An integrated framework for effective reduction of occupational radiation exposure in a nuclear power plant

Pergamon Ann. Nucl. Energy, Vol. 25, No. 17, pp. 1429-1440, 1998 0 1998 Elsevier Science Ltd. All rights reserved

PII: SO306-4549(98)00042-5 Printed in Great Britain 0306-4549/98 $19.00 + 0.00

An Integrated Framework for Effective Reduction of Occupational Radiation Exposure in a Nuclear Power Plant

Joo Hyun Moon and Hak Soo Kim

Center for Advanced Reactor Development, Korea Electric Power Research Institute, Taejon, South-Korea

YoungHoChoandChangSunKang

Department of Nuclear Engineering, Seoul National University, Seoul, South-Korea

(Received 16 March 1996)

Abstract- For effective reduction of occupational radiation exposure in a nuclear power

plant, it is necessary to identify repetitive high radiation jobs during maintenance and

refueling operation and comprehensively assess them. An integrated framework for

effective reduction of occupational radiation exposure is proposed in this study. The

framework consists of three parts; data collection, statistical analysis, and ALARA

findings. A PC-based database program, INSTORE, is used for data collection and

reduction, and the Rank Sum Method is used in identifying high radiation jobs. As a case

study, the data accumulated in Kori Units 3 and 4 have been analyzed. The results of this

study show that the radiation job classifications of SG related work have much effect on

annual ORE collective dose in Kori Units 3 and 4. As an example of AIARA findings,

hence, the improvements for the radiation job classifications of SG related work are

summarized. 0 1998 Elsevier Science Ltd. All rights reserved

Keywords : occupational radiation exposure; radiation jobs; ALARA findings; rank sum method.

1. Introduction The effective reduction of occupational radiation exposure (ORE) has always been one of major

concerns in designing and operating a nuclear power plant. For the advanced nuclear power plants, it

is even more important, since one can minimize the expense in the design stage. To keep levels of

ORE as low as is reasonably achievable (ALARA), collective as well as individual doses are to be

evaluated in view of the economics of design improvements. In this context, a numerical guide of

annual collective dose limit in person-Sv per year is set forth as one of design objectives in an

advanced nuclear power plant. In addition to this dose limit, devoted efforts should be demonstrated to

meet the ALARA criterion for further reduction of the collective dose through an extensive cost-benefit

analysis.

Most of ORE incurs during maintenance and refueling operation. Hence, it is necessary to identify

1429

1430 J. H. Moon ef al.

repetitive high radiation jobs during maintenance and refueling operation, and systematically review

them in order to explore most cost-effective ways to reduce the collective dose.

An integrated framework for effective reduction of ORE is proposed in this study. In the framework,

(1) ORE database is built using historical data and (2) high radiation jobs are identified using statistical

analysis technique. As a case study, the data accumulated in Kori Units 3 and 4 have been analyzed.

We are planning to utilize this framework to evaluate the ORE in compliance with its ALARA

requirements for the advanced nuclear system under development in Korea called Korea Next

Generation Reactor (KNGR) (Korea Electric Power Corporation, 1995).

2. Framework of ORE Evaluation Figure 1 shows schematically the framework to evaluate the ORE to effectively reduce the exposure

through a cost-benefit study. There are three major parts in the framework; data collection, statistical

analysis, and ALARA findings, which are integrated as a package for effective reduction of ORE in a

nuclear power plant.

2.1. Data Collection

As shown in Figure 1, the part of data collection consists of five subparts; 1) selection of reference

plants, 2) review of their radiation job breakdowns. 3) development of radiation job classifications as in

detail as possible, 4) collection of ORE data, and 5) construction of database program.

In selecting reference plants, there are preliminary information factors to be considered such as

reactor type and power. It is recommended that the reference plants have at least five-year operating

or four-full-power-year operating experiences (U. S. Regulatory Commission, 1979).

For the reference plants, all the radiation-related maintenance and refueling jobs are reviewed, and

the structure of radiation job classification is established. The breakdown should be as detailed as

possible to cover all radiation jobs performed in a nuclear power plant. The radiation jobs are firstly

classified into main jobs. Each main job is further broken down in detail to radiation job classifications.

