Atmospheric dispersion modeling for an accidental release from the Pakistan Research Reactor-1...

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Annals of Nuclear Energy 32 (2005) 1157–1166

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NUCLEAR ENERGY

Atmospheric dispersion modeling foran accidental release from the Pakistan

Research Reactor-1 (PARR-1)

S. Shoaib Raza *, M. Iqbal

Nuclear Engineering Division, Pakistan Institute of Nuclear Science and Technology,

P.O. Nilore, Islamabad, Pakistan

Received 23 February 2005; accepted 22 March 2005

Available online 23 May 2005

Abstract

Atmospheric dispersion modeling and radiation dose calculations have been performed for

a postulated accidental airborne radionuclide release from the Pakistan Research Reactor-1

(PARR-1) appropriate to a power upgrade to 10 MW. Estimates of releases for various radio-

nuclide groups are based upon US-NRC regulatory guide 1.183. Committed Effective Doses

(CEDs) to the public at various downwind distances were calculated using a health physics

computer code ‘‘HotSpot’’ developed at the Lawrence Livermore National Laboratory, Uni-

versity of California, USA. The doses were calculated for various atmospheric stability classes,

viz., Pasquill categories A–F with site-specific averaged meteorological conditions. The mete-

orological data on atmospheric stability conditions, mean wind speed and the frequency dis-

tribution of wind direction based on data collected near the reactor site have also been

analyzed and are presented here.

The results indicate that a person located within a downwind distance of about 500 m from

the reactor would receive more than the permissible CED under the analyzed severe accident

scenario. Analysis of one typical year of wind data indicates that the predominant wind direc-

tion is East–North East (ENE), which occurs at the site for more than 15% of the time.

� 2005 Elsevier Ltd. All rights reserved.

0306-4549/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.anucene.2005.03.008

* Corresponding author. Tel.: +92 51 9290231/3354; fax: +92 51 9290275.

E-mail addresses: [email protected]; [email protected] (S.S. Raza).

1158 S.S. Raza, M. Iqbal / Annals of Nuclear Energy 32 (2005) 1157–1166

1. Introduction

Atmospheric dispersion and radiation dose calculations for accidental releases of

radioactive gases and volatiles are an important contribution to licensing require-

ments for the selection of site for a nuclear reactor. The reactor-operating licenseis obtained from local regulatory authorities in accordance with internationally

adopted criteria (IAEA, 1979).

Under normal operating conditions, nuclear reactors, especially research reactors,

do not release any significant quantity of radioactive material to the atmosphere.

However, under accident conditions with severe core damage, a significant fraction

of the radionuclide inventory in the core may be released to the atmosphere. An ear-

lier document, WASH-1400 (NRC, 1975) used release fractions of 0.9, 0.7, 0.4, 0.4

and 0.05 for noble gases, halogens, alkali metals, tellurium and the Ba–Sr group,respectively. An IAEA document on research reactors used 100%, 50% and 1% re-

lease fractions for noble gases, halogens and particulates, respectively (IAEA,

1992). These release fractions were used in the previous radiological consequences

analysis for PARR-1 (FSAR, 2001). The exclusion area boundary for PARR-1

was calculated to be 750 m, based upon a 3 Sv criterion of 2-h thyroid-dose from

iodine isotopes.

In this work, we have performed the updated radiation dose calculations for

accessing any increased radiological consequences of the postulated severe accidentat PARR-1 after its power upgrade to 10 MW. An alternate source term (NRC,

2000) with release fractions based upon NUREG-1150 (NRC, 1989) was used.

The use of this new source term criterion is required by 10 CFR 50.67, on a volun-

tarily basis. This criterion assumes release fractions of 1, 0.4, 0.3, 0.05 and 0.02 for

noble gases, halogens, alkali metals, the tellurium group and the Ba–Sr group,

respectively (NRC, 2000), to determine the exclusion area from a limit on committed

effective dose (CED) of 0.25 Sv. We have used the dose conversion factors based

upon most recent recommendations of ICRP (ICRP 72, 1996; IAEA SS-115,1996). A health physics computer code ‘‘Hotspot’’, which employs a Gaussian plume

model (GPM) was used in this work.

In the following section, site-specific meteorological conditions (that were used in

the GPM) are presented. The source term released to the atmosphere is presented in

Section 3. The postulated severe accident conditions and the modeling assumptions

are presented in Section 4. Calculations of CED results are presented and discussed

in Section 5. Section 6 presents our conclusions.

