Dose Rate Visualization of Radioisotope Thermoelectric Generators RECEl VED

BEC 0 5 1995

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Dose Rate Visualization of Radioisotope Thermoelectric Generators R. A. Schwarz S. F. Kessler T. A. Tomaszewski

Date Published September 1995

To Be Presented at Space Technology & Applications International Forum (STAIF-96) Albuquerque, New Mexico January 7-1 1 , 1996

Prepared for the U.S. Department of Energy Assistant Secretary for Environmental Management

Westinghouse P.0 Box 1970 Hanford Company Richland, Washington

Management and Operations Contractor for the U.S. Department of Energy under Contract DE-AC06-87RL10930

C4pyrigh-t h s a By acceptance of this -'de, the publisher andlor redpimt admowledges the U.S. Government's right to retain a nonexdusive, roym-free license in and to any copyright covering this paper.

Approved for public release

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DISCLMP.CHP (1-91)

DOSE RATE VISUALIZATION OF RAD IO ISOTOP E THERMOELECTRIC GENERATORS

Randolph A. Schwarz, Steve F. Kessler, and Tom A Tomaszewski Westinghouse Hanford Company

P. 0. Box 1970 Ri chl and, WA 99352

(509)376-5977, 376-9953 , and 376-4858

Abstract

Ad\ an d visualization techniques can be used t o investigate gamma ray and neutron dose ra tes around complex dose ra te intensive operations. A method has been developed where thousands of dose points are calculated using the MCNP (Monte Carlo N-Particle) computer code (Briesmeister 1993) and then displayed t o create color contour p l o t s of the dose ra te for complex geometries. Once these contour p l o t s are created, they are sequenced together creating an animation t o dynamically show how the dose ra te changes with changes in the geometry or source over t i me.

INTRODUCTION

Advanced visualization techniques were used t o investigate gamma ray and neutron dose rates around the Radioisotope Thermoelectric Generator (RTG) '

transportation system being designed for the shipment of RTG's from Mound, O h i o t o the Kennedy Space Center in F l o r i d a . The radioactive material contained in the RTG's i s plutonium oxide consisting of 80 t o 86 atom percent 238Pu which produces heat as i t decays.

The RTG produces a significant neutron and gamma ray source term. The neutron source i s dominated by the spontaneous fission of the p l u t o n i u m isotopesh6the gamma ray source term i s dominated by 208T1 w h i c h i s a decay product of Pu . To minimize the gamma ray source term, the amount 'of 236Pu must be minimized. A typical RTG will have less t h a n 1 ppm o f 236Pu.

Because of the high dose rates associated w i t h the RTG's i t i s necessary t o have a method t o accurately predict the dose ra tes fo r handling the RTG's. To meet this need a computer program has been developed tha t allows a user t o select a shielding configuration and source term and calculated anticipated dose r a t e exposures t o personnel.

DOSE RATE CALCULATION

The MCNP computer code was used t o calculate, the gamma and neutron dose rates using the source term for the General Purpose. Heat Source (GPHS) RTG documented in the Safety Analysis Report for Packaging (Barklay 1993). The GPHS source configuration consists of 18 GPHS modules stacked ver t ical ly . Each GPHS module contains two impact shel ls w i t h each impact shell containing two iridium clad fuel pe l le t s encased in graphite. . Included i n Figure 1 i s a plot o f the MCNP model fo r the source conf i gurati on.

MCNP models were created for three shielding configurations. .The f i r s t Include in Figure

The shielding around the GPHS RTG configuration i s for a fu l ly shielded transportation package. 1 shows an MCNP plot of t h i s configuration.

Cross Section of GPHS Module

indium clad

Graphite I

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1 I

Water 1 / !I

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FIGURE 1. MCNP Model Showing the RTG Source and S h i e l d i n g C o n f i g u r a t i o n .

package consis ts o f two bel ls , an external bell which contains a 3.2 cm thick water jacket for neutron shielding which i s encased in s tee l and an inner s tee l bell 1.9 cm thick.

The second configuration consists o f a GPHS RTG with the inner s teel shield i n place and w i t h the outer shield raised 28.6 cm. The outer shield will be raised for i n s t a l l i ng and removing the hold down b o l t s fo r the inner shield.

The th i rd configuration consists of a GPHS RTG with no external shielding. This represents an i n i t i a l B o t h the inner and outer bel ls have been removed.

configuration prior t o instal l ing the shielding for'transporting the package.

The configurations were chosen t o be representative of operational conditions expected when transporting the RTG packages. Both neutron and gamma ray calculations were made for each shielding configuration.

DOSE RATE VISUALIZATION

A computer program was written t,o read in the MCNP data and generate color contour p l o t s o f the dose r a t e around an RTG fo r the different shielding configurations. The computer program reads the dose rates associated w i t h each shielding configuration calculated by the MCNP computer code then creates a contour p l o t for each time u n i t selected. The user can display e i ther the gamma, neutron, o r to ta l dose rate.

The user se lec ts a shielding configuration for the RTG o r multiple RTG's t o simulate' an actual storage, shipping or handl ing operation. The user can then inser t into the program the expected locations o f personnel and the amount of time the personnel will be i n each location t o calculate the to ta l dose tha t will be received by personnel.

The user can s e t u p a number o f frames t o simulate an operation, such as I

removing an RTG from a truck and then removing the shields. These frames-can then be sequenced together t o create an animation tha t can be used t o calculate the to ta l dose f o r the personnel present w i t h i n 'the animation. Individual color frames have proven t o be useful pre-planning t o o l s t o communicate understanding a b o u t radiation levels and res t r ic t ion zones t o personnel who will be involved w i t h the work.

Figure 2 shows a p l o t of the graphical user interface tha t i s used t o create the contour plots. The main p o r t i o n of the interface consists o f a p l o t window showing a contour p l o t of the dose rates for the specified geometry. In t h i s case the geometry consists of a shielded RTG.

The geometry i s constructed using the "setup" b u t t o n which al lows the user t o define the location and shielding configuration fo r each RTG i n each frame. Once these configurations are defined, the user can then use the control b u t t o n s shown a t the t o p l e f t o f Figure 2 t o r u n an animation.

The user i s provided w i t h numerous o p t i o n s shown on the l e f t side of Figure 2. The "people" b u t t o n allows for the insertion o f personnel i n t o the contour p l o t . These personnel will show u p a s s t i ck figures on the p l o t . Other o p t i o n s allow the user t o zoom, unzoom, pr int , specify contour levels , s e t the shield

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conf igu ra t ion , and i n d i c a t e the of dose r a t e t y p e (neutron, gamma, o r t o t a l ) d e s i r e d .

In Figure 2 a person has been i n s e r t e d i n t o the f i g u r e bes ide the RTG. The t o t a l accumulated dose i s c a l c u l a t e d a t the c ross po in t between the arms and the body. The person is loca ted a t 100 cm from the center of the RTG and his t o t a l accumulated dose i s 98 mrem. This accumulated dose assumes the person i s a t this l o c a t i o n f o r 1 hour.

F igure 3 shows the contour p l o t t h a t i s generated when the o u t e r s h i e l d i s r a i s e d 28.6 cm. Notice t h a t the contour l ines now i n d i c a t e an increased source near the bottom o f the RTG where there i s less sh ie ld ing . The person i n Figure 3 is a t the same l o c a t i o n a s i n Figure 2, except now his accumulated dose i s 101 mrem.

The computer program can a1 so d i spl ay the s t a t i s t i c a l e r r o r s a s soc ia t ed w i t h the d a t a i n the p l o t . Because the dose r a t e i s c a l c u l a t e d us ing the Monte Carlo N P a r t i c l e (MCNP) computer code, each da ta po in t has a s t a t i s t i c a l unce r t a in ty a s s o c i a t e d w i t h i t . The "Show Error" button will produce a c o l o r contour p l o t o f the s t a t i s t i c a l unce r t a in ty a t each d a t a po in t . The "Show Grid" but ton will show the l o c a t i o n of the d a t a po in t s used t o c r e a t e the contour p l o t . Both the "Show Error" and "Show Grid" opt ions can be used t o understand i f anomalies i n the p l o t a r e due t o a r e a l e f f e c t o r a s t a t i s t i c a l f l u c t u a t i o n .

SUMMARY

Advanced v i s u a l i z a t i o n techniques can be used t o br ing complex s h i e l d i n g c a l c u l a t i o n s t o a l eve l t h a t can be used f o r opera t iona l dose r a t e assessments, j o b planning, and personnel s a f e t y awareness.

Acknowl edqments

Funding f o r the work repor ted i n the paper was provided by The United S t a t e s Department o f Energy (DOE).

References

Barklay, Chadwick D. (1993) "Safety Analysis Report f o r Packaging (SARP) f o r the Mound 1 kW Package," MLM-MU-91-64-001, Rev. 6.

Br i e sme i s t e r , J u d i t h F. (1993) "MCNP-A General Monte Carlo N-Part ic le Transport Code, LA-12625-M.