Neutron methods – a summary

Neutron methods are an inherently complex business. Neutron methods are used specifically to account for actinides and mostly for fissile material. Neutron methods are significantly affected by material characteristics and for wastes usually involve relatively high uncertainties. There are no simple neutron methods—each requires a knowledgeable physicist to manage the measurement campaign to ensure accurate interpretation of results.

Most mainstream methods rely on ³He proportional detector tubes. Spectroscopic methods are not in wide use nor would they typically be reliable, given that neutrons are not emitted at fixed energies. Additionally, most wastes possess a degree of moderating

properties, so all but the smallest of packages of neutron transparent materials corrupt the neutron energies well before the neutrons are captured for analysis. The source of neutrons cannot be determined simply by identifying ways they are detected. Some minimal amount of knowledge of the waste stream characteristics is required.

Neutron methods involve a combination of measurement process and matrix correction techniques. Any combination is possible. Unlike gamma measurement processes, all matrix correction techniques are inherently complex and the technique chosen needs to be closely matched to the waste stream.

Tables VIII and IX provide brief descriptions of neutron NDA methods and associated matrix correction techniques.

Table V. GAMMA MEASUREMENT PROCESS Dose rate Useful for a very stable nuclide vector; it does not specificallyquantify individual radionuclides. Simplest ofall gamma measurements and very fast process. Gross gamma counting Can be very fast and makes use of large scintillation detectors (plastic or NaI). Again, useful for very stable nuclide vectors; it is not nuclide specific. Simple process. With large detectors can be very sensitive, much more than dose rate measurement. Total spectrumThis name is a bit awkward; it is relatively the same as gross gamma counting. Spectroscopy functions can enhance gross gamma counting by using upper end discrimination to avoid naturals, visual checks of spectrum to highlight unexpected peaks, etc. Window method or SCAUseful when there is interference from naturals or other gamma emitters and you wish to focus on a key nuclide. Biggest advantage is speed. This does not require complex amplifiers or software, which quickly use up signal processing time. Multiple peaks (MCA) Specific for individual nuclides. Can independently quantify each gamma emitter. Requires more knowledgeable operator to interpret results. Beware of restricted libraries that may hide the presence of unexpected nuclides. Better to always have a warning for unidentified peaks.

INCREASING COMPLEXITY

HRGS vs. LRGS (CdZnTe) Resolution determines how clearly different nuclides can be resolved from each other. Higher resolution detectors required when nuclide vectors are complex and many gammas of close energies are emitted. There is a corresponding penalty in maintenance burden and cost. LRGS detectors are more sensitive on a size for size basis and are much less expensive. Intermediate resolution detectors are showing promise, but large crystals cannot yet be grown, so process is often not applicable.

Spect roscopy Modelling coupled with HRGS (e.g. ISOCS)

Currently being aggressively marketed as a revolutionary technique. Very complex and difficult to operate. A technique that is easy to abuse. Requires an operator with knowledge and experience equivalent of a university graduate. A technique that requires the utmost respect. It has its uses but is very difficult to manage. Tomography Potentially the most accurate technique; it corrects in 3 dimensions. Very powerful and no longer limited by computer processing time. Now it is limited by 3-D resolution of scan. More expensive and potentially time consuming process. Gamma imaging (gamma camera) Scans an area and overlays visual camera image with a survey of dose rate . Some systems incorporate spectroscopy; overlays are nuclide specific. Very powerful survey tool that helps identify where more focused surveys should be taken. Can save planning time and surveyor dose. Not usually used as final quantitative survey; uncertainties can be large when surveying a large area at a significant distance.

Table VI. TYPE OF GAMMA SCAN Total package May be one measurement or the average of several measurements taken around the package. The total package is assumed to have average properties. May involve rotation on a turntable. Single detector Time consuming process; separate results are given for separate regions of the package. Multiple detectors Same as for single detector but measurement time increased due to more detectors. Correspondingly more expensive. Vertical Sensible to vertical variation of activity concentration.

INCREASING COMPLEXITY

AngularThe Angular Scanning mode consists of collecting spectra of a waste form in a determined horizontal section (slice) at predefined angular positions. The angular scanning obtains information on the angular and/or radial non-uniformity of the distribution of activity and/or matrix properties.

