VERIFYING NUCLEAR WASTE TILE-HOLES USING GAMMA RADIATION SCANNING

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1 FULL ARTICLE Nuclear waste management facilities at Chalk River Laboratories (CRL) use below-ground tile-holes to store solid waste from various activities such as medical isotope production. After long periods of isotopic decay, some of the waste has decayed down to low activities and can be transferred to low-level waste storage facilities. This paper presents a method to verify the radiation level of the waste inside tile-holes by performing gamma radiation scans along the depth of waste storage tile-holes. Such measurements allow for noninvasive verification of tile-hole contents and provide input to the assessment of radiological risk associated with removal of the waste. Using the radiation profile system, the radiation level of the radioactive waste may be identified based on the radiation profile. This information will support planning for possible transfer of this waste to a licensed waste storage facility designed for low-level waste, thus freeing storage space for possible tile-hole re-use for more highly radioactive waste. CRL-developed small diode-based gamma radiation sensors have been used in these radiation scans. The diode sensors were deployed into verification tubes adjacent to the tile-holes to measure the radiation profile. Over 10 tile-holes have been scanned using this technique since VERIFYING NUCLEAR WASTE TILE-HOLES USING GAMMA RADIATION SCANNING James Johnston*, Shuwei Yue, and Jeremy Stewart Canadian Nuclear Laboratories, Chalk River, ON K0J 1J0, Canada Article Info Keywords: Tile-holes, waste management, verifying nuclear waste, gamma radiation scanning. Article History: Received 30 August 2017, Accepted 13 February 2018, Available online 13 November DOI: *Corresponding author: james.johnston@cnl.ca Nomenclature A ampere, the base unit of electric current CNL Canadian Nuclear Labs CRL Chalk River Laboratories Gy gray, a derived unit of ionizing radiation dose I-V current vs. voltage P-N the P (positive) N (negative) junction is a boundary or interface between 2 types of semiconductor material, P-type and N-type, inside a single crystal of semiconductor. The P-type contains an excess of holes, while the N-type contains an excess of electrons. Si silicon WMA Waste management area 1. Introduction Waste management areas (WMAs) at Canadian Nuclear Laboratories (CNL) Chalk River Laboratories (CRL) have many tile-holes [1] for storing solid high-level waste. Development of a gamma radiation field scanning instrument was necessitated by the need to move decayed waste from existing tile-holes to a more appropriate location to accommodate new high-level waste. The instrument is required to be able to fit into narrow verification tubes adjacent to the tile-holes (see Figure 1), have a fairly wide range of sensitivity to gamma fields, must be inexpensive in case it becomes contaminated, and must be disposed of and provide a radiation profile along the length of the tile-hole. Commercially available devices used for waste management operations are typically designed to measure fields <0.01 gray (Gy)/h for the protection of WMA personnel. Off-the-shelf equipment for measurements of fields >0.1 Gy/h for research, production or waste management facilities, or process monitoring is not available. Custom-built devices for measuring high gamma radiation fields are typically expensive and bulky. An instrument based on both the silicon (Si) rectifier diode and Si photodiode have been developed for this purpose. These sensors are inexpensive and small enough to be deployed in the narrow verification 1

2 FIGURE 1. Structural drawing of below-ground solid nuclear waste storage tile-hole at CNL CRL. tubes or within guide tubes inserted into the verification tubes. The sensitivities of these sensors cover 10 mgy/h to thousands Gy/h, and are linear over a large range. Previous studies [2, 3] did not deal with the environmental conditions for the applications. Measurements were completed on the ionizing radiation and temperature effects on positive negative (P N) junction diodes and are discussed in this paper. The instrument system described in this paper allows for online measurements of radiation fields as the sensor is deployed along the depth of verification tubes to generate a gamma radiation field profile of the tile-holes. 2. Tile-Holes Nuclear waste management facilities at CRL use belowground tile-holes [1] to store highly radioactive solid waste. It is always desirable to move waste to a more appropriate location if possible. Administrative records exist detailing the contents of each tile-hole, including when waste was deposited and the near-contact radiation fields from waste at the time of storage. Radiation field profiles generated using the Si diode based sensor system will be used to verify administrative records and identify tile-holes whose contents have decayed to very low levels. This very low level radioactive waste can be extracted from the tile-holes and transferred to a more appropriate licensed above-ground storage facility. The empty tile-holes will then be available for future storage of high-level waste. Figure 1 shows the structure of the tile-holes at CNL s WMAs. The tile-holes are sealed with a removable cap and concrete plug. A verification tube is built beside each tile-hole for performing certain measurements and analysis, as removing the cap and seal plug is often not preferred. Taking readings from the verification tube is a much safer and less involved procedure than accessing the tile-hole itself; however, the accuracy of the readings from such measurements has not been tested. There is a significant amount of concrete between the tile-hole interior and the verification hole which acts as shielding. Efforts are underway to calculate the shielding effect, hence the relationship between fields inside the tile-hole and those measured in the verification tube. It is intended to confirm this relationship in the future by direct measurement. 3. Instrument System Description 3.1. The principles of solid-state detectors The P N junction is a critical component of solid-state electronics. When P-type and N-type material come into contact, the junction between the 2 acts in a different way than either material on its own. Current flows in 1 direction, or forward bias, but not in the other direction, or reverse bias, thus producing the characteristics of a diode. The nonreversing behavior occurs from the nature of the charge transport process in each of the P-type and N-type materials. The open circles in Figure 2 in the P-type region of the junction indicate holes or deficiencies of electrons in the lattice and perform like positive charge carriers. The solid circles in Figure 2 in the N-type region of the junction represent the available electrons from the N-type doped material. Electrons diffuse across the P N junctiontocombinewith holes, forming a depletion region. 2

3 FIGURE 2. The silicon P N junction and the depletion region. When a P N junction is created, some of the free electrons in then-typeregiondiffuseacrossthejunctionandcombine with holes to form negative ions, leaving behind positive ions at the donor impurity sites Si diode as radiation sensor The most common type of sensors used for measuring radiation dose rate includes gas ionization chamber, scintillation detector, and semiconductor device. Of all these radiation detector types, Si diodes have the ability to be used in high radiation fields without a bias voltage. Figure 3 compares the current vs. voltage (I V) characteristics measured for a small commercially available diode in the laboratory (no radiation field) that measured in a 550 Gy/hour exposure rate radiation field inside a 60 Co gamma-cell. As seen in Figure 3, the effect of the radiation FIGURE 3. Current vs. voltage (I-V) characteristics of a silicon diode in and out of a radiation field. is to shift the I V curve in the negative or reverse current direction. The magnitude of the shift is proportional to the dose rate. As seen in Figure 3, exposure to a radiation field causes a reverse current, usually known as photocurrent, even when the diode is not biased Two types of diode sensors Two types of diode sensors have been developed at CNL. One type is Si rectifier diodes, which are effective for measuring radiation fields from 0.1 Gy/h to 100 Gy/h andhavebeendeployedsuccessfullyatmanyfacilities at CRL [4, 5]. Using the same concept, a large-area photodiode has been demonstrated to function as a radiation sensor in unbiased current mode [6]. The particular model of photodiode was selected for its narrow profile that makes it possible to assemble it in a thin capsule, which allows access into small orifices for measurements. Additionally, the Si volume of the junction, or depletion region, in this diode is nearly a 100 times that of the rectifier diode, resulting in much higher sensitivities that can measure a radiation field down to 1 mgy/h. The volume of the depletion region in the diode is the effective area for measuring radiation. As such, the photodiode has a greater effective area and a greater sensitivity as a result. The photo on the left in Figure 4 shows the rectifier diode based sensor that has an approximate assembled outside diameter of 4 mm. The right side photo in Figure 4 is the relatively large photodiode sensor which has a 3 mm 30 mm photodiode potted inside a section of heat shrink. The capsule provides a light tight seal for the photodiode to minimize photocurrent due to visible light and protects it from contamination and corrosion. Both types of diode sensor assemblies were used in gamma radiation scans for tile-holes and the sensor assemblies are 3

4 FIGURE 4. Two types of diode sensor assemblies (without PolyFlo tubing). identical except the end tip as shown in Figure 4. The sensor assemblies were required to be rigid to deliver the sensor into the verification tube. The cable runs through PolyFlo tubing and the sensor is attached at the end of the tubing to provide the rigidity as shown in Figure 5. The diode gamma sensor system consists of 3 parts: the sensor assembly, a guide tube, and a signal readout system. The sensor assembly consists of the sensor inside PolyFlo tubing and a low-noise, shielded, coaxial signal cable encased in a PolyFlo sheath to provide rigidity. A BNC connector at the end of the signal cable is the interface to acquire data from the sensor. Refer to Figure 5 for a drawing of the sensor assembly. The components and materials for the sensors are purchased separately, assembled, and their sensitivities measured at CNL. The sensitivities of these sensors were measured using acalibratedco-60gammabeamsourceandgammacell (1 point calibration for Si rectifier diode). The result of this study is presented in Figure 6 and shows a very linear relationship between the dose rate of the exposed field and the photocurrent measured from the sensors. The rectifier diode sensor had a sensitivity of pa(mgy/h) 1, whereas the photodiode sensor had a sensitivity of 5.58 pa(mgy/h) 1. This is about 150 times greater than the sensitivity of the rectifier diode based sensor Deployment and signal readout system The sensor is deployed inside a guide tube to ensure a clear and straight path and to provide an additional layer of protection from loose contamination as shown in Figure 7. As part of the system, the guide tube is a 5.33 m long stainless steel tube with 1.27 cm outer diameter and cm wall thickness. The tube has a sealed rounded tip to aid the insertion into tile-holes and, if necessary, to navigate around the contents of the tile-hole. The top end of the tube is open and fitted with a Swagelok mating attachment. A choice of a 90 bend elbow or a 60 bend elbow is available to mate with the open end of the straight tube using corresponding Swagelok fittings. The sensor is inserted through the open end of the elbow and manually pushed or pulled to position the sensor. The PolyFlo sheath is rigid enough to be pushed to the move the sensor, while being flexible enough to go through the provided elbow attachments. Figure 8 shows the sensor delivery system and readout electronics, which provides the sensor position readings and radiation measurements. The readout electronics were developed at CRL, and consists of an amplifier module and an interface to position encoder. A laptop computer communicates with the readout electronics and records the radiation measurements and position readings as pairs. The human machine interface is a LabVIEW application that displays and logs the measurements. FIGURE 5. Sensor assembly. 4

5 FIGURE 6. Dose rate calibration, (top) rectifier diode sensor; (bottom) photodiode sensor Sensor directionality The rectifier diode sensor shown in Figure 4 has a cylindrical shape and did not demonstrate different sensitivities when it was rotated toward the source. However, the photodiode sensor within the assembly is rectangular and has a ceramic backing. Hence, it must be assumed that the sensor is not equally responsive to radiation from all radial directions. The radial directional sensitivity of the sensor was studied by maintaining it at a fixed location relative to a calibrated gamma beam source and rotated along its longitudinal axis. The sensor suffers a signal loss of up to 22% from the direction of peak sensitivity to that of low sensitivity. Figure 9 shows the sensitivity to direction relationship, with 0 being an arbitrary but marked position on the sensor casing. The position of peak sensitivity has been marked on the casing for identification during field use. Two back-to-back photodiodes have been used in applications to increase sensitivity. There was a reduced signal loss from directionality as a result Sensor sensitivity Three properties affecting the sensitivity of a diode are the effectiveareaofthedepletion region, the damaging effects of ionizing radiation, and temperature Sensor effective area The volume of the depletion region in the diode is the effective area for measuring radiation. As such, the photodiode has a greater effective area and a greater sensitivity as a result. Section 3.3 discusses how the depletion region of the photodiode is nearly 100 times that of the rectifier diode, generating much higher sensitivities. This is confirmed by 5

