USE OF NOVEL NEUTRON COUNTING SYSTEMS TO SATISFY DIVERSE AND CHALLENGING MEASUREMENT NEEDS WITHIN THE DOE COMPLEX

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1 USE OF NOVEL NEUTRON COUNTING SYSTEMS TO SATISFY DIVERSE AND CHALLENGING MEASUREMENT NEEDS WITHIN THE DOE COMPLEX ABSTRACT C.H. Orr J.P. Ronaldson S. Jones G. Mottershead BNFL Instruments Ltd. Pelham House Calderbridge Cumbria CA20 1DB UK BNFL Instruments Inc. 278 D P Road Los Alamos New Mexico USA The US Department of Energy (DOE) has many nuclear licensed sites whose Decontamination and Decommissioning (D&D) commitments pose numerous significant challenges for the assay and characterization of nuclear materials. This paper discusses and summarises three such examples. Firstly, the assay of uranium contaminated boxed waste by a Uranium Waste Monitor at the East Tennessee Technology Park (ETTP), the purpose of which is to consign the waste as Low Level Waste (LLW) by providing the 235 U inventory prior to shipment from the site. The waste comprises components of large gaseous diffusion converters, turbine compressors, electric monitors, valves and piping. The principal technical challenge was the development of an ability to reliably correct for the significant cosmic ray generated neutron spallation background from the assay data to leave the neutron count rate from uranium contamination. This was successfully achieved by the use of neutron mult iplicity electronics together with careful site selection, a high counting detection efficiency and the use of neutron shielding materials. The second example is the in-situ assay and imaging of plutonium concentrations within plutonium contaminated gloveboxes, tanks and other process equipment within buildings undergoing deactivation and decommissioning at the Rocky Flats Environmental Technology Site (RFETS). DISPIM 3D Imaging technology has been applied for criticality safety control purposes and its ability to be configurable for variable geometries. Its novel imaging capability for locating concentrations of residual plutonium improves safety and dose uptake by facilitating the earlier and more efficient removal of plutonium. Improved assay accuracy also means that criticality clearance limits are less likely to be exceeded, resulting in a positive effect on disposal strategy and cost effectiveness. The major technical difficulty was a large cosmic spallation background in the DISPIM assemblies and in the matrix material of the measured items. This was overcome by the development and implementation of two background subtraction techniques. Thirdly, the assay of crates of varying size reclaimed from burial pits at area TA54 of the Los Alamos National Laboratory (LANL) site using a passive crate counter, known as the Large Item Neutron Counter (LINC). The system employs both totals neutron and coincidence neutron counting techniques and proprietary imaging algorithms to provide an initial upper limit measurement of plutonium contamination. The LINC system also provides neutron source location information for both personnel safety and to maximize the efficiency of decontamination activities. The major technical challenge was to design a system to meet the strict performance requirements for a wide range of crate sizes. This was achieved primarily by the ability to position the detector packages to produce an optimized measurement arrangement for each size of crate.

