Reactor and Accelerator Health Physics

Question 1

List five considerations when estimating the ¹³¹I airborne concentration in containment 24 hours after shutdown.

Answer 1

• ¹³¹I concentration in the reactor coolant
• Reactor coolant system leak rate to the atmosphere
• Containment free air volume
• Containment atmosphere charcoal filters cleanup flow rate
• Containment atmosphere pressure reduction ventilation rate

Question 2

List four factors that should be considered in the pre-job analysis for a containment entry after shutdown in order to keep the worker’s total effective dose equivalent ALARA.

Answer 2

• Potential internal radiation exposures
• Potential external radiation exposures
• The effect of adding shielding to reduce external exposures, compared to the exposures received by personnel who establish the shielding
• The effect that respiratory protection has on reducing internal exposure compared to any possible increase in external exposure due to any increased stay times

Question 3

State three methods for reducing the reactor coolant system ⁵⁸Co cleanup time.

Answer 3

• Increase the reactor coolant cleanup flow rate.
• Add new resin to the reactor coolant system demineralizer OR regeneratie the existing resin prior to cleanup.
• Increase the size of the RCS demineralizer resin beds to improve cleanup efficiency.

Question 4

State two benefits of adding H₂O₂ to the RCS at the onset of a refueling.

Answer 4

• External radiation exposure to all workers will be reduced.
• Internal radiation exposure to all workers exposed to any internal surface of the RCS contaminated with ⁵⁸Co will be reduced.

Question 5

What two actions would you take before allowing a mechanic to enter a room with a known reactor coolant leak?

Answer 5

• Obtain an air sample near the release point from the valve and also in ambient air. Analyze the results and verify the worker’s anticipated exposures
• Obtain external gamma radiation measurements in the room and in the vicinity of the leaking valve to calculate the worker’s anticipated external gamma radiation exposure.

Question 6

List six events/conditions that could lead to unusual exposures either in the primary or in the secondary beam areas of an accelerator facility.

Answer 6

• Increase in beam current ⇒ Proportional increases in the dose rates in all areas
• Increase in particle beam energy ⇒ Produce more and higher energy neutrons and possibly more gamma radiation, causing higher dose rates in all areas.
• A redirection of a higher percentage of the beam from primary to secondary areas ⇒ Increase dose rates in secondary areas
• Reduction of the beam directed to secondary areas ⇒ Increase in beam current and dose rates in primary areas
• Alteration or removal of local shielding in primary/secondary areas ⇒ Higher doses in either area.
• Violation or interruption of interlocks ⇒ Would result in high exposure if personnel gained access to areas protected by interlocks.

Question 7

What are three sources of radioactive gas in the reactor coolant system?

Answer 7

• Neutron activation of dissolved gases.
• Fission gases that leak from inside the fuel rods.
• Fission gases produced by fission of “tramp” uranium on the outside surfaces of fuel rods.

Question 8

Specify three mechanisms by which ³H is produced in a PWR.

Answer 8

• Through ternary fission
• Through activation of deuterium present in some of the water molecules, which is present in hydrogen at an abundance of about 0.015%
  ²H(n, γ)³H
• Through activation of the chemical shim (boric acid) and then fission of the excited product
  ¹⁰B(n, α)^*7Li
  ^*7Li → ⁴He + ³H

Question 9

Why does a BWR have lower equilibrium concentrations of radioactive gases in the reactor coolant than a PWR (assuming similar fuel load and power history)?

Answer 9

The BWR coolant has lower concentrations of radioactive gases because gases are stripped from the steam when it is condensed back to water in the coolant.

Question 10

How is hydrogen produced in a reactor coolant system?

Answer 10

• The radiolytic dissociation of the water molecule by intense radiation field in the reactor core
• H₂O + radiation → H₂ + O

Question 11

What are two methods used to prevent the hydrogen concentration from reaching explosive concentrations in the reactor coolant system?

Answer 11

• Recombination of the H₂ with oxygen to form H₂O
• Dilution of the hydrogen concentration with air before it reaches the lower explosive limit

Question 12

Which radionuclide would be the source of the highest on-site external dose rate during BWR reactor operation?

Why is it not a problem during reactor shutdown?

Answer 12

¹⁶N

¹⁶N decays with a very short half-life of 7 seconds

Question 13

Where would the highest accessible dose rate likely occur in a BWR?

Answer 13

On contact with the steam line leading from the containment building to the turbine building

Question 14

What is the production mechanism for ¹⁶N in a reactor coolant system?

Answer 14

Fast neutron reaction with oxygen in the water molecule

¹⁶O(n, p)¹⁶N

Question 15

How would you expect researchers/operators and maintenance workers to differ in the β/γ exposures from working at an electron accelerator facility?

Answer 15

Researchers and operators

• Receive low levels of β/γ exposure from activated components
• Receive n/γ radiation leakage through shielding OR direct radiation from the target only when the machine is operating

Maintenance workers

• Not normally present during operation
• Only receive β/γ exposure from activated components while performing maintenance operations in the vicinity of the target

Question 16

At what electron energy would you need to begin worrying about neutron production in most materials? Why?

What are implications at higher energies?

Answer 16

• Neutron production in most materials by the reaction ^AX(γ, n)^A-1X starts at about 8 MeV, which is the binding energy of the last neutron in the formation of ^AX.

Implications

• The electron will undergo radiative energy losses producing bremsstrahlung photons up to its kinetic energy.
• Neutron yields and energies increase significantly at higher electron/photon energies ⇒ greater demand for neutron shielding.

Question 17

Why is knowing the neutron spectrum in areas occupied by accelerator personnel so important?

Answer 17

• If the spectrum is known, proper fluence to dose conversion factors can be evaluated, making a more accurate estimation of neutron dose.
• Additionally, type and magnitude of the responses of neutron personnel dosimeters depend strongly on the neutron energy.

Question 18

List one radioactive and one non-radioactive contaminant commonly produced in the air around an accelerator target area.

Answer 18

Radioactive

• ¹³N with 9.97 min T_1/2
• Produced by ¹⁴N(γ, n)¹³N

Non-radioactive

• O-zone (O₃)
• Results from ionization of air by electrons or photons

Question 19

List and describe five parameters of significant importance to estimate the emission of radiation from an accelerator.

Answer 19

Average beam current ⇒ Radiation output varies linearly with average beam current

Beam particle energy ⇒ As energy increases radiation yields from target typically increase

Beam particle type ⇒ Different particles produce different effects which need to be accounted for. Electrons – Bremsstrahlung, Positive ion – neutrons, energies > 8 Mev – photoneutrons

Atomic number of target nuclei ⇒ Typical bremsstrahlung yields increase for electron machines as atomic # increases

Target thickness ⇒ Radiation yields vary with target thickness, typically reaching a peak at a specific thickness

Question 20

List five considerations for selecting shielding materials for an accelerator.

Answer 20

• Hydrogen content (neutron shielding)
• Type of radiation
• Energy of radiation
• Atomic number
• Mass density

Question 21

What is the principle radiation of concern for the following criteria?

Potential drop accelerator
Proton/deuterons accelerated
Beam energy 1 – 10 MeV

Answer 21

Neutrons and Bremsstrahlung

Question 22

What is the principle radiation of concern for the following criteria?

Electron linear accelerator
Electrons accelerated
Beam Energy 1 – 10 MeV

Answer 22

Neutrons and Bremsstrahlung

Question 23

What is the principle radiation of concern for the following criteria?

Electron linear accelerator
Electrons accelerated
Beam energy > 10 MeV

Answer 23

Neutrons and Bremsstrahlung

Question 24

What is the principle radiation of concern for the following criteria?

Cyclotron accelerator
Protons and deuterons accelerated
Beam energy 10 – 50 MeV

Answer 24

Neutrons and Gamma-rays

Question 25

What is the principle radiation of concern for the following criteria?

Betatron accelerator
Electrons accelerated
Beam energy 1 – 50 MeV

Answer 25

Neutrons and Bremsstrahlung

Question 26

List six factors that need to be considered in planning to release a portion of radioactive coolant water to sanitary sewers.

