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Kessler, Stephen

Publications and source records attributed to Kessler, Stephen.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Criticality Scoping Calculations for a Multi-Function Dissolver Insert

As one of its missions, the Savannah River Site seeks to be a leader in the safe and productive disposition of special nuclear materials. One of the premier programs is the disposition of used nuclear fuel, primarily from research reactors, through dissolution, purification, and down blend into low enriched uranium for commercial power reactors. Prior to use, this fuel contained highly enriched uranium (HEU). In an effort to advance this expertise, this study has been performed to demonstrate the feasibility to disposition a number of special geometry HEU fuels. These fuels come from a number of different domestic and foreign research reactors which had geometries considerably different from round or square bundles of plates or rods. As such, they cannot be dispositioned in current H-Canyon dissolver inserts so a new, multi-function dissolver insert may be considered. While this is not a current mission, it is believed that this study's scoping calculations demonstrate the feasibility of the site taking such a mission. At the H-Canyon facility, a dissolver is a large stainless steel pot, loaded with a nitric acid solution into which uranium-alloy fuel is dissolved. The dissolver uses a device called an insert to feed the fuels to the acid solution in a controlled manner. Most fuel forms fit into one of the existing inserts which include one with ten {approx}5-inch diameter circular wells, one with four {approx}4.5 inch by {approx}15 inch slab wells, and one specifically designed for the Oak Ridge National Laboratory High Flux Isotope Reactor (HFIR). This study considered five bounding fuel candidates for scoping the potential new insert. The fuels include a generic annular plate fuel (APF), the Tower Shield Reactor (TSR) fuel elements, the Reactor A Haute Flux (RHF) core, Knolls Atomic Power Laboratory (KAPL) reactor fuel, and SLOWPOKE cores. These are expected to bound a range of proposed fuel candidates. Scoping studies were made to establish geometric restrictions for the new dissolver insert and to determine if these fuels could safely be dissolved. Homogenous models were used for the scoping study due to time constraints. Computational modeling was performed using KENOVI in the SCALE 6.1 code package.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

A Chemistry Based Criticality Safety Evaluation of the HM-Process Mixer Settlers

The criticality safety of Savannah River Site's H Canyon two cycle uranium extraction and purification process, called the HM process, is examined. Explicit Monte Carlo models of the multi-stage mixer-settlers are generated for the first time using the KENO-VI module of the SCALE 6.1 code package and spanning normal operating conditions and a wide range of potential upset conditions with stagewise compositions based directly on SEPHIS Mod-4 chemistry models of the same conditions. To achieve industrial scale processing, mixer-settlers are often not geometrically favorable to prevent criticality so other means must be used for criticality safety, namely control of fissile concentration and mass. Historically, the worst case upset is a reflux condition resulting from a stream flow rate, temperature, or composition that is off from the normal operating conditions. A reflux is considered to be the state in which the fissile material is not proceeding in the desired direction, i.e. is stripping rather than extracting or vice versa. The criticality safety analyses which set the current operational limits were conducted many years ago with less sophisticated codes and only loosely tied criticality to chemistry. Those analyses conservatively assumed that a reflux changed the behavior of the mixer-settler such that the fissile content collected continuously in only three stages of the mixer-settler, at a rate based on the SEPHIS modeling, until criticality occurred. This implies that under some condition the distribution coefficient of the fissile material either goes to infinity or zero, which is not physically achievable. In such a scenario, the operator would have some amount of time to react and correct the problem before sufficient fissile material built up in the mixer-settler and caused a criticality. In reality, an upset in a stream would cause the process to transition to a new, though undesired, equilibrium state of operation. The distribution coefficients would change, but would not diverge. In this work, aqueous and organic streams are modeled for both the mixer and the settler region of each stage of each bank using chemical distribution data from SEPHIS and solving the chemical equations for a wide range of operational parameters spanning normal and upset conditions. Conservative assumptions are made regarding enrichment and reflection and the whole bank multiplication factor is evaluated using discrete stage-wise compositions. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Incredibility of a Postulated Criticality Event for Enriched Uranyl Nitrate Storage

At the H-Canyon outside facilities, highly enriched uranyl nitrate solutions collected from leaks or during deliberate maintenance activities is collected into 5-gallon polyethylene bottles. The current criticality safety evaluation for the facility postulates that a criticality event is possible if the solutions are allowed to concentrate through evaporation, freezing, storing multiple bottles in a geometrically unfavorable array, or any combination of these conditions. Using chemical data on uranyl nitrate freezing from Savannah River National Laboratory, protected limits on concentration and enrichment, and normal operational restrictions, criticality was shown to be incredible under all normal and credible abnormal conditions. A bounding normal solution is analyzed under normal conditions and shown to be substantially subcritical. A bounding credible abnormal solution is defined as 9.2 g U{sup 235}/L and 73 wt. % enriched and 0 M excess nitric acid. The concentration and enrichment are limitations already protected by the facility for other processes. Because acidity is not specifically restricted, the most reactive lower bound is selected. The credible abnormal worst placement is in a concrete basin reflected on three sides. There the bottle(s) would be subject in the worst cases to flooding (full water reflection) and postulated concentration through evaporation or freezing. Computational modeling was performed using KENOVI in the SCALE 6.1 code package. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

Incredibility of a Postulated Criticality Event for Enriched Uranyl Nitrate Storage

At the H-Canyon outside facilities, highly enriched uranyl nitrate solutions collected from leaks or during deliberate maintenance activities is collected into 5-gallon polyethylene bottles. The current criticality safety evaluation for the facility postulates that a criticality event is possible if the solutions are allowed to concentrate through evaporation, freezing, storing multiple bottles in a geometrically unfavorable array, or any combination of these conditions. Using chemical data on uranyl nitrate freezing from Savannah River National Laboratory, protected limits on concentration and enrichment, and normal operational restrictions, criticality was shown to be incredible under all normal and credible abnormal conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

A Chemistry Based Criticality Safety Evaluation of the HM-Process Mixer Settlers

The criticality safety of Savannah River Site’s H Canyon two cycle uranium extraction and purification process, called the HM process, is examined. Explicit Monte Carlo models of the multi-stage mixer-settlers are generated for the first time using the KENO-VI module of the SCALE 6.1 code package and spanning normal operating conditions and a wide range of potential upset conditions with stage wise compositions based directly on SEPHIS Mod-4 chemistry models of the same conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

Criticality Scoping Calculations for a Multi-Function Dissolver Insert

As one of its missions, the Savannah River Site seeks to be a leader in the safe and productive disposition of special nuclear materials. One of the premier programs is the disposition of used nuclear fuel (UNF), primarily from research reactors, through dissolution, purification, and down blend into low enriched uranium for commercial power reactors. Prior to use, this fuel contained highly enriched uranium (HEU). In an effort to advance this expertise, this study has been performed to demonstrate the feasibility to disposition a number of special geometry HEU fuels. These fuels come from a number of different domestic and foreign research reactors which had geometries considerably different from round or square bundles of plates or rods. As such, they cannot be dispositioned in current H-Canyon dissolver inserts so a new, multi-function dissolver insert may be considered. While this is not a current mission, it is believed that this study’s scoping calculations demonstrate the feasibility of the site taking such a mission.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