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Monte Carlo Analysis for On-board Experiment
of the LLW Shipping Vessel Seiei Maru
Toshimitsu Sanui
and
Kohtaro Ucki, Atsuto Ohashi and
Nobuteru Nariyama
Tadasm Sasao,
Takayoshi Fhsc
Ship Research Institute
6-38-1 Shinkawa, Mitaka, Tokyo 1St, Japan
81一 422-4 1-31 37
Nuclear Fuel Transport Co.,uci.
1-1-3 Shiba Dainion, Minato-Ku, Tokyo 1lJ5,
Japan
81-3-3 43 8-324 1
ABSTRACT
On-board experiments were carried out in the LLW shipping vessel Seiei Maru, in 声ch 360
corrtahiers were loaded m six ho犀s, and the gamma-ray cIose-eqIllvaleflt rates were incasurca mainay on
the hatch covers of 血 holds, in the accommodation rooms, and a10 on the -quay at Mutsu Ogawara
Fm.
The Monte Carlo code MCNP 4A with t加 Next Event Surface Crossing estimator was employed to
reduce t加丘ad血d standard dcviatio皿 C血div山 in the cakulations・ In order to briロg Out the
characteristics of the Monte Carlo method, the gamma-ray source was g加en at every container 血 whith
8 LLW drums were contained, and 60 containers h a hold were modeled at every container. The
homogenized mode of which 60 containers were homogenized throu帥 the ho届 of LLW source region
was also taken up to compare the 丁esult with the heterogeneous model・
The Monte Carlo analysis with the heterogeneous model could produce fairiy good agreement as
compared with the measured dose-equivalent rates 。ロ t加 hatch cover・ On the 0th巳Ihand, the an山sk
吐h the homogenized model overestimated the measured values 叶 a factor of 2.25 to 7.5. The
mritribution of the g日mma-ray 血予St血ic was estimated "the Mmllc Carlo calculation・ In
consequ。ロcc, it accounted approximately for 11% 血 the gamma-ray dose-門uiva lent rate under opening
a hatch cover of the ship.
I.INTRODUCnON
On-board experiments were carried Out in a LLW (low level radioactive wastes) shipp叱 vessel, the
Seici Mare, 加 whkh 360 contamci's were loaded m Six holds, aiia gamma-ray aose-eqUIValeflt rates were
measured mainly on the hatch covers of the holds and in the accommodation rooms. Furthermore, the
shine on the quay when the
「めution of the sky・
gamma-ray doses were measured to estimate the cont
Seici Maru entered Mutsu Ogawara port.
The continuous enerty Monte Carlo code MCNP 4A' with the NESX estimator was used to analyze
the measured gamma-ray dose-eq u加ale口trates on the hatch covers ot the holds. 血計cad Or me 1-'Oint
Detectol・est加atoLt加 NESx(Next Itvent Surtace し丁05510gJ estimator was emP10ra・ aria me
subroutine TALLYD was revised to use the NESX estimator 血 the MCNP 4A code. In order to bring
Out the characteristics of the Monte Carlo method, the gamma-ray source was given at every container
in which 8 LLW drums were contained, and the 61) containers 血 a hold were modeled at every container
IO37
in the present Monte Carlo ana恥is. On the other hands, the homogenized model of which 印
containers were homogenized through the hold of the LLW source rcgioロ，was also l 止en up to con甲are
the result with the heterogeneous model. In the homogeロized ロlodeL the gamnia-raysources wm
uniformly distributed in the homogenized region. According垣 the homogeneous model can be
employed in the point kernel code QAD-CG2, the one-dinienioual liansport code ANISN3, and also the
two-dimensional code DOT 3.54, However, the heterogcneous model is ditEcult to calculate exac町
except りthe Monte Carlo method. In the QAD-CG code, the source geometry is able to model as
precis山 as the MCNP 4A, but the QAD-CU cannot give the source strength and energy speetnirn at
every container like the MCNP 4A.
II. ON-BOARD EXPERIMENT IN THE SEIE1 MARU
1. Outline of the Seici Maru
hm
The LLW shipp叱 vessel Scici Maru has the following outlines.
Construction
】 Sept. 1991
Total length
lOOm
・
l m
Width
: 16m
: 8UI
DCpth
8 l
Deadweight
: 3,000 tons
Service Speed : 13 knots
Holds
: 7 holds
LLW ama血era:Total 384 amainers, 血 which 8 LLW drums arc contained
(No. 2, No. 3・・・・ No. 7 holds load 60 containers, No. I ho日 loads 24).
The LLW ship has been plying between each nuclear power station and Mutsu Ogawara port where
the nuclear-fuel-cycle facilities are located. The on-board experiment was carried out to measure the
gamma-ray dose-equivalent rates m and out of the ship 血 Dec. 1993.
2. On-Board Experiment
The gamma-ray dose-equivalent rates were measured mainly on the hatch covers of the holds, 血 the
accommodation area of the ship, and also on the quay m Mutsu Ogawans port when the Seici Marc
entered there. The measured dose points on the center line of the Seiei Maru are shown 血 Fig. 1, a加
the corresponding measured dose-equivalent rates are surmuarized 加 Tablel. In Tablel,the values at
the dose point 7 to 30 are the maximum ones on each measured Une. There are 5 meatured points '
the port side to the starboard on the measured line. The dose point 31 is located under t加 No.3,～ー
hatch cover and the dose was measured when the hatch cover was opened 血 Mutsu Ogawara port. so
that the measured vame was much higher than the other points. In all the measurements, the
scintillation survey meter of the ALOKA was employed. maccordance with the manu叫 the survey
meter has a measurement error of 士 15% in the measured values. According与 the measurement error
is indicated 血 tables and gurcs.
In Table l,the maximum dose point on the hatch cover is at No. 17 of No. 5 hold, and the
corresponding dose-equivalent rate is 1.17 U Sv/h. The maximum dose-equivalent rate on the hatch
coverof the No. 3 hold is 1.00pSv/h at No. 23 p0血,and the dose on the back side of 血 hatch cover
is 193 U Sv/h at No. 31. The dose attenuation ratio of 1/20(LOU Sv/h ! 19.7 U Sv/h)is due to the
】038
J
的 Hい ・試mけ
(ii) The dimension of each container is 320 X 176 H 107h cm and it is covered with 0.16 emE of steel
In the heterogeneous mode! of a hold, the 60 containers are modeled individually, and air 血 and around
the container k not homogeロized. Accordingly, the gammaー「ay streaming through the air is takeロ into
account in the Monte Carlo ariaかsis.
2. Compa山on and Discussion of Exぎriment and Monte Carlo Anal蝉S
As decdbed before, the measurements of the dose-equivalent rates were carried out mainly on the
hat血 covers. Accordingly, Monte Carlo calculations were concentrated upon the hatch covers.
,ure 3 shows the comparison of the gamma-ray dose-equivalent rates between the measurements and
、一一．」e Monte Carlo calculations on the hatch cover of the No. 4 hold. The fsd's (fractional standard
deviation) of all the calcutations were within 10%,and the calcu tat ion time in each model, i. e., the
het cr0geneous model and the homo即nized model, was set iロ 500 mm. 加 the HP 755 workstation・
As compared with the experiment, the Monte Carlo results are overestimated 計組 the dose po血s in
・
5 to
取．
3. The Monte（コ arlo results with the heterogeneous model 日m Overestiロlatedりa factor of 1
2.5 and 句 a factor of 2.25 to 7.5 with the homogenized model・ However, the calculated profiles of the
dose-equivalent me distributions are almost the same as t加 experiment・ The reasons of the
overes山nat加n are as 加llows【
肥terogeneous model.
(2) The gamma-ray sources are distributed uniformly in the source region. According加 Some of the
Sources are generated 丘om nearer locations than that of the real source ones and also than that of the
"eterogeneous model. As a result, the homogenized model also leads to more large doses than that
both the measurements and the heterogeneous model.
⑤ The data on the gamma-ray source strength and radioisotopes of the LLW drums are offered by the
nuclear power plant, and the data are also utilized to the saf七 ty assessmm of the LLW disposal.
Therefore, some safety margin was to be taken into the source data. Accordingly, the Monte Carlo
results were overestimated the measurements.
3. Contribution of Sky-Shine
The dose-equivalent rates were measured when the hatch cover was opening and closing at 50 al一
distance from the side surface of 山e Seiei Maru on the quay. The natural background subtracted doses
were between 0.01 and 0.03 ft Sv/h 血 which the gamma-rays con血g from the LLW source, the ship
shine were included. According恥 it is di価cult to take out the ロet contnljution
structures, and the sky・
of the sky-shine from the present measured d叶 a, so that the contribution of it was estimated from the
1O]9
紺隠 刊乱）
1油
刊細掃側側』
(1) In the homogenized mod叫山e gamma-ray source regioロ of the LLW was homogenized oyer the
面 containers with air in and around them. Then the density of the source region is 0.612 glcm2. On
the other hand, as shown in Fig. 2, 曲e 4 LLW drums are homogenized 血 each block, however the
conliguration of the 60 containers m whichlmwo blocks arc contained m each 【
c m血ロer, and ak 血 and
around the containers are taken into the calculations without being homogenized. The dens抑 of each
block is 1.69 g/cm3. Consequent与 even though the gamma-ray streaming through the air gap in and
紅ouロd the containers, due to the difference of the den町血 the sou丁 Ce re'on, the Monte Carlo results
with the homogeiiized model are to produce more large dose-equ加alent rates than that of the
shie旧hgeffect of the hatch cover, which is constrilcted with l cm-thick steel 十 16cm-thick concrete 十I
cm-thick steel. The maximum dose point m the accommodation area k at No.lon t加 wheel house
deck, and the conespond叱 value is 0.05"Sv/h. The controlled dose-equivalent rate in the
accommodat加n area k l・8"Sv/b 加 the J叩ariese sea・
transport に“
g 肌ion. Accom叱妬 the
magnitude of 0.05 ii SvIh is 1/36 of the regulation.
The measured dose-equivalent rates were 曳tween 0.04 a耐 0.06 ftSv/h at 5Dm-distance from the
side sudhce of the Seiri Maru in Mutsu Ogawara port, and those measured values were not affected 町
the hatch cover opening or cbs叱．Accordingly, the contribution of the sky-shine could not be cleared
毎 the measurement, so it was estimated 叶 the Monte（証In calculation. The net dose-equivalent rates
were between 0.01 and 0.03)2 Sv/h at the dose point.
Each container contains 8 LLW drt」ロ］a, which are made of concrete. The most of the 2 amin
sources in a LLW drum are 加Co and 】’7 CS. There is a little source of s8 Co in the LLW. Hふeye'
magnitude of the'8 Co is 10% of the 'Co at the most, and its half-life is 7(L8 days. The LLW drums
transported"the present shipment are more than 5 years passed. Accordinuly. the eamma.rav
剖mrces were recognized [0 be ーし0 and ー【ー‘肌 sinpment・ The m昭nitude of the gamma-my sourcesk
indicated 血 Tablez There was no containermthe No. 1 血 hold 血 the present shipment.
皿 . MONTE CARLO ANALYSIS OF EXPERIMENTS
1. Modeling of Source Tenn and Structures in Vessel
In order to bring out the best of the Monte Carlo method, as illustrated 血 Fig. 2, 8 LLW drums in a
container were d加M
“血o two blocks, ・
i 。．,each block has 4 drums. Cons門uc血．
gamma-I可
source interis抑，
and energy spectrum of the LLW could m given in each block. This detailed model h
called the "heterogeneous model" 血 this pape丁・ The homogenized model of the LLW storage ho届 was
also analyzed"the Monte Carlo method to compare l 加 result with the heterogeneous model. The
homogenized model can be calculated りthe point kernel code QAD-CG, the one-dimensional cod:
ANIS凡 ad 司so the two-dimensional code DOT 3.5.
The homogenized and the heterogeneous models are summarized as follows.
(1) Homogenized Model
(1)The gamma-ray sources in a hold are distributed in a homogenized region of 660 X 880 Xw
cm.
