Charles W. Solbrig

Electrorefiner liquid cadmium cathode crucible thermal shock

Abstract A liquid cadmium cathode is used in an electrorefiner to remove plutonium and minor actinides from spent nuclear fuel by pyroprocessing. Liquid cadmium in a beryllia crucible, originally at 35°C, is lowered into 500°C salt electrolyte to begin reprocessing. Crucible cracking from thermal stress would release cadmium into the liquid salt causing electrorefiner failure. This studys purpose was to predict if the ceramic crucible would fail. A handbook method showed it would. An analytical model eliminating two large conservatisms predicted no failure. A beryllia crucible preheated to 321°C was successfully immersed in electrorefiner salt without failure. The conclusion is that handbook methods can be severely conservative in predicting thermal stress failures for immersion in low thermal conductivity fluids.

Modeling Solidification-Induced Stress in Ceramic Waste Forms Containing Nuclear Wastes

Abstract The goal of this work is to produce a ceramic waste form that permanently occludes radioactive waste. This is accomplished by absorbing radioactive salts into zeolite, mixing with glass frit, heating to a molten state at 915°C to form a sodalite glass matrix, and solidifying for long-term storage. Less long-term leaching is expected if the solidifying cooling rate does not cause cracking. In addition to thermal stress, this paper proposes a mathematical model for the stress formed during solidification, which is very large for fast cooling rates during solidification and can cause severe cracking. A solidifying glass or ceramic cylinder forms a dome on the cylinder top end. The temperature distribution during solidification causes the solidification stress and the dome resulting in an axial length deficit. The axial stress, determined by the length deficit, remains when the solid is at room temperature with the outer region in compression and the inner region in tension. Large tensions will cause cracking of the specimen. The temperature deficit, derived by dividing the length deficit by the coefficient of thermal expansion, allows solidification stress theory to be extended to the circumferential stress. This paper derives the solidification stress model, gives examples, explains how to induce beneficial stresses, and compares theory to experimental data.

Stabilizing Glass Bonded Waste Forms Containing Fission Products Separated from Spent Nuclear Fuel

Abstract A model has been developed to represent the stresses developed when a molten, glass-bonded brittle cylinder (used to store nuclear material) is cooled from high temperature to working temperature. Large diameter solid cylinders are formed by heating glass or glass-bonded mixtures (mixed with nuclear waste) to high temperature (915°C). These cylinders must be cooled as the final step in preparing them for storage. Fast cooling time is desirable for production; however, if cooling is too fast, the cylinder can crack into many pieces. To demonstrate the capability of the model, cooling rate cracking data were obtained on small diameter (7.8 cm diameter) glass-only cylinders. The model and experimental data were combined to determine the critical cooling rate which separates the non-cracking stable glass region from the cracked, non-stable glass regime. Although the data have been obtained so far only on small glass-only cylinders, the data and model were used to extrapolate the critical-cooling rates for large diameter ceramic waste form (CWF) cylinders. The extrapolation estimates long term cooling requirements. While a 52-cm diameter cylinder (EBR-II-waste size) can be cooled to 100°C in 70 hours without cracking, a 181.5-cm diameter cylinder (LWR waste size) requires 35 days to cool to 100°C. These cooling times are long enough that verification of these estimates are required so additional experiments are planned on both glass only and CWF material.

USING CENTER HOLE HEAT TRANSFER TO REDUCE FORMATION TIMES FOR CERAMIC WASTE FORMS FROM PYROPROCESSING

The waste produced from processing spent fuel from the EBR II reactor must be processed into a waste form suitable for long term storage in Yucca Mountain. The method chosen produces zeolite granules mixed with glass frit, which must then be converted into a solid. This is accomplished by loading it into a can and heating to 900 C in a furnace regulated at 915 C. During heatup to 900 C, the zeolite and glass frit react and consolidate to produce a sodalite monolith. The resultant ceramic waste form (CWF) is then cooled. The waste form is 52 cm in diameter and initially 300 cm long but consolidates to 150 cm in length during the heating process. After cooling it is then inserted in a 5-DHLW/DOE SNF Long Canister. Without intervention, the waste takes 82 hours to heat up to 900 C in a furnace designed to geometrically fit the cylindrical waste form. This paper investigates the reduction in heating times possible with four different methods of additional heating through a center hole. The hole size is kept small to maximize the amount of CWF that is processed in a single run. A hole radius of 1.82 cm was selected which removes only 1% of the CWF. A reference computation was done with a specified inner hole surface temperature of 915 C to provide a benchmark for the amount of improvement which can be made. It showed that the heatup time can potentially be reduced to 43 hours with center hole heating. The first method, simply pouring high temperature liquid aluminum into the hole, did not produce any noticeable effect on reducing heat up times. The second method, flowing liquid aluminum through the hole, works well as long as the velocity is high enough (2.5 cm/sec) to prevent solidification of the aluminum during the initial front movement of the aluminum into the center hole. The velocity can be reduced to 1 cm/sec after the initial front has traversed the ceramic. This procedure reduces the formation time to near that of the reference case. The third method, flowing a gas through the center hole, also works well as long as the product of heat capacity and velocity of the gas is equivalent to that of the flowing aluminum, and the velocity is high enough to produce an intermediate size heat transfer coefficient. The fourth method, using an electric heater, works well and heater sizes between 500 to 1000 Watts are adequate. These later three methods all can reduce the heatup time to 44 hours allowing production to be doubled and a more uniform heating.Copyright

