The Assessments and Comparisons of RASCAL and EPZDose on Atmospheric
Dispersion and Dose Consequences in Radioactive Material Release Accidents
Shaohsuan Chen1,2, Jongrong Wang 1,2, Chunkuan Shih1,2, YuTing Ku2, Yu Chiang2, ShaoWen Chen2,
WenSheng Hsu3
1:

Nuclear and New Energy Education and Research Foundation
Institute of Nuclear Engineering and Science, National Tsing Hua University
3:
Nuclear Science & Technology Development Center, National Tsing-Hua University.
2:

Address : No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, R.O.C.
Key words; Dose, EPZDose, RASCAL, SBO

AbstractRASCAL has been developed by NRC for making dose projections during radiological incidents andemergencies. It evaluates the atmospheric releases from nuclear power plants, spent fuel storage pools and casks,
fuel cycle facilities, and radioactive material handling facilities. EPZDose is a dose assessment code designed
and developed by NTHU. It calculates the dose consequences according to the input time-varying source term
and the meteorology conditions and is capable of demonstrating the dose dispersion progress. In this study, wecompared the atmospheric dispersion and the dose consequences evaluated by RASCAL and EPZDose. First, weused RASCAL to simulate a long term SBO accident for Maanshan NPP and exported the calculated released
source terms. Then we converted the source term files generated by RASCAL to the input source term format ofEPZDose. Finally, we used RASCAL and EPZDose to evaluate the dose consequences of the simulated
accidents, respectively under two postulated meteorology conditions and then compared the assessment results atcertain hours after the start of the release. The accumulated thyroid dose distribution image calculated by
RASCAL and EPZDose appeared fairly consistent.
1. Introduction
Taiwan is located at the circum-Pacific seismic
belt and experiences earthquakes frequently. A severe
earthquake may cause damage to the nuclear power
plants or the related facilities and incur different
dangerous levels of radiation. While the severe
nuclear power plant accidents that cause fission
products to be released from the reactor core happen,
the authorities will need to make a protective action
decision shortly based on the evaluation of the doseconsequences inside the planning zone. Therefore, arapid assessment of dose projections tool is very
important for responding to radiological emergencies.
RASCAL and EPZDose are two tools for dose
consequences estimation. RASCAL is developed by
the USNRC for rapid assessment of dose projections
during radiological emergencies based on plant andenvironmental conditions. The RASCAL graphicaluser interface (GUI) consists of seven tools. The
Source Term to Dose (STDose) primary tool is themain tool for the user to input information aboutplant conditions or accident conditions in order toestimate the projected radiation doses from a plume
to people downwind.[2] It can evaluate the released
source term caused by the accidents of nuclear power
plants, spent fuel storage pools and casks, fuel cycle

facilities, and radioactive material handlingfacilities.[1] EPZDose is an offsite dose assessment
code designed and developed by NTHU. It canrapidly simulate the offsite radiation dispersion
process and the dose results based on the input source
terms and the meteorology conditions and display thedosedispersion
progresswithcontinuous
animations.[5]Fig. 1 RASCAL GUI

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Fig. 2 RASCAL STDose GUI
In this paper, we compare the assessment results ofground level accumulated thyroid dose calculated byRASCAL and EPZDose for a Maanshan power plant
LTSBO (long term station blackout) accident.
Maanshan power plant is located at south Taiwan,
equipped with a twin-unit Westinghouse 3-loop PWR(Pressurized Water Reactor), and currently operates
at rated core thermal power of 2822 MWt for eachunit.
We first used RASCAL to simulate an LTSBO
event case for Maanshan nuclear power plant, andthen exported the calculated source term andconverted it to meet the input source term format ofEPZDose. Next, we used EPZDose to do thesimulation with the previously calculated source term
and then compared the dose consequences evaluatedresults of these two codes under two different
meteorology conditions.
Since the information of Maanshan nuclear power
plant is not in the site database of RASCAL, there isno map of this site installed and the doses footprint
result images generated for Maanshan are on blankbackgrounds. To compare the dose consequence withthe images, we exported the accumulated dose
footprint images (gis shapefiles) of the simulated
LTSBO event and displayed them using the free GIS
software-QGIS.[2]The following paragraphs will describe the source
term generating mechanism, the dispersion and dose
calculation models of these two tools, the descriptionof the simulation case and finally the comparison
results.
2. Source Term
A source term is defined as the activity of eachradionuclide released to the environment as afunction of time. RASCAL evaluates the source term
based on the methods and information described insome publications such as NUREG-1228, “SourceTerm Estimation during Incident Response to Severe
Nuclear Power Plant Accidents”, NUREG-1465,“Accident Source Terms for Light-Water Nuclear
Power Plants, Final Report,” and NUREG/CR-6451“A Safety and Regulatory Assessment of Generic
BWR and PWR Permanently Shutdown Nuclear
Power Plants” [1].

