A recently developed neutron diagnostic probe system has the potential to satisfy a significant number of van-mobile and fixed- portal requirements for nondestructive verification of sealed munitions and detection of contraband explosives and drugs. The probe is based on a unique associated-particle sealed-tube neutron generator (APSTNG) that interrogates the object of interest with a low-intensity beam of 14-MeV neutrons generated from the deuterium-tritium reaction and that detects the alpha-particle associated with each neutron. Gamma-ray spectra of resulting neutron inelastic scattering and fission reactions identify nuclides associated with all major chemicals in chemical warfare agents, explosives, and drugs, as well as many pollutants and fissile and fertile special nuclear material. Flight times determined from determined from detection times of the gamma-rays and alpha-particles yield a separate tomographic image of each identified nuclide. The APSTNG also forms the basis for a compact fast-neutron transmission imaging system that can be used along with or instead of the emission imaging system; a collimator is not required since scattered neutrons are removed by ``electronic collimation`` (detected neutrons not having the proper flight time to be uncollided are discarded). The small and relatively inexpensive APSTNG exhibits high reliability and can be quickly replaced. Proof-of-concept experiments havemore » been performed under laboratory conditions for simulated nuclear and chemical warfare munitions and for explosives and drugs.« less

It is pointed out that new developments in hodoscope radiation detection technology offer a wide range of capabilities for arms control treaty verification (ACTV) applications. This concept uses an array of radiation detectors to image or detect objects inside opaque containments. Hodoscope systems can detect neutrons and/or gamma-rays. The systems can be based on transmission of radiation through the objects, can detect radiation stimulated in the objects, or can detect intrinsic object radiation. Laboratory measurements to demonstrate a range of potential applications have been performed. Gamma-ray transmission hodoscopes can be used to inspect canisters, rail cars, etc. to monitor objects such as rocket motors. The use of relatively weak isotopic radiation sources makes it unnecessary to employ expensive and high-intensity accelerators. The heavy metal of nuclear warheads is characterized by strong gamma-ray absorption, and these materials could be counted by low-resolution tomography. Absorbers located in line with objects will themselves be detected, and sources located in the object region will be subtracted out as background. Intrinsic gamma-ray radiation from warheads can also be detected in a passive-instrument mode.< >

The verification of nuclear arms control agreements is likely to entail provisions for detection, identification, and imaging of objects that are known to have or might contain fissile masses. Determining the number of nuclear warheads in a MIRV or finding out whether a cruise missile has a nuclear or conventional war head are examples. To do so requires tech-niques that make use of either inherent radiation associated with fissile materials used in weapons or their interrogation with external sources. Because of the possibility of evasive measures, some application of more intrusive active interroga-tion techniques might be necessary, in which case the minimum delivery of radiation doses would be preferred or mandated. Fast neutrons are most promising for interrogation because of their penetrability and because of the distinctive character-istic radiation that can be induced in the object being inspected. The information that is collected, which might include a radiograph, must be tailored to the situation, providing neither too much nor too little information. These and other requirements that are public information are examined, particularly as they relate to potential terrestrial and space applications.

The accurate determination of fissile content is essential to domestic and international safeguards of nuclear fuel. Experience gained with the fast-neutron hodoscope at the U.S. Department of Energy reactor TREAT indicates a sensitivity of 1 g of fuel out of a kg. The actual unanticipated absence of less than 1 g of mixed oxide was found in one instance; in fact, all prior forms of quality control, including mass measurements, x-ray radiography, and neutron radiography had failed to detect the deficit. Although the means utilized for uncovering this particular deficiency is a large fixed installation, the generic principles of using a neutron-induced source for digital radiography with an array of fast-neutron detectors operated in a scanning mode are applicable to many other situations. For example, a modest-size detector array could be installed at reactor or neutron sources to achieve equivalent sensitivity with longer operating times.

In all the transients in the PFR/TREAT series, fuel motion had been monitored by the fast-neutron hodoscope. This paper treats the enhancements in hodoscope operation and data analysis since the start of the PFR/TREAT tests. The hodoscope has a maximum viewing height of 1.2 m. Data collection intervals for the series have been in the order of 1 ms, depending on the duration of the transient. Mass-displacement resolutions of about 0.1 g are achievable for the single-pin tests and 1 g for 7-pin tests. The hodoscope system can accommodate the full dynamic range of power.

