NMT Designs and Fabricates Standards for Nuclear Materials Assay

How good is a nondestuctive assay (NDA) measurement? A measurement done by an NDA instrument is only as good as the standard that is used to calibrate the instrument. Standards are needed to calibrate various NDA instruments such as neutron coincidence counters, gamma-ray counters, and calorimeters. These instruments measure a variety of nuclear materials being produced in the DOE nuclear community. The measurements help alleviate problems associated with shipper/receiver differences and the measurement and storage of residues and waste. Los Alamos National Laboratory has taken a lead role in the fabrication of uranium and plutonium standards, along with other actinides such as neptunium and americium. These standards have been fabricated for several laboratories within the DOE complex.

Figure 6: SGS can standards. Such containers must be handled easily in a glove box and compatible with the instrument they were designed for.

Planning the fabrication of standards requires very precise detailing. Designs encompass components such as precise weighing, destructive analysis of samples, specialized contain-ers, diluents, and the use of post-fabrication NDA measurements to confirm that the standards meet all preliminary expectations before they are used in instrument calibration.

Each NDA instrument varies in the amount of material it is able to measure. Segmented gamma scanners (SGSs) and neutron counters are able to generate precise measurements from 0 grams to 250 grams. Calorimeters, depending on chamber size, are able to measure from 0 grams to 5 kilograms. With these limits in mind, various amounts of nuclear materials are needed to accommodate the calibration ranges of the instruments. The standards also need to demonstrate linearity in the entire range of NDA measurements. For a given set of standards, the instrument reading should be linearly dependent on the quantity of the materials being measured.

Standards should be fabricated of material similar to that of the actinide being measured. We do not calibrate an instrument with pure plutonium metal if we are measuring oxides or waste. Measurements on the SGS are done on residues or low-density materials. Neutron instruments measure high-density materials such as metals, piping, residues not exhibiting high alpha-neutron emission, glass, and leaded gloves.

Every standard fabricated is created from a highly pure actinide material. The material is roasted, sieved, blended, and sampled. The analysis of the sample consists of isotopic compositions (enrichment), actinide percent purity, and levels of impurities associated with the material, such as iron or lead. A variety of the standards have been subjected to multiple assays by several analytical laboratories. The multilaboratory analysis is used to minimize the bias of any one laboratory assay and to ensure the homogeneity of the blended batch. Multiple samples also give results that help to develop good statistical comparisons among samples and laboratories.

Another important element in the fabrication of calibration standards is the containers that hold the standards. The container shape/size configuration can affect the radiation and thereby the NDA measurements. Each standard-fabrication assignment is carefully analyzed during the planning stage for the construction of a container that is easily handled in the glove box and compatible with the instrument it was designed for. SGS can standards are specialized cans eleven inches in height and four inches in diameter (Figure 6). These are ideal dimensions to help alleviate problems in end-effects and for gamma-ray transmission through the standard. SGS standards consist of oxide diluted with diatomaceous earth, which is used to homogenize and transfer the oxide uniformly throughout the container.

SGS drum standards consist of twenty four-liter polyethylene bottles, each contain- ing a known amount of oxide diluted in diatomaceous earth. These are stacked in a 55-gallon drum to simulate a homogeneous drum standard. New standards for neutron shuffler drums consist of oxide diluted with diatomaceous earth in one-inch diameter zirconium vials (Figure 7). The vials are stacked in a 55-gallon drum and can be varied for instrument calibration simply by adding or deleting vials to the drum. New calorimeter standards are now being fabricated with 12% plutonium-240 oxide. These will be distributed throughout the complex and are measured throughout the year to compare calorimetery measurements among labs. They also will provide us with a higher-wattage standard to complement the 6% plutonium-240 wattage standards that already exist. These standards consist of two mechanically sealed, "food-packed" cans containing two kilograms of high-burnup oxide.

Figure 7: Zirconium tubing for shuffler standards. Oxide is diluted with diatomaceous earth in these one-inch diameter vials.

With the problems associated with shipping materials today, details on dimensions of the standards need to be compatible with the shipping containers. Containerization must meet all requirements for shipping; therefore, certification of packaging demands double- or triple-encapsulated containers depending on what Department of Transportation drums are used.

Designs are greatly affected by these restrictions. It is therefore imperative that research be done before standards are fabricated. This will alleviate future problems in supplying a compatible standard used in measuring the various types and amounts of actinide materials and complying with all shipping requirements.

There has been and always will be a continuous demand for calibration standards throughout the NDA community. With new and better technology, Los Alamos will be the forerunner in producing these needed standards for the DOE complex and for other actinide measurement users.

The main designers of the standards work are T. Hsue, S. M. Simmonds, J. K. Sprinkle, and P. M. Rinard, all of NIS-5, and V. L. Longmire and S. M. Long of NMT-4.