Study On Radiation Of Boron Carbide In 300 # Research Reactor

Boron carbide ceramics was used as control assembly material in SPRR-300(Swimming Pool Research Reactor-300), boron carbide ceramics has changed in property and swelling after long-term irradiation during 30 years lifetime of the reactor. Helium, results in the capture of a neutron by 10B, causes the swelling of boron carbide. It was found that the irradiation swelling of boron carbide mainly decided by neutron fluence level and irradiation temperature by affecting the amount of helium produce and release in the pellet. Unfortunately, neutron fluence rate and temperature of control rod could not be measured experimentally directly. Neutronics analysis and reactor thermal hydromechanics were done to obtain these factors. After acquisition of neutron fluence rate and heat generated in Boron carbide pellet, we can use FLUENT to work out the maximum temperature inside boron carbide pellet under condition that reactor power was 3 MW. We measured the swelling of 5 different neutron fluence samples, the relation between swelling and neutron fluence rate was found, at last, mechanism of swelling in boron carbide was disscussed.In case of calculating the neutron fluence rate of safety rod, especially the top end of safety rod, a problem must be overcome, that was less count when neutron transport at long distance in little volume. We applied different variance reduction techniques in the computation to solve it. Several variance reduction methods were tested, and finally an efficient variance reduction solution for this problem was found. The reactor computation model was changed to describe water in detail with deferent levels, so that deferent cell importance could be designate respectively. More neutron tracks were traced into the destination cell by this variance reduction solution. As a result, the neutron fluence rate was done successfully.A symmetry model of boron carbide was achieved by modeling software, ICEM. We can get the maximum temperature of boron carbide by using FLUENT with appropriate grid and input data of heat generated in boron carbide provided by MCNP. As a result maximum temperature in boron carbide pellets were less than 1000℃, so we can know that helium generated in the capture of neutron with 10B was mainly sustain in the pellet which cause the swelling of boron carbide material.Five sampls with different neutron fluence rate from 2.00×1017 to 5.08×1021 n/cm2 was choosed to measure the density according to the neutron fluence rate computation results, Density and volume changed after irradiation by neutrons was found for the samples, the volume swelling will be larger with the higher neutron fluence rate samples. Because of the low neutron fluence rate in Safety Rod, the samples took from the Safety Rod have less swelling than others, meanwhile the samples took from control Rod have bigger swelling for its higher neutron fluence rate. Sample’s swelling rate is higher with the increase of neutron fluence rate, maximum volume swelling rate was 2.905% for 5# sample, which is the biggest volume swelling rate for all samples.At last, mechanism of swelling in boron carbide and key factors such as neutron spectra, neutron fluence and neutron capture by 10B which influence the swelling rate was disscussed.