Chapter 10 HRTD - Nuclear Regulatory CommissionChapter 10 ¾Contamination Surveys ¾Contamination...

Chapter 10

Contamination Surveys

Contamination Monitoring

HRTDHuman ResourcesTraining & Development

- Slide 1 -H-201 - Health Physics Technology

g

Review the 10 CFR Part 20 requirements for surveys

Objectives


Discuss the purpose of performing surveys.

Distinguish between surveys and monitoring.

Discuss why there can be differences between licensee and NRC survey results.

Describe the different types of contamination likely to be encountered

Explain how the extent and magnitude of the contamination can be evaluated

Objectives


contamination can be evaluated

Describe the techniques used in performing contamination surveys

Calculate the contamination levels present given relevant parameters

Part 20 - Surveys

Subpart F--Surveys and Monitoring

Sec. 20.1501 General.

(a) Each licensee shall make or cause to be made, surveys that--(1) May be necessary for the licensee to comply with the


regulations in this part; and(2) Are reasonable under the circumstances to evaluate--

(i) The magnitude and extent of radiation levels; and(ii) Concentrations or quantities of radioactive material;

and(iii)The potential radiological hazards.

(b) The licensee shall ensure that instruments and equipment used for quantitative radiation measurements (e.g., dose rate and effluent monitoring) are calibrated periodically for the radiation measured.

Radiation Surveysvs


vs.Contamination Surveys

Radiation Level MonitoringFrom a Point Source

GM detector:

May measure cpm but we want mR/hr

Ionization chamber:

Measures mR/hr which is what we want


Non-fixed (removable) – means radioactive contamination that can be removed from a surface during normal conditions

Fixed – means radioactive contamination that

Surface Contamination


cannot be removed from a surface during normal conditions

- 49 CFR 173.403 definitions for transport

Point vs Dispersed Source

For a point source, the measured value will decrease as the survey meter is moved farther from the source due to


inverse square for photons

For charged particles, measurement is dependent on range of the particles

Point vs Dispersed Source

For a dispersed source, the measured value will greatly depend on the geometry. The closer the survey meter is to the source, the more accurate the measurement will be relative to the actual amount of contamination present since only the photons or particles directly under the probe will be detected As the probe is raised above the


probe will be detected. As the probe is raised above the surface, the range of the particles becomes a factor as well as the “crosstalk” between particles outside the “footprint” of theprobe which arecounted andthose within the“footprint”which escape.

Calculatingdpm

cpm = Total Efficiency * dpm

where total efficiency is composed of the following:


IE = intrinsic efficiency of instrument (counts/hit)GE = geometric efficiency (hits/particle emitted)

Y = yield of radionuclide (particlesemitted/disintegration)

(a “hit” implies that a photon or particle enters the detector but it may or may not result in a “count”)

cmin

csec

1

area of detector (cm2)x =

c 1 hx =

Intrinsic Efficiency (IE)


This can be converted to mR/hr by using the photon fluence graph on page Misc-40 which tells us how many photons per cm2 per sec equals one R/hr.

cm2 - secIE

ch

cm2 - secx =

Factors for Calculatingcpm from dpm

where:

= x x xcm

IE GE Y dm



Y = yield of radionuclide (particlesemitted/disintegration)

(a “hit” implies that a photon or particle enters the detector but it may or may not result in a “count”)

Factors for CalculatingFactors for Calculatingcpmcpm from from dpmdpm

= x x x= x x xccmm

ddmm

cchh

pp

dd

hhpp


= {(IE= {(IE11 x GEx GE11 x Yx Y11) + (IE) + (IE22 x GEx GE22 x Yx Y22) + …} x) + …} xccmm

ddmm

For the general case of several different types of radiations:For the general case of several different types of radiations:

For the simple case ofone type of radiation:

cm

dm=x

(IE x GE x Y)

1

Factors for Calculatingdpm from cpm

h


where:


Y = yield of radionuclide (particles emitted/disintegration)

cm

1 dmc

hpd

hp

x xx =

c d1 = c d

Factors for Calculatingdpm from cpm


For the general case of several different types of radiations:

cm mc

d

1x = cm cx =

{(IE1 x GE1 x Y1) + (IE2 x GE2 x Y2) + …}

cm

dm=x

1

Geometric Efficiency (Ideal)

or2B 4B


GE = h/p

If the instrument is placed on top of the source (e.g., a portable survey instrument), then the GE = 0.5 h/p or less.

However, if the detector surrounds the source (e.g., a “well counter”), then theGE = 1 h/p or less.

“Beta Efficiency = 35% as a percent of 2B emission rate”

The detector “counts” 35% of all the particles emitted in

Example from Vendor Brochure


The detector counts 35% of all the particles emitted in the upward direction so its intrinsic efficiency is 35% (i.e., it counts 35 out of every 100 particles that hit the detector). Since only 50% of the particles are emitted upwards (geometry), the detector “counts” only about 17.5% of ALL the particles emitted from the source.

c ch p

“Beta Efficiency = 35% as a percent of 2B emission rate”

Example from Vendor Brochure


Remember, if your ultimate goal is to detect the amount of surface contamination (:Ci or dpm), then you MUST be able to account for ALL of the particles emitted so you can get the number of disintegrations.

0.35 x 0.5 x 1 = total efficiency = 0.175 ch

cd

hp

pd

If the detector is not in contact with the source (i.e., it is not as close as possible to the source), then some of the particles travelling upwards may not hit the detector. In that case, the GE has two components:

Geometry (Real)


GE = = xhp

hu

up

where u/p is the fraction of the particles emitted upwards (normally 0.5 for ideal situation) and h/u is the fraction of the upward particles that actually hit the detector which could be any fraction from 1 to 0 depending on how close or far the detector is from the source.

Geometry

GE = = xhp

hu

up


For example, if an alpha detector is placed on the source the GE would equal 0.5 (u/p = 0.5 and h/u =1). But if the same detector were raised about 2 inches, the GE would equal 0. Even though u/p would still be 0.5, h/u would be 0 since none of the upward particles would reach the detector.

A radiation safety technician, carrying a survey meter with a beta/gamma GM pancake probe, enters an unoccupied laboratory where 125I iodinations are performed to conduct a routine weekly survey. When the technician moves the probe into the hood, the instrument

Problem 10-1


goes off scale on the lowest scale. What could be causing this and what should be done?

A technician enters a laboratory where small sources of americium are used for research. The technician is carrying a ZnS alpha scintillation survey meter. The technician scans the counter top with negative results. The technician then stoops down to check the floor and

Problem 10.2


notices that the instrument registers a positive result whenever it crosses a seam between floor tiles. What could this mean?

An individual is drawing a dose of 99mTc from a vial. The individual is wearing gloves and the syringe is shielded. After drawing the dose and administering it to a patient in a dosing room, the technician removes the gloves and discards them in radioactive trash. The technician then

Problem 10.3


proceeds to an imaging room where another patient is just completing a scan. When the technician moves his/her arm under the camera to adjust something, the imaging screen lights up like a Christmas tree. What could this mean?

The surface contamination limits listed in USNRC Regulatory Guides are given in terms of dpm per 100 cm2. You are performing a surface contamination survey using an alpha probe which has a scale calibrated in cpm. The sensitive area of the probe is 60 cm2. Briefly explain what

Problem 10.4


p y pyou would do to make your measurement(s) consistent with the limits.

A technician uses a GM pancake probe to monitor a tabletop for beta/gamma surface contamination. The probe has a sensitive area of 15 cm2. The technician surveys an area measuring about 400 cm2 and obtains an average reading of 2,300 cpm. The background is

Problem 10.5


100 cpm. The instrument has a total efficiency (intrinsic and geometric) of 10%. What is the contamination level in terms of dpm per 100 cm2?

Although these helpful hints refer to the old NRC Reg Guide 10.8 and a previous version of 10 CFR Part 35, they still illustrate the

t f d ticoncept of conducting surveys at varying frequencies depending on the level of hazard.

There is an 131I spill at a nuclear pharmacy. The spill area measures about 3 m in diameter. The count rate using a GM pancake probe is 1 x 107 cpm with a 20% intrinsic efficiency, i.e., 2 out of every 10 betas hitting the

Problem 10.6


detector are counted (assume the gamma efficiency is very small compared to the beta efficiency). The probe face area is 50 cm2. What is the estimated dose rate to the gonads about 1 meter above the ground?

You are using an air ionization chamber with the beta window open. You are measuring a relatively high dose rate. You close the beta window. The dose rate is reduced by ½. What does this mean?

Problem 10.7


You have performed a survey of the surface of the exterior wall of a shielded exposure room. All of your measurements were less than 1 mrem/hr. As you walk away from the wall you note that the exposure rate appears to be increasing to about 3 mrem/hr. How do

Problem 10.8


pp gyou explain this?

An NRC inspector, using an air ionization chamber, measures 40 mR hr-1 at a rope barrier set up by a radiographer. The area outside the rope barrier is designated an unrestricted area. Do you concur with this designation? Briefly explain why or why not

Problem 10.9


this designation? Briefly explain why or why not.

END OF


CHAPTER 10

Chapter 10 HRTD - Nuclear Regulatory CommissionChapter 10 ¾Contamination Surveys ¾Contamination...

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