Boron Equivalent Measurement Of Molten Salt At A Photoneutron Source

There are many kinds of materials that can be used in nuclear reactors,such as graphite used as moderator,and/or reflector,molten salt used as fuel carrier or coolant,alloy used as structural material.These materials are arranged in or around the reactor core,undergoing intense neutron irradiation,high temperature and pressure,and facing a series of challenges of issues of nuclear,mechanical,chemical,irradiation and economic.In terms of reactor physics,it is necessary to focus on the nuclear performance.In order to reduce the neutron consumption,critical concentration,critical mass or critical dimension,it is necessary to use moderator,reflector and structural materials with smaller neutron absorption cross section.The absorption of thermal neutrons by these impurities is expressed as boron equivalent,which is represented for the purity of nuclear materials.Boron equivalent is one of the important indicators of nuclear materials.Both the existing reactors and the generation IV reactors have the requirement of boron equivalent for those materials with large and important consumption.For example,the boron equivalent of graphite must be less than 80 ppm for molten salt reactor(MSRE),and less than 1.3 ppm for hightemperature gas-cooled reactor.In the thorium-based molten salt reactor(TMSR),the boron equivalent of graphite and molten salt(without lithium)must be smaller than 2 ppm.Therefore,it is very important to evaluate and measure the boron equivalent of nuclear materials with a high accuracy.Boron equivalent can be determined by the elemental trace analysis method or by measurement of thermal neutron macroscopic absorption cross section.The former method is a common method,which includes Inductively Coupled Plasma Mass Spectrometry(ICP-MS,Laser Ablation Inductively Coupled Plasma Mass Spectrometry(LA-ICP-MS,Glow Discharge Mass Spectrometry(GD-MS,and so on.The main advantage of this method is that it can accurately give the composition and content of impurities of a small sample.However,it also faces some challenges,such as introduction of the uncertainty during the processing of sample dissolution or injection and without ability to provide the average boron equivalent for bulk materials.The latter method is also used for the BE measurement,but only a few experiments have been carried out up to now.The main advantage of this method is that it can give the total boron equivalent for both small and big samples without special treatment.However,the measurement accuracy of the existing methods is low,even for a long measurement time,and the effect of energy spectrum cannot be determined.Therefore,it is very important to develop a rapid and accurate boron equivalent measurement method for bulk nuclear materials.The photoneutron source driven by a 15 MeV electron LINAC with high neutron intensity,good direction,and almost similar energy spectrum to thermal molten salt reactors,is firstly used for the boron equivalent measurement.This method can effectively improve the accuracy of boron equivalence measurement,but with a series of challenges.First,it is necessary to develop a suitable theoretical method for the photoneutron source according to its characteristics.Then,due to the high background of neutron and ?-ray,additional shielding needs to be constructed to further reduce the neutron and ?-ray backgrounds for the boron equivalence measurement.Finally,boron equivalent measurement is a trace measurement,which requires higher calibration samples,and the measurement environment is complex,so it is necessary to establish a set of appropriate data correction process to improve the reliability of boron equivalent measurement.Based on an elaborate consideration aiming to the above factors,the boron equivalent measurement of graphite and molten salt materials based on the photoneutron source is completed for the first time.The main contents of the paper are organized as follows:1)First of all,the measurements of boron equivalent in the existing experiments are based on isotope neutron sources,which are not applicable for the measurement on the photoneutron source.According to the basic theory of the transmission method,the relation between ?ln()and boron equivalent is established for measurement,and the data correction is also proposed.2)Secondly,boron equivalent is measured through the relative change of thermal neutron counts between the calibration samples which requires a low neutron and ?-ray backgrounds.However,due to the compact layout of the photoneutron source,all devices are arranged in a shared hall,leading to a high background of neutron and ?-ray.The local shields are needed to reduce the backgrounds.A two-step subsection simulation based on MCNP is performed for the new background shielding design,which can reduce the computing time and improve the precision of simulation.3)Thirdly,a large number of graphite was used as moderator in the molten salt reactor due to its smaller absorption cross section.Boron equivalent calibration experiment based on the photoneutron source was carried out with nuclear graphite.In order to test the reliability of the experimental process,the thermal neutron scattering cross section of graphite are calculated and compared with the evaluated data and the existing experiment results.Through the checking and correcting of experimental data,the boron equivalent calibration parameters of graphite are obtained,and the uncertainty is analyzed.4)Finally,according to the standard measurement process of boron equivalent of nuclear graphite,the boron equivalent measurement of beryllium fluoride molten salt was carried out.Because of the particularity of beryllium fluoride sample,it is necessary to analyze the influence of seal box by simulation.With the same data processing as the graphite boron equivalent measurement,the boron equivalent calibration experiment of beryllium fluoride molten salt is also completed at the photoneutron source.The boron equivalent of graphite and molten salt materials is measured for the first time based on the photoneutron source.The standard measurement process of boron equivalent is established,which can be used for rapid measurement of boron equivalent of large volume and complex materials,and therefore provides an alternative approach to the elemental trace analysis method.