Thermal Analysis of ZPPR High Pu Content Stored Fuel

The Zero Power Physics Reactor (ZPPR) operated from April 18, 1969, until 1990. ZPPR operated at low power for testing nuclear reactor designs. This paper examines the temperature of Pu content ZPPR fuel while it is in storage. Heat is generated in the fuel due to Pu and Am decay and is a concern for possible cladding damage. Damage to the cladding could lead to fuel hydriding and oxidizing. A series of computer simulations were made to determine the range of temperatures potentially occuring in the ZPPR fuel. The maximum calculated fuel temperature is 292°C (558°F). Conservative assumptions in the model intentionally overestimate temperatures. The stored fuel temperatures are dependent on the distribution of fuel in the surrounding storage compartments, the heat generation rate of the fuel, and the orientation of fuel. Direct fuel temperatures could not be measured but storage bin doors, storage sleeve doors, and storage canister temperatures were measured. Comparison of these three temperatures to the calculations indicates that the temperatures calculated with conservative assumptions are, as expected, higher than the actual temperatures. The maximum calculated fuel temperature with the most conservative assumptions is significantly below the fuel failure criterion of 600°C (1,112°F).

Cadmium Personnel Doses in an Electrorefiner Tipping Accident

This chapter estimates airborne cadmium concentrations caused by the facility design base earthquake (DBE) at the INL Fuel Conditioning Facility which damages the MK-IV electrorefiner (ER) vessel so that cadmium spills out onto the floor. In addition, the seismically qualified safety exhaust system (SES) is assumed to fail. The SES is a safety grade system that is large enough to keep the flow through any DBE caused breach into the cell. But with SES inoperative, failure of non -seismically qualified cell boundary penetrations allows release of cadmium vapor to the facility workers, site workers, and general public. Consequence categories are designated by estimating airborne concentrations at specific personnel locations and comparing them to applicable exposure guidelines. Without the failure of the SES, there would be negligible doses to all workers and the general public.

Use of Similarity Analysis on Experiments of Different Size to Predict Critical Cooling Rates for Large Ceramic Waste Forms

Ceramic waste forms (CWF) are produced to store fission products for the long term. They are cast into cylindrical shape at high temperature (925°C). Rapid cooling of the product is desirable for product turnaround, but cooling has the potential to crack the coalesced product into many pieces due to thermal stress. This paper investigates the rapid-cooling process with a borosilicate-glass component of the CWF used as a surrogate. The critical cooling rate of formed cylinders (the rate which separates the damage from the no-damage region) has been determined. This paper extends previous experimental data and analysis to production temperature as a step in the extrapolation of the data to production CWF’s. The glass solidifies in the range of 650°C to 625°C. The previous tests (7.8-cm diameter) were all run starting from a solid (625°C or less) to provide a basis for the higher temperature cases. Thermal stress cannot build up until solidification begins to occur. The current tests (7.8 and 9.9cm diameter) were run from the liquid temperature of 925°C. A theoretical model has been developed to analyze the data. The model includes heat transfer and the stress developed from the thermal gradients. Similarity analysis based on this model is used to produce dimensionless charts which allow data of different initial temperatures and diameters to be analyzed. The new data corroborated the previous estimate of the critical cooling rate and analytical-model projection for the minimum in-furnace cooling times for two production size CWF’s that will be stored in Yucca Mountain (70 hours for the 52-cm diameter and 35 days for the 181.5-cm sizes). To further reduce these times, an analytical prediction was made which shows that the formed cylinder can be removed from a furnace at a temperature of 320°C without any danger of cracking.

Thermal Stress in HFEF Hot Cell Windows Due to an In-Cell Metal Fire

This work investigates an accident during the pyrochemical extraction of Uranium and Plutonium from PWR spent fuel in an argon atmosphere hot cell. In the accident, the heavy metals (U and Pu) being extracted are accidently exposed to air from a leaky instrument penetration which goes through the cell walls. The extracted pin size pieces of U and Pu metal readily burn when exposed to air. Technicians perform the electrochemical extraction using manipulators through a 4 foot thick hot cell concrete wall which protects them from the radioactivity of the spent fuel. Four foot thick windows placed in the wall allow the technicians to visually control the manipulators. These windows would be exposed to the heat of the metal fire. This analysis determines if the thermal stress caused by the fire would crack the windows and if the heat would degrade the window seals allowing radioactivity to escape from the cell.

Converting Maturing Nuclear Sites to Integrated Power Production Islands

Nuclear islands, which are integrated power production sites, could effectively sequester and safeguard the US stockpile of plutonium. A nuclear island, an evolution of the integral fast reactor, utilizes all the Transuranics (Pu plus minor actinides) produced in power production, and it eliminates all spent fuel shipments to and from the site. This latter attribute requires that fuel reprocessing occur on each site and that fast reactors be built on-site to utilize the TRU. All commercial spent fuel shipments could be eliminated by converting all LWR nuclear power sites to nuclear islands. Existing LWR sites have the added advantage of already possessing a license to produce nuclear power. Each could contribute to an increase in the nuclear power production by adding one or more fast reactors. Both the TRU and the depleted uranium obtained in reprocessing would be used on-site for fast fuel manufacture. Only fission products would be shipped to a repository for storage. The nuclear island concept could be used to alleviate the strain of LWR plant sites currently approaching or exceeding their spent fuel pool storage capacity. Fast reactor breeding ratio could be designed to convert existing sites to all fast reactors, or keep the majority thermal.