2 EAST TENNESSEE TECHNOLOGY PARK (ETTP) URANIUM WASTE MONITOR Background On August 25 th 1997 the US Department of Energy (DOE) and BNFL Inc. signed a contract for the East Tennessee Technology Park (ETTP) Three Building Decontamination, Decommissioning and Recycle project at the former K25 site at Oak Ridge, TN. This fixed price contract is to dismantle, remove and decontaminate the process equipment and support systems within the three gaseous diffusion plant buildings K29, K31 and K33. Some recovered metals will be recycled whilst the remainder will be consigned as low level waste (LLW) and transported in Intermodal waste containers to the Envirocare radwaste disposal facility in Utah. Strict regulations and limits exist for both the transportation and disposal of this LLW. In particular nuclear safety considerations place a 350g 235 U limit on the total inventory of the Envirocare facility. To ensure that this and similar transport limits are not breached (and each waste consignment is accompanied by a manifest detailing all other radionuclides of concern) BNFL Instruments developed a uranium waste monitor capable of accurate uranium assay for an Intermodal or Sealand container. This development is described in the following sections. Waste Characteristics Waste in each LLW consignment largely consists of stainless steel components and scrap, with other metals size reduced as necessary to achieve an optimum packing fraction in Intermodal waste containers. The dimensions of an Intermodal are approximately 20' x 8' x 8' with each container likely to hold an average of 33,000 lb (15 tons) waste. The distribution and degree of dispersion of ETTP low enriched uranium (LEU) contamination within any individual Intermodal was assumed to be random and inhomogeneous in order to truly represent the real world situation. Radionuclide 'fingerprint' data from historical records and the radiochemical analysis of representative (new) samples taken from each plant area are used to assist in the process of interpreting NDA measurement data. Key fingerprint data for this purpose includes: 235 U enrichment 234 U enrichment Chemical form of uranium (i.e. UO 2 F 2, UF 6 etc.) Activity ratios for all other radionuclides present in the waste from the particular plant area. Measurement Technique A number of radiometric measurement methods were considered against criteria which included practicability, assay accuracy and lifetime costs. 235 U has limited passive radiation emissions. Foremost amongst these is the abundant gamma emission at kev, routinely used for assay of radwaste bags and drums from the operational or decommissioning phases of uranium enrichment/fuel fabrication facilities. However the utilization of this radiation was not selected due to the ready absorption of this radiation by even modes t quantities of metallic waste. Similar problems exist for the gamma radiation from 238 U and its daughters [especially 234m Pa]. Penetration and accurate (external) measurement of uranium gamma radiation from an LEU concentration at the centre of an Intermodal containing 15 tons of metallic waste was not, therefore, considered to be reliable. By contrast fast neutrons are highly penetrating radiation (especially in metal matrices) due to their lack of charge. Although the spontaneous fission generated neutron emission of LEU is relatively small the (alpha, neutron) interaction between uranium alpha emitters and surrounding light nuclei such as fluorine do generate a measurable neutron signal. This penetrating neutron radiation from the (α,n) interactions of UO 2 F 2 and UF 6 therefore form the basis of uranium LLW measurements in K25 Intermodals destined for Envirocare. Major Challenges Several major challenges were faced by the BNFL Instrument s Technical Department at this stage. The neutron signal was known to be typically neutrons per second from 300g 235 U as LEU in the chemical form UO 2 F 2. To measure this signal reliably would require a neutron counting system of dimensions which matched that of an Intermodal container whilst still achieving 5-10% total neutron counting efficiency. The team utilized a versatile and efficient detector module known as DISPIM as the basic 'building block' of this system this comprising two 1 metre long 3 He neutron detectors clad in an optimized polyethylene and stainless steel package.

3 Figure 1 DISPIM Neutron Counting Module Using a mathematical (MCNP) model of the counting arrangement, optimum locations were found for 80 such DISPIM modules around an Intermodal container. For this arrangement an average neutron detection efficiency of 7% was achieved. The uniformity of response for a uranium concentration in the chamber is expected to be better than +/- 20%, making the chamber measurements quite independent of the location of uranium within expected waste loads. Figure 2 Schematic of the Uranium Waste Monitor for Intermodals Concrete Chamber Intermodal DISPIM Module Chamber Door PC, Monitor & Keyboard Gantry Rails Not to Scale As with the measurement of any small radiation signal it is vitally important to control ambient background levels such that they are as low as possible and remain steady during the complete measurement cycle of each Intermodal. The best measurement location available close to the decommissioning operations was identified by a neutron background site survey using a small slab neutron counter and counting electronics. Building 761 at ETTP was found to be an appropriate location with low ambient background and sufficient space around the proposed monitor location within the building to construct additional neutron shielding if necessary. Our final challenge concerned the additional source of background neutrons caused by cosmic ray interactions with the waste steel matrix of each filled Intermodal during measurement. Test neutron measurements in the UK of steel masses up to 15 tons showed a valuable correlation between the waste mass and additional neutron fluxes detected, either as single or multiple neutron events. Of particular interest were the multiple events which are generated as a result of cosmic ray interactions occurring as showers producing many time correlated neutrons in large metal masses. The use of neutron multiplicity electronics has therefore been implemented as the method of choice to both identify and remove this undesirable source of background neutrons. Finally, interpretation of the net neutron emission rate to grams UO2F2 or UF6 in each Intermodal is then a matter of predicting the specific neutron emission (n/sec/g) from the radiochemical analysis of uranium samples from the originating area of plant. This fingerprint data is then further ut ilized with the measured uranium estimate to provide the 235 U mass and the activity estimates of all other radionuclides present, e.g. 99 Tc etc.

4 Table 1 Anticipated Assay Performance of the ETTP Uranium Waste Monitor Current Status Total neutron counting efficiency = 7% Matrix/geometric uncertainty = ± 20% of mean Average Intermodal contents = 15 tons stainless steel Mean counting time = Up to 2 hours Minimum Detectable Mass = 30-50g 235 U Work to prepare building 761 at ETTP for the construction of the uranium waste monitor chamber is now well advanced with cosmic ray shielding being provided by the building structure and additional concrete above and to the sides of the chamber enclosure. All DISPIM neutron detection modules have been manufactured and await installation within the assay chamber. Following installation of the DISPIM modules a full programme of inactive and active commissioning is planned. When completed all Intermodals filled with decommissioning steelwork from the 3 building project will be monitored within this chamber for a period of 1-2 hours prior to dispatch to Envirocare for final disposal. ROCKY FLATS ENVIRONMENTAL TECHNOLOGY SITE (RFETS) DISPIM 3D IMAGING SYSTEM Introduction The DISPIM 3-dimensional (3D) Imaging system was developed specifically in response to a request from Siemens for a system to assay the plutonium and uranium content of gloveboxes in a redundant fuel fabrication plant at Siemens plant at Hanau, Germany. The plant contained many vessels and process items that could potentially hold residual fissile material. Prior to dismantling it was necessary to perform a fissile measurement survey to (i), help plan the decommissioning programme and (ii), ensure criticality safety during the dismantling process. The material held up in the Siemens plant is typically mixed oxide (MOX) with an isotopic composition of ~66% 239 Pu, 23% 240 Pu. In addition some of the process gloveboxes utilized neutron absorbing and shielding materials such as raschig rings, which incorporate boron, and plexi-glass panels. The DISPIM 3D Imaging system, as shown in Figure 1, was developed and manufactured and underwent an extensive series of tests at Sellafield prior to shipping to Siemens in Figure 1 - DISPIM 3D Imaging System

5 After the successful development and production of this system, a further enquiry was received from the Rocky Flats Environmental Technology Site (RFETS) for a system to assay plutonium contaminated gloveboxes, tanks, and process equipment. This material was typically plutonium oxide with an isotopic composition of ~94% 239 Pu, 6% 240 Pu. After an initial set of trial measurements was successfully performed, the system was supplied for use at RFETS. Following supply, both DISPIM Imaging systems were used for further measurements under more realistic environmental conditions. During this period it became clear that under certain circumstances both systems could be influenced by environmental factors which caused the measurement performance to be significantly poorer than specified. In the case of the Siemens application, the specific problems noted were reflections from nearby structures and floors, and neutron multiplication effects and variations in efficiency for the detection of 252 Cf and Pu source neutrons. An additional problem was noted at RFETS; that of a positive bias due to cosmic spallation neutrons. The higher altitude of the RFETS site and lower 240 Pu eff value of the assay material were responsible for this previously hidden effect. This paper describes the basic measurement techniques of the DISPIM 3D Imaging system and the further development work that has been performed to eradicate these problems. It also reports on the improvements in performance that have been achieved and demonstrated during subsequent test measurements. Measurement Technique Of the types of radiation emitted from plutonium, only neutrons are sufficiently penetrating to escape from process vessels or gloveboxes without significant loss due to attenuation within the plutonium itself or the construction materials of most primary containments. Two passive neutron counting methods are commonly used for plutonium measurement; total neutron counting and neutron coincidence counting. In total neutron counting, the neutrons from spontaneous fission and (alpha, n) events are counted, whilst in neutron coincidence counting, the time-correlated neutrons from spontaneous fission events are recorded separately. In the context of in-situ glovebox or vessel measurements, the neutron coincidence signal is sensitive only to neutron pairs resulting from spontaneous fission events and not sensitive to single neutrons from (α, n) reactions or room background. In addition, in-situ neutron coincidence counting exploits the rapid decrease in coincidence detection efficiency outside the enclosure defined by the coincidence counter array to provide a high degree of discrimination against background neutrons generated by plutonium in adjacent gloveboxes. However, the precision with which this signal can be measured depends strongly on the detection efficiency that can be achieved. The detection efficiency of in-situ measurement systems is severely compromised by the deployment requirements. For this reason the DISPIM 3D Imaging system measures both the neutron coincidence signal and the more precise total neutron signal. The DISPIM system incorporates up to 40 individual neutron counting modules each comprising two 500mm long, 50mm diameter 3 He neutron detectors encased in a rectangular block of high density polyethylene. Outer dimensions are approximately 700mm by 200mm by 110mm and each module weighs about 20kg. These modules are deployed in assemblies of four, horizontally mounted around the accessible surfaces of each assay item. The configuration and number of modules varies from item to item depending on accessibility and size. A mobile deployment mechanism is usually used to support each DISPIM assembly in the preferred positions adjacent to the item to be assayed. The deployment mechanism allows the modules to be raised to the optimum measurement height whilst ensuring maximum stability to satisfy plant safety requirements. The system s electronics and data recording facilities are housed in a sealed cubicle positioned remotely to prevent contamination of its components and minimize operator dose uptake during use. The number and arrangement of modules is usually chosen to minimize variations in counting efficiency with plutonium (or MOX) location. The mean counting efficiency of the DISPIM system therefore changes from measurement to measurement since the counting geometry is different each time the DISPIM system is assembled around a glovebox or other process vessel.

6 The system is supplied with a 3D map of detection efficiency points around each detecting module. Given information on the deployment and geometry of the assay item, the DISPIM system automatically reconstructs a calibration map for each measurement configuration. Once the position of each detecting module in the DISPIM array has been accurately specified in the system s software, a mathematical model of the specific glovebox arrangement is generated. This model accurately predicts the response of each module for any distribution of sources within the assay item. The measured total neutron response of each DISPIM module and the coincidence response of each assembly of four modules are determined over a measurement period usually lasting several hours. From suitable starting conditions, the mathematical model selects p lutonium concentration locations and predicts all module count rates. The agreement obtained between the predicted and actual measured count rates of each module is then evaluated using a chi-square test. Single sources and multiple sources are iteratively located in the model by a non-linear least squares approach until the chi-square test reaches a minimum value. Once the minimization process has completed, the model parameters indicate the relative activity of each plutonium concentration and its position on 3D co-ordinate axes. Siemens Deployment Following Site Acceptance Tests performed at the Siemens MOX facility at Hanau in Germany, a positive bias was noticed in the results of Pu measurements. The problem was investigated to see whether the height of a DISPIM assembly affected the efficiency. A series of measurements were carried out using one DISPIM assembly with a 252 Cf source placed at various distances from it. The assembly was raised to various heights above floor level and the efficiency measured. The results are summarized in Table 1 below:- Table 1 - Efficiency of DISPIM Assembly to a Source at Distances of 72.5cm and 25.0cm Assembly Module Efficiency (%) at Distances of 72.5 and 25.0 cm Height above Module m01 Module m02 Module m03 Module m04 Sum of Modules Floor (cm) 72.5 cm 25.0cm 72.5 cm 25.0cm 72.5cm 25.0cm 72.5cm 25.0cm 72.5cm 25.0cm The results from this series of measurements clearly show that the efficiency of the DISPIM system increases as the assembly is lowered. The effect is most noticeable in the lowest module (m04). This increase in efficiency is due to floor reflections and results in a potential relative increase in efficiency of 8% in the entire assembly. On a single module it is as high as 16%. This effect not only results in an overestimate of the Pu hold-up of a measured item, it also affects the accuracy of the imaging because the effect is not uniform over all the modules in an assembly. The DISPIM system s software contains a set of constants, normally set to unity, that are used to compensate for the effects of variations in relative efficiency between modules. These constants can be increased, in this case to compensate for the increase in relative efficiency due to the height of the measured item above the floor, in any particular measurement scenario. For example, if there is a relative increase in the detection efficiency of 10%, the value of the constant is increased to When the m easured count rate is divided by this constant, it is effectively reduced to the expected free-space count rate. Measurements have been performed at Siemens that have confirmed the effectiveness of this technique.

7 Rocky Flats Deployment The DISPIM 3D Imaging system was demonstrated to Rocky Mountain Remediation Services (RMRS) at RFETS in September Test measurements were performed to show how the DISPIM system should be deployed and operated and to demonstrate its performance for the following assay items:- containing low levels (~1g) of total Pu hold-up, containing uniform or dispersed contamination, containing U as well as Pu, containing high levels (~1-2kg) of total Pu hold-up, containing boronated glass (neutron poison), with limited access in non-imaging mode, in high background conditions. The DISPIM system was being compared against high and low resolution gamma spectrometry based measurement techniques. The DISPIM system s performance was also evaluated for situations where gamma based measurements are less reliable such as items with a high metallic content. To test the performance of the DISPIM system, RMRS personnel selected a number of different items to be measured. Several of these are summarized in Table 2 below:- Table 2 - RMRS Demonstration Measurement Items Measurement Item Item Description RMRS Estimated Pu Hold-up Measurement Challenge Glovebox J-40 Large wooden crate containing Low Pu level, large size, 0 25g decontaminated glovebox uniform contamination Glovebox SR14 Glovebox containing Pu + U Presence of U, high nitrate solution ~75g background Tank 0705 Raschig ring filled lead lined tank ~25g Limited access, raschig rings The DISPIM results of J-40 measurements indicated that the majority of the hold-up was associated with a filter housing which was suspected to be contaminated. Uniformly distributed hold-up was also indicated around the glovebox. DISPIM results indicated a total hold-up of 99.2gPu with a 2σ confidence interval of (70.5, 128.2) gpu. The best estimate hold-up associated with the filter housing was 55.1gPu with a 2σ confidence interval of (42.9, 67.4) gpu. This is equivalent to a measurement uncertainty of ± 23%. The measurement of this glovebox was taken in two parts as the box was too large to be assayed as one. A consequence of this is cross-talk between the two measurements, where signal from one half of the glovebox is present during the measurement of the other half and vice versa. Although the imaging results do help to separate the mass present in each section, contamination close to the border of one of the two sections does contribute to the measured mass of the other section. Previous RMRS gamma based measurements of this glovebox have estimated a total hold-up of 23gPu with a 2σ confidence interval of (5, 36) gpu. DISPIM measurement results of glovebox SR14 indicated that the majority of the hold-up present was associated with a slop pot near one end towards the base of the glovebox. Some uniformly distributed hold-up was again indicated around the glovebox. Total hold-up of 137.2gPu was indicated with a 2σ confidence interval of (112.1, 162.4) gpu. The results also indicated that the best estimate of U235 hold-up in the glovebox was 33.3g 235 U with a 2σ confidence interval of (27.2, 39.4) gpu. Earlier RMRS gamma based measurements of the same glovebox have estimated a total hold-up of 70 ± 64gPu and 17 ± 14g 235 U. DISPIM results of Tank 0705 indicated a total hold-up of 124.8gPu with an upper limit of 499.2gPu. The large uncertainty of this measurement was due to the limited access around the tank. This resulted in insufficient information for the DISPIM system to provide an imaging result. No matrix correction was applied to this measurement. As the tank contained borosilicate raschig rings, which absorb thermal neutrons, some absorption may have taken place but this was thought to be insignificant as the tank contained no moderating materials such as water. Previous gamma based measurements of the tank have estimated a total hold-up of 14.4gPu.

8 It is clear that all the DISPIM measurements above give higher mass estimates than the HRGS results performed by RMRS. At the time this was thought consistent with the opinion that the HRGS measurements were likely to be an underestimate of the true hold-up. No measurements of known standards were performed at this time. In December 1998 a series of measurements was taken of known Pu standards. The DISPIM system consistently and considerably overestimated the amount of Pu within several measurement items including steel standards crates. A problem with the DISPIM system was confirmed by an overnight measurement of a crate which contained soil but no Pu standard gave ~35g total Pu. Analysis of the raw measurement data concluded that the problem was some form of background source giving a high real coincident count rate in the DISPIM system. The magnitude of the background real coincident signal suggested that cosmic spallations were the most likely cause. RMRS performed the following measurements to determine the relative proportions of background from the steel crate and the DISPIM assemblies:- A measurement of a steel filled crate with no Pu standards, An identical measurement but with the crate absent. These measurements suggested the following background Pu mass contributions:- DISPIM system 36 to 45g Steel filled crate 22 to 33g These figures are consistent with the theoretical values calculated earlier. The results indicate that more than half of the effect is caused by cosmic spallations within the DISPIM assemblies themselves. Calculations using cosmic induced neutron production rates were also performed which confirmed the results of the measurements. Background Correction Cosmic Spallation in the DISPIM System For obvious reasons the background contribution from the measured item can not be easily subtracted by a background measurement and subtraction routine. However, the contribution from the DISPIM system itself can be subtracted by performing a background measurement routine. The system is deployed in the same area as the measured item, in the same configuration, but with nothing in the measurement envelope. The measured background count rates can then be subtracted from the actual measured item count rates. This will remove the cosmic background contribution from the measurement, assuming the cosmic background rate remains approximately constant over the time and position of the two measurements. This stage of the correction was tested using measurements performed by RMRS. When the real coincidence count rates for the DISPIM system and assemblies were corrected for the background, the results showed a marked improvement as shown by Table 3 below:- Table 3 - Results of Background Correction of Measured Data Measurement ID 690(Empty480) 719(Open Air) 765(Open Air25g) 782(Empty480) 240 Pu eq mass (g) Imaging Result No No No No Pu mass (g) Background Measurement ID 734(Open Air2) 734(Open Air2) 749(Open Air3) 749(Open Air3) Corrected 240 Pueq mass (g) Imaging Result No No Yes No Corrected Pu mass (g) Pu mass subtracted (g) In all cases a significant Pu mass contribution, consistent with that caused by spallations within the DISPIM system, has been removed from the final mass estimate. In the case of measurement 719(Open Air), the Pu mass estimate has dropped below 10g. This is despite the fact that the background measurement was t aken without the presence of large amounts of Pu in the background, which were present during the original measurement. The Pu standard in measurement 765(Open Air25g) was successfully imaged after background correction, which was not the case before the correction. These results are encouraging and show the potential of this background correction technique.

9 Background Correction Cosmic Spallation in the Matrix Material of the Measured Item To correct the remaining contribution from cosmic spallations in the measured item a method of discriminating between the cosmic signal and the signal from the measured Pu is required. Most of the practical methods rely on the cosmic events occurring discretely in time. The simplest of these uses the current segment rejection technique. This is based on rejection of statistical reals coincident rate outliers, applied to very short segments of 10 to 60s duration. If a cosmic event occurs within a segment the real coincidence count rate in that segment will be significantly higher than that in a segment with signal only from the measured Pu. Both background correction techniques have been implemented on the RMRS DISPIM system. The DISPIM with the new cosmic background subtraction software has been tested on previously assayed waste drums containing significant quantities of weapons grade plutonium. The system overestimated the plutonium content on average between 13 and 20 percent, which makes this a valuable instrument for hold-up estimation and will allow operators to better adhere to criticality limits during D&D. Conclusion The DISPIM 3D Imaging systems supplied to Siemens and RMRS have been the subject of much testing and further development. Several improvements have been applied such that both systems perform to specification in a wider range of environments that was the case originally. The removal of the effects of reflections from external surfaces, self-multiplication and cosmic spallation have all been successfully demonstrated. LOS ALAMOS NATIONAL LABORATORY (LANL) LARGE ITEM NEUTRON COUNTER (LINC) Introduction In 1998, Los Alamos National Laboratory (LANL) contracted with BNFL Instruments Inc. (BII) for the development and installation of a Large Item Neutron Counter (LINC). This system incorporates imaging passive neutron techniques. The LINC was installed at a temporary site in June 1999 at Technical Area 54 (TA54) and commissioning and calibration were completed on-site in November of the same year. As part of the operations of the LANL TA54 Decont amination and Volume Reduction System (DVRS), the LINC is expected to detect and provide upper limit quantification of 240 Pu in gloveboxes contained in plywood crates varying in size to a maximum of 10x10x40 ft 3. The system is also expected to be capable of segregating LLW in B25 crates. Two representative matrices, a mock-up of a 5x5x9 ft 3 crated glovebox and a B25 containing dry combustibles and glass, were used as the basic guide for the calibration of the system and for determining the requirements for the Acceptance Test Plan, conducted in March The Acceptance Test Plan included sensitivity and accuracy requirements for the LINC, which utilizes arrayed 3 He detectors. The system passed its testing without exception. Upon installation, additional measurements and MCNP modeling were used to expand the calibration range of the system. The system is now operational at its temporary site, and is set to be used throughout the coming year while the DVRS facility is completed. LANL TA54 DVRS Facility The DVRS is set for installation at LANL s low level waste (LLW) disposal facility late in Once operational, this facility will process waste contained in crates reclaimed from burial pits at TA54, Figure. 1. In 1998, LANL began plans for the DVRS. This facility will decontaminate and compact such materials as gloveboxes and repackage the waste for permanent disposal as either LLW or transuranic (TRU) waste. Several measurement systems are required for integration with the facility process to ensure safe operations. Before the crates enter the facility, a screening measurement must be completed in order to ensure that facility safe operating limits are not exceeded by the introduction of a waste crate. To this end, LANL has contracted with BII for a LINC.

10 Figure 1 - Reclaimed Waste Crates at LANL TA54 Once the LINC has completed the screening measurement on a given crate, the crate will be moved into the permacon facility for decontamination. Real time monitoring of the facility hold-up levels allows operations to proceed safely and indicates the need for clean out activities when threshold levels are reached. LANL also contracted with BII for this monitoring system known as the Neutron Area Hold-up Monitor (NAHM). The NAHM was successfully tested and delivered in November The NAHM will undergo installation and commissioning upon completion of the DVRS facility. Items removed from the reclaimed crates will be individually cleaned out before compaction. In order to determine whether clean out activities have been successful and to determine the location of hotspots that may still exist on or in a given item, a third system will be required for the facility. The measurement method this system will employ has not yet been determined, but provisions have been made for its installation as part of the permacon facility. After decontamination activities have been completed, the items will be moved into a shredder and compactor, designed by Max Saturn, for volume reduction. This system will produce pucks sized for optimal placement into 55-gallon drums. Because one of the goals of the facility is to maximize the number of drums that may be disposed of as LLW, it will be necessary to assay the pucks individually before packing them into the drums. The pucks will be placed into drums based on their activity levels so that the total for a given drum will classify the waste as LLW. The drums will then be sealed and assayed for the final quantification of the activity. Drums will either be disposed of as LLW at the LANL disposal site, or as TRU waste and will be bound for WIPP. Currently, LANL plans to utilize one of their existing drum systems for these measurements. The majority of waste expected to be processed by the DVRS facility will be decommissioned gloveboxes and other metal items such as scrap and decommissioned equipment. Since these items are packaged in plywood and fiberglass reinforced plywood (FRP) crates, it will be necessary to package the crate and packing materials. This waste is expected to be largely uncontaminated and will most likely be LLW. The materials will be packed into B25 crates and will be measured by the LINC system. In this capacity, the LINC is expected to screen these B25 crates, classifying them as LLW or TRU at the 100 nci/g level.

