ASTER. Dose Rate Visualization of Radioisotope Thermoelectric. Generators. RECElVED BEC OF THIS DOCUMEMT IS UNLlMIi L, Hanford Company

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1 Dose Rate Visualization of Radioisotope Thermoelectric RECElVED Generators BEC Prepared for t h e US. De.partment of Energy Assistant Secretary for Environmental Management Westinghouse Hanford Company Richland, Washington Management and Operations Contractor for the U.S. Department of Energy under Contract DE-AC06-87RL10930 Copyright Licwm By acceptance of th'i ardde. the pubbher andlor rm-pient acknowledges the U.S. Government's tight to retain a nonexdusive, roydty-free license in and to any copytigh: covering 6 s paper. Approved for public release OF THIS DOCUMEMT IS UNLlMIi L, T?iSTRIBUTION OF THIS DOCUMENT IS UNLIMITEDd p WTRIBUTION ASTER

2 Portions of this document may be illegible in electronic image pro dud^. b a g s are produced from the best available original doculilerrt,

3 - _---_->-~ WHC-SA FP 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 Hanford Company P.0 Box 1970 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 t o retain a nonexdusive, roym-free license in and to any copyright covering this paper. Approved for public release -r-

4 LEGAL DISSCLAIMER This report was prepared as an account of work sponsored by an agency of t h e United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party's use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or senn'ce by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors. The views and opinions of euthors expressed herein do not necessarily state or reflect those of the United States Governmenfor any agency thereof. This report has been reproduced from the best available copy. \ Rinted in the United Stater of America DISCLMP.CHP (1-91)

5 DOSE RATE VISUALIZATION OF RAD IO ISOTOPE THERMOELECTRIC GENERATORS Randolph A. Schwarz, Steve F. Kessler, and Tom A Tomaszewski Westinghouse Hanford Company P. 0. Box 1970 Ri chl and, WA (509) , , and Abstract Ad\ an d visualization techniques can be used t o investigate gamma ray and neutron dose r a t e s around complex dose r a t e 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 r a t e f o r 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 r a t e 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 l e s s 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 r a t e s f o r handling the RTG's. To meet this need a computer program has been developed t h a 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 f o r Packaging (Barklay 1993). The GPHS source configuration consists of 18 GPHS modules stacked vertically. Each GPHS module contains two impact shells w i t h each impact shell containing two iridium clad fuel p e l l e t s encased in graphite.. Included i n Figure 1 i s a plot o f the MCNP model f o r the source conf i gurati on. MCNP models were created for three shielding configurations..the f i r s t configuration i s f o r a f u l l y shielded transportation package. Include in Figure 1 shows an MCNP plot of t h i s configuration. The shielding around the GPHS RTG '

6 Cross Section of GPHS Module indium clad Graphite I I- 11 Water I /!I I I 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.

7 package consists o f two bells, an external bell which contains a 3.2 cm thick water jacket for neutron shielding which i s encased in s t e e l and an inner s t e e l bell 1.9 cm thick. The second configuration consists o f a GPHS RTG with the inner steel shield i n place and w i t h the outer shield raised 28.6 cm. The outer shield will be raised f o r i n s t a l l i n g and removing the hold down b o l t s f o r the inner shield. The t h i r d configuration consists of a GPHS RTG with no external shielding. Both the inner and outer bells have been removed. This represents an i n i t i a l configuration prior t o installing 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 f o 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 t h e r the gamma, neutron, o r t o t a l dose rate. The user s e l e c t s a shielding configuration for the RTG o r multiple RTG's t o simulate' an actual storage, shipping or handling operation. The user can then i n s e r 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 t o t a l dose t h a 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 removing an RTG from a truck and then removing the shields. These frames-can then be sequenced together t o create an animation t h a t can be used t o calculate the t o t a 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 r e s t r i c t i o n 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 t h a 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 allows the user t o define the location and shielding configuration f o 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 c k figures on the p l o t. Other o p t i o n s allow the user t o zoom, unzoom, print, specify contour levels, s e t the shield I

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10 configuration, and indicate the of dose rate type (neutron, gamma, or total) desired. In Figure 2 a person has been inserted into the figure beside the RTG. The total accumulated dose is calculated at the cross point between the arms and the body. The person is located at 100 cm from the center of the RTG and his total accumulated dose is 98 mrem. This accumulated dose assumes the person is at this location for 1 hour. Figure 3 shows the contour plot that is generated when the outer shield is raised 28.6 cm. Notice that the contour lines now indicate an increased source near the bottom of the RTG where there is less shielding. The person in Figure 3 is at the same location as in Figure 2, except now his accumulated dose is 101 mrem. The computer program can a1 so di spl ay the statistical errors associated with the data in the plot. Because the dose rate is calculated using the Monte Carlo N Particle (MCNP) computer code, each data point has a statistical uncertainty associated with it. The "Show Error" button will produce a color contour plot of the statistical uncertainty at each data point. The "Show Grid" button will show the location of the data points used to create the contour plot. Both the "Show Error" and "Show Grid" options can be used to understand if anomalies in the plot are due to a real effect or a statistical fluctuation. SUMMARY Advanced visualization techniques can be used to bring complex shielding calculations to a level that can be used for operational dose rate assessments, job planning, and personnel safety awareness. Acknowl edqments Funding for the work reported in the paper was provided by The United States Department of Energy (DOE). References Barklay, Chadwick D. (1993) "Safety Analysis Report for Packaging (SARP) for the Mound 1 kw Package," MLM-MU , Rev. 6. Briesmeister, Judith F. (1993) "MCNP-A General Monte Carlo N-Particle Transport Code, LA M.