Holly R. Trellue

Radioactive and nonradioactive waste intended for disposal at the Waste Isolation Pilot Plant

Transuranic (TRU) waste generated by the handling of plutonium in research on or production of US nuclear weapons will be disposed of in the Waste Isolation Pilot Plant (WIPP). This paper describes the physical and radiological properties of the TRU waste that will be deposited in the WIPP. This geologic repository will accommodate up to 175,564 m{sup 3} of TRU waste, corresponding to 168,485 m{sup 3} of contact-handled (CH-) TRU waste and 7,079 m{sup 3} of remote-handled (RH-) TRU waste. Approximately 35% of the TRU waste is currently packaged and stored (i.e., legacy) waste, with the remainder of the waste to be packaged or generated and packaged in activities before the year 2033, the closure time for the repository. These wastes were produced at 27 US Department of Energy (DOE) sites in the course of generating defense nuclear materials. The radionuclide and nonradionuclide inventories for the TRU wastes described in this paper were used in the 1996 WIPP Compliance Certification Application (CCA) performance assessment calculations by Sandia National Laboratories/New Mexico (SNL/NM).

Integrated Nondestructive Assay Systems to Estimate Plutonium in Spent Fuel Assemblies

Abstract An integrated nondestructive assay (NDA) system combining active (neutron generator) and passive neutron detection and passive gamma (PG) detection is being analyzed in order to estimate the amount of plutonium, verify initial enrichment, burnup, and cooling time, and detect partial defects in a spent fuel assembly (SFA). Active signals are measured using the differential die-away (DDA), delayed neutron (DN), and delayed gamma (DG) techniques. Passive signals are measured using total neutron (TN) counts and both gross and spectral resolved gamma counts. To quantify how a system of several NDA techniques is expected to perform, all of the relevant NDA techniques listed above were simulated as a function of various reactor conditions such as initial enrichment, burnup, cooling time, assembly shuffling pattern, reactor operating conditions (including temperature, pressure, and the presence of burnable poisons) by simulating the NDA response for five sets of light water reactor assemblies. This paper compares the performance of several exploratory model-fitting options (including neural networks, adaptive regression with splines, iterative bias reduction smoothing, projection pursuit regression, and regression with quadratic terms and interaction terms) to relate data simulated with measurement and model error effects from various subsets of the NDA techniques to the total Pu mass. Isotope masses for SFAs and expected detector responses (DRs) for several NDA techniques are simulated using MCNP, and the DRs become inputs to the fitting process. Such responses include eight signals from DDA, one from DN, one from TN, and up to seven from PG; the DG signal will be examined separately. Results are summarized using the root-mean-squared estimation error for plutonium mass in held-out subsets of the data for a range of model and measurement error variances. Different simulation assumptions lead to different spent fuel libraries relating DRs to Pu mass. Some results for training with one library and testing with another library are also given.

Planetary surface reactor shielding using indigenous materials

The exploration and development of Mars will require abundant surface power. Nuclear reactors are a low-cost, low-mass means of providing that power. A significant fraction of the nuclear power system mass is radiation shielding necessary for protecting humans and/or equipment from radiation emitted by the reactor. For planetary surface missions, it may be desirable to provide some or all of the required shielding from indigenous materials. This paper examines shielding options that utilize either purely indigenous materials or a combination of indigenous and nonindigenous materials.

Delayed-Gamma Simulation Using MCNPX

Abstract Monitoring issues related to activation and fission processes occur in many health physics, instrumentation and equipment design, nuclear forensics, and homeland security applications. Gamma radiation that is emitted during these processes as a result of the radioactive decay of reaction by-products [delayed gammas (DGs)] provides unique signatures that are useful for interrogation and information acquisition. Thus, it is of compelling interest to have a simulation tool that can be used to conduct studies to provide insights into the activation and fission processes. Beginning with version 2.5.0, MCNPX has been undergoing major upgrades to facilitate DG simulations. We illustrate the upgrades for a simple multiparticle reaction model involving 60Ni and for 235U photofission caused by 12-MeV photons.

ENDF70: A Continuous-Energy MCNP Neutron Data Library Based on ENDF/B-VII.0

Abstract Following the release of ENDF/B-VII.0 evaluations, an ACE-formatted continuous-energy neutron data library called ENDF70 for MCNP has been produced at Los Alamos National Laboratory. This new library contains data for 387 isotopes and three elements at five temperatures: 293.6, 600, 900, 1200, and 2500 K. It can be obtained as part of the MCNP5 1.50 release. The new library was created using ENDF/B-VII.0 neutron evaluations and primarily version 248 of NJOY99. A processing script was created that set up the input files for NJOY and employed checking codes to test the content of the processed data. A sample MCNP run was performed for each isotope and temperature, and cross sections for each isotope were plotted to make sure there were no major problems. The processed ACE libraries did not always pass all quality assurance tests. For example, energy-balance problems were identified for several evaluations having negative heating numbers or inconsistencies between total and partial heating. Similarly, some problems were found with unresolved resonance probability tables, resulting in probability tables being excluded from the final library for several materials. Certain evaluations were modified and reprocessed as a result of the quality assurance tests, and some data points in the final ACE files were changed because they were too small or had other problems. The new ENDF70 library provides MCNP users with the latest ENDF/B data available. This collection of data includes a larger range of isotopes and temperatures than previously released, which will be beneficial in numerous applications. The upgrades included as part of ENDF/B-VII.0 and, hence, ENDF70 should improve calculations.

