Study On The Photoneutrons In FLiBe Molten Salt Reactors

Delayed neutron is the key that the reactors can be controlled.Generally,the delayed neutrons are released by the fission products soon after the fission event.The yield of the delayed neutrons is mainly decided by the fissile nuclides.The effective fraction of the delayed neutrons is also affected by the construction,materials and sizes,etc.For the reactors with a large amount of Be or D,the delayed photoneutrons which is generated by??,n?reaction are also parts of the delayed neutrons.Due to the longer decay time,the reactors with delayed photoneutrons may be sluggish.The delayed photoneutrons may become the main concern at certain cases.In recent year,Shanghai Institute of Applied Physics of Chinese Academy of Science leads the TMSR project which aims at developing Molten Salt Reactors?MSRs?.The short-term goal includes a 10MWt solid-fueled MSR?TMSR-SF1?and a2MWt liquid-fueled MSR?TMSR-LF1?.MSR is one of the promising generation four reactors which are focusing on the Th-U cycle,the economy,security,sustainability and non-proliferation.The MSR is divided into two types according to the fuel.One is solid-fueled and the other is liquid-fueled.LiF-BeF₂ is widely used in MSR designs for the outstanding neutronic characteristics,such as TMSR-SF1 and TMSR-LF1.Since there is lots of Be in the core,it is necessary to analyze the photoneutrons in MSRs and its effect on the transient process and reactivity measurement which would be helpful for the operation or the experiments.At the beginning,the history and the status of MSRs are reviewed;the researches about photoneutrons in the other reactors are introduced.Because there are large differences in the core structures,the effective fraction and the effect of the photoneutrons differ a lot.The traditional neutron transportation model without the photons considered may be not enough to give the photoneutrons full consideration in MSR.In this work,the coupled neutron–photon transportation model are obtained on the particle conservation rule which is used to obtain the neutron transportation model.Then the software and the nuclear data used in this work are introduced after the introduction of the methods to solve the transportation equations.The coupled point reactor neutron–photon kinetics are gained based on the perturbation theory.The parameters in the kinetics are defined strictly;the algorithms and the approximation used are presented together.Based on the kinetics,the basis formulas such as the in-hour equations for the reactor control and experiment are deduced.In this work,the kinetics is solved by MATLAB code.The ODE build-in functions are used for the stiffness of the kinetics.With the simplified TMSR-SF1 model,the kinetics parameters were calculated.The fraction of the delayed photoneutrons was analyzed based on two kinds of methods.With the delayed photoneutrons,four types of transient processes including the critical,subcritical,the negative reactivity insertion and the positive reactivity insertion were studied and the effects on the reactivity measurement are particularly analyzed.At last,the inherent neutron sources including the??,n?,??,n?and the spontaneous fission neutrons were calculated according to different power histories based on one of the primary model of TMSR-LF1.The inherent neutron sources were concerned for the mixing the light nuclides?F,Li and Be?and the heavy nuclides?Th and U?.The possibility to recritical the reactor was analyzed on assumption of different power histories.Then,the effect of the photoneutrons was analyzed together with the fuel circulating and the inherent neutrons.The results show that,the delayed photoneutrons can not be omitted in reactivity measurement after the saturation,or it may give results with large error,even wrong results;the delayed photoneutrons causes longer wating time for the period measurement;for the liquid-fueled reactor,although the fraction of the delayed photoneutrons is higher than the solid fueled molten salt reactor,the circulating of the fuel salt would decrease the effect.And the main concern in measurement would be the inherent neutron source other than the photoneutrons.