Operator Generic Fundamentals Components - Sensors and Detectors 2

Operator Generic Fundamentals Components - Sensors and Detectors 2

Terminal Learning Objectives
At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80 percent on the following Terminal Learning Objectives (TLOs): Describe the operation of radiation detectors and conditions which effect their accuracy and reliability. Describe the operation of personal radiation monitoring instruments and conditions which effect their accuracy and reliability. Describe the operation of neutron detectors and conditions which effect their accuracy and reliability. TLOs

Radiation Detectors TLO 1 – Describe the operation of radiation detectors and conditions that effect their accuracy and reliability. 1.1 Describe the following radiation detection concepts and terms: Electron-ion pair Specific ionization Stopping power Alpha (α) Beta (β) Gamma (γ) Neutron (n) 1.2 Describe the theory of operation of a gas-filled detector to include: How electric field affects ion pairs How gas amplification occurs Name the regions of the gas amplification curve Describe the interactions taking place within the gas of the detector Describe the difference between alpha and beta curves It is important to have an understanding of how these sensors and detectors measure plant parameters and how they are prone to failure. Recognizing the indications associated with failed sensors and detectors is an essential skill for plant operators. Familiarity with instrument failure modes will ensure proper interpretation of plant parameters during abnormal operating events, allowing operators to take appropriate mitigating actions. TLO 1

Enabling Learning Objectives for TLO 1
1.3 Describe the operation of a proportional counter to include: Radiation detection Quenching Voltage variations 1.4 Given a block diagram of a proportional counter circuit, state the purpose of the following major blocks: Proportional counter Preamplifier/amplifier Single channel analyzer/discriminator Scaler Timer TLO 1

Enabling Learning Objectives for TLO 1
1.5 Describe the operation of an ionization chamber to include: Radiation detection Voltage variations Gamma sensitivity reduction 1.6 Describe how a compensated ion chamber compensates for gamma radiation. 1.7 Describe the operation of a Geiger-Mueller (GM) detector to include: Quenching Positive ion sheath 1.8 Describe the operation of a scintillation counter to include: Three classes of phosphors Photomultiplier tube operation TLO 1

Radiation Detection Concepts
ELO 1.1 – Describe the following radiation detection concepts and terms: electron-ion pair, specific ionization, stopping power, alpha (α), beta (β), gamma (γ), and neutron (n). Radiation detectors sense the presence and level of radiation Also determine power level of the reactor Radiation results from Fission Activation of particles exposed to the neutron flux of the reactor Provides indication, alarms, and input for automatic functions Radiation detection is important because of the effect that radiation has on personnel and equipment Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.1

Electron-Ion Pair Ionization is process of converting an atom or molecule into an ion by adding or removing electrons Products of a single ionizing event are called an electron-ion pair Positive ion Negative electron Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.1

Specific Ionization Number of ion pairs formed by a given type of radiation as it travels through matter, dependent on Mass – the greater the mass, the more interactions per given distance Charge – has the greatest effect on specific ionization Higher charge increases number of interactions per given distance Increasing number of interactions produces more ion pairs Energy of the particle – as energy of a particle decreases, it produces more ion pairs for the same amount of distance traveled Electron density of matter – increased density increases the number of interactions ELO 1.1

Stopping Power Energy lost per unit path length depends on type and energy of particle and on properties of material it passes The more ion pairs produced per unit of distance travelled, the quicker the stopping power (shorter the distance travelled) Alphas (++) travel shorter distances than betas (-) A related concept is “quality factor”; the high linear energy transfer of the alpha is more likely to damage a given cell, so a gray (100 rads) of alpha does 20 times the damage as a gray of gammas, 2,000 rem compared to 100 rem. ELO 1.1

Radiation Types Alpha Particle
Consists of two protons and two neutrons bound together into a particle identical to a helium nucleus Highly ionizing form of particle radiation with low penetration Produced from radioactive decay of heavy metals and some nuclear reactions Specific ionization of an alpha particle is high Tens of thousands of ion pairs per centimeter in air Travels a relatively straight path over a short distance ELO 1.1

Radiation Types Beta Particle
Electron or positron ejected from the nucleus of a beta-unstable radioactive atom Single negative or positive electrical charge and a very small mass High-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei ELO 1.1

Radiation Detector Video
Radiation Types Gamma Ray Photon of electromagnetic radiation with a very short wavelength and high energy Emitted from an unstable atomic nucleus with high penetrating power Three methods of attenuating gamma-rays: Photoelectric effect Compton scattering Pair production Link to Video: Play video and request feedback at the end of the video, hold questions until the end of the ELO. Radiation Detector Video ELO 1.1

Radiation Types - Gamma Attenuation
Photoelectric Effect Occurs when a low energy gamma strikes an orbital electron Total energy of gamma is expended in ejecting electron from its orbit Result is ionization of atom and expulsion of a high-energy electron Most predominant with low- energy gammas Figure: Photoelectric Effect ELO 1.1

Compton Scattering Elastic collision between an electron and a photon Photon has more energy than is required to eject the electron from orbit All energy from photon cannot be transferred, photon must be scattered Result is a high energy beta a gamma of lower energy Figure: Compton Scattering ELO 1.1

Pair Production High energy gamma passes close enough to a heavy nucleus Gamma disappears, energy reappears in form of electron and positron Transformation of energy into mass must take place near a particle to conserve momentum Kinetic energy of recoiling nucleus is very small Excess photon energy appears as kinetic energy of pair Sometimes the electron and positron spiral into each other (attractive force), resulting in another gamma. Figure: Pair Production ELO 1.1

Radiation Types – Neutron
Have no electrical charge Nearly the same mass as a proton Hundreds of times larger than an electron, but one quarter the mass of an alpha particle Source is primarily nuclear reactions Fission Decay of radioactive elements ELO 1.1