Segmented scan

Swivelling/ horizontalThe Horizontal or Swivel scan modes merely aim at determining information on the radial distribution of activity and/or matrix properties. At a fixed height of the waste package, spectra are collected at a number of predefined horizontal positions (horizontal scanning) or at a number of predefined swivel orientations of the detector with respect to the waste form (swivel scanning). In both scanning modes (horizontal or swivel scanning) and due to the detector collimation, only a ring is visible for the detector. Depending on the stepsize used for the horizontal or swivel scan, the imaginary rings defined by the collimated detector may overlap.

Table VII. MATRIX CORRECTION TECHNIQUE FOR GAMMA METHODS Operator selectOperator, through knowledge of package contents, selects appropriate calibration parameters. Package densityPackage is weighed and fill height entered by operator; average density is computed by instrument and result corrected based on average density.

Pas sive t echniques

Differential peakDifferent energy peaks (e.g. Cs-134 and Cs-137) from package contents are analyzed and matrix density is computed. Requires real activity to be present in enough quantity to make computation reliable.

INCREASING

CO MPLEXITY Transmission measurement A source is shone through contents and attenuation from the other side of the package is measured. Liability in managing the source, heavy shielding required, more maintenance intensive, etc.

Active techni ques

Tomography A more complex transmission measurement and thus more time consuming but much more accurate correction in 3-D without the large segment averaging uncertainties associated with SGS transmission correction.

Table VIII. NEUTRON MEASUREMENT PROCESS Passive total counting Every neutron emitted is counted. Not specific to any individual actinide. Can be very sensitive due to statistical precision but easily upset by interference from other neutron emitters. Passive coincidence counting (PNCC)Specific to actinides that decay by spontaneous fission. If fissile material (or more appropriately, the fissionable isotopes of fissile species) is of interest, then interference from other spontaneous fission isotopes (e.g. Cf or Cm) is possible. Most often used to assess plutonium content by direct measurement of Pu-240 and inferred Pu-239 content by prior knowledge of the isotopic ratio.

Pas sive Techni ques

Multiplicity counting In principle may not require separate calibrations for different material types if one of the principle parameters is accurately known. Each of the critical parameters usually varies somewhat. Potentially a powerful technique but requires careful attention. Active coincidence counting (ANCC)An ever-present random neutron source induces prompt fission in fissile isotopes. Coincidence electronics rejects the random neutrons and only counts those from the fission. Useful for fissile isotopes, primarily U-235 and Pu-239. Cf-shuffler A Cf-252 source is rapidly placed near the package and temporarily induces fission in fissile isotopes. The source is rapidly withdrawn and the delayed fission neutrons counted. The process is often repeated several times to achieve the required statistical precision. Again, useful for fissile isotopes, primarily U-235 and Pu-239. Larger packages often require excitation at several locations; the induced fission tends to be quite localized.

Act ive Techni ques

Differential die-away (DDA) A powerful neutron generator tube induces fission in fissile material with a burst of neutrons. The difference in time for the flux to decay back to normal for the package and an empty chamber determines the quantity of fissile material present. Potentially extremely sensitive. Only suitable for non-moderating materials. Very significant maintenance burden. Combined Passive/Active TechniquesWhere wastes contain both uranium and plutonium, a combined active/passive method can be used to individually quantify each component. The passive result indicates the Pu-240 and U-238 (if present in large quantity) and the active result indicates U-235 and Pu-239 content. Mathematical combination of the results combined with knowledge of the uranium enrichment and plutonium isotopic ratio allow for computation of the separate uranium and plutonium concentrations. This is a powerful solution that is in common practice. Systems and software are more complex and require more knowledgeable users.

Table IX. MATRIX CORRECTION TECHNIQUE FOR NEUTRON METHODS Operator select Add a source Flux probes Ring ratio Multiplicity Imaging algorithms

Each of these is too complex to summarize in this table. Very detailed descriptions and related issues are given in Section 3 of the Good Practice Guide referenced in the generic issues at the front of this summary. In general, each method suffers when neutrons can undergo significant moderation in the waste matrix prior to reaching a detector. Thus, larger packages or presence of neutron absorbers severely limit each method.