Neutronic and Logistic Proposal for Transmutation of Plutonium from Spent Nuclear Fuel as Mixed-Oxide Fuel in Existing Light Water Reactors

Abstract The use of light water reactors (LWRs) for the destruction of plutonium and other actinides [especially those in spent nuclear fuel (SNF)] is being examined worldwide. One possibility for transmutation of this material is the use of mixed-oxide (MOX) fuel, which is a combination of uranium and plutonium oxides. MOX fuel is used in nuclear reactors worldwide, so a large experience base for its use already exists. However, to limit implementation of SNF transmutation to only a fraction of the LWRs in the United States with a reasonable number of license extensions, full cores of MOX fuel probably are required. This paper addresses the logistics associated with using LWRs for this mission and the design issues required for full cores of MOX fuel. Given limited design modifications, this paper shows that neutronic safety conditions can be met for full cores of MOX fuel with up to 8.3 wt% of plutonium.

Salt-Cooled Modular Innovative Thorium Heavy Water-Moderated Reactor System

Abstract This paper describes a new reactor concept: the Salt-cooled Modular Innovative THorium HEavy water-moderated Reactor System (SMITHERS), which addresses the goals of (a) evolving deployment needs, (b) increasing overall fuel burnup, (c) reducing proliferation risk, and (d) providing high-efficiency power generation. The reactor is modular and thus scalable from a few to hundreds of megawatts(thermal). The concept further burns used fuel from light water reactors (LWRs) without aqueous separations, reducing costs and proliferation pathways relative to current reprocessing plants. The additional burning of LWR fuel reduces proliferation risk by reducing global inventories of plutonium from used fuel in a way that does not isolate weapons-useable material and that increases the amount of power produced per ton of mined uranium. Improved fuel utilization through the potential use of thorium provides cost benefits by increasing neutron economy and enabling operation at higher efficiencies. Neutron economy is increased by using the lower neutron energies associated with large quantities of heavy water moderation and/or thorium for innovative reactor control and constant long-term power generation (i.e., sustainability). Finally, the proposed reactor also generates high-temperature coolant discharge in the form of liquid salt without coolant pressurization for external process heat applications such as oil extraction. Salt offers significant improvement over existing coolants such as light water and heavy water, which require pressurization to operate at high temperatures, adding to the cost and complexity of reactor operation. SMITHERS designs discussed in this paper either burned a full core of used fuel, ThO2 with 1.2 wt% PuO2 or other fissile material, or a combination of the two.

Uncertainty Quantification for New Approaches to Spent Fuel Assay

Abstract Estimating plutonium (Pu) mass in spent nuclear fuel assemblies (SFAs) helps inspectors ensure that no Pu is diverted. Therefore, nondestructive assay (NDA) methods are being developed to assay Pu mass in SFAs. Uncertainty quantification is an important task in most assay methods, and particularly for SFA assay. A computer model (MCNPX) is being used to predict isotope masses and the spatial distribution of masses in virtual SFAs for 64 combinations of initial fuel enrichment (IE), fuel utilization [burnup (BU)], and cooling time (CT) values. Additional MCNPX modeling for the same 64 virtual SFAs provided the expected detector responses (DRs) for several NDA techniques such as the passive neutron albedo reactivity method and the 252Cf interrogation with prompt neutrons method. A previous paper describes one uncertainty quantification approach involving Monte Carlo (MC) simulation using individually any of six new NDA options together with IE, BU, and CT. This paper provides an interpretation of the MC approach that is suited for a numerical Bayesian alternative, separately assesses the impact of MCNPX interpolation error, and compares several options to use subsets of IE, BU, CT, and one DR.

A composite position independent monitor of reactor fuel irradiation using Pu, Cs, and Ba isotope ratios

When post-irradiation materials from the nuclear fuel cycle are released to the environment, certain isotopes of actinides and fission products carry signatures of irradiation history that can potentially aid a nuclear forensic investigation into the materials provenance. In this study, combinations of Pu, Cs, and Ba isotope ratios that produce position (in the reactor core) independent monitors of irradiation history in spent light water reactor fuel are identified and explored. These position independent monitors (PIMs) are modeled for various irradiation scenarios using automated depletion codes as well as ordinary differential equation solutions to approximate nuclear physics models. Experimental validation was performed using irradiated low enriched uranium oxide fuel from a light water reactor, which was sampled at 8 axial positions from a single rod. Plutonium, barium and cesium were chemically separated and isotope ratio measurements of the separated solutions were made by quadrupole and multi-collector inductively coupled mass spectrometry (Cs and Pu, respectively) and thermal ionization mass spectrometry (Ba). The effect of axial variations in neutron fluence and energy spectrum are evident in the measured isotope ratios. Two versions of a combined Pu and Cs based PIM are developed. A linear PIM model, which can be used to solve for irradiation time is found to work well for natural U fuel with <10% 240Pu and known or short cooling times. A non-linear PIM model, which cannot be solved explicitly for irradiation time without additional information, can nonetheless still group samples by irradiation history, including high burnup LEU fuel with unknown cooling time. 137Ba/138Ba is also observed to act as a position independent monitor; it is nearly single valued across the sampled fuel rod, indicating that samples sharing an irradiation history (same irradiation time and cooling time) in a reactor despite experiencing different neutron fluxes will have a common 137Ba/138Ba ratio. Modeling of this Ba PIM shows it increases monotonically with irradiation and cooling time, and a confirmatory first order analytical solution is also presented.

Robotic Spent Fuel Monitoring – It is time to improve old approaches and old techniques!

This report describes various approaches and techniques associated with robotic spent fuel. The purpose of this description is to improve the quality of measured signatures, reduce the inspection burden on the IAEA, and to provide frequent verification.