Significance and Use
This test method is useful for quantifying fissile (for example, 233U, 235U, 239Pu and 241Pu) and spontaneously-fissioning nuclei (for example, 238Pu, 240Pu, 242Pu, 244Cm, 248Cm, and 252Cf) in waste and scrap drums. Total elemental mass of the radioactive materials can be calculated if the relative abundances of each radionuclide are known.
Typically, this test method is used to measure one fissile isotope (for example, 235U or 239Pu).
This test method can be used to segregate low level and transuranic waste at the 100 nCi/g concentration level currently required to meet the DOE Waste Isolation Pilot Plant (WIPP) waste acceptance criterion (5, 8, 9).
This test method can be used for waste characterization to demonstrate compliance with the radioactivity levels specified in waste, disposal, and environmental regulations (See NRC regulatory guides, DOE Order 435.1, 10 CFR Part 71, 40 CFR Part 191, and DOE /WIPP-069).
In the active mode, the DDT system can measure the 235U content in the range from <0.02 to >100 g and the 239Pu content, nominally between <0.01 and >20 g.
In the passive mode, the DDT system is capable of assaying spontaneously-fissioning nuclei, over a nominal range from 0.05 to 15 g of 240Pu, or equivalent (5, 10, 11, 12, 13).
This test method should be used in conjunction with a waste management plan that segregates the contents of assay items into material categories according to some or all of the following criteria: bulk density of the waste, chemical forms of the plutonium or uranium and matrix, (α, n) neutron intensity, hydrogen (moderator) and absorber content, thickness of fissile mass(es), and the assay item container size and composition. Each matrix may require a different set of calibration standards and may have different mass calibration limits. The effect on the quality of the assay (that is, minimizing precision and bias) can significantly depend on the degree of adherence to this waste management plan.
The bias of the measurement results is related to the fill height, the homogeneity and composition of the matrix, the quantity and distribution of the nuclear material, and the item size. The precision of the measurement results is related to the quantity of the nuclear material, the background, and the count time of the measurement.
For both matrix-specific and wide-range calibrations, this test method assumes the calibration material matches the items to be measured with respect to homogeneity and composition of the matrix, the neutron moderator and absorber content, and the quantity, distribution, and form of nuclear material, to the extent they affect the measurement.
The algorithms for this test method assume homogeneity. Heterogeneity in the distribution of nuclear material, neutron moderators, and neutron absorbers has the potential to cause biased results (14).
This test method assumes that the distribution of the contributing radioisotopes is uniform throughout the container and that lumps of nuclear material are not present.
Reliable results from the application of this test method require waste to be packaged so the conditions of Section 5.5 can be met. In some cases, site-specific requirements will dictate the packaging requirements with possible detrimental effects to the measurement results.
Both the active mode and the passive mode provide assay values for plutonium. During the calibration process, the operator should determine the applicable mass ranges for both modes of operation.
Scope
1.1 This test method covers a system that performs nondestructive assay (NDA) of uranium or plutonium, or both, using the active, differential die-away technique (DDT), and passive neutron coincidence counting. Results from the active and passive measurements are combined to determine the total amount of fissile and spontaneously-fissioning material in drums of scrap or waste. Corrections are made to the measurements for the effects of neutron moderation and absorption, assuming that the effects are averaged over the volume of the drum and that no significant lumps of nuclear material are present. These systems are most widely used to assay low-level and transuranic waste, but may also be used for the measurement of scrap materials. The examples given within this test method are specific to the second-generation Los Alamos National Laboratory (LANL) passive-active neutron assay system.
1.1.1 In the active mode, the system measures fissile isotopes such as 235U and 239Pu. The neutrons from a pulsed, 14-MeV neutron generator are thermalized to induce fission in the assay item. Between generator pulses, the system detects prompt-fission neutrons emitted from the fissile material. The number of detected neutrons between pulses is proportional to the mass of fissile material. This method is called the differential die-away technique.
1.1.2 In the passive mode, the system detects time-coincident neutrons emitted from spontaneously fissioning isotopes. The primary isotopes measured are 238Pu, 240Pu, and 242Pu; however, the system may be adapted for use on other spontaneously-fissioning isotopes as well, such as kilogram quantities of 238U. The number of coincident neutrons detected is proportional to the mass of spontaneously-fissioning material.
1.2 The active mode is used to assay fissile material in the following ranges.
1.2.1 For uranium-only bearing items, the DDT can measure the 235U content in the range from about 0.02 to over 100 g. Small mass uranium-bearing items are typically measured using the active mode and only large mass items are measured in passive mode.
1.2.2 For plutonium-only bearing items, the DDT method measures the 239Pu content in the range between about 0.01 and 20 g.
1.3 The passive mode is capable of assaying spontaneously-fissioning nuclei, over a nominal range from 0.05 to 15 g 240Pu equivalent.
1.4 This test method requires knowledge of the relative abundances of the plutonium or uranium isotopes to determine the total plutonium or uranium mass.
1.5 This test method will give biased results when the waste form does not meet the calibration specifications and the measurement assumptions presented in this test method regarding the requirements for a homogeneous matrix, uniform source distribution, and the absence of nuclear material lumps, to the extent that they effect the measurement.
1.6 The complete active and passive assay of a 208 L drum is nominally 10 min or less but either mode can be extended to meet data quality objectives.
1.7 Some improvements to this test method have been reported (1, 2, 3, 4). Discussions of these improvements are not included in this test method although improvements continue to occur.
1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.9 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 8.
