붕화금속 나노분말이 함유된 연성 방사선 차폐재료 개발

Abstract

This research was conducted to develop a soft material including metal boride nanoparticles for radiation shielding. The optimum conditions for the synthesis of nanoparticles were analyzed by numerical simulation of RF thermal plasma for synthesizing metal nanoparticles. The metal boride nanoparticles were produced by using an RF thermal plasma system. In addition experiment was conducted on the radiation shielding characteristic of soft materials which are hydrogels including nano and micro powders. In case of the 5 coil turn for the RF plasma torch, the high temperature area over 9,000 K was distributed widely along the torch center and the high temperature area became smaller with increasing the torch radius. In the case of 20 mm for the torch radius, the high temperature area over 9,000 K was distributed widely from an injector to the torch exit. Meanwhile, the velocity of the plasma is higher than other design conditions resulting a relatively short residence time of raw material. The plasma velocity had decreased by increasing the number of coil turn. In plasma numerical simulation results about operation conditions, the high temperature area was shifted from torch center to the torch wall with increasing the injector length. Therefore, it is considered that the increasing of injector length is a bad condition for nanoparticle synthesis. In the case of 30 mm for the injector length, temperature at the torch center is about 6,000 K. On the other hand, it was found that the center temperature was 9,000 K when the injector length is from 10 to 20 mm. The length of injector which is located at the axis of the plasma generation area affects the distribution of plasma temperature while it has small effect on plasma velocity distribution. The high temperature area of 10,000 K is moved to the torch center region with increasing the sheath gas flow rate. At 70 L/min for sheath gas flow rate, the highest temperature area was found. Also, the plasma velocity is naturally increased with increasing the total flow rate. Raw powders used for synthesizing the metal boride nanoparticles are boron, nickel, titanium, and tungsten. In the case of titanium boride, boron and titanium were mixed at the ratio of 1:5. The RF thermal plasma was operated at 40 kW for plate power, 500 torr for chamber pressure, 10 L/min for powder feeding flow, 60 L/min for central gas flow, 80 L/min for sheath gas flow, 180 L/min for quenching gas. The size of synthesized titanium boride nanoparticles distributed in 20~110 nm with 42 nm for D50 which is a representative value of the particle size. In order to measure the gamma ray shielding accurately for the soft material, one side of the lead shield was closely adhered to the source and the other side was closely adhered to the sample material. The gamma ray sources were Cs-137(0.662 MeV) and Co-60(1.17 and 1.33 MeV). As the thickness increased by 5 mm for each sample, the transmittance was decreased by 6 ~ 18%. The lowest transmittance of the hydrogel including lead oxide micro powder was 37.97%. The hydrogel including lead oxide micro powder had the highest attenuation coefficient of 0.296 cm-1. Attenuation coefficients of pure hydrogel and hydrogel including iron oxide and tungsten oxide micro powders were measured at 0.216, 0.246 and 0.251 cm-1, respectively for Cs-137 source. In γ-ray shielding experiment with Co-60, the highest attenuation coefficient was 0.181 cm-1 with the hydrogel including lead oxide micro powder.