Attachment 5 Computation of Delayed Fission Product Gamma Ray Dose Rates from NCSU PULSTAR Reactor Using a Monte Carlo Number Albedo Approach B.E. Hey ABSTRACT Hey, Brit Elkington. Computation of Delayed Fisson-Product Gamma-Ray Dose Rates From NCSU PULSTAR Reactor Using a Monte Carlo Number Albedo Approach.(Under the direction of Dr. J. M. Doster.) The dose rate due to decaying fission-product gamma-rays escaping the PULSTAR reactor 10 minutes after a loss of water accident was investigated. A Monte Carlo simulation of the accident was developed using the departmental VAX-11/730 mini-computer. Dose rates were calculated at several hundred point detector locations in and around the Burlington facility so that isodose lines (lines of constant dose rate) could be established. Work preliminary to the simulation included the construction of a table of gamma-ray number albedos for use in sampling and dose estimation. It was found that many areas would be exposed to gamma radiation fields of significant intensity. The dose rate computed 20 feet directly over and in line-of-sight of the core was 230 +- 10 Rem/hr. The maximum dose rate computed on the bay floor was 175 +- 12 mRem/hr. Gamma-ray streaming through bay doors and windows resulted in maximum dose rates of 250 +- 13 mRem/hr and 175 +- 9 mRem/hr in the control room and loading dock respectively. The exposure occurring outside the reactor building was due primarily to skyshine and caused dose rates on the order of 4 +- 0.3 mRem/hr next to the building. The dose rate calculated for offices adjacent to the reactor bay were found to be negligible due to attenuation through the bay walls. The number albedos as well as the Monte Carlo accident simulations were benchmarked against available experimental and calculated data found in literature. Calculated total number albedos agreed with those given by Berger and Raso (9) generally to within 5 per cent. Comparisons between calculated dose rates and those measured at the Livermore Pool-Type Reactor (4) generally show agreement to within 50 per cent and less. COMPUTATION OF DELAYED FISSION-PRODUCT GAMMA-RAY DOSE RATES FROM NCSU PULSTAR REACTOR USING A MONTE CARLO NUMBER ALBEDO APPROACH by BRIT E. HEY A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science DEPARTMENT OF NUCLEAR ENGINEERING Raleigh 1 9 8 4 APPROVED BY: Committee Page ii BIOGRAPHY Brit Elkington Hey was born in Lansing, Michigan on November 27, 1957. At the age of five his family moved to Fort Worth, Texas where he lived for two years. Again his family moved to Houston, Texas where he graduated from Scarborough Senior High School in 1976. In August 1977 the author entered North Carolina State University <NCSU> in Raleigh and received a Bachelor of Science degree in Nuclear Engineering in May 1982. In August 1982 he obtained a fellowship and enrolled in the Nuclear Engineering Graduate School at NCSU where he is currently pursuing a Master of Science degree in Nuclear Engineering. In January 1980 the author entered the cooperative education program and was employed by Carolina Power and Light Company during alternate semesters as an undergraduate in NCSU. He is now employed by the Gulf States Utilities Company located in Beaumont, Texas. The author is married to the former Teresa Anne Gresham. Mrs. Hey is fully employed in the raising of their two children, Heidi and Houston. Page iii ACKNOWLEDGEMENT The author wishes to express his deepest gratitude to Dr. Joseph Michael Doster, the chairman of his Advisory Committee. His advice and council given in the preparation of this thesis proved both sound and invaluable. The author would also like to express his appreciation to his wife, Teresa, for her patience and understanding and to his little daughter, Heidi, for her sacrifices given unknowingly. Last but not least the author extends a very dear thanks to his friend, Nita Miller, for her support and encouragement during the course of this study. Page iv TABLE OF CONTENTS Page LIST OF TABLES • LIST OF FIGURES vi INTRODUCTION . • 1 LITERATURE REVIEW 6 GAMMA-RAY NUMBER AND DOSE ALBEDOS 16 Introduction • • • • . • • . • • • • • • • • 16 Definitions • • . • . • . • . • . • 18 Generation of Albedos Using Monte Carlo • • . • • • 23 Comparison of Results to Literature •..•.•.. 30 Use of Gamma-Ray Albedos in Sampling and Estimation • . • . • . 33 A MONTE CARLO MODEL OF DELAYED FISSION PRODUCT GAMMA-RAYS • • • • • • • • • • • • • • • • 42 Introduction and General Discussion • • • 42 Source Intensity and Energy Distribution • 49 Importance Sampling From Source Angular and Spatial Distributions • . • 54 Geometrical Considerations . • • • 65 Estimating the Dose • • • 68 BENCHMARKING • • 72 A Comparison to a Simple Analytical Solution • . 72 A Comparison to the Livermore Experiment • 79 RESULTS AND CONCLUSIONS 86 LIST OF REFERENCES • . • 99 Page v LIST OF TABLES Page 1. NCSU PULSTAR Reactor data . 3 2. Coefficients for least square fits to dose albedo parameters • . • . • 22 3. Total number albedo comparison 32 4. Total dose albedo comparison 32 5. Relative dose rate magnitudes for different times after shutdown . 52 6. Dose rate comparison to analytical results <mRem/hr) . • . 78 7. Dose rate comparison to measured values . 82 Page vi LIST OF FIGURES Page 1. Reactor building . 5 2. Geometry depicting particle reflection from a surf ace . 18 3. Geometry used in albedo computations 25 4. Direction cosines and angles of reflection 38 5. Total fission-product gamma-ray activity versus time after shutdown for reactor operating at 1 watt for 1000 hours . 53 6. Geometry of core and biological shield 55 7. Relative axial thermal flux distribution . 61 8. Geometry of simplified problem . 74 9. Lawrence Livermore reactor building . 80 10. Computed dose rates on bay floor <mRem/hr> 92 11. Computed dose rates on bay floor using the C-H formula <mRem/hr> . 93 12. Computed dose rates on reactor platform <Rem/hr> 94 13. Computed dose rates in control room <mRem/hr> . 95 14. Computed dose rates in loading dock <mRem/hr> . 96 15. Computed dose rates on bay roof <Rem/hr> . 97 16. Computed dose rates outside reactor building ( mRem/hr > • • • • • • • • • • • • • • • • • 98 INTRODUCTION For a pool-type research reactor a sudden loss of water from the tank is one of the more credible accidents (4). The North Carolina State University <NCSU) PULSTAR Reactor is susceptable to such an accident. Table 1 lists its operating characteristics. The loss of water covering the core could occur since the tank liner has penetrations which represent possible failure modes, permitting the draining of the tank into the reactor room basement. It is estimated that the time required to drain the tank through tank liner penetrations varies from 2.4 minutes for the 12 X 12 inch beam tube to 12.3 minutes for the 6 inch beam tube facility <20). The power density of the core is low enough to allow air cooling by natural convection after a sudden full power shutdown. Therefore failure of the fuel pin cladding would not result from loss of tank water (20). The problem arises in the external radiation dose resulting from decaying fission-products within the core. Gamma-rays, which contribute more than 97 per cent to the dose (4), scatter within the core, biological shield, building structure, and air at rates high enough to cause significant dose rates in many areas. Page 2 The objective of this study is to provide information relevant to several basic questions. They are: What dose rates are to be expected from this loss of water accident? What areas must be evacuated? What routes are the best to take? and; At what time can those evacuated areas be reentered? This study and its recommendations are to be incorporated into the Safety Analysis Report for the NCSU PULSTAR Reactor and become a permanent part thereof. The data most important to the formulation of safety recommendations are the dose rates found in and around the reactor building. As the dose rates are continuously decreasing due to radioactive decay of the fission products, all values computed in this study correspond to a reference time of 10 minutes after shutdown. The areas considered are the bay floor, reactor platform, control room, loading dock, offices, bay roof, and outside the reactor building. These locations are shown graphically on the isometric figure 1 with the exception of the offices. These are located immediately adjacent to and north of the reactor building. Page 3 Table 1: NCSU PULSTAR Reactor data <20> Power 1 MW steady state, 2200 MW pulse 13 Maximum Thermal 1.69 x 10/6 steady state Flux 3.72 x 10 pulse Fuel Sintered uo2 pellets 4 per cent enriched Cladding Zircaloy-2 Moderator-Reflector- Light water Coolant Representative Core 359 Kg uo2 , 12.6 Kg U-235 Excess Reactivity 5 per cent dk/k Temperature Coef. -8 x 10-5 dk/°F (cold,clean, rods out) Control Rods Ag-In-Cd (80-15-5> 3 shim safety, 1 pulse Core Dimensions 15 7/8 x 15 x 25 inches Core Volume U02 0.3823 Fractions Gap 0.0155 Water 0.4339 Zr-2 0.0802 Al 0.0881 Energy Production Reactor on 189 hrs/mo in 1983 44 hrs at 1 MW December 1983 Shielding High density and ordinary concrete Pool Volume 14,900 gals Reactor Bay height 55 ft Dimensions width 37 ft length 94 ft Page 4 The most accurate way of obtaining the dose rate information is to actually drain the reactor pool and measure the dose. However this being impractical the best approach is a Monte Carlo simulation of the gamma-ray transport following a loss of water accident.