2. Meteorology at the PARR-1 site

Wind roses based upon tower measurements at 10 and 60 m above local ground

level, which is about 760 m above sea level, are presented in Figs. 1 and 2. The hourly

wind speed data were determined for 16 directional sectors using a cup anemometer.

The radial length of a thick line indicates occurrence frequency for that direction,

with mean annual wind speed indicated in parentheses. The values in the central

Fig. 1. Wind rose at 60-m height for 16 directional sectors.

S.S. Raza, M. Iqbal / Annals of Nuclear Energy 32 (2005) 1157–1166 1159

circle indicate percent of time that calm conditions exist. The data presented in Figs.1 and 2 indicate that the dominant wind direction was ENE. The wind with this

direction occurred for 15.7% of total time with an average wind speed of 2.0 m/s

at 10 m and 2.9 m/s at 60 m. The record indicates that maximum monthly wind speed

at 10 m, in the dominant wind direction, was 8.2 m/s, which occurred in the month of

August. At this height, mostly the wind is mild with a speed ranging from 1 to 2 m/s.

At 60 m, the maximum monthly wind speed was 9.2 m/s with the annual average

ranging from 2.0 to 3.0 m/s for all directions. Average occurrence of calm conditions

(wind speed less than 0.5 m/s) at 10 m is about 2.3% of the total time with a mini-mum of 1.1% in July and a maximum of 4.2% in December. At 60 m, these values

are 2.0%, 0.9% and 3.5%, respectively.

Stable inversion conditions (percent of total time for a particular month) were

experienced for 36.5% of the total time, with 56.1% in December (maximum) and

21.1% in July (minimum). Day time inversions are generally of shorter duration than

night time inversions. Temperature measurements indicated that January was the

coldest month with a maximum of 17.6 �C and a minimum of 2.6 �C, while June

was the hottest month with a maximum of 38.7 �C and a minimum of 23.7 �C.Measurements of precipitation indicate a mean monthly rainfall for the months of

July and August (monsoon season) of 267 and 310 mm, respectively. For the winter

Fig. 2. Wind rose at 10-m height for 16 directional sectors.


rainy season (December–March) the rainfall varies from 37 to 90 mm/month. During

the rest of the months of the year mostly the weather is dry.

3. Source term and accident scenarios

The source term for the fission product inventory in the reactor core for an aver-

age burn-up of about 21% (initial 235U content) was calculated using the computercode ORIGIN. The release fractions applied to the calculated core activity were ta-

ken from US-NRC regulatory guide 1.183 (NRC, 2000) having values of 1, 0.4, 0.3,

0.05 and 0.02 for noble gases, halogens, alkali metals, the tellurium group and the

Ba–Sr group, respectively. It may be mentioned here that the earlier calculations

used release fractions of 1.0, 0.5 and 0.1 for noble gases, halogens and particulates,

respectively (FSAR, 2001).

For core inventory calculations, it was assumed that the reactor had been oper-

ated on continuous basis at its full power of 10 MW to achieve an average burn-up of 21%, at the time of accident. This assumption led to maximum possible fission

products inventory in the core, which was 3.17 · 1017 Bq. Table 1 shows the activity

Table 1

Radionuclide inventory in the core, release fraction and isotopic activity released to the atmosphere