Answer 26

• Time for delay in release to allow for complete decay of the short-lived radionuclides (e.g., ¹⁵O, ¹³N, ¹¹C).
• Total aticipated activity of each radionuclide in the water to be released.
• Activities of any other radionuclides previously released into the sanitary sewer in the month/year in question.
• Monthly volume of sanitary sewage for calculating monthly average concentrations of each radionuclide.
• Sum of anticipated monthly average concentrations in the sanitary sewer relative to the stated limits, whose sum should not exceed unity.
• Limits of total activities of released radionuclides in any year.

Question 27

Would neutron dose yields be higher or lower if copper is used as an accelerator target instead of tungsten?

Give two reasons

Answer 27

• Lower
• Binding energy per nucleon is higher in copper and lower in tungsten.
• The neutrons are produced by photoneutron interactions in the target ⇒ Because copper has a lower Z than tungsten, the bremsstrahlung yield will be lower.

Question 28

List five factors that need to be considered when evaluating materials for use as neutron shielding

Answer 28

• Cost
• Weight
• Inclusion of nuclides that have high inelastic scattering cross sections to reduce energy of fast neutrons
• Inclusion of hydrogen which has a high elastic scattering cross section to reduce energy of fast and intermediate range neutrons
• Inclusion of a nuclide that has a high thermal neutron absorption cross section and does not lead to significant capture gamma radiation and induced radioactivity in the shield

Question 29

A diver is working underwater on a valve that contains a hotspot. Identify 5 controls which could be applied to ensure the diver remains below regulatory limits

Answer 29

• Control job duration
• Monitor dose rate in the vicinity of the diver with an underwater detector
• Have the diver maintain a maximum reasonable distance from the hotspot (work from the side, away from the spot)
• Provide localized shielding to reduce dose rates (cover hot spot with lead)
• Provide direct instruction/advice to the diver

Question 30

List 5 actions that you would taken upon discovery of contamination on a radiation worker

Answer 30

• Remove all clothing and survey the whole body for contamination. Look for cuts.
• Attempt to quantify radiation levels with appropriate instrument (thin window GM detector) and sample radioactivity with gentle wipe or sticky tape
• Decontaminate the worker
• From wipe sample, identify the radionuclides
• From information collected, estimate the dose to the skin

Question 31

What is the most likely problem a GM instrument will experience when operating in the vicinity of the beam interaction area?

Answer 31

• Dead time of 50 – 100 µsec
• Reading on GM instrument will correspond to the pulse rate of the accelerator
• Pulse width of 1 µsec is much less than the GM dead time

Question 32

What is the most likely problem an ionization chamber instrument will experience when operating in the vicinity of the beam interaction area?

Answer 32

• Mean current is measured to yield the exposure rate.
• Recombination of ions within the ion chamber will occur if the instantaneous dose rate within the 1 µsec pulse width for the accelerator is very high.
• Recombination will give an erroneous reading below the actual average exposure rate

Question 33

List five sources of industrial hazards associated with operation of an accelerator facility

Answer 33

• Electrical shock
• Noxious gases (e.g., ozone, NOx)
• Cryogenic hazards (e.g., liquid nitrogen)
• RF/microwave hazards from RF sources used to supply energy to beam particles
• All radiation sources – neutron/gamma radiation from beam particle reaction in target and beam line components

Question 34

List five ionizing radiations that can be produced during operation of an accelerator

Answer 34

• Neutron
• Gamma photons
• Characteristic X-ray
• Bremsstrahlung photons
• Beta particles (from neutron activation reactions)

Question 35

Criticality Dosimetry

What are the five factors that determine the effective multiplication factor (keff)?

Answer 35

• e ⇒ Ratio of # of neutrons slowing below U-238 threshold to # of neutrons per fission
• p ⇒ Probability of resonance capture escape from 1 keV to 5 eV
• f ⇒ Fraction of thermal neutrons absorbed in uranium atoms
• η ⇒ # of fission neutrons per neutron captured in uranium
• L ⇒ Probability of neutron not leaking outside the assembly

Question 37

Criticality Dosimetry

How can you prevent criticality accidents?

Answer 37

1. Prohibiting the mass of individual pieces of uranium from exceeing a specified maximum size
2. Controlling the shape of a piece of fissionable material (sphere is the worst because of lowest surface area)
3. Administrative control of the immediate surrounding area with regard to materials that can feflect or moderate neutrons (including low Z materials such as hydrogen, water, wax, plastic)