6 This effect is discussed in detail in Holmes Siedle and Adams [7]. Results of total absorbed dose testing to 0.5 MGy with the rectifier diode and the photodiode completed at CNL are presented in Figures 10 and 11. The data in Figures 10 and 11 indicate that both diodes lost about 20% of their original photocurrent in the first 0.1 MGy of absorbed dose, over 40% by 0.4 MGy and approximately 50% by 0.5 MGy. Thus, the sensitivity of the diodes degrades to roughly 50% after receiving a total dose of 0.5 MGy. This is a very large dose and is not an issue for most applications. It is observed that they both demonstrated consistent behaviour. Since the degradation appears well behaved, it can be modelled as a function of absorbed dose and corrected for in radiation detection applications. FIGURE 7. Guide tube assembly for deployment of sensor. looking at the data from Figures 10 and 11 where the current output of the photodiode in photovoltaic mode with no dose applied of 2.2 μa is almost 100 times larger than the current output of the rectifier diode in photovoltaic mode with no dose applied of 23 na Sensitivity to ionizing radiation Ionizing radiation affects the electron-hole pairs in semiconductor material. The production and subsequent trapping of the holes in oxide films causes serious degradation in metal oxide semiconductor and bipolar devices. FIGURE 8. Sensor delivery and readout system Sensitivity to temperature Testing of the effect of temperature, coupled with the effect of radiation exposure was completed at CNL on the rectifier diode and the photodiode. The I V curves were generated for each of the diodes at a set of temperatures while being irradiated inside a 60 Co gamma-cell are presented in Figure 12. The diodes are used in photovoltaic or unbiased mode to measure gamma radiation fields so no voltage is applied to the diodes. As seen in Figure 12, at zero bias voltage, the current is not sensitive to temperature because all the curves at different temperatures merge into 1 single point at zero volts. When taking radiation measurements at 25 C, or relatively low temperatures, the variation in the bias voltage causes very little current change so there are fewer constraints on the measurement instrument characteristics such as impedance or offset voltage because the curve is nearly horizontal. However, when taking radiation measurements at high

7 FIGURE 9. FIGURE 10. Rotational sensitivity of Si photodiode sensor. Loss of photocurrent in rectifier diode. FIGURE 11. Loss of photocurrent in photodiode. temperatures, any small variation in the bias voltage can cause a huge change in the output current because the curves are steep at 0 V. To not be affected by the temperature, the measurement instrument must have a near zero impedance and a near zero offset voltage to maintain the zero bias condition. 7

8 FIGURE 12. I V curves recorded at a range of temperatures inside the gammacell. 4. Radiation Field Profile of Waste Storage Tile-Holes 4.1. Radiation profile measurement and results Radiation profile measurements have been performed since October The objective was to lower the sensor into the verification tubes beside the tile-holes to measure the radiation fields detectable through the tile-hole shielding and verify the general fields from selected tile-holes. The sensor was lowered to the bottom of the verification tube and then brought up in 15 cm intervals, while recording the sensor signal. The graphs presented in this section plot the measured dose rate versus the height from the bottom of the verification tube, which is nominally the same level as the bottom of the tile-hole. Figure 13 presents the results of a radiation profile scan performed in a tile-hole containing medical radioisotope production waste. The contents of this tile-hole were expected to be at a fairly high level, and this is reflected in the radiation profile with the peaks around 500 mgy/h. These values were measured in the verification tube; the radiation levels within the tile-hole itself are therefore higher. This tile-hole contained waste in multiple canisters. The radiation peaks in the profile lined up with the canister positions very well. More radiation profiling was completed on 3 tile-holes located in WMA-B in 2015 that contained irradiated fuel. 8