Radiation Types – Neutron
Difficult to stop with relatively high penetrating power May collide with nuclei causing one of the following reactions: Inelastic scattering Elastic scattering Radiative capture or fission Special surveys needed for “streams” during containment entries at power. ELO 1.1

Detector Theory of Operation
ELO 1.2 – Describe the theory of operation of a gas-filled detector to include: how electric field affects ion pairs, how gas amplification occurs, name the regions of the gas amplification curve, describe the interactions taking place within the gas of the detector, and describe the difference between alpha and beta curves. Instruments measuring radiation provide a measurement of dose or dose rate Dose is a total accumulated exposure Dose rate is the amount of exposure per unit of time Radiation detectors detect a specific type(s) or energy range Detectors use ionization and electron pairs produced to measure the radiation energy Most detector designs have Positive probe – collects electrons (negative ions) Negative can – collects positive ion Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.2

Gas-Filled Detector Gas-filled cylinder with two electrodes
Cylinder itself may act as one electrode Thin wire along axis of cylinder acts as other electrode Gases used since their ionized particles can travel more freely than those of a liquid or a solid Typical gases used are: Argon Helium Boron-tri-fluoride used to measure neutrons ELO 1.2

Gas-Filled Detector Central electrode, or anode, collects negative charges Anode is insulated from chamber walls and cathode which collects positive charges Voltage is applied to the anode and chamber walls Charged particle passing through gas-filled chamber Ionizes some of gas along its path of travel C Figure: Gas-Filled Detector Diagram ELO 1.2

Gas-Filled Detector Positive anode attracts electrons, or negative particles Detector wall, or cathode, attracts the positive charges Collection of these charges reduces voltage across capacitor Causing pulse across resistor that is recorded by an electronic circuit Voltage applied to anode and cathode determines electric field and its strength C Figure: Gas-Filled Detector Diagram ELO 1.2

Gas-Filled Detector As detector voltage is increased, electric field has more influence upon electrons produced Sufficient voltage causes a cascade effect that releases more electrons from cathode Forces on electron are greater and its mean-free path between collisions is reduced at this threshold Total number of electrons collected by anode determines change in charge of the capacitor C Figure: Gas-Filled Detector Diagram ELO 1.2

Gas-Filled Detector Change in charge is directly related to total ionizing events which occur in gas Ion pairs initially formed by incident radiation attain a great enough velocity to cause secondary ionization of other atoms or molecules in gas Resultant electrons cause further ionizations Multiplication of electrons is gas amplification C Figure: Gas-Filled Detector Diagram ELO 1.2

Detector Voltage Applied Voltage
Pulse height and number of ion pairs collected are directly related Ion pairs collected versus applied voltage Two curves are shown (next slide): one curve for alpha particles and one curve for beta particles (for example); each curve is divided into several voltage regions Alpha curve is higher than the beta curve from Region I to part of Region IV due to the larger number of ion pairs produced by the initial reaction of the incident radiation Alpha particle will create more ion pairs than a beta since alpha has a much greater mass Keep in mind that the curve is shown with two curves to distinguish the difference in pulses of alphas and betas (alphas are larger). A similar relationship will be shown in a later slide for the pulse height of a neutron versus a gamma in the source range detector (neutron pulse is larger). ELO 1.2

Detector Voltage Recombination Region (Region I)
At voltages less than V1 Ions move slowly toward electrodes Ions tend to recombine to form neutral atoms or molecules Not all “primary” ionizations collected Few detectors used in this region The “compensating” section of the old Westinghouse Intermediate Range detectors operated in this range. Most have been replaced with Fission Chambers operating in the Ionization range. Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Ionization Region (Region II)
Ion pairs collected relatively constant from V1 to V2 Voltage high enough to collect all “primary” ionizations Least sensitive region Most accurate region Used for radiation detectors Intermediate and Power Range neutron detectors Newer Fission Chamber Source Ranges also operate here. Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Proportional Region (Region III)
Ion pairs collected increases linearly as voltage increases Increased voltage imparts high velocity to electrons High velocity electrons cause gas amplification Secondary ionizations Gas amplification factor is proportional to applied voltage In this region ALL of the primary ionizations are still collected, but a proportionally higher amount of secondary ionizations are collected as voltage is raised. Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Proportional Region (Region III)
Gas-filled detectors operating at high voltages within the proportional region have effect called the positive space charge Pulse amplitude from an ionizing event is reduced because positive ions form a cloud around the positive electrode reducing the electric field strength limiting secondary ionizations Yields less accurate neutron count rate ELO 1.2

Detector Voltage Limited Proportionality Region (Region IV)
Additional secondary ionizations due to higher voltage Change not proportional to voltage Causes Townsend avalanche to spread along anode Not used for detector operation Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Geiger-Mueller Region (Region V)
Pulse height is independent of type of radiation Cannot distinguish between types of particles Entire can ionizes when any particle is detected Most sensitive region Least accurate Some dead time while quench gas clears detector V4 is the threshold voltage Where number of ion pairs level off and remain relatively independent of the applied voltage This is called the Geiger plateau which extends over a region of 200 to 300 volts Threshold normally ~1000 volts Gas amplification factor depends on specific ionization of radiation to be detected Figure: Gas Amplification Curve ELO 1.2

Detector Voltage Continuous Discharge Region (Region VI)
Steady discharge current flows Applied voltage is so high that once ionization takes place in the gas, there is a continuous discharge of electricity Detector cannot be used for radiation detection Figure: Gas Amplification Curve ELO 1.2

Proportional Counter Theory
ELO Describe the operation of a proportional counter to include: Radiation detection, Quenching, Voltage variations. Uses a slightly higher voltage between anode and cathode Primary ionizations cause secondary ionizations Electrons move towards positive anode Positive ions move towards negative cathode (can) Each pulse corresponds to one gamma ray or neutron interaction Number of electrons produced is proportional to energy of incident particle Figure: Proportional Counter Instrument Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.3

Proportional Counter Linear relationship between the number of ion pairs collected and the applied voltage. Can detect: alpha, beta, gamma, or neutron radiation in mixed fields fill gas will determine what type of radiation will be detected Argon and helium are the most frequently used fill gases allow for the detection of alpha, beta, and gamma radiation When detection of neutrons is necessary detectors use boron trifluoride gas ELO 1.3

Proportional Counter Circuit
ELO 1.4 – Given a block diagram of a proportional counter circuit, state the purpose of the following major blocks: proportional counter, preamplifier/amplifier, single channel analyzer/discriminator, scaler, and timer. Proportional counters measure charge produced by each particle of radiation To make full use of the counter’s capabilities, it is necessary to measure the number of pulses and the charge in each pulse Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.4

Proportional Counter Circuitry
Capacitor converts charge pulse to a voltage pulse Voltage is equal to amount of charge divided by capacitance of capacitor Preamplifier amplifies voltage pulse Amplifier circuit further amplifies signal Single channel analyzer determines pulse size Figure: Proportional Counter Circuit ELO 1.4

Proportional Counter Output passes to a scaler that counts number of pulses it receives A timer gates the scaler so it counts pulses for a predetermined length of time Knowing number of counts per a given time interval allows calculation of the count rate (number of counts per unit time) Proportional counters can also count neutrons by introducing boron into the chamber Most commonly by combining it with tri-fluoride gas to form boron tri-fluoride When a neutron interacts with a boron atom, an alpha particle is emitted Counter can be made sensitive to neutrons and not to gamma rays with the discriminator ELO 1.4

Proportional Counter Gamma rays can be eliminated because neutron-induced alpha particles produce more ionizations than gamma rays produce Gamma ray-induced electrons have a much longer range than dimensions of the chamber Alpha particle energy is greater than gamma rays produced in a reactor Neutron pulses are much larger than gamma ray-produced pulses ELO 1.4

Proportional Counter Using a discriminator, the scaler can be set to read only larger pulses produced by a neutron A discriminator is basically a single channel analyzer with only one setting Figure: Discriminator Characteristics ELO 1.4

Proportional Counter Circuit
Knowledge Check – NRC Bank A BF3 proportional counter is being used to measure neutron level during a reactor startup. Which of the following describes the method used to ensure that neutron indication is not being affected by gamma reactions in the detector? Two counters are used, one sensitive to neutron and gamma and the other sensitive to gamma only. The outputs are electrically opposed to cancel the gamma-induced currents. The BF3 proportional counter measures neutron flux of sufficient intensity that the gamma signal is insignificant compared to the neutron signal. In a proportional counter, gamma-induced pulses are of insufficient duration to generate a significant log-level amplifier output. Only neutron pulses have sufficient duration to be counted by the detector instrumentation. In a proportional counter, neutron-induced pulses are significantly larger than gamma pulses. The detector instrumentation filters out the smaller gamma pulses. Correct answer is D. Correct answer is D. Bank Question P16 Analysis: When a neutron interacts with BF3, a positively charged lithium and alpha particle are emitted and accelerate toward the negatively charged cathode, along with several electrons, which are accelerated towards the positively charged anode. The acceleration of the ions creates secondary and tertiary ionizations. This process is known as gas amplification and explains why detector current increases exponentially with voltage in the proportional region. Gamma radiation also will ionize the BF3 gas and produce smaller pulses than neutron interactions. In the proportional region neutron-induced pulses are significantly larger than gamma pulses. The source range nuclear instrument filters out the smaller gamma pulses through a process known as Pulse Height Discrimination. ELO 1.4

Ionization Chamber ELO 1.5 – Describe the operation of an ionization chamber to include: radiation detection, voltage variations, and gamma sensitivity reduction. Detect radiation when voltage is adjusted to ionization region Charge obtained is result of collecting ions produced by radiation This charge will depend on type of radiation being detected Two distinct disadvantages compared to proportional counters Less sensitive Slower response time Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.5

Ionization Chamber Flat plates or concentric cylinders may be used in an ionization chamber Flat plates preferred due to: Well-defined active volume Ensures ions will not collect on insulators causing distortion of electric field Ionization chamber construction allows for integration of pulses produced by incident radiation ELO 1.5

Ionization Chamber Use relatively low voltage between anode and cathode Only charges produced in initial ionization event collected Weak output signal corresponds to number of ionization events Higher energies and intensities of radiation will produce more ionization Results in a stronger output voltage Figure: Simple Ionization Circuit ELO 1.5

Ionization Chamber Beta particles will pass between plates and strike atoms in air Sufficient energy beta particles cause an electron ejection from air A beta particle may eject 40 to 50 electrons for each cm traveled Ejected electrons often have enough energy to eject more electrons from the air Total number of electrons produced is dependent Energy of beta particle Energy of gas between plates ELO 1.5

Ionization Chamber Can be used to detect gamma rays
Ammeter only sensitive to electrons; gamma rays must interact with the atoms in air between the plates to release electrons via: Compton scattering, Photoelectric effect, Pair production Energy of incident gamma converted into kinetic energy of ejected electrons Ejected electrons move at very high speeds and cause other electrons to be ejected from their atoms Electrons collected by positively charged plate and measured by ammeter ELO 1.5

Ionization Chamber Can be used to detect neutrons
Neutrons have no charge, therefore cause no ionizations Inner surface of ionization chamber is covered with a thin coat of boron The following reaction can takes place: Neutron is captured by a boron atom and an energetic alpha particle is emitted Detectors in this region are not affected by small changes in voltage Constant output for varying voltage ELO 1.5