The calculation method of source terms inRASCAL includes four steps. First, it calculates theactivity of each radionuclide that is present in thecontainer (e.g., the nuclear power plant or spent fuelstorage). Second, it calculates the release fraction of
the inventory of each radionuclide for the accident
being evaluated. Third, the product of the above twoterms is multiplied by a reduction factor (e.g., forreduction by filters). Lastly, the source term of eachradionuclide released to the atmosphere during a
specific time step is obtained by multiplying theleakage fraction for that time step and the result fromthe third step[1].
EPZDose can calculate the contribution of doses
for 52 nuclides and the source terms generated byRASCAL including more than 60 nuclides[1][5].
Although not every radionuclide generated byRASCAL is calculated for dose with EPZDose,according to the "Cumulative Importance to Dose"table in RASCAL, more than 99% of the doses arecontributed by the nuclides calculated by EPZDose.

3. Dispersion Model
In a nuclear accident, the fission products are firstreleased from the reactor core and then leak into theatmosphere and disperse, depending on themeteorological conditions such as the wind direction,wind velocity and stability category. Both RASCAL
and EPZDose consider the radioactive materials to bereleased in a discrete manner and separated as aseries of puffs that contain certain amount ofactivities(in Ci) associated with different kind ofnuclides and describe the atmospheric dispersion ofradioactive materials based on the Gaussian puff
model[1][3]. The atmospheric dispersion parametersin the solution of Gaussian puff model are functionsof either the distance from the release point or thetime since release. They may also be functions ofatmospheric stability and surface roughness.
RASCAL uses dispersion parameters that are basedon travel time and the turbulence parameters(stability class) that are responsible for dispersion[1].
EPZDose calculates the dispersion parameters of apuff based on the moving distance of it and thePasquill’s stability conditions. If a puff has beentransported under different stability status, EPZDosewill adjust the dispersion parameters of it according
to its moving distance of each stability status.
3. Graphical Dose Assessment Display
For the doses calculation, RASCAL provides fourchoices for the calculation distance (16km, 40km,80km, and 160km). After finishing the event
conditions and meteorology setting, the user canselect one of the four distance choices to set theassessment geography range. The code accomplishes
all the calculations of an event for the selecteddistance and provides the assessment results display
option in "Detail Result of Dose Calculations" GUI.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

The user should always select 16km for the initial run.
If the user is interested in the doses at distances
beyond 16 miles, then he/she should run thecalculations again using the other distance choices[2].
The assessment result can be displayed as footprint
images or numeric tables. The footprint images areshown upon a map for only built-in sites. The user
can export the images and display them using GIS
software[2].

Fig. 3 Detailed Results of Dose Calculations GUI
EPZDose assesses the dose rates and accumulated
doses of whole body and thyroid based on the input
source term and meteorology conditions (e.g., thewind direction, wind velocity, Pasquill’s stability
conditions, and fallout fraction). The backgroundmap of the event sites can be retrieved through theinternet for 8 different zoom levels of the map, from
3km to 367km, centered at the release point.
EPZDose calculates and displays the result
simultaneously[4]. The assessment results are shownusing dose dispersion animations. While the dose
dispersion process is displayed, it can automaticallyswitch the zoom levels of the map according to thefurthest transport distance of released puffs. The codealso allows the user to manually switch the map
zoom levels during or after the simulations.

