32) Division scheme of fuel in the reactors with slow neutrons

A thermal reactor is a nuclear reactor that uses slow or thermal neutrons. ("Thermal" does not mean hot in an absolute sense, but means in thermal equilibrium with the medium it is interacting with, the reactor's fuel, moderator and structure, which is much lower energy than the fast neutrons initially produced by fission.)

Most nuclear power plant reactors are thermal reactors and use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutronsto low velocity thermal neutrons.

The nuclear cross section of uranium-235 for slow thermal neutrons is about 1000 barns, while for fast neutrons it is in the order of 1 barn. Therefore thermal neutrons are more likely to causeuranium-235 to fission than to be captured by uranium-238. If at least one neutron from the U-235 fission strikes another nucleus and causes it to fission, then the chain reactionwill continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass.

Thermal reactors consists of the following:

• Neutron moderator to slow down the neutrons. In light water reactors and heavy water reactors it doubles as the nuclear reactor coolant.

• Nuclear fuel, which is a fissile material, usually uranium.

• Reactor vessel that is a pressure vessel containing the coolant and reactor core.

• Radiation shielding to protect people and the environment from the harmful effects of ionizing radiation.

• Containment buildings which is designed, in any emergency, to contain the escape of radiation.

• Instrumentation to monitor and control the reactor's systems.

7. Radioactive capture cross section. Radioactive capture - the capture of a particle, as a neutron, by a nucleus, inducing the emission of electromagnetic radiation, as a gamma ray. Radiative capture reactions: . The most important stellar capture reactions: . ..For examples: ¹²C(n,γ)¹³C; ¹¹C(p,γ)¹²N. The starting point to calculate transition per unit time from initial state |i〉 to final state |f 〉 is the Fermi's golden rule. It assumes that the transition occur due to a perturbation and

is given, to first order in the perturbation, by

where ρ is the density of final states (number of states per unit of energy)and 〈i| H’| f 〉 is the matrix element of the perturbation H' between the final and initial states. Fermi's golden rule is valid when the initial state has not been significantly depleted by scattering into the final states. Cross sections for capture reactions

Energy dependence of capture cross sections. Charged particles. Energy dependence is mainly determined by the entrance channel wave function Fl(kr,η). At small stellar energieswhereI is the modified Bessