C UFtj FISSILE MASS FLOW SIMULATION AT

OAK RIDGE NATIONAL LABORATORY

J. T. Mihalczo J. March-Leuba T. E. Valentine J. K. Mattingly

T. Uckan. J. A. McEvers

Oak Ridge National Laboratory* P.O. Box 2008

Oak Ridge, TN 3783 1-6004 (423) 574-5577

Presented at the Institute of Nuclear Materials Management

Phoenix, Arizona

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UFa FISSILE MASS FLOW SIMULATION AT OAK RIDGE NATIONAL LABORATORY

J. T. Mihalczo J. March-Leuba T. E. Valentine J. K. Mattingly

T. Uckan J. A. McEvers

Oak Ridge National Laboratory P.O. Box 2008

Oak Ridge, TN 3783 1-6004 (423) 574-5577

ABSTRACT

A system for monitoring fissile mass flow in slurries, liquid and gaseous streams has been developed at Oak Ridge National Laboratory. The basis for the measurement is the activation of a fissile stream by neutrons and subsequent detection of delayed radiation produced by fission products resulting from such activation. This development of source modulation correlation measurements for fissile mass flow was first supported by US DOE Nuclear Energy Division in September, 1995, after the concept was proposed earlier in 1995. This paper describes recent simulation measurements with the first prototype of the system for fissile mass flow measurements with HEU UF6 gas for use in blenddown facilities. The theoretical model predictions from basic data for the simulation apparatus were only 15% higher than those actually measured, which is a remarkable agreement considering all of the physical phenomena involved. Thus, the calibration factor for the HEU gas from these simulation measurements is only 0.85 rather than unity which would mean perfect agreement. This amounts to using a completely mathematical based model as a calibration for a nondestructive assay method and having only a 15% difference from measurement. The simulation of HEU gas flow confirms extremely weil the understanding of the physical phenomena associated with this measurement system.

INTRODUCTION

A system for monitoring fissile mass flow in slurries, liquid and gaseous streams has been developed at the Instrumentation and Controls Division of the Oak Ridge National Laboratory. The basis for the measurement is the activation of a fissile stream by neutrons and subsequent detection of delayed radiation produced by fission products resulting from such activation. The development of source modulation correlation measurements for fissile mass flow was first funded by the US DOE Nuclear Energy (NE) Division in September 1995 after the concept was proposed by ORNL earlier in 1995. An initial demonstration of the configuration was performed in 1996 and was described at the 37th Annual Meeting of INMM in July 1996.' In November 1996 this method was chosen by NE for

implementation. This paper describes recent measurements of simulated HEU UF6 gas flow using this system.

DESCRIPTION OF APPARATUS

The apparatus shown in Fig. 1 consists of a polyethylene moderator block with 252Cf source to provide thermal neutrons mounted around a 10-cm-ID iron pipe. Between the moderator block and the pipe, a neutron absorbing shutter is moved periodically to provide a modulated source of neutrons entering the pipe. A detection system is located on the pipe approximately one meter away and consists of four BGO detectors and the associated electronics. These particular detectors are used to detect delayed fission gamma rays with energies above 300 keV. A low density uranium deposit is provided through the use of a 243-cm-long fission chambe? that slides back and forth through the source-modulator-detection system. This chamber is coupled to a variable speed chain drive. In these simulations; moving a 243-cm-long fission chamber simulated flowing UF6 gas. This chamber has eight 25-cm-long sections with a deposit of 90 mg 235U per cm of length in each deposit. Each of these sections is separated by a 3-cm-long insulator. This results in an average deposit of 80 mg 235U per cm of length which is comparable to the fissile density in a 10 cm diameter pipe (66 mg O f Hg. Therefore, the chamber has a 235U density per unit of length that is only 20% higher than the

per cm of pipe length ) containing UF6 gas (90% 235U), at a pressure of 40 mm

U density of the gas. 235

Description of Simulation Sequence

The sequence of operation for this simulation apparatus is: start motion of the fission chamber, open shutter thereby activating a portion of the fission chamber, close the shutter, continue motion through the detector assembly while counting the activity, stop and wait, recycle back to the start position through the source moderator with the shutter closed, and repeat the process. In these simulations, the fission chamber is passed repeatedly through the source modulator and through the detector modules. This sequence is repeated many times until the desired statistical precision is obtained. This simulation with the fission chamber does not have fission product loss effects? because all fission products produced by activation remain in the chamber and thus move along with the fission chamber. For simulation of the actual flow, this fission product range effect results in activation that is too large by a factor of 3 in the simulation apparatus. In addition, the fission chamber can not be used to simulate the laminar flow which reduces the signal of the detectors by a factor of approximately 3 in UF6 gas at a pressure of 40 mm Hg.4 Since the fission products move with the fission chamber, the fission chamber motion simulates turbulent flow where gas moves as a slug. This results in the simulation apparatus having a factor of 3 too much signal over the gas. In addition, the apparatus has an additional effect of buildup of fission products from previous activations due to the repeated recycling of the chamber through the source modulator. This is included in the modeling for the simulation apparatus and can be adjusted somewhat by stopping the chamber after the active portion passes through the detectors before recycling it back for the next activation cycle. In addition to this. the fission chamber is exposed to the source as it moves back to the starting position by passing through the source moderator with the absorbing shutter closed. This introduces an additional background in the chamber along the portion moving through the source when the shutter is closed making the signal after the main peak slightly higher than the one

before the peak. Although this apparatus has characteristics (e.g. recycling the activated material) that an application does not have and lacks some of the characteristics (e.g., fission product losses and laminar flow effects) that an application does have, these characteristics are well understood. The latter two phenomena are included in the models for the application, but recycling is not.

Simulation of HEU Line Now let us consider the simulation of the HEU gas at 40 mm. The response of the detectors is approximated by the following product,

number of laminar fission product ( i i z t y ) (d1:s:ty) ( detectors 1 (velicity ) (flow eflects) ( losses

In the simulation apparatus the detector response is approximated by the following product,

( source) [ 2 3 5 ~ ) (numberof) ( buildup of intensity density detectors fission products

Because these expressions are products, the values of the individual parameters of Eq. 2 can be varied to give a numerical value close to that of Eq. 1. In this way, the slightly higher uranium density present in the apparatus (compared to HEU gas) can be compensated for by reduced source size. Also, the reduction of the signal in the application from laminar flow can be compensated for in the apparatus by reducing source size or the number of detectors and other adjustments such as delaying the chamber to reduce the signal before counting by letting additional decay to occur.

Consider the first expression [Eq. 11 for the application, whose value is proportional to

586 (3)

where the source intensity is represented by 20 pg 252Cf, the density by 66 mg 235U/~m of pipe, the number of detectors by 4, laminar flow effects by a reduction of approximately 3, and fission product range effects by a reduction of approximately 3. In both the gas and the simulation apparatus. the velocity is the same and is 5.8 cm per sec. Now consider what the values in Eq. 2 have to be to nearly equal 586/velocity. The source intensity used in the HEU simulation is reduced to 0.7 pg 252Cf (ie., a single small source in one of the source positions). The fission chamber average density is 80 mg 235U/cm of length which is slightly higher than the application; the number of detectors remains at 4; and fission produced buildup effects increase the signal by a factor of 3. The value of the second expression [Eq. 21 is then:

This product [Eq. 31 is 672helocity which is 15% higher than the gas which has a value of 586/velocity. Therefore a reasonable simulation of the HEU gas flow at 40 mm Hg is accomplished

with a single 0.7 pg source, 4 detectors, the fission chamber, and the actual velocity and the spacing of the source modulator and the detectors of one meter. The velocity and spacing in the simulation are the same as the gas. The conditions of this simulation are also summarized in Table 1. The gas in the application is simulated using these parameters.

Table 1. Simulation Test Parameters in the ORNL Apparatus

PARAMETER VALUE COMMENTS

Cf Source in one position only

Moderator-detector distance 1.07 m -almost same

Fission chamber velocity - 5.8 cm/s same velocity

uZ5 density 80 mg/cm -20% more than HEU gas density

Number of detectors 4 same

0.7 l-43 -3.5% of gas system source

To more appropriately simulate the gas application, the background counts directly from the source fission and neutron induced gamma rays at the detector must also be included for the gas application which has 20 pg of "2Cf. With the 20 pg in the source moderator, the measured background at the detectors (spaced one meter from the sources) for the gas is -5000 counts-per-second (cps). To include this factor in the simulation, a 137Cs source was placed adjacent to a fifth detector far removed from the apparatus in such a way as to produce a statistically varying count rate with average value of 5000 cps. During simulations, the counts from the detectors on the pipe of the apparatus and the background detector were acquired and summed to produce the measured signal for the simulation. Also, during simulations the shutter was open for 3 seconds and the irradiated portions of the fission chamber moved through the detection system and this process repeated at forty seconds intervals. In the gas application this process will be repeated at 20 second intervals so thirty cycles in the simulation would equate to a measurement time in the gas of 10 minutes.

Agreement with Model The correlated counts in the detector as a function of time in this simulation at 4 different times during the simulation are shown in Fig. 2 for the equivalent of 10 minutes of measurement time or 30 cycles of the shutter motion in the gas. The correlated counts were obtained by subtracting the average background rate. The data shown in Fig. 3 is for the total length of the simulation run, which consisted of 1300 cycles which is the equivalent of 7 hrs of data for the HEU gas. These data were obtained by also subtracting the additional background count which comes from the 235U of the fission chamber without activation as it passes through the detectors (which is due to pileup of 186 KeV gamma rays from 235U above the 300 KeV threshold for detection) and the constant background from the sources. This additional background value as a function of time was acquired by performing the simulation without any sources in the modulator block and comes from the structure of the fission chamber associated with the nonuniformity of the 235U deposit. The measured

peak count rate in the simulation is 80 cps which is close to the expected values in the HEU gas of 100 cps at 40 mm. This agreement is good considering the simplified approximations (Eqs. 1-4) used for obtaining the parameters of the simulation with the fission chamber. Because the maximum count rate in the simulation is lower than the gas, the simulation is slightly conservative. Two curves are shown; the experimental data with all background subtracted, and the background subtracted value as calculated by the model for the simulation apparatus. Use of the model for the simulation apparatus produces a calculated curve (Figs. 2.3) only 15% higher than that measured. The calculated curves of Figs. 2 and 3 have been adjusted down 15% to agree with the simulation measurement. This adjustment of the model to achieve the amplitude agreement is small considering all the basic data utilized in the model calculations. This difference of only 15% between the model and measurement for the simulation apparatus is a remarkable agreement. The ability of the model to predict within 15%, depends on knowing: the "'Cf source intensity, the "'Cf spectra; the slowing down of neutrons by the moderator, transmission of neutrons into the pipe, induced fissions in the fission chamber and their spatial distribution along the fission chamber (field of view of the source), time decay of the fission products and their buildup, spectrum of delayed gamma rays, absolute detection efficiency, field of view of the detectors, and an understanding of the operation of the detection systems including the data acquisition and processing hardware. This amounts to using a completely calculation based model as a calibration for a nondestructive assay method and having only a 15% disagreement. The calculated shape of the peak agrees with the measured shape and thus shows that the calculated and measured activation spatial distribution along the chamber and detection field of view are in excellent agreement. This 15% error is mostly due to fission product buildup effects as the same 235U fission chamber is repeatedly cycled through the system to simulate flow.

The velocity as determined from the nuclear measurement is 5.9 cm/s while that obtained with a stopwatch and a ruler is 5.8 cdsec. Both are in excellent agreement. The velocity from the data of Fig. 2 and 3 is not obtained from the peak of the activation pulse but rather from fitting the model to the simulator data.4 The detected peak is wider than the open time of the shutter (3 seconds) because of the field of view of the detector (15 cm or 2.5 seconds for this velocity) and the width of the source activation region (15 cm or 2.5 seconds for this velocity) produced by the modulator design.

The agreement between the model and the simulation provides a high degree of confidence in the understanding of this method and validates the theoretical model except for laminar flow and fission product range effects which are well understood. The model is then used to predict the statistical uncertainty in the measurements at the plants.

Conclusions The following conclusions have been reached from the simulation of the HEU line with the Oak Ridge Apparatus: (1) simulation of the HEU gas is a slightly more difficult situation to measure than the gas application because the count rate in the simulation is lower than that expected in the gas, (2) simulation of the HEU gas confirms extremely well the understanding of the physical phenomena associated with this measurement system, (3) theoretical model predictions from basic data for the

simulation apparatus were only 15% higher than those actually measured, and (4) the calibration factor for the HEU gas from these simulation measurements is only 0.85 rather than unity which would mean perfect agreement. This amounts to using a completely calculation based model as a calibration for a nondestructive assay method and having only a 15% difference from measurement.

REFERENCES 1. J. T. Mihalczo, J. A. March-Leuba, T. E. Valentine, R. A. Abston, J. K. Mattingly, and

J. A. Mullens, “Source Modulation-Correlation Measurements for Fissile Mass Flow in Gas or Liquid Fissile Streams,” Proceedings of Institute of Nuclear Materials Management Annual Meeting, July 1996, Naples, Florida.

2. J. T. Williams, J. T. Mihalczo, and C. W. Ricker, “A High-Sensitivity, Position-Sensitive Fission Chamber for Subcriticdty Measurements of Spent Fuel,” Nuclear Instruments and Methods in Physics Research A299, pp 187-190 (1990).

3. J. K. Munro, T. E. Valentine, R. B. Perez, J. K. Mattingly, J. March-Leuba, and J. T. Mihalczo, “Fission Product Range Effects on HEU Fissile Gas Monitoring for UF6 Gas,” Proceedings of 38th Annual Meeting Institute of Nuclear Materials Management, Phoenix, AZ, July 20-24,1997.

4. J. March-Leuba, J. K. Mattingly, J. A. Mullens, T. E. Valentine, J. T. Mihalczo, and R. B. Perez, “Methodology for Interpretation of Fissile Mass Flow Measurements,” Proceedings of 3 8th Annual Meeting Institute of Nuclear Materials Management, Phoenix, AZ, July 20-24,1997

Simulated .- Fission __ Chain Drive

for Fission source Process Piping -.. Detector

Moderator Assembly \ / Assembly / 1 Chamber

Fig. 1 ORNL Flow Simulation Apparatus Showing Instruments and 235U Fission Chamber

10 m in Block 1

-Calculation + Measured Data

-- - _ - _____

1

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Fig. 2 Count Rate vs Time with Background Subtracted from HEU Gas Simulation of 10 Minutes (4 Different 10 Minute Segments of Data out of a 1300 Cycle Run)

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Fig. 4 Estimated Mass Flow and It's Uncertainty From The Model For The HEU Input Line To The Blend Down or Blcnd Point