Ionization Chamber Neutrons may also be detected using gas boron tri-fluoride (BF3) instead of air in the ion chamber Neutrons react with boron to produce alpha particles When detecting neutrons Beta particles shielded by detector walls Gamma rays cannot be shielded Discrimination can eliminate gamma Reducing sensitive volume of chamber without reducing boron coated area, reduces sensitivity to gammas ELO 1.5

Compensated Ion Chamber
ELO 1.6 – Describe how a compensated ion chamber compensates for gamma radiation. Consists of two separate chambers One chamber is coated with boron Other chamber is not coated Coated chamber is sensitive to both gamma rays and neutrons Uncoated chamber is sensitive only to gamma rays Net output of both detectors is read on a single ammeter Polarities arranged so currents oppose one another Reading indicates difference between the two currents Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.6

This is the old Westinghouse CIC Intermediate Range. The compensating (unlined) chamber operates in the recombination range, so its output is determined by Vb. Figure: Compensated Ion Chamber ELO 1.6

Boron coated chamber is the working chamber Uncoated chamber is the compensating chamber When exposed to a gamma source: Working chamber battery sets up current flow that deflects meter in one direction Compensating chamber battery sets up current flow that deflects meter in opposite direction Compensating chamber cancels current due to gamma rays Compensation required for Intermediate Range NIs Neutron population relatively low When operating in Power range Neutron to gamma ratio so high, no compensation required OE; if the compensating voltage is not correct for the time in life, the net reading will be wrong. This has frequently caused the IR reading to “hang up” high enough to require manually energizing the SR about 15 minutes after a Rx trip. ELO 1.6

Geiger-Mueller Detector
ELO Describe the operation of a Geiger-Mueller (GM) detector to include: radiation detection, quenching, and positive ion sheath. GM detectors produce larger pulses than other types of detectors Discrimination is not possible Pulse height is independent of the type of radiation Geiger-Mueller region has two important characteristics: Number of electrons produced is independent of applied voltage Number of electrons produced is independent of the number of electrons produced by the initial radiation Radiation producing one electron will have same size pulse as radiation producing hundreds or thousands of electrons Reason for this characteristic is related to the way in which electrons are collected K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors K1.19 Use of portable and personal radiation monitoring instruments Counting systems that use GM detectors are not as complex as those using ion chambers or proportional counters ELO 1.7

Gamma produces an electron Electron moves rapidly toward positively charged central wire As electron nears wire, its velocity increases Velocity great enough to cause secondary ionizations Larger pulse C Figure: GM Detector ELO 1.7

As applied voltage is increased, number of positive ions near central wire increases Positively charged cloud (called a positive ion sheath) forms around central wire Positive ion sheath reduces field strength of central wire, preventing further electrons from reaching wire Positive ion sheath makes the central wire appear much thicker and reduces field strength C Figure: GM Detector ELO 1.7

Phenomenon is the detector’s space charge Positive ions migrate toward negative chamber picking up electrons As in a proportional counter, transfer of electrons can release energy Causing ionization and liberation of an electron To prevent secondary pulse, quenching gas is used Usually an organic compound C Figure: GM Detector ELO 1.7

Geiger-Mueller Detector Summary
GM counter produces many more electrons than proportional counter More sensitive device Often used to detect low-level gamma rays and beta particles Electrons produced collected rapidly, usually within fraction of microsecond Output is pulse charge large enough to drive meter without amplification Cannot distinguish radiation of different energies or types Same size pulse is produced regardless of amount of initial ionization Not adaptable for neutron detectors Used for portable instrumentation due to: Sensitivity Simple counting circuit Ability to detect low-level radiation ELO 1.7

Knowledge Check – NRC Bank Which one of the following describes the reason for the high sensitivity of a Geiger-Mueller tube radiation detector? Changes in applied detector voltage have little effect on detector output. Geiger-Mueller tubes are thinner than other radiation detector types. Any incident radiation event causing primary ionization results in ionization of the entire detector gas volume. Geiger-Mueller tubes are operated at relatively low detector voltages, allowing detection of low energy radiation. Correct answer is C. Correct answer is C. NRC Bank Question – P215 Analysis: A. WRONG. Even though detector voltage changes have minimal impact on detector output, this is NOT why GM tubes are more sensitive. B. WRONG. Geiger-Mueller tubes are generally thicker per-unit length. C. CORRECT. In the Geiger-Mueller region, the detector voltage is so great that a single ionizing causes the entire tube to become ionized (regardless of that particle’s energy). Even though GM detectors cannot discriminate against gamma or neutron pulses, they are very sensitive to low levels of radiation. D. WRONG. A Geiger-Mueller operates at several thousand volts and cannot discriminate between types of incident radiation. ELO 1.7

Scintillation Detector
ELO 1.8 – Describe the operation of a scintillation detector. Uses a scintillation crystal (phosphor) to detect radiation and produce light pulses Process called “luminescence” Radiation interacts in scintillation crystal energy transferred to bound electrons of the crystal’s atoms If a photon or beta particle hits the crystal, it produces visible light Related KA - K1.18 Theory and operation of ion chambers, GM tubes and scintillation detectors ELO 1.8

A photomultiplier tube senses flashes converts them into an electrical signal Constructed by coupling a suitable scintillation phosphor to a light- sensitive photomultiplier tube Figure: Scintillation Detector ELO 1.8

Photomultiplier is a vacuum tube containing A photocathode Series of electrodes called dynodes Light from a scintillation phosphor liberates electrons Photoelectrons strike first dynode and liberate several new electrons Second-generation electrons are attracted to second dynode Amplification continues through 10 to 12 stages Large enough pulse to measure Figure: Photomultiplier ELO 1.8

Scintillation detector advantages Efficiency High precision High counting rates Can be used to determine the energy, as well as the number of the exciting particles (or gamma photons) Photomultiplier tube output is very useful in radiation spectrometry Determination of incident radiation energy levels The amount of light produced is proportional to the energy of the incoming particle. ELO 1.8

Knowledge Check – NRC bank Scintillation detectors convert radiation energy into light by a process known as... gas amplification. space charge effect. luminescence. photoionization. Correct answer is C. Correct answer is C. NRC Bank Question – P15 Analysis: A. WRONG. Radiation interacts with phosphors, not BF3. B. WRONG. Radiation interacts via luminescence, not space charge effect. Space charge effect is when the output is diminished by positive ions hanging around positive probe capturing electrons. C. CORRECT. Scintillation detectors operate on the principle that when phosphors encounter radioactive particles, electrons within the phosphor are elevated to a higher valence level. This energy is remitted as visible light (this concept is known as luminescence), which is converted to an electrical signal by a type of semiconductor known as a photocathode. This signal is then multiplied by the photo multiplier tube, and the count rate can be measured. D. WRONG. Photo-ionization radiation detectors are used to detect ultraviolet radiation and have limited application at nuclear power plants. ELO 1.8

Knowledge Check – NRC Bank Which one of the following contains the pair of radiation detector types that are the most sensitive to low-energy beta and/or gamma radiation? Geiger-Mueller and scintillation Geiger-Mueller and ion chamber Ion chamber and scintillation Ion chamber and proportional Correct answer is A. Correct answer is A. NRC Bank Question - P4906 Analysis: By process of simple elimination, Choice “A” would have to be correct because ion chambers operate at the lowest voltage of all detectors and are therefore the least sensitive. GM tube operate at high voltages resulting in gas amplification (secondary ionizations. Scintillation detectors have photo-multiplier tubes that also cause secondary ionizations. The larger the pulse, the higher the sensitivity. ELO 1.8

Personnel Radiation Monitoring
TLO 2 – Describe the operation of personal radiation monitoring instruments and conditions which effect their accuracy and reliability. 2.1 Describe the use of portable personnel radiation monitoring instruments. 2.2 Describe the operation of the following personnel radiation detection devices, including advantages and disadvantages of each: Thermoluminescent dosimeter (TLD) Self-reading pocket dosimeter (SRPD) Electronic dosimeter Film badge K1.19 Use of portable and personal radiation monitoring instruments TLO 2

Portable Radiation Monitoring
ELO 2.1 – Describe the use of portable personnel radiation monitoring instruments. Ensure you are qualified to use portable personnel monitoring instruments Prior to using, verify it is working properly Each survey instrument is required to be calibrated Check calibration due date on the instrument calibration sticker If calibration has expired inform health physics personnel and/or shift supervisor Review the detector video shown earlier below, if desired: Related KA - K1.19 Use of portable and personal radiation monitoring instruments Radiation Detector Video ELO 2.1

Portable Detector Use Instrument should be visually inspected for damage or defects Cords should be in good shape, not kinked or frayed Probe should be intact Indicating scale indicates a reasonable background reading Battery strength should be verified high enough for proper operation Accomplished by placing the meter in the battery check position If battery check not satisfactory, meter should not be used Battery check not required if instrument connected to AC power source ELO 2.1

Portable Detector Use Verify source check current
Most meters are source checked by health physics ensures proper operation and indication within a specified range A sticker on the detector may indicate the last source check ELO 2.1

Dosimetry and Types of Radiation Detected
ELO 2.2 – Describe the operation of the following personnel radiation detection devices, including advantages and disadvantages of each: thermoluminescent dosimeter, self-reading pocket dosimeter, electronic dosimeter, and film badge. Thermoluminescent dosimeter Normally detect gamma and neutron radiation accumulated doses Can detect beta, depending on construction Self-reading pocket dosimeter detects gamma radiation Electronic dosimeter detects gamma and x-ray radiation Film badges measures and records gamma rays, x-rays, and beta particles ELO 2.2

Thermoluminescent Dosimeter
Two types of dosimeter of legal record (DLR) currently used Thermoluminescent dosimeter (TLD) Optically stimulated luminescent dosimeter (OSLD) Measures ionizing radiation exposure by Heating a crystal in the detector Measuring the amount of visible light emitted from the crystal Amount of light emitted is dependent upon the radiation exposure Calcium fluoride crystal records gamma exposure Lithium fluoride crystal records gamma and neutron exposure Related KA - K1.19 Use of portable and personal radiation monitoring instruments Note: Many plants have switched to or are using OSLDs, which use aluminum oxide to absorb the radiation energy and a laser rather than heat to release the stored energy and measure the amount of ionizing radiation received (remaining material will focus on TLDs) Figure: Typical TLD ELO 2.2

Radiation interacts with crystal Electrons in the crystal's atoms to jump to higher energy states Electrons trapped due to impurities in crystal Usually manganese or magnesium When crystal is heated, electrons give up stored energy Trapped electrons to drop back to their ground state The energy is released in a photon Equal to energy difference between trap state and ground state Released light is counted using photomultiplier tubes Number of photons counted is proportional to quantity (energy) of radiation ELO 2.2

Used for Environmental monitoring Personnel exposure monitoring Often worn for a period of time (3 months or less) Worn in chest area on trunk of the “whole body” for the body dose ELO 2.2

Advantages Linearity of response to dose Relative energy independence Sensitivity to low doses Reusable Disadvantages No permanent record or re-readability is provided Immediate on spot reading not possible ELO 2.2

