Neutron capture therapy with 235U seeds.
A combination of brachytherapy and neutron capture therapy has been evaluated using 235U metal seeds and external neutron beam irradiation. When thermal neutrons are absorbed by 235U, high-energy neutrons and gamma rays are produced and some of these deposit energy in surrounding tissue. A Monte Carlo program, using the code MCNP, has been used to evaluate two sizes of 235U seeds in a water phantom. The results of flux suppression around the seeds and dose distributions are illustrated and discussed. The results show that high doses can be delivered in a relatively short time by using 235U seeds with neutron capture therapy. This therapy with multiple needles or seeds can be envisioned as a substitute for traditional brachytherapy to give an effective killing dose.
Published in Medical physics
The Chernobyl accident was the most traumatic event in the entire history of civil nuclear power. For some months we could only speculate and guess about the causes of the accident but in August 1986, the IAEA organised international conference in Vienna at which a group of Russian scientists and engineers made comprehensive presentations on what had happened. The accident of the Russian RBMK nuclear reactor, occured on 26 April 1986, was due to the three main design drawbacks: 1. The reactor had a positive void coefficient and, below 20 % power, a positive power coefficient, which made it intrinsically unstable; 2. The shutdown systems were too slow in its operation in the event; 3. There were no physical controls to prevent the staff from operating the reactor in its unstable regime or with safeguard systems seriously disabled or degraded. The accident was triggered by a turbo-generator experiment, when the reactor core contained water at just below the boiling point. When the experiment began, half of the main coolant pumps were slowed down and the flow reduction caused the water in the core to start boiling vigorously. The bubbles of steam that formed absorbed neutrons much less strongly than the water. They displaced and the number of neutrons in the core started rising. This situation increased the power of the reactor, more steam was produced and thus less neutrons were absorbed due to the phenomenon known as positive feedback. The reduction of neutron absorption cause
The moderator and coolant flows between the fuel elements (or rods) moderating the neutrons and carrying away the heat. The region inside the nuclear reactor where the fuel elements undergo fission to generate heat is called the nuclear reactor core. - Moderator: Slows down fast neutrons to thermal energy range. Moderators must be light materials to slow down neutrons without causing capture. Some moderators are: water, carbon, heavy water. - Coolant: Absorbs and removes the heat produced by nuclear fission. In most current commercial-scale nuclear reactors, purified regular water, called light water, is used as the coolant. Some other coolants are: heavy water, carbon dioxide or helium gas, or molten metals such as sodium, lead, or bismuth. - Control Rods: Absorbs neutrons, designed to reduce the amount of neutrons available to continue the chain reaction. The control rods, interspersed between the fuel elements in the reactor core, can be inserted into or out of the core as needed to control conditions or shut down the reactor. Some materials used for control rods are: boron, silver, indium, cadmium, or hafnium.
(primarily uranium-235 or plutonium-239) absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further fission. Reactors
A nuclear reactor is a device used to sustain a controlled fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production, and research. Fissile nuclei (primarily uranium-235 or plutonium-239) absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further fission. Reactors stabilize this, regulating neutron absorb
The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.
The fastest method for adjusting the levels of fission-inducing neutrons in a reactor is by movement of the control rods. Control rods are made of so-called neutron poisons and therefore absorb neutrons. When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces – often the moderator. This action results in fewer neutrons available to cause fission, which can reduce the reactor's power output. Conversely, extracting the control rod will result in an increased rate of fission events, which can increase power.
The physics of radioactive decay also affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "prompt neutrons") released immediately upon fission. The fission products that produce delayed neutrons have half-lives for their decay by neutron emission that range from milliseconds to as long as several minutes; considerable time is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise, the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction would be too short to allow for intervention. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There…
[The shape of the absorbed dosage in neutron irradiation of a water phantom].
The paper is concerned with the results of experimental and estimated investigations into the spatial distribution of an absorbed dose and the spectrum of neutrons during irradiation of a water phantom by a P-3 beam of a BP-10 reactor. The ratio of densely ionizing and rarely ionizing components of an absorbed dose as well as the ratios between neutrons of different energetic groups were shown to undergo considerable changes with the penetration of reactor neutrons into the depth of a tissue-equivalent medium. The obtained results serve as basic data in various biomedical investigations using reactor neutron beams, including the planning of their use in cancer therapy.
Published in Meditsinskaia radiologiia (1990)
[A comparative assessment of the changes in absorbed neutron doses and in the frequency of chromosome aberrations in samples of human blood lymphocytes by the depth of the water phantom during irradiation by the biomedical BR-10 reactor beam].
Distribution of the chromosome aberration frequency in human blood lymphocyte samples and absorbed doses have been compared by the water phantom depth during irradiation with 1.5 Gy neutrons (mean energy of 0.85 MeV). There is a good concordance of their depth distribution. The half-fall layer of the absorbed dose within the tissue-equivalent medium is similar (approximately 5 cm) with both measurements done. The aberration frequency in the biological samples placed outside the radiation field in the phantom increases which indicates that the neutron been bounds are indistinct upon passing the tissue-equivalent medium.
Published in Radiobiologiia
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