rods, the fuel cladding will start to be heated.
Later, the fuel itself will start to melt and cause alarge release. An LTSBO (long term stationblackout) event is a very serious accident thatleaves the fuel rods uncovered and causes large
amounts of fission products to be released fromthe reactor core.
To compare the dose assessment results ofRASCAL and EPZDose, we used RASCAL tosimulate an LTSBO event of the Maanshan nuclear
power plant. This event was assumed the Maanshan
plant was shut down and scrammed due to an
earthquake such that the offsite power was lost toleave the plant in a station blackout situation and thecore became uncovered. Besides, the ECCS andRCIC were also not operable. Eight hours later, anSGTR (steam generator rupture) occurred and thefission product began to leak through it. The makeup
flow was about 50 gpm; the rupture location wasabove the water level; the steaming rate was about
75,000 lbs/h and the release point was the safety
relief valve. For this case, we assumed the core wasnot recovered.

Two kinds of meteorology conditions arecalculated. For meteorology condition 1, weassume no rain, the condition remaining
unchanged with the wind velocity of 2m/sec, thewind from the south-west, and the stability
category of E (Slightly stable). Fig. 6 shows theaccumulated thyroid doses assessment results, 10hours after the release, calculate by RASCAL andEPZDose, respectively.

Fig. 5 Accumulated Thyroid Doses undermeteorology condition 1 at 10 hours after the firstrelease

Fig. 4 EPZDose Main GUI
4. Assessment Results Comparison

For meteorology condition 2, we assume norain, the initial wind velocity of 4m/sec, the initialwind from the south-west, and the stability
category D. In 5 hours, the weather changes
gradually with the wind velocity of 3m/sec, thewind from the west, and the stability category F.
Fig. 6 and Fig. 7 show the assessment results ofaccumulated thyroid doses at 10 hours and 8 hours,respectively.

Almost all the radioactive material at anuclear power plant is in the fuel rods. The fuelrods keep producing heat by radioactive decay.
If the cooling water no longer covers the fuel

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Consequences with EPZDose", PBNC2016,
Beijing, China, April 5-9, 2016.
[5] Shao-Hsuan Chen, Jong-Rong Wang andChunkuan Shih, "An Offsite Dose Evaluation
Code for Radioactive Material Release Accidents",
PBNC2014,
NO:118,Vancouver,
BritishColumbia, Canada, August 24-28, 2014.
Fig. 6 Accumulated Thyroid Doses undermeteorology condition 2 at 8 hours after the firstrelease

Fig. 7 Accumulated Thyroid Doses undermeteorology condition2 at 16 hours after the firstrelease
4. Conclusion

In this paper we introduced two dose
consequence estimation tools - RASCAL andEPZDose. RASCAL can evaluate the source termof the user-defined nuclear power plant or of therelated nuclear facilities accidents. We used
RASCAL to simulate an LTSBO event of theMaanshan nuclear power plant and export thegenerated source term which is then used in
EPZDose so that we can compare the assessment
results calculated by these two tools. Two different
meteorology conditions had been simulated. The
footprints of the evaluated accumulated thyroid
doses at some hours after the start of release of
these two tools are pretty close. Since RASCAL
calculates the decay of nuclides between the source
and the receptors but EPZDose does not, the redregions indicating the accumulated thyroid dosevalue higher than 250 mSv in the footprint
calculated by EPZDose are a little larger than those
calculated by RASCAL, which thus also shows theassessment results are pretty consistent.
5. References[1] U.S. NRC: RASCAL 4: Description of Models
and Methods, U.S. NRC, 2012.
[2] U.S. NRC: RASCAL 4.3: User’s Guide, U.S.
NRC, 2015[3] Yi-Hsiang Cheng, Chunkuan Shih, Sow-Chyuang
Jiang, and Tong-Li Weng, “Improvement ofAccidental Dose Consequences Simulation
Software for Nuclear Emergency ResponseApplications,” Annals of Nuclear Energy, Vol. 35,917-926, 2008.
[4] Shao-Hsuan Chen, Jong-Rong Wang andChunkuan Shih, "Modeling Fukushima Dose

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan