Project Subjects
Rational Radiation Safety Technology
Background and Objective
A principle of radiological protection for members of the public is to reduce their exposure due to manmade radionuclides as low as reasonably achievable even in the low-dose region below 1 mSv.y-1 (about 0.11
μSv per hour), that is the dose limit for public. This leads to a radiological safety system to require complianceof various safety standards in the extremely low-dose region below 1 mSv.y-1. On the other hand, in the ﬁeld ofradiation safety standards in transport of radioactive materials, transport of radioactive waste in a large size container is expected to be realized going with a lot of decommissioning of nuclear reactors.
In this project, reasonable concept and methodology to judge compliance of radiation safety standards is
proposed through enhancement of techniques of radiation measurement and safety assessment.

Main results
1. Risk due to man-made radioactive nuclide based on background (BG) cancer risk in Japan
Probability distribution of BG cancer risk (already-existing risk) based on a concept of the InternationalCommission on Radiological Protection (ICRP) and maximum assumed risk distribution due to man-made radioactive nuclides assuming a dose limit for public (the very highest risk distribution when dose limit is compliedwith) were estimated using Japanese mortality data by prefecture (Fig. 1). As a result, it became clear that morethan 98% of assumed risk distribution due to radioactive nuclides is smaller than standard deviation of BG
cancer risk distribution that is equivalent to 1 σ. This result revealed that the risk for members of the public is
sufﬁciently small and lower than the risk that is within the deviation of the BG cancer risk, if there is appropriate
radiation protection compliance with the dose limit of 1 mSv.y-1.
2. Development of the activity distribution estimation system for transport of large radioactive waste
Large radioactive waste, which is generated from the decommissioning of a nuclear power plant, can betransported within the manner of the usual radioactive waste transport procedure when it is demonstrated that its
speciﬁc activity (activity concentration) is distributed throughout. Thus, we have developed the activity distribution estimation system which utilizes photogrammetry, Monte Carlo calculation and gamma-ray measurement
techniques, sequentially (Fig. 2).
To clarify the system ability, accuracy of the BG count rate estimation was evaluated experimentallyusing mock large radioactive waste and radiation standard sources. Moreover, the relationship between the waste
packaging conditions, radiation measurement conditions and the uncertainty in the speciﬁc activity were estimated as a function of the ﬁlling rate. As a result, it was revealed that the system can give the BG count rate within
accuracy of ±30%, and that the system is a practical method since it can conﬁrm the transport requirement with afew gamma-ray measurements. The effect of the system on the manufacturing process of a large radioactive waste
package was also estimated. Regarding the typical radioactive waste whose ﬁlling rate is 11%, it was indicatedthat the time increase in the radioactive waste manufacturing process is approximately 10% (Table 1) [L10008].
Other reports

Jpn. J. Health Phys., 45 (2), 172-176 (2010).

26

02_1Nuclear4_27_2010.indd 26

11/10/03 16:03

Nuclear Technology
BG cancer risk
BG cancer risk

Probability
Probability

0.150.15
Doselimit
publicDoselimit
forforpublic1mSv/y
1 mSv/y0.10
0.10

1σ1σof of
BG cancer risk
Assumedmaximum
Assumedmaximum
riskrisk BG cancer risk

0.05
0.05

due
man-madedue
to toman-made
radioactive
nuclidesradioactive
nuclides

0.00
0.00
-6 -6
1010

-5-4-3-2 -2-5-4-31010
1010
1010
-1 -1
Risk
defined
ICRP
Risk
definedbyby
ICRP(y (y) )1010

Fig. 1 Comparison between BG cancer risk in Japan and maximum assumed risk distribution due toFig
1 Comparisonbetween
cancerin Japanmaximum
assumeddistribution
Fig1 Comparison
between
BGBG
cancerriskriskin Japan
andandmaximum
assumedriskrisk
distribution
duedueto toman-made radioactive nuclides
man-maderadioactive
nuclidesman-made
radioactive
nuclidesProbability distribution of BG cancer risk can
be considered
as a normal
distribution. The arithmetic mean is 1.2 x 10-3 y-1
Probability
distributionofBG
cancercanbeconsideredasanormal
distribution.
arithmetic
mean
isassumed
Probability
distributionofBG
cancercanbeconsideredasanormal
distribution.
TheThearithmetic
is 1.2x1.2 x-5 risk-1 risk-1
and its-3standard
deviation is 7.4 x 10 y . Risks equivalent
to 1 mSv.y (doselimit -1for public)andthe meanmaximum-3 -1 -1
-5 -5
-1 -1
-1
10y
and
its
standard
deviation
is
7.4
x
10y.
Risks
equivalent
to1mSv.y(doselimitforpublic)
and
the
10y
and
its
standard
deviation
is
7.4
x
10y.
Risks
equivalent
to1mSv.y(doselimitforpublic)
andtherisk distribution due to man-made radioactive nuclides are smaller than the risk equivalent to 1 σ of BG cancer risk.
maximum
assumedrisk
distribution
to man-maderadioactive
nuclidessmallerthe risk
equivalentmaximum
assumedrisk
distribution
duedueto man-maderadioactive
nuclidesare aresmallerthanthanthe risk
equivalent
to 1 to 1
BG
cancerrisk.
σσofofBG
cancerrisk.
Tableincreasetheradioactive
radioactive
waste
Table
11Timeincreasein theradioactive
waste
Table
1 TimeTime
increase
ininthewaste
packagemanufacturing
process
packagemanufacturing
process
packagemanufacturing
process
Waste
placing
process
Waste
placing
process
Specific
activity
distributionestimation
Specific
activity
distribution
estimation(containersize: size:1.6m 1.6m
x 1.6m)(containerx 1.6m)

Photogrammetry
systemPhotogrammetry
systemUnit Unit
NumberIncrease ratio ratio
Filling
Placing
MeasurePlacingof Number
Measureof theIncrease
placement
placementrate Fillingofof theprocess
ment
rate
processmentweightMeasuremanufacturing
weightMeasure(%) (%) (h) (h)*1time (h)time(%)manufacturing*1(ton)(ton)mentsments time (h)time(%)

Radiation
detectors
Radiation
detectors

5
Mock-disposal
container
Mock-disposal
container

0.1 0.11020

52.5 2.52

10

5.0 5.0
10.0 10.0

320

6

2

0.33 0.33

14% 14%3

0.50 0.50

61.00 1.00

10% 10%
10% 10%Whentheradioactive
radioactive
isinplaced
the disposal*1*1*1When
the theradioactivewasteiswasteplacedtheindisposal
When
waste
is placedtheindisposal

container,
useof remote-control
methodis projectedcontainer,
the theuseof remote-control
methodisprojectedcontainer,
the useof remote-control
methodis projected
since
its
activitylevel
iscomparatively
high.
since
its activity
levellevelis comparatively
high.high.
since
its activity
is comparatively

Possibletreatment
weightof a manipulator
was assumed
treatment
weightof a ofmanipulator
was wasassumed
to to to2 activity
The
activity
distribution
estimation
The
distribution
estimation
system Possible
Fig. 2 Fig.
Possibletreatment
weighta manipulator
assumedFig.
2 The
activity
distribution
estimation
be0.1
ton(100kg)
inthisstudy.
ton ton(100kg)
in thisstudy.
systema largeradioactive
waste
packagefor fora forlarge
radioactive
waste
packagebe 0.1(100kg)
in thisstudy.
systema largeradioactivewaste
package be 0.1

InIn
thethecasethatactivityof ofradioactiveis measured,
the theaccuracyof thespecific
activityIn
thecasethat
thespeciﬁc
activityof radioactive
wastewaste
is wastemeasured,
the accuracy
ofaccuracy
the speciﬁc
deteriorates
case
thatthethespecificspecific
activity
radioactiveis measured,of activity
thespecific
activity
becauseoftheincreaseoftheuncertainty
intheshieldingeffect,which
iscausedbytheunknownradiation
sourceposideteriorates
becauseoftheincreaseoftheuncertainty
intheshieldingeffect,which
iscausedbytheunknowndeteriorates because of the increase of the uncertainty in the shielding effect, which is caused by the unknown
tion. Thus, in this system, gamma-ray measurement is carried out after the placement of a certain amount of radioactive
radiation source position. Thus, in this system, gamma-ray measurement is carried out after the placement of aradiation
sourceThus,
thisinsystem,
gamma-ray process,measurement
is carriedout afterof awaste
to suppress
theposition.
uncertainty.
Forinthis,the manufacturing
althoughthe placement
andthetheplacement
measurementcertainamountrepeated,of radioactive
wasteto suppress
thetimeuncertainty.
For this, in the
manufacturing
process, although
arealternately
theeffectofthemeasuringonthemanufacturing
process
isquitesmall.
certain amount of radioactive waste to suppress the uncertainty. For this, in the manufacturing process, although
the placement and the measurement are alternately repeated, the effect of the measuring time on thethe placement and the measurement are alternately repeated, the effect of the measuring time on the
manufacturing process is quite small.
manufacturing process is quite small.

2

02_1Nuclear4_27_2010.indd 27

227

11/10/03 16:17