(il) Even though concrete density of the solidified LLW is 2.2 g/cmm, due to homogenized be叫
concrete of the LLW, steel of Ihe LLW drum and the container, air h and around the container, the
density of the homogenized region is reduced to 0.612 g/cmm.
(2) Heterogeneous Model
〔 i) As ifiustrated in Fig. 2, each container consists of two blocks of 270X 51 X 89h cm. Each block
has homogenized the 4 LLW drums, and the density is 1.69 g/cm3. In the present calculation, the
source condition of the two blocks is the same.
1040
Monte carlo calculations with the homogenized model of the LLW source re'OTt. Iロ the fit訊
calculation, air was considered up to 1,500 m around the shhand it was up to 50 m around the sh加 in
the second calculation. In the Monte Carlo calculations, the hatch cover of the Seiei Maru was opened.
The result of the first calculation was 5.24 Hlr（島d 】 0.035)/.tSv/Ji and the second was 4.64X1(12
輿：0.036 ) /4 Sv/h at the detector Pロhl.The calculated dose-equivalent mc of 5.24 X t0ユ t Sv/h お
overestimated " a factor 吐 1.7 to 5.2 as corn四red with the measured values, and the contribution of
the sky-shine is estimated to be approximateWll%of the total dose at the measured point. The
tendency of the overes山nation h the Monte carlo results is the same as the results on the hatch cover.
W.ONCLUSIONS
、一,The follow叱 remarks were ohtained as compa丁ed with the on-hoard experiments and the Monte
αrio mis"is.
1. The Monte Carlo analysis with the heterogeneous model of the LLW source region could produce
isfrly good agreement as compared with the measured dose-equivalent rates on the hatch cover. On
I血 ot加「 hand, the Mo口te Carlo mis恥is with the homo即nized model overestimated the lllmsured
values 句a factor of 2.25 to 7ふ
2. Estimating from the present study, the dose-equivalent rate on the hatch cover is to be kas than 2.5
"Sv/h 吐 the ilユ aximum dose point, wlユ ercas l加 I
昭旧吐ion 血iit k 2,000"Sv/h. According, the
safaty 1actor (regulation liniit/ measured or calculated valuりis more than 80(1. On the shielding point
of view, it is confirmed t加t 山e shie亙lag des加n of the Seiri Maru is safe enough. The maximum
dosc -equivaleロtrate at 50 rn-distance from th巳 ship on the quay is no more than 0.05"Sv/h. This
value corTesponds to 0・438 mSv/y・ AccordinglL radiation exposure to 山e public is also safe enough
mthe ship.
3. The contribution oft加 gamma-ray sky-shine was obtained w皿 and without air around the detector
located on the quay at the port りthe Monte（ユrio calculation. In consequence, it accounted
approx加ately 血『 11% 血 the gamma-ray dose-equivalent rate under opening a hatch cover of the ship.
REFERENCES
LF. Brietrneisteら Editor, UMCNPThI・
A General Monte Carlo N-Particle Transport Code, Version
14A", Los Alamos National Laborato与LA-]2625-M (1993).
2. 晒LC. Hopkins, et al., "QAD-CGa（加mbinatorial Geometry Version of QAD-P5A, a Point Kernel
（加de for N巳mro口 and OatmnョRay Shfeld叱 Calculations二 Oak Ridgc Natiロロal Laboratory,
CCC-307 (July, 1977).
3. W.W. Eagle, Jr., et al., "A Users Manual for ANISN, a One-Dimensional Discrete Ordinates
Transport Code with Anisofropic Scattering'二 K-1963 (1967).
4. W.k Rhodes, and F.R. Mynatt, 'The DOT皿 Two-Dimensional Discrete Ordinates Transport
Code", ORNL-TM-4280 (1973).
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凸「 L.LW re9Ion In a hotd. Dimensions are all nentirneires
SelelMaru ：ロn No,4 Hold Hatch Coven
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hatch sever of time SdelM，『剖 ,
Meneured poInts are 51 3 rn-diStance from the bow side.
1044
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