Development Of L-edge Densitometry And Its Applications In The Reprocessing Of Spent Nuclear Fuel

The accurate measurements of uranium,neptunium and plutonium are very important for nuclear material accounting and control analysis in the reprocessing of spent nuclear fuel.Radiation methodologies are usually used for concentration determination of urainum,neptunium and plutonium with none-destructive analysis.The concentration of the measuring object can be determined by the energy and radioactive intensity of the emiting rays.There are many radiation methodologies being used in nuclear material accounting and control analysis,including alpha ray counter / spectrometry,gamma ray spectrometry,gamma ray absorption method,X-ray fluorescence method and hybrid K-edge / K-XRF(HKED).Alpha ray counter / spectrometry are based on the alpha radioactivity of plutonium isotope.The plutonium isotope ratios are obtained by measuring the alpha ray intensity of different energies producing by the corresponding plutonium isotope.However,the alpha ray counter / spectrometry are easily affected by alpha radionuclide,and the alpha ray spectrometry should be used with mass spectrometry.Gamma ray spectrometry and gamma absorption method are used for concentration determination based on the radioactive intensity of gamma ray.Plutonium isotopes can emit gamma ray of different energies.The concentration of the isotope depends on the intensity of the corresponding characteristic energy of gamma ray.Gamma ray absorption method is realized by using radioisotope source to irradiate the measuring object.The transmission spectrum is related to the concentration.Isotopes analysis softwares,such as PC/FRAM and MGA,are needed in gamma ray spectrum for isotopic abundance calculation.Gamma ray absorption method is often used in on-line analysis and it can not be used for qualitative analysis.X-ray fluorescence method is used for concentration determination of uranium,neptunium and plutonium by measuring the intensity of the characteristic X-ray which is generated spontaneously or by being excited.Though it can determinate the concentration accurately and rapidly,it can not be used for high concentration samples(such as 100g/L).HKED is widely used in nuclear material accounting.It can not only determine the uranium concentration of high concentration samples accurately,but also can determine uranium and plutonium simultaneously in mixed samples.The digital L-edge densitometry is designed based on the K-edge techniques.It is applied for the concentration analysis of uranium,neptunium and plutonium with FAST-SDD detector.For this design,the multi-channel pulse amplitude analyzer(MCA)is simplified to analogy-to-digital convertor(ADC),filed-programmable gate array(FPGA)and micro control unit(MCU)to realize nuclear signal acquisition and digital pulse shaping.Firstly,the detector output signal is shaped to exponential decay signal by C-R circuit,and then it is amplified in the front-end circuit for ADC acquisition.The obtained signal after ADC is transmitted to FPGA for digital pulse shaping and amplitude discrimination,creating energy spectrum.Finally,the energy spectrum is sent to the upper software through the MCU.There are two parallel signal processing paths inside the FPGA,the fast and slow channels.The fast channel is optimized to measure the true incoming count rate and the slow channel is optimized to create energy spectrum with 12.8?s trapezoidal pulse shaping.In order to achieve digital adjustment of the offset and amplitude of nuclear signal,the offset adjust circuit and programmable gain amplifying circuit are designed in the front-end circuit.The adjustment is completed by the combination of MCU and digital-to-analog convertor(DAC).Considering the on-line applications of the digital L-edge densitometry,a CAN bus interface which can be used for long distance transmission is developed in the MCU.Trapezoidal pulse shaping algorithm is utilized in the L-edge densitometry to improve energy resolution and count rate.The influence of the distributed resistance and capacitance in the hardware on the rising edge of nuclear signal is equivalent to a result of R-C network.Its mathematical model is established to restore the rising time of the input nuclear pulse to improve the shaping performance.According to the falling edge of the shaped pulse,the pile-up pulse can be discriminated.By measuring the rising time and width of the shaped pulse,the pile-up pulse in which the amplitude of secondary pulse is recovered can be identified.This method can record the pile-up pulse,increasing count rate.The inverse method is applied in the fast channel to obtain the unit impulse signal generated by the incident ray in the detector.The total count rate of the unit impulse signal is used as the true incoming count rate for count losses correction in the slow channel.Compared to the trapezoidal pulse with short shaping time constant,the unit impulse signal has smaller width.Thus,it can eliminate the effects that the pile-up pulse has on the correction results.The working curves of uranium,neptunium and plutonium measurement are established by preparing standard aqueous solutions with different concentrations.The repeatability,accuracy and stability of the instrument are verified by both standard solutions and organic solutions.The results indicate that the absolute relative error of the instrument is less than 4%.The RSD of the solutions whose concentration is above 10g/L is better than 0.5%.The designed digital L-edge densitometry can be used for control analysis of nuclear material.