9 FIGURE 13. FIGURE 14. Figure 14 presents the radiation profile from one of the tileholes. This particular type of tile-hole has verification tubes located inside the concrete cylinder, whereas most of the tile-holes have verification tubes outside the concrete cylinder. Since there was no detail geometry information available for the waste, the profile cannot be explained. 5. Conclusions Radiation profile of tile-hole containing medical isotope waste. Radiation profile of tile-hole containing low level strontium waste. Nuclear waste storage facilities at CNL use below-ground tile-holes to store solid waste from a number of nuclear operations. It is always desirable to move waste to a more appropriate location if possible. Radiation field measurements performed along the depth of occupied tile-holes can verify administrative records and identify low level waste for transfer. A diode-based gamma radiation scanning system has been developed and used to profile the gamma radiation fields along the depth of solid nuclear waste storage tile-holes at CNL. The sensor assembly consists of the sensor itself, a signal transmission cable, cable connector, and a PolyFlo sheath for the cable. The sensor is deployed inside a stainless-steel guide tube to provide a straight path free of obstacles and an additional layer of protection from loose contamination. The directionality, sensitivity, and linearity of the sensor response have been well studied and documented. 9

10 The diode sensor system was used to generate radiation field profiles along the depth of multiple waste storage tile-holes at CNL s WMA-B in Chalk River. The radiation measurements were executed by deploying the sensor into verification tubes adjacent to the tile-holes. The measurements were consistent with expected radiation fields in the tile-holes based on administrative knowledge of the contents. This technique hasproventobeabletoaccuratelydeterminethenearcontact radiation fields of low level tile-hole contents to assess the possibility of withdrawal and transfer, to extend the use of existing tile-holes. This system could also be used at other locations at CNL to measure the contents of tileholes or any space or infrastructure requiring a radiation profile with limited access with unknown gamma fields. REFERENCES [1] Atomic Energy of Canada Limited, 1997, Canada Enters the Nuclear Age, McGill-Queen s Press, Montreal, QC, Canada. [2] M. Bruzzi, 2001, Radiation Damage in Silicon Detectors for High-Energy Physics Experiments, IEEE Transactions on Nuclear Science, 48(4), pp doi: / [3] S.C. Klevenhagen, 1997, The Non-Linearity of the Temperature Response of Silicon P-N Junction Radiation Detectors Operated in the DC Mode, Physics in Medicine & Biology, 22, pp doi: / /22/2/015. [4] B. Sur, S. Yue, and A. Thekkevarriam, 2007, Radiation Exposure Rate and Liquid Level Measurement Inside a High Level Liquid Waste (HLLW) Storage Tank, 28th Annual Conference of the Canadian Nuclear Society, Saint John, NB, Canada, 3 6 June 2007, Canadian Nuclear Society, Toronto, ON, Canada. [5]B.Sur,S.Yue,andG.Jonkmans,2009, A DetectorSystemFor Measuring High Radiation Fields, Sixth American Nuclear Society International Topical Meeting on Nuclear Plant Instrumentation, Control, and Human-Machine Interface Technologies (NPIC&HMIT2009), Knoxville, TN, USA, 5 9 April 2009, American Nuclear Society, La Grange, IL, USA. [6] A.Das,S.Yue,B.Sur,J.Johnston,M.Gaudet,M.Wright,etal.,2010, Gamma Radiation Scanning of Nuclear Waste Storage Tile-Holes, 31st Annual Conference of the Canadian Nuclear Society, Montreal, QC, Canada, May [7] A. Holmes-Siedle and L. Adams, 2002, Handbook of Radiation Effects, 2nd ed., Oxford University Press, New York, NY, USA. 10

inverse collimator-based radiation imaging detector system

inverse collimator-based radiation imaging detector system TECHNICAL NOTE Abstract A radiation imaging system has been developed using the concept of inverse collimation, where a narrow shielding pencil is used instead of a classical collimator. This imaging detector