Nuclide Group Core inventory Release fraction Activity released

Kr-83 m Noble gas 5.38E + 14 1 5.38E + 14

Kr-85 m Noble gas 3.72E + 15 1 3.72E + 15

Kr-87 Noble gas 7.36E + 15 1 7.36E + 15

Kr-88 Noble gas 1.02E + 16 1 1.02E + 16

Kr-89 Noble gas 1.39E + 16 1 1.39E + 16

Xe-131 m Noble gas 2.92E + 15 1 2.92E + 15

Xe-133 Noble gas 1.00E + 16 1 1.00E + 16

Xe-135 m Noble gas 5.76E + 15 1 5.76E + 15

Xe-135 Noble gas 1.88E + 16 1 1.88E + 16

Xe-137 Noble gas 1.76E + 16 1 1.76E + 16

Xe-138 Noble gas 1.79E + 16 1 1.79E + 16

I-131 Halogen 3.55E + 15 0.4 1.78E + 15

I-132 Halogen 1.24E + 16 0.4 4.96E + 15

I-133 Halogen 1.93E + 16 0.4 7.72E + 15

I-134 Halogen 2.24E + 16 0.4 8.96E + 15

I-135 Halogen 1.81E + 16 0.4 7.24E + 15

Sr-89 Br–Sr group 3.83E + 16 0.02 7.66E + 14

Sr-90 Br–Sr group 1.77E + 14 0.02 3.54E + 12

Y-91 Lanthanide 4.65E + 15 0.0002 9.30E + 11

Zr-95 Lanthanide 5.08E + 15 0.0002 1.02E + 12

Zr-97 Lanthanide 1.69E + 16 0.0002 3.38E + 12

Nb-95 Lanthanide 5.41E + 15 0.0002 1.08E + 12

Ba-140 Br–Sr group 6.51E + 15 0.02 1.30E + 14

Ce-141 Cerium group 4.86E + 15 0.0005 2.43E + 12



Pr-143 Lanthanide 6.05E + 15 0.0002 1.21E + 12

Nd-147 Lanthanide 2.47E + 15 0.0002 6.18E + 12

Ru-103 Noble metal 2.45E + 15 0.0025 6.13E + 12

Rh-105 Noble metal 2.34E + 15 0.0025 5.85E + 12

Ru-106 Noble metal 2.77E + 14 0.0025 6.93E + 11

Te-127 m Tellurium group 1.88E + 14 0.05 9.40E + 12



Te-132 Tellurium group 8.22E + 15 0.05 4.11E + 14

Cs-137 Alkali metal 1.63E + 14 0.3 4.89E + 13

Total 3.17E + 17 4.45E � 01 1.41E + 17


inventories of important radionuclides in the reactor core at the time of accident,

along with their release fractions and the activity released to the atmosphere.

Two release scenarios were considered. In the first scenario, all releases were as-

sumed to occur at a stack height of 61 m. The radionuclides released from the core

were assumed to be mixed within the containment free volume and escaped from the

building through the ventilation system at an air exhaust rate of 25,000 m3/h. A pre-

vious analysis indicated that, at this exhaust rate, a significant reduction in radionu-

clide release rate occurred with time due to high early containment cleaning byventing (Raza et al., 2004). In the second scenario, the containment exhaust system


was closed, the building became pressurized and the radionuclides were released at

ground level through cracks and penetrations in the structure.

The radionuclide activity released to the atmosphere was immediately picked up

by the wind and transported downwind according to the site meteorology. It was as-

sumed that the wind was blowing with a mean speed of 2.9 m/s at 61 m and 2.0 m/snear ground surface, in the predominant direction (see Section 2). Slightly unstable

(Pasquill�s category C) atmospheric conditions, as observed frequently at the site,

were assumed for dispersion calculations. However, other stability classes, Pasquill�sA–F were also considered for comparison purposes.

4. Radiation dose calculations

The computer code ‘‘Hotspot’’ was used to calculate 50 years committed effective

dose (CED). The radiation dose has been calculated taking into account the contri-

bution from 36 radionuclides, which were considered important for plume dose cal-

culations (FSAR, 2001). Atmospheric dispersion and airborne radionuclide

concentrations were calculated in the code using the Gaussian plume model

(GPM). Details on the GPM are given elsewhere (IAEA, 1982; Turner, 1994; Raza

and Avila, 2002). Time-integrated dose, D, to a receptor at ground level is calculated

by multiplying the time integrated radionuclide concentration, v, with appropriatedose conversion factor (DCF), and breathing rate (BR). A summation is provided

over all the radionuclides:

Dðx; y; zÞ ¼XN

i¼1

viðx; y; xÞ �DCFi � BR. ð1Þ

Complete radiological data on the dose conversion factors and other related param-

eters are given elsewhere (ICRP 72, 1996; IAEA, 1996; ICRP 60, 1991). The breath-

ing rate is taken to be 3.33 · 10�4 m3/s for an average human being under conditionsof light exercise (ICRP 66, 1994).

5. Results and discussion

Radiation dose calculations were performed using the site-specific meteorological

conditions with a wind speed of 2 m/s at ground level and 2.9 m/s at the stack/release

height of 61 m (buoyancy and exit momentum effects were neglected). Under the se-vere accident conditions assumed, about 45% of the total core activity of the volatile

radionuclides was released to the atmosphere (Table 1).

