from the furnace immediately. Visual and metallurgical examination of the rupture site was done later. A total of seven elements were tested in the above manner

[en] The purpose of this study was to determine the time to rupture of both irradiated and unirradiated Mk-II fuel elements operating above the eutectic temperature of 715⁰C

[en] Subassembly bowing in an LMFBR can affect core neutronics, subassembly handling, and possibly create a potential safety hazard if the phenomenon can not be characterized and predicted. Bowing may be induced by thermal gradients, differential swelling, and/or intersubassembly loading. Neutronic variations, such as long or short term power-reactivity decrement (PRD) variations, can be indicators of a bowing problem. But, because of other influences on the PRD it is very dificult to isolate the affect of bowing. As a result, subassembly bowing was not considered a problem in EBR-II until subassembly handling problems occurred. A typical handling problem, caused by top end deflection of the bowed subassembly, is centering of the subassembly removal mechanism over the subassembly. Another problem is excess loads of up to 375 kg that have been required to remove bowed subassemblies from their grid position. This latter problem is due in part, to the magnitude of bow and subassembly stiffness in addition to the overall subassembly cluster tightness caused by duct swelling and/or the occurrence of multiple bowing. Observed interferences in storing a bowed subassembly in the storage basket can be used as a qualitative gauge for bowing magnitude. In EBR-II a storage basket tube can accomodate a subassembly bow of 2.3 to 3.1 mm depending on the position in the basket. All of the bowed type 304 stainless steel subassemblies that created handling difficulties were given a post-irradiation examination. Damage that may have been caused by interference loading was found to be limited to surface scratches. The measured bow of these subassemblies range from 1.9 mm for a fueled subassembly to 5.7 mm for a neutron-source-rod thimble

[en] Chemomechanical interactions inside metal-clad fuel elements are defined as those fuel-cladding mechanical interactions (FCMI) that are influenced by or result from chemical reactions between constituents of the irradiated fuel system. The purpose of the present paper is to interpret some recent experimental and analytical results in terms of chemomechanical reaction mechanisms, with special emphasis on the modeling of breached LMFBR oxide fuel pin behavior

[en] The chemical reaction of liquid sodium with uranium fuel forms a trisodium uranate compound. Property characterization of this compound is essential to irradiation performance modeling of breached fuel pins. Thermal expansion and diffusivity of high density Na₃UO₄ sintered bodies were measured and analyzed. These data show that the thermal properties of Na₃UO₄ are significantly different than UO₂ to alter the thermal performance characteristics of fuel pins

[en] Bowing was not considered a problem in EBR-II until subassembly handling problems occurred. A typical handling problem, caused by top end deflection of the bowed subassembly, is centering of the subassembly removal mechanism over the subassembly. Another problem is excess loads of up to 375 kg that have been required to remove bowed subassemblies from their grid position. All of the bowed type 304 stainless steel subassemblies that created handling difficulties were given a post-irradiation examination. Correlation of the in-core bowing phenomena with the bowing data base required both an axial and radial thermal and flux characterization, consideration of the subassembly design, interpretation of swelling and creep behavior and a simple workable model that could incorporate these factors. A thermal hydraulics code, 'CLUSTER' was used to calculate the operating temperatures at sixty radial locations for each of nineteen axial sections along the hex duct. For this analysis a simple beam method was used with postulated radial restraint bounds in simulating intersubassembly loading. The results of this analysis showed that the bow at a neutron fluence of >2.7x10²²n/cm², E>0.1 MeV, in these structurally varying subassemblies was highly thermal gradient dependent. (Auth.)