Self-Reading Pocket Dosimeter
Measures dose, not dose rate Usually worn in conjunction with (and near) a TLD or OSLD Chest area to measure whole body dose Contains a small ionization chamber Central wire anode in ionization chamber Moveable quartz fiber attached to wire anode Anode charged to a positive potential Charge is distributed between the wire anode and quartz fiber Electrostatic repulsion deflects quartz fiber Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.2

Gamma radiation in chamber produces ionization Alpha and beta particles cannot pass through metal casing Positively charged central anode attracts electrons produced by ionization Electrons reduce net positive charge Moveable quartz fiber moves toward original position Movement is directly proportional to amount of ionization Figure: Direct Reading Dosimeter and Charger Does not contain a window thin enough to measure betas ELO 2.2

Point a light source to read position of the fiber Fiber is viewed on a translucent scale Reads up to 200 milliroentgens Some designs higher scale The Roentgen is an obsolete unit of charge. One Rad of energy generates about one Roentgen of charge in air. One Rad/Roentgen of 1 MeV gammas causes one Rem of biological damage. New metric unit of energy and damage are the Gray and the Sievert respectively. Figure: Pocket Dosimeter Internals ELO 2.2

Advantages Provides immediate reading of radiation accumulated dose Reusable Disadvantages Limited range Does not provide a permanent record Can be discharged by dropping or bumping Charge leakage, or drift, can also affect accuracy If dropped, exit area immediately and inform health physics Recall last good reading and time Must be charged with DC voltage to “zero” the device prior to use ELO 2.2

Knowledge Check – NRC Bank A nuclear plant worker normally wears a thermoluminescent dosimeter (TLD) or similar device for measuring radiation exposure. When a self- reading pocket dosimeter (SRPD) is also required, where will the SRPD be worn and why? Below the waist near the TLD to measure radiation from the same source(s). Below the waist away from the TLD to measure radiation from different sources. Above the waist near the TLD to measure radiation from the same source(s). Above the waist away from the TLD to measure radiation from different sources. Correct answer is C. Correct answer is C. NRC Bank Question – P6806 Analysis: Self reading pocket dosimeter (SRPD)’s should be work above the waist to accurately measure radiation to the chest, representative of the whole body, not the extremities. SRPD’s should also be worn within 6 inches (if possible) to the TLD to ensure the SRPD measures radiation from the same source as the TLD. ELO 2.2

Knowledge Check – NRC Bank Which one of the following types of radiation is the major contributor to the dose indication on a self‑reading pocket dosimeter? Alpha Beta Gamma Neutron Correct answer is C. Correct answer is C. Bank Question P714 Analysis: Gammas are the most highly penetrating type of radiation listed, thus penetrate the aluminum “can” and cause ionization inside the chamber. Alphas (and most Betas) would be shielded by the aluminum case. Neutrons are non-charged particles and therefore require some sort of medium (BF3) to cause ionizations. ELO 2.2

Electronic Dosimeter Another type of pocket dosimeter (replaced SRPDs)
Records dose information and dose rate Constructed using a Geiger-Mueller counter that measures Gamma X-ray Digital counter displays Accumulated exposure Dose rate Related KA - K1.19 Use of portable and personal radiation monitoring instruments ELO 2.2

Electronic Dosimeter Can include an audible alarm feature
Can provide a continuous audible signal when a preset exposure has been reached Allows higher maximum readout before resetting is necessary Advantages Minimizes reading errors associated with direct reading pocket dosimeters Reliability Ability to indicate accumulated dose as well as dose rate Audible response Disadvantages Much higher cost than SRPDs ELO 2.2

Film Badges Film badges no longer commonly used for personnel monitoring Used to measure and record radiation exposure or accumulated dose due to: Gamma rays X-rays Beta Contains a piece of radiation- sensitive film Film packaged in a light proof, vapor proof envelope Prevents light, moisture or chemical vapors from affecting film Figure: Film Badge Related KA - K1.19 Use of portable and personal radiation monitoring instruments The “CR39” solid detector mounted in some TLDs functions similarly for high range (accident) gamma and neutron doses. ELO 2.2

Film Badges Special film used which is coated with two different emulsions One side coated with a large grain fast emulsion Sensitive to low levels of exposure Other side coated with a fine grain slow emulsion Less sensitive to exposure ELO 2.2

Film Badges Advantages Provides a permanent record
Able to distinguish between different energies of photons and can measure doses due to different types of radiation Accurate for exposure greater than 100 millirem Disadvantages Third party must develop it because a processor must read it Prolonged heat exposure can affect the film Exposures of less than 20 millirem of gamma radiation cannot be accurately measured Can be very costly and inaccurate at lower doses ELO 2.2

Nuclear Instrument Detectors
TLO 3 – Describe the operation of neutron and failed fuel detectors and conditions which effect their accuracy and reliability. 3.1 Describe the purpose and operation of the following nuclear instruments: Source Range Intermediate Range Power Range Fission chamber 3.2 State the effect core voiding, core loading pattern, and environmental effects could have on neutron detection and power indication. 3.3 Describe the theory and operation of failed fuel detectors. K1.17 Effects of core voiding on neutron detection K1.18 Theory and operation of ion chambers, G-M tubes and scintillation detectors K1.20 Theory and operation of failed-fuel detectors Nuclear instrumentation is important to be able to provide power level indication in all ranges of power operations from start up to full power. Instruments also provide alarms and trips based on power measurements. TLO 3

ELO 3.1 – Describe the purpose and operation of the following nuclear instruments: Source Range, Intermediate Range, Power Range, Fission chamber. A PWR generally has two types of neutron detection systems installed: Out-of-core (Excore) NIs – used for reactor power monitoring and reactor protection and control Source, Intermediate, and Power ranges Detect neutron leakage Secondary calorimetric used to calibrate from BOC to EOC In-core (Incore) NIs – used for flux mapping to determine hot channel factors May be movable or stationary Fission chamber Neutron detector types and regions of operation might vary between plant types. This section describes the “generic” types used by most plants. If your plant type varies, you can discuss as necessary. ELO 3.1

Three ranges are used to monitor the power level of a reactor throughout the full range of reactor operation Source range Intermediate range Power range Source range normally uses a proportional counter Intermediate and power ranges use ionization chambers Compensated ion chamber for intermediate range Uncompensated ion chamber for power range Neutron to gamma ratio so high that gammas are insignificant ELO 3.1

Ranges overlap Proper overlap monitored as power increases Source Range Counts Per Second (CPS) Intermediate Range Amps (or percent power) Power Range Percent power Figure: Three Range Overlap ELO 3.1

Nuclear Instrument Detectors - SR
Monitor/indicate when reactor is shutdown and initial phase of reactor startup Neutron flux level Rate of change of neutron flux level Normally consists of two redundant count rate channels Composed of a high-sensitivity proportional counter Associated signal measuring equipment Typically used over a counting range of 0.1 to 106 counts per second Output displayed on meters in terms of the logarithm of the count rate Measures rate of change of the count rate as well Startup rate or reactor period ELO 3.1

Protective functions and interlocks Some plants have High Flux Trip on SR counts Allows blocking High Flux Trip on normal reactor startup When blocked might de-energize high voltage to detector extend detector life Some reactor designs allow source range detectors to be moved from normal operating positions once the flux level increases above the source range ELO 3.1

B10 lined or BF3 gas-filled proportional counters are normally used as source range detectors Proportional counter output is in form of one pulse for every ionizing event Series of random pulses varying in magnitude representing neutron and gamma ionizing events Figure: Source Range Channel ELO 3.1

Pulse height may only be a few millivolts Too low to be directly used without amplification Linear amplifier amplifies input signal by a factor of several thousand Pulse Height Discriminator excludes passage of pulses that are < a certain level excludes noise and gamma pulses ensures only neutrons are counted Pulses sent to pulse integrator where they are integrated to give a signal that is proportional to logarithm of count rate ELO 3.1

Nuclear Instrument Detectors - IR
Usually two redundant channels Each channel made up of Boron-lined or boron gas-filled compensated ion chamber Compensated ion chamber (CIC) detector Compensates for signals from gamma flux Provides a rate of change measure of neutron level Displayed in terms of startup rate in DPM High startup rate on either channel may initiate a protective action May be a control rod withdrawal inhibit and alarm or a high startup rate reactor trip Compensation was discussed in an earlier ELO. ELO 3.1

Figure: Intermediate Range Detector ELO 3.1

Compensated ion chamber output is an analog current ranging from to amperes Instead of AMPS, can also be calibrated to read % power Log n amplifier is a logarithmic current amplifier that converts the detector output to a signal proportional to the logarithm of the detector current Logarithmic output is proportional to the logarithm of the neutron level Figure: Intermediate Range Detector ELO 3.1

Differentiator measures rate change of logarithm of neutron level Measures reactor period or startup rate Startup rate in intermediate range is more stable because neutron level signal is subject to less sudden large variations Often used as an input to the reactor protection system Reactor protective interface provides signals for protective actions Control rod withdrawal interlocks Startup rate reactor trips ELO 3.1

Nuclear Instrument Detectors - PR
Normally consists of four identical linear power level channels which originate in eight uncompensated ion chambers Usually 4 Upper and 4 Lower detectors Uncompensated ion chambers used because gamma compensation is unnecessary Neutron-to-gamma flux ratio is high Number of gammas is insignificant compared to number of neutrons Output of each channel is directly proportional to reactor power Typically covers a range from 0–125% of full power Output displayed on meter in terms of power level in percent of full rated power ELO 3.1

Gain of each instrument is adjustable, which provides a means for calibrating the output Adjustments normally determined by using a plant heat balance Protective actions may be initiated by high power level on any two channels Two detectors in each channel are functionally connected in parallel, so measured signal is sum of the two detectors Output drives linear amplifier which amplifies signal to useful level Figure: Power Range Channel ELO 3.1

Reactor protective interface provides signals for protective actions Protective action signals provided Signal to reactor protection system at a selected value (normally 10% reactor power) to disable the high startup rate reactor trip Signal to protective systems when reactor power level exceeds predetermined values Signal for use in the reactor control system Signal to the power-to-flow circuit ELO 3.1

Nuclear Instrument Detectors – Fission Chamber
Usually used for In-Core detector system May be moveable or fixed in-cores Moveable detectors take “snapshots” at various core heights Flux mapping Ensures core within thermal design limits (hot spot) Detector coated with highly enriched U-235 and an argon gas Neutrons in the core cause fissions in the detector Fission fragments ionize the argon gas Ionizations provide distinguishable pulses Systems vary between plants and plant types so this material is generic in nature. ELO 3.1

Nuclear Instrumentation Detectors
Knowledge Check – NRC Bank A proportional detector with pulse height discrimination circuitry is being used in a constant field of neutron and gamma radiation to provide source range neutron count rate indication. Assume the pulse height discrimination value does not change. If the detector voltage is decreased significantly, but maintained within the proportional region, the detector count rate indication will __________ and the detector will become __________ susceptible to the positive space charge effect. decrease; less decrease; more remain the same; less remain the same; more Correct Answer: A Correct answer in A. NRC Bank Question – P7613 Analysis: If the detector’s operating voltage is decreased, the detector output will decrease in the proportional region. Therefore, the amount of secondary ionizations will decrease. The positive space charge effect is when “larger” positive ions hang out near the positive probe resulting in capture of electrons and less negative electrons making it to the positive probe. This tends to lower the output reading. As a result of decreasing the voltage, there will be less positive charges hanging out near the positive probe resulting is less of a positive space charge effect. ELO 3.1