The establishment of radiation job classification is an important preparatory activity for collecting data

and organizing the database structure. Once the structure of radiation job classification is developed,

all the collected data are assigned to each corresponding job classification.

The process of collecting ORE data should be well planned before-hand, especially, for the plants

which are not well equipped with appropriate database programs. The lack of organized data could be

augmented by frequent site visits and discussions with plant staff members, review of various

documents as well as evaluation of routine annual reports of ORE dose submitted to regulatory bodies.

The database program should be set up for efficient evaluation of raw ORE data. To maximize the

benefits of these data from past experiences, a PC-based database program, INSTORE (Cho et al.,

Reduction of occupational radiation exposure

1998) is used in this study.

1431

classifications as I” detail as ;y,,

Collection of ORE data

Review of identfied high collect~e # 1-1

Figure 1. Diagram of an Integrated Framework to ldentlfy and Manage High-Does Jobs at a Nuclear Power Plant

2.2. Statistical Analysis

In a nuclear power plant, all radiation jobs have to be performed according to highly-organized job

procedures. This allows to consider radiation job as a scenario which is a set of conditions to

eventually lead to radiation exposures. There are 3 major variables to determine collective dose; dose

rate, job time, and crew number. These variables can be considered as random variables since these

variables are subject to change each time radiation job occurs. Hence, the resultant collective dose for

each radiation job can be considered as random variable. This fact is a basic assumption of this study

to apply statistical analysis technique to the data in ORE database, in order to identify high dose jobs.

In a view of radiation protection, our immediate concern is to obtain enough information on each

radiation job to enable the high dose jobs to be identified. The number of radiation jobs needs to be

reduced to manageable amount so that they can be scrutinized more extensively. In this regard, the

information about the general exposure characteristics and the relative importance of each radiation

1432 J. H. Moon et al.

job is important. To grasp a general exposure characteristics of each radiation job, several percentiles

are sampled from the collective dose data of each radiation job. To get the relative importance of each

radiation job, the concept of ‘rank” is employed.

The part of statistical analysis consists of four subparts; 1) assignment of each radiation job to a

radiation job classification, 2) formulation of a distribution function for each radiation job classification,

3) identification of high radiation job classifications using the rank sum method (RSM) (Moon et al.,

1998) and 4) verification and validation of the results using the Friedman test (Hollander and Wolfe,

1973; Gibbons and Chakraborti 1992).

Each maintenance and refueling job in the database is assigned to a specific radiation job

classification. Then, a set of collective dose data will be compiled for each radiation job ctassification,

which create a distribution function of collective dose within the given radiation job classification.

Hence, there automatically exist N numbers of distribution function if N numbers of radiation job

classification are specified.

For a given distribution percentile, there exist N numbers of collective doses. The collective doses

having the same distribution percentile are ranked in ascending order. Starting from 1, the rank

assigned to the radiation job classification with the smallest collective dose value, the ranks

successively ascend one-by-one in order of increasing magnitude of collective dose. The ranks are

summed for each radiation job classification, and the sums are ranked for final assessment. This

method is called the RSM, which identifies high radiation job classifications.

It is necessary to check the RSM for verification and validation. It is required to check whether the

ranks drawn by this method have the characteristics of homogeneity and represent the characteristics

of a population. Since the RSM is based upon ordinal data, one of non-parametric statistical methods

such as the Friedman test is preferable in discriminating the homogeneity of the samples in case

many samples are interrelated each other.

2.3. ADI RA Findings

There are four subparts in ALARA findings; 1) review of identified high collective dose jobs pursuant to

ALARA requirements, 2) identification of means of reducing ORE, 3) assessment of the means using

the cost benefit analysis, and 4) adoption of cost-effective means of ORE reduction.

Each high collective dose job as identified should be closely scrutinized to investigate the adequacy

of work procedure, the robustness of radiation protection equipment, and the friendliness of working

conditions such as accessibility, available workspace, and mean space exposure rate. The close

scrutiny will suggest various means of reducing ORE including alternatives. The means could simply

include the enhancement of work procedures or working conditions. Sometimes, extensive design

improvements could be required including state-of-art technologies. All the available alternatives will

be evaluated using the cost-benefit analysis, and the adoption of cost-effective means of ORE

Reduction of occupational radiation exposure 1433

reduction will be decided

2.4. Case Study

Koti Units 3 and 4 are selected as reference plants. Kori Units 3 and 4 are typical Westinghouse-type

PWR’s of 950 fvlWe each and have been operating since 1985 and 1986, respectively.

2.4.1. Data Collection

The ORE data collected in the database are over the past ten years from 1986 through 1995 (Han-i1

Atomic Energy Corporation, 19861995; Korea Electric Power Corporation, 1986-1995). In total, there

are 4,335 ORE data are available. Each ORE data includes the information such as plant unit name,

job date, radiation work permit number, main job and code title, detailed job code and title, full job

description, dose rate, job time and crew number. The ORE data are classified into 26 main jobs,

which are further subdivided into 74 detailed radiation job classifications. Table 1 summarizes the

radiation job classifications used for this study.

Among 74 radiation job classifications, however, there are nominal jobs which were not performed

at outage of the reference plants and other jobs which hardly contain enough data to evaluate

collective dose. In this study, hence, these job classifications have been excluded and the rest 62 job

classifications are considered. These data are assembled in a PC-based ORE database program,

INSTORE, which is composed of easy-to-use program modules.

2.4.2. Statistical Analysis

The collective ORE dose data assigned to a radiation job classification are sorted in order of

increasing magnitude, which actually create a distribution function. For convenience of this study, 9

different classes of percentile value are chosen from each distribution function as follows: 10, 20, . . . .

90 percentiles with 10 percentile increment. In this case, for each of 62 radiation job classifications. 9

values of collective ORE dose are selected from the corresponding distribution function.

Let {SJ} be a set of collective ORE dose data for radiation job classification j in which the data are

arranged in ascending order of collective dose as follows:

{S’} = {s’; S,$ < s,l I I s,l }; j = 1, 2, . . . . 62 (1)

where S,d, S,l, . . . . S,’ are the collective ORE doses (annually normalized) of 10, 20, . . . . 90 percentiles,

respectively, which have been classified to radiation job classification j. Using a matrix notation, a

collective ORE dose matrix is formed as follows:


isI = (2)

For a given percentile value 5, let’s arrange the elements of column 5 of the matrix in ascending

order of collective dose, assign the rank of 1 for the element with the smallest collective dose value,

and subsequently increase the ranks of rest successively one-by-one in order of increasing magnitude

of collective dose. In addition, the ranks with the same collective dose are averaged.

Let R: denote the rank of S,l. I f we replace the elements, S,l with R,’ in Equation (2), the rank matrix

becomes

(3)

where R; is within the range of [I, 621.

Let CR: denote the sum of elements in row j, i.e., ZR: = c R: , the sum of ranks for radiation job ‘f=lO

classification j over all 9 percentile values. Including the sum of ranks in each row, Equation (3)

becomes

[R,‘, R;, . . . R&ICR;

u-1 . CR - RfO Rio . . . R& CR;

. -. (4)

Let the elements of the far right-hand side column of Equation (4) be sorted in order of increasing

magnitude, and let RI (j = 1, 2, .._, 62) be the rank of element. The rank sum column vector which

expresses the rank of each job classification becomes

[B] =c&’ R* ... R62)

By adding one more column of RJ ‘s, Equation (4) becomes

[Rs R;, R;, . . . R& CR, R’

I= RfO Rzo . . . R,‘, CR; R2

if; R;; *.

