Activation of the Structural Materials of Fast-Neutron Generators
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ACTIVATION OF THE STRUCTURAL MATERIALS OF FAST-NEUTRON GENERATORS
R. N. Rodionov, A. O. Kovalev, D. V. Portnov, and Yu. A. Kashchuk
UDC 539.16.07
The activation of a neutron generator is modeled for two operating scenarios: continuous for 16 h and 100 h at 4 h/day for one month. The distribution of the equivalent γ-ray dose rate was calculated for different times after the generator was shut down. Different grades of steel were considered for the material of the neutron generator flange. The time dependence of the γ-ray dose rate was calculated for each variant of steel. For long holding times, the steel 316L(N)-IG exhibits the lowest dose rate.
Neutron generators are widely used in different fields of science and technology as sources of fast neutrons and they are an alternative to nuclear reactors, particle accelerators, and radioisotopic sources. They are used in the oil and gas production industry for well-logging, in analytical applications for activation analysis, and safety systems for detecting narcotics and explosives as well as in medical research [1]. Currently, there exist in the world several manufacturers producing compact generators with yield 108–1011 sec–1. Sealed neutron tools are used as the source. An ion acceleration section with acceleration voltage, as rule, not exceeding 250 keV, a Penning ion source, a tritium target, and a system for filling and storing a deuterium-tritium gas mixture are arranged inside the tube. In powerful neutron generators, such as RTNSII (USA), SNEG-13, NG-12 (Russia), or FNS (Japan), accelerating voltages up to 400 kV, more powerful sources of ions (RF discharge, duaplasmatron), and complicated rotating, cooled, tritium targets were used to obtain output above 1012 sec–1. These facilities are used to study questions pertaining to radiation materials science and the radiation resource of materials and components, to measure the cross sections of reactions driven by fast neutrons, and other problems. Currently, as scientific problems are solved and as a result of the high cost of operation, almost no operating powerful neutron generators remain in the world. Nonetheless, there is still a demand for them, for example, for actual benchmark experiments, use of 14 MeV neutrons as a primary standard, radiation therapy using fast neutrons, and other needs. A NG-12I powerful pumped neutron generator with output >1012 sec–1 is successfully operating at the Ural Center for Neutron Therapy as a source of 14 MeV neutrons. Over 10 years of operation, more than a thousand cancer patients have been treated at the center, and the demand for such treatment is several-fold greater than the center’s capacity [2]. Formulation of the problem. For medical applications of neutron generators, it is assumed that structural materials are selected and optimized so as to decrease the activation level of the target unit and correspondingly the patient dose load during beam adjustment and preparatory operations. Moreover, powerful neutron generators operate for several decades. Long-lived rad
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