Core Voiding and Loading Effects
ELO 3.2 – State the effect core voiding, core loading pattern, and environmental effects could have on neutron detection and power indication. In PWR, neutron instrumentation used for power monitoring is located external to core measures neutron leakage from core Under normal operations power level, coolant temperature, boron concentration, and core enrichment loading can affect leakage If the flux at the core edge is affected neutron instrumentation may also be affected Voiding in the core will also impact reading Related KA - K1.17 Effects of core voiding on neutron detection TMI experienced confusion when core started to become uncovered. Basically they saw an increase in SR counts and thought some sort of positive reactivity event was occurring. This section explains this phenomenon. ELO 3.2

Core Voiding If a nuclear reactor experiences a loss of coolant accident Less moderator to moderate neutrons Neutrons travel further More neutrons will leak out This loss of moderation also adds negative reactivity reduction in Keff Even though Keff is decreasing a higher percentage of neutrons are leaking out SR NI readings would initially increase Eventually Keff decreases so far that SR counts start decreasing ELO 3.2

Core Voiding – Counts vs Voids
As voiding increases from 0% to 100% Counts increase, then decrease As voiding decreases from 100% to 0% Counts still increase, then decrease % void Counts 100 Low High This is merely a representation of how counts change with respect to voiding. Exactly how much voiding is required for counts to start to lower is not a concept tested by the NRC. That is why a representation of 50% voiding is used because it works for all existing bank questions. ELO 3.2

Core Voiding If core is refilled after voiding:
Neutron count rate will initially increase more neutrons become available for fission due to restored moderator As refilling continues neutron count rate will decrease due to more neutrons being reflected into core Less leaking from core to interact with detectors ELO 3.2

Core Voiding and Loading Effects
Knowledge Check – NRC Bank A reactor is shut down at 100 cps in the source range when a loss of coolant accident occurs. Assuming the source neutron production rate remains constant, how and why will excore source range detector outputs change as homogeneous core voiding increases from 20 percent to 40 percent? Increases, because more neutron leakage is occurring. Decreases, because less neutron leakage is occurring. Increases, because Keff is increasing. Decreases, because Keff is decreasing. Correct answer is A. Correct answer is A. NRC Bank Question – P1612 Analysis: A. CORRECT. Initially, excore source/startup range neutron indications will increase. This is because steam in the downcomer region (which is much less than dense than subcooled water) significantly lowers the fast-non-leakage probability (Lf). Therefore, due to less neutron moderation occurring in the downcomer, more neutrons reach the excore source range neutron level, causing an increase in detector signal. As the core voiding increases (no magical point, but for GFES purposes around 50 – 60% voiding), the core can not maintain criticality due to the loss of neutron moderator. Thus, the loss of the neutron source begins to compensate for the decrease in the downcomer region; fewer neutrons become available for subcritical multiplication and count rate decreases. Since the stem of this question specifically stated “from 20% to 40%”, the dominant effect is the increase in neutron indication due to increased leakage. B. WRONG. Counts will be increasing during this phase of core voiding because more leakage is occurring. C. WRONG. Counts are increasing because less moderation in the downcomer; however, Keff is actually decreasing due to the negative void coefficient of reactivity. D. WRONG. Even though Keff is decreasing, counts will be increasing during this phase of core voiding. ELO 3.2

Failed Fuel Detectors ELO 3.3 – Describe the theory and operation of failed fuel detectors. Proportional counters and/or fission chambers, located in RCS letdown flow Determine the presence of fission product activity delayed neutrons Both can be indicative of failed fuel Related KA - K1.17 Effects of core voiding on neutron detection ELO 3.3

Failed Fuel Detectors Examples of Failed-Fuel Detector Systems are:
Gross Failed-Fuel Detector System Located in RCS hot leg and has about a 60-second delay from core to detector Letdown Monitors could be isolated on low PZR level Types of activity monitored include gross gamma activity Cs-137 and iodine activity based upon dose equivalent I-131 Basically, ANY fission product gases Recall fission product yield curve ELO 3.3

Failed Fuel Detectors Knowledge Check
During reactor power operation, a reactor coolant sample is taken and analyzed. Which one of the following lists three radionuclides that are all indicative of a fuel cladding failure if detected in elevated concentrations in the reactor coolant sample? Lithium-6, cobalt-60, and argon-41 Iodine-131, cesium-138, and strontium-89 Nitrogen-16, xenon-135, and manganese-56 Hydrogen-2, hydrogen-3, and oxygen-18 Correct answer is B. Correct answer is B. NRC Bank Question – P3714 Analysis: Recall the Fission Product Yield curve discussed in – Neutrons: Fuel cladding failure will result in the presence of fuel and fission fragments inside the RCS. Iodine-131, cesium-138, and strontium-89 are all U-235 fission fragments and their abundance in the RCS is indicative of a fuel-clad failure. ELO 3.3

NRC KA to ELO Tie KA # KA Statement RO SRO ELO K1.17
Effects of core voiding on neutron detection 3.3 3.5 3.2 K1.18 Theory and operation of ion chambers, G-M tubes and scintillation detectors 2.6 2.8 1.2, 1.7, 1.8 K1.19 Use of portable and personal radiation monitoring instruments 3.1 2.1, 2.2 K1.20 Theory and operation of failed-fuel detectors 2.5 2.7

Operator Generic Fundamentals Components - Sensors and Detectors 2

Similar presentations

Presentation on theme: "Operator Generic Fundamentals Components - Sensors and Detectors 2"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Operator Generic Fundamentals Components - Sensors and Detectors 2

Similar presentations

Presentation on theme: "Operator Generic Fundamentals Components - Sensors and Detectors 2"— Presentation transcript:

Similar presentations

About project

Feedback