11 Measurement Principals The LINC was developed by BII, Figure 2, and employs passive neutron counting and imaging techniques for the characterization of crated gloveboxes. The operator must identify the matrix type and crate size before a measurement can be completed. Currently, the LINC can measure two matrix types and thirteen crate sizes. Additional matrix types and crate sizes can be added as necessary for the DVRS operations. Figure 2 - LINC Detector/Package Configuration Once a crate has been set at its beginning position between the two detector faces or slabs, the crate must be measured in one-foot increments or grabs. The crate is manually moved with positioning information provided to the operator through a laser-positioning device. The number of grabs completed for a measurement is dependent on the length of the crate. A crate that is nine feet long will require nine grabs of data. The LINC contains 20 3 He tubes, 10 in each slab. Because the crates bound for the DVRS facility vary widely in size, the LINC detector packages can be moved depending on the width of a crate. The maximum possible separation for the LINC slabs is eleven feet. This allows for a maximum crate width of ten feet. Additionally, the LINC detector packages can be positioned in either an up or down position. The down position is used for crates up to eight feet in height. The up position is used for crates up to ten feet in height. Positioning of the detectors for a measurement is dependent on the crate size and is determined during calibration of the system. The 3 He tubes, in conjunction with a programmable multi-channel coincidence module (PMCCM), are used to perform neutron coincidence analysis to provide an upper limit of the 240 Pu eff present in the waste. In addition, imaging data is obtained from each of the detectors as the crate is moved past them in one-foot increments. The imaging allows source positions to be determined for greater personnel safety during crate unpacking procedures and higher efficiency during decontamination activities. Acceptance Test Requirements and Results The LINC was required to meet accuracy and sensitivity limits based on the operational requirements for the system. Two matrix types were defined in the Acceptance Test Plan that would simulate a 5 x5 x9 crated glovebox, Figure 3, and dry combustibles in a B25 container.

12 Figure 3 - LANL TA54 DVRS LINC with Crated Glovebox An additional matrix type, HEPA filters, and additional crate sizes have been defined during installation and commissioning activities. These have been added to the LINC calibration library using a combination of experimental calibration data and Monte Carlo modeling techniques. The procedure for the accuracy testing involved using a 252 Cf source of sufficient strength as a surrogate for 240 Pu. For each measurement, the appropriate source was placed in the 5 x5 x9 representative matrix and measured. The process was repeated for three source positions. Positions were chosen randomly. The width of the crate corresponds to the x-coordinate, the height corresponds to the y-coordinate, and the length corresponds to the z-coordinate. Table 1 and Table 2 contains the complete set of performance requirements and the measured performance for the LINC system. All of the Pu equivalent values are calculated assuming weapons grade Pu (WGPu) isotopics of 6% 240 Pu and 94% 239 Pu. Table 1 Pu Mass Accuracy Performance Matrix 5 x5 x9 Crated Glovebox 5 x5 x9 Crated Glovebox 5 x5 x9 Crated Glovebox Position (X,Y,Z ) Pu Mass Required Pu Mass Used Pu Mass Measured Absolute Accuracy Required Absolute Accuracy Measured (38, 45, 81.5) 200 g 97.2 g 98.3 g 25% 1% (38, 21, 54) 200 g 97.2 g 84.6 g 25% 13% (46, 45, 54) 200 g 97.1 g 81.8 g 25% 16%

13 Table 2 Imaging Accuracy Performance Matrix 5 x5 x9 Crated Glovebox 5 x5 x9 Crated Glovebox 5 x5 x9 Crated Glovebox Pu Mass 10 g 10 g 10 g Position (X,Y,Z ) Position Measured Absolute Accuracy Required Absolute Accuracy Measured X Y Z X Y Z X Y Z Sensitivity measurements for the LINC were completed in both the 5 x5 x9 Crated Glovebox and the B25 Dry Combustibles. Because the requirements for the B25 are specific to screening at the 100 nci/g level, the requirement is an activity density. The Pu equivalent has therefore been converted to activity using the nominal conversion for WGPu. The weight for the B25 matrix was 815 kg. For both cases, six replicate background measurements were used along with a volume-weighted conversion factor determined during calibration. The conversion factor converts response to WGPu equivalent mass. Table 3 contains the complete set of sensitivity performance requirements and the measured performance for the LINC system. Table 3 Matrix Detection Limit Required Detection Limit Measured (95% Confidence) (95% Confidence) 5 x5 x9 Crated Glovebox 1 g 0.97 g B25 Dry Combustibles 100 nci/g 50 nci/g Conclusion The LINC has passed its acceptance testing without exception and has thus far performed as expected. Currently, the LINC installation and commissioning has been completed at a temporary site with measurements planned throughout the year As part of the DVRS facility, it will function as an integral part of the decontamination process. The LINC is an excellent tool for sites with inventories of crates varying widely in size and content. Its capabilities make it possible to maximize the efficiency of facility activities and to maximize personnel safety and protection.