Nuclear and non-nuclear applications of cineradiography are reviewed, with emphasis on fuel-motion diagnostic instrumentation that is used for in-pile nuclear-reactor safety studies. The primary instrument for this purpose has been the fast-neutron hodoscope, which achieves quantitative monitoring of time, location, mass, and velocity of fuel movement under the difficult conditions associated with experiments in transient reactors. Nominal 1-ms, 0.1-g, 1-mm resolution have been accomplished during reactor transients of over 104 in power. Although alternative diagnostic devices—such as coded apertures, flash radiography, and pinhole imaging—have been developed, they have not matched the performance of the hodoscope; the evaluation of the relevant parameters appears in the paper. Because discriminating detectors can be placed in tandem in hodoscope channels, other test material constituents—such as cladding steel and sodium coolant—can be dynamically distinguished. Material-motion diagnostic systems can also be used for time-integrated radiography and direct time- and space-resolved fuel-pin power monitoring. High-resolution diagnostic systems are now operating at two reactors, and studies and tests have been carried out for application to several others.

New requirements for time resolution, test duration, field of view, and recording redundancy in dynamic digital radiographic imaging of fuel motion in TREAT and TREAT-Upgrade (TU) in-pile experiments have been formulated. This has necessitated the design and fabrication of a new hodoscope high-speed data acquisition system. Recently an array of proportional counters was installed to operate in tandem with the Hornyak-button array. The full implementation of this new array, together with the increased field-of-view needed for future TU 37-pin experiments, required a separate recording system operting in parallel with that for the Hornyak buttons. The new recording system was required to have substantially higher capacity than the earlier recording system in use, in order to record sufficient data channels and samples with adequately small collection intervals, for some new types of experiments.

The most important parameter in calibrating reactor-safety experiments at TREAT is test-fuel energy deposition during irradiation transients. The coupling of the reactor power to the test fuel depends on conditions that vary from experiment to experiment and vary during the transient. In order to more accurately determine the power coupling and better understand its time and space dependencies, data has been obtained by the fast-neutron hodoscope and compared to experiment calibrations, reactor instrumentation data, and theoretical calculations.

The fast-neutron hodoscope is a cineradiographic device that monitors fuel motion within thick opaque test capsules during power transients at the Transient Reactor Test Facility reactor which simulate LM FBR accident conditions. By means of a collimator and a detector array, emissive neutron radiographic digital data is collected in time-resolved and time-integrated (scan) modes. The data is digitally reconstructed into radiographic images and used directly for quantitative analysis. Spatial resolution is adequate in most cases (due in part to fuel-motion correlation), but is marginal for a few experiments. To enhance the resolution of existing collimators, hardware and software techniques are being applied, including collimator repositioning, decreasing scan increments, noise reduction, and fitting procedures involving fuel-motion models and deconvolution. Significant improvements in spatial resolution have been obtained.

The use of computer-related devices has resulted in significant improvements in word processing, permitting better accuracy and more efficient turnover of paperwork in an office. A selective combination of these components comprises a computer-assisted writing and editing system for professionals at Argonne National Laboratory, used primarily for technical reports. Text is entered and corrected at...

The fast-neutron hodoscope at the Transient Reactor Test Facility detects fuel motion in thick opaque capsules during in-core destructive transient tests. Counts from several hundred detectors, each with rates up to a megahertz, are collected at intervals as short as a millisecond for up to tens of seconds. The large amount of data must be decoded, normalized, represented in suitable forms, and analyzed. A computer-controlled magnetic disk data acquisition system has been installed which provides shorter data collection intervals, simplifies decoding, and permits immediate data analysis. Data normalizations and representations have been developed which significantly increase the dynamic range, yield sensitive quantitative indications of fuel mass motion, and render the data intuitively comprehensible. The improved hodoscope system performance level is demonstrated by results from a recent transient test, Pinex-2, which show the quantitative evolution of fuel mass motion from 1515 MW peak reactor power through a 1 MW post-scram radiation level.

TREAT F-series tests are being conducted to provide data on fuel motion in an LMFBR during a hypothetical loss-of-flow accident. Fuel and fuel-boundary conditions in an LMFBR subassembly following sodium voiding and dryout under loss-of-flow conditions are simulated in each F-series test. Simulation is achieved with a single fuel element surrounded by an annular nuclear-heated wall in a dry (no sodium) test capsule. The area inside the heated wall was selected to represent the area inside the perimeter of an LMFBR coolant channel. Test F1 was conducted with an irradiated fuel element to investigate the effect of fission gas on fuel motion at design power levels following cladding melting and drainage. The principal conclusion from Test F1 is that fission products retarded, but did not prevent, eventual fuel collapse. The collapse was retarded by a fuel/fission-product froth that prevented fuel collapse until the fission products separated from the partially molten fuel. The fuel motion observed in F1 represents a particular type of fuel (burnup of 2.35 at.%, power rating of 394 W/cm) transient heated at design power rating.