. . . R$Zi~R62

(6)


Table 1 Radiation Job Classification of A Tvoical PWR Plant

Main Job Code and Title

A Reactor Job

B SG Manway Job

C SG ECT Job

D SG Tube Job

E SG Nozzle Dam Job

F SG Lancing Job

G SG Related Job

SW

Detailed Job Code and Title Al Preparatory Job, A2 Reactor Disassembling, A3 Fuel Withdrawal, A4 Fuel Inspection, A5 Fuel Loading A6 Reactor Assembling, A7 Reactor Inspection, A6 Others Bl Manway Open, 82 Manway Close, 83 Others Cl Preparatory Job, C2 Inspection, C3 Template Construction & Removal, C4 Equipment Installation & Movement, C5 ECT, C6 Others Dl Preparatory Job, D2 Inspection D3 Template Construction & Removal, D4 Equipment Installation & Movement, D5 Plugging, D6 Sleeving, D7 Equipment Decontamination, D8 Others El Dam Construction, E2 Dam Removal, E3 Others

Fl Preparatory Job, F2 H/H Job, F3 Lancing, F4 Equipment Removal 8. Decontamination F5 Others

Hl Preparatory Job, H2 RCP Motor Job, H RCP Check & Maintenance Job H3 RCP Seal & MFB Job, H4 RCP TVCS Job,

H5 RCP DACS Replacement, H6 Others

I PZR Check & Maintenance Job

J RHR Check & Maintenance Job

K In-Service Inspection

L Containment Leak Test

M In-Core Job

Kl RT, K2 PT, K3 MT, K4 ET, K5 UT, K6 VT, K7 Others

Ml Thimble Job, M2 DFMS System Job, M3 Thermocouple Job, M4 Detector Job

N RTD Check 8 Maintenance Job

0 Snubber Check 8 Maintenance job PI BB System, P2 BH System, P3 BG System,

P Valve Check & Maintenance job P4 BM System, P5 BC System, P6 HB System, P7 HC System, P8 Others

Q P/P Check & Maintenance Job

R Heat Exchanger Check & Maintenance job

S Filter Job

T Evaporator Job

U Decontamination 6. Laundry Job

V Waste Related Job

W Radiation Safety Control

X System Operation

Y Waste Drum Deposit Job

Z Others


The elements of the far tight-hand side column in Equation (6) indicates the ranks for final assessment.

Table 2 summarizes the elements of Equation (S), which are the results of the RSM. The last column

of Table 2 shows the ranks of radiation job classification. The rank of El is the highest, which

represents the steam generator (SG) nozzle dam construction job.

The Friedman test of ranks is organized as shown in the following matrix equation:

The Friedman test of the data given in Table 2 using SPSSPC+ shows that the test statistic S is

computed to be 7.1516. This value is compared with x2W,0051 which is defined as the upper 0.05

percentile point of the x2 distribution with degree of freedom of 9. The comparison of two values shows

that S is less than x~~~,,,~) = 16.92, which means that 10 samples (9 percentile values plus I rank sum

value) have the characteristics of homogeneity and well represent the characteristics of a population.

2.4.3. ALA RA Findings

The results of this study show that the radiation job classifications of SG related work have much

effect on annual ORE collective dose in Kori Units 3 and 4. As an example of ALARA findings, hence,

the improvements for the radiation job classifications of SG related work are summarized. The job

classifications of SG related work to be considered are tube job, eddy current test job, and nozzle dam

job. In addition, SG man-way job is also selected for evaluation since it is always on the critical path in

performing the above-mentioned tube, eddy current test, and nozzle dam jobs.

For each job classification of the above-mentioned SG related work, the adequacy of work

procedure, the robustness of radiation protection equipment, and the friendliness of working conditions

are closely scrutinized. In most cases, it is found that a substantial reduction of ORE could be

achieved by enhancing work procedures, employing additional design features, or improving working

conditions.

The specific means of reducing ORE are identified. The means could simply include the

enhancement of work procedures and working conditions, or some design improvements. All the

available means including their alternatives are supposed to be evaluated using the cost-benefit

analysis, and based upon the results of the analysis, the adoption of cost-effective means of ORE

reduction should be decided. In this study, we have not performed an extensive cost-benefit study.

Rather, a preliminary list of improvements is derived based on simply the cost-effectiveness of each

improvement, generally expressed in terms of man-rem reduction per unit cost and summarized in

Table 3. The means identified as cost-effective in the table are as follows:


l photographic technique for SG tube plugging inspection

l mockup training for SG related jobs

l multistud tensioner/detensioner and handling device for SG manway cover

l SG tube inspection and repair robot

Table 2 Summary of the Statistics for Percentiles of Each Detailed Job Scheme


Table 2 Summary of the Statistics for Percentiles of Each Detailed Job Scheme(continued)


3. Conclusions and Recommendations The effective reduction of ORE in a nuclear power plant could be achieved by analyzing existing ORE

data, drawing high radiation jobs from the data using statistical analysis, and finally adopting

appropriate means to reduce ORE based upon ALARA findings. Therefore, a framework which

integrates data collection, statistical analysis, and ALARA findings is proposed in this study. A PC-

based database program, INSTORE, is used for data reduction, and the RSM is used in identifying

high radiation jobs. In order to demonstrate the applicability of the framework, it is actually applied to

Kori Units 3 and 4. This framework is also being applied to evaluate whether the KNGR design meets

the criterion ALARA for its ORE. For completeness, it is strongly recommended to add the scheme of

cost-benefit study in the framework in the future.

Table 3 Summary of Means of Reducing ORE of SG Related Works

Main Job Current State

Guidance for Title Improvements Means of Reducing ORE

- The entrance of worker to - Reduction in the - Use of remote control high radiation area in SG personnel required to device such as robot for

SG Tube building is the key factor to perform an activity tube inspection and

Plugging Job high collective dose repair - Use of remote photo-

graphic technique for SG tube plugging

- In high radiation area, - Reduction in the - Use of remote equipment for ECT is personnel required to controller for inspection installed by worker himself perform an activity and ECT

- Temporary platform is used - Reduction in setup and - Installation of SG ECT Job - It takes long time to install takedown requirements permanent platform

temporaty platform if required to perform an necessary activity

- Improved planning for - Mockup training performing an activity

- The activities are - Reduction in the job time - Introduction of

SG Nozzle frequently delayed due to advanced and light

Dam Job nozzle dam of heavy nozzle dam weight - Improved planning for - Mockup training

performing an activity - The manway cover is - Reduction in the - Use of multistud

opened and closed through personnel required to tensionerldetensioner handling manway cover perform an activity and handling device for bolts SG manway cover

- Bolt-fastened incidents - Improved access - Replacement by SG Manway frequently occurs maway cover of quick Open/Close - Long job time leads to high opening hatch type

exposure of workers - Employing the design - Access way for equipment to widen access way fol

seems to be narrow equipment - Improved planning for - Mockup training

performing an activity


ACRONYMS ALARA As Low As Reasonably Achievable

KNGR Korea Next Generation Reactor

ORE Occupational Radiation Exposure

RSM Rank Sum Method

SG Steam Generator

REFERENCES Cho, Y. H., et al (1998) INSTORE: A PC-Based Database Program of Occupational Radiation

Exposure for a Nuclear Power Plant, to be published in Journal of Korean Nuclear Society, 1998.

Gibbons, J. D. and Chakraborti, S. (1992) Non-parametric Statistical Inference, Marcel Dekker.

Han-II Atomic Energy Corporation (1986-l 995) Report of Radiation Safety Management - Kori Units 3

and 4 Maintenance Report.

Hollander M. and Wolfe, D. A. (1973) Non-Parametric Statistical Methods, John Wiley & Sons.

Korea Electric Power Corporation (1986-1995) Annals of Radiation Management.

Korea Electric Power Corporation (1995) Utility Requirements Document of the Korean Next

Generation Reactor.

Moon, J. H., et al (1998) Use of Rank Sum Method in Identifying the Maintenance Jobs for Cost-

Effective ORE Reduction in Compliance with ALARA, to be published in Journal of Korean Nuclear

Society.

U. S. Regulatory Commission (1979) Occupational Radiation Dose Assessment in Light-Water

Reactor Power Plants Design Stage Man-Rem Estimates, Regulatory Guide 8.19.

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An integrated framework for effective reduction of occupational radiation exposure in a nuclear power plant