Estimation of the effects of a lead vest on dose reduction for NPP workers using Monte Carlo calculations KIM JEONG-IN

Contents Introduction Materials and Methods Results and Discussion Future Works Conclusion

Introduction In the field of medical diagnosis or treatments using radiations, lead vests or aprons are widely used to protect the patients from unwanted irradiation. In nuclear power plants, it is recommended that the workers should wear a lead vest to reduce the dose for working in high radiation area. Various models have been designed and changed for convenience and effectiveness for dose reduction

Introduction Personal dosimeters like TLDs, Film badges have been used to estimate the doses of radiation workers The dosimeters cannot give the absolute values Measured values should be modified by comparing the reference conditions with conversion factors In ICRP 74, These conversion factors are tabulated for various particles, energies and typical phantom geometries.

Introduction The dose of a worker with a lead vest cannot be estimated directly from the personal dosimeter Many trials using two or more dosimeters (located in/outside the shield, head and chest respectively) were done. But it still has limitations. In the concepts of ALARA, conservative approach is not always the best choice. Through this research, the personal doses with/without a lead vest and the effectiveness were evaluated by Monte Carlo methods

Materials & Methods [ lead vest ] Used in KHNP nuclear site since 2004 Two front sides were folded 2.5cm*32cm*1.5mm lead plates were placed About 9.3kg

Materials & Methods [ The phantom ] MIRD-V Simplified human body with critical organs Front side : 3mm, back side : 1.5mm

Materials & Methods [ Monte Carlo Method ] Most of the natural phenomena are random nature. If we understand the underlying principles, we can simulate the process by Monte Carlo method Utility of Monte Carlo technique –Sampling –Particle behavior (molecules, radiation) –Reactions –uncertainty

Materials & Methods [ Monte Carlo Method ] Merits? –Can handle complicated 3-D geometries –Can visualize simulated particle behavior –Can give uncertainty estimates Drawbacks –Long computing time (geometry tracking) –Limited number of detectors

Materials & Methods [ Monte Carlo Method ] AP, PA, RLAT, LLAT Parallel photon beam (0.2, 0.4, 0.6, , 1.2MeV) MCNP-5 Monte Carlo transport code PA AP RLAT LLAT

Results [ AP ]

Results [ PA ]

Results [ RLAT ]

Results [ LLAT ]

Discussion The results have some limitations to apply due to the differences between phantom and real body In most cases, the lead vest decreases the organ doses about 30%. For low energy (<400keV), the lead vest is very effective to reduce the dose but it is not so good for high energy photon shielding (less than 20%). Both the shielding effect of lead vest and the delay of working time due to heavy clothing should be considered.

Discussion For thyroids, the doses to high energy photons increased by 5% on the contrary These results comes from scattered radiation (Build-up)

Future Works Monte Carlo results are not always good answers. For verification, real measurements should be done and compared For the optimal method of dose measurements using personal dosimeters and dose conversion factors from personal dosimeters

Future Works Other type of phantom models should be used to calculate the dose. More precise values and build up effects can be calculated.

Conclusion Using Monte Carlo method, the effects of a lead vest on dose reduction for various irradiation geometries were examined. In most cases, it had 30% dose reduction effect. But some problems should be considered were also found. Real measurements and applications to advanced man- like mathematical phantoms are required. This study may be applied to the better design of personal shielding and dose estimation procedures for practical use.

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