Scenario 1. In the first scenario, the released activity was assumed to be mixed in

the containment free volume (15,000 m3 ) and was released to the atmosphere

through the normal ventilation/exhaust system at a stack height of 61 m. Fig. 3

shows the calculated results for the CED as a function of downwind distance for

all atmospheric stability classes (Pasquill A–F). These doses would be received by

Fig. 3. Committed effective dose (CED) as a function of downwind distance for Pasquill stability classes

A–F with a release height of 61 m.


a person at ground level who remained within the plume for the entire duration of

cloud passage. As the release occurred at 61 m stack height, the doses first increase

with distance, reach a maximum value and then decrease. The location of the max-

imum dose gives the plume-touch down distance, which was about 0.25 km for sta-

bility class C with a maximum CED of 0.25 Sv. It can be observed that between

0.15–0.50 km from the release point the assessed CED values were greater than

0.1 Sv. It can also be observed that the plume touchdown distance (point of maxi-

mum dose) increases with increasing stability, however the maximum dose valuereduces with increasing stability.

Scenario 2. In the second scenario, the activity was released at ground level. Fig. 4

shows the calculated results for CED as a function of downwind distance for all

atmospheric stability classes (Pasquill A–F). As the release occurred at ground level,

Fig. 4. Committed effective dose (CED) as a function of downwind distance for Pasquill stability classes

A–F for ground level release.

Table 2

Committed effective dose (CED) and thyroid (organ) dose as a function of downwind distance for a

ground level release in stability category C conditions

Distance (km) CED (Sv) Thyroid dose (Sv) Arrival time (h:min)

0.100 6.5E + 00 2.3E + 02 00:01

0.200 1.6E + 00 5.5E + 01 00:02

0.300 7.0E � 01 2.3E + 01 00:03

0.400 3.9E � 01 1.3E + 01 00:04

0.500 2.5E � 01 7.9E + 00 00:05

0.600 1.7E � 01 5.3E + 00 00:06

0.700 1.2E � 01 3.7E + 00 00:08

0.800 9.3E � 02 2.8E + 00 00:09

0.900 7.2E � 02 2.1E + 00 00:10

1.000 5.8E � 02 1.7E + 00 00:11

1.200 3.9E � 02 1.1E + 00 00:13

1.450 2.6E � 02 6.7E � 01 00:16

1.500 2.5E � 02 6.2E � 01 00:17

2.000 1.3E � 02 2.9E � 01 00:23


the maximum dose occurs at the release point and the dose decreases with increasing

distance downwind (Table 2). The limiting dose of 0.25 Sv for the exclusion area

boundary occurs at 0.5 km. For all distances beyond 0.8 km, ground level doses

are below 0.1 Sv. Fig. 5 shows a 2-D picture of the area under the plume with three

regions marked with dose contours of 0.25, 0.05 and 0.01 Sv. The areas over which

these doses were exceeded were 0.065, 0.29 and 1.2 km2, respectively. It should be

Fig. 5. Contours of the committed effective dose (CED) for Pasquill stability class C for a ground level

release.


noted that the staff residential colony does not fall within this area, whatever the

wind direction. Radiation protection measures like sheltering and use of prophylac-

tic iodine tablets may be required in the area immediately surrounding the exclusion

zone, as determined by actual dose measurements during a severe accident situation.

6. Conclusions

It was concluded from this study that with PARR-1 power upgrade to 10 MW,

and with the use of the alternate source term criterion, there is no increase in the po-

tential radiological impact on the public. There was no increase in the exclusion area

boundary. In fact the area within 1–2 km around PARR-1 is already under admin-

istrative control of the licensee with total control on entrance and exit. There are nomembers of the public residing within this area, except a small residential colony for

essential/emergency staff at the site located at a distance of about 1 km towards the

north-east. The personnel living in this area (who are radiation workers and their

families) can be evacuated (if necessary) using institute-owned transport (buses

and wagons), although there would be no need to evacuate them for the severe acci-

dent scenario considered in this work. No modifications in the existing site emer-

gency plans are required. According to the new CED criterion, the exclusion area

boundary comes out to be 500 m, which is within the already adopted distance of750 m based on thyroid dose criterion.

Acknowledgements

The authors thank Mr. Showket Pervez, Head NED, PINSTECH for his interest

in this work. The authors are also thankful to the reviewers specially Dr. Michael

Charles Thorne, Director, Mike Thorne and Associates Limited, for the technicalcomments and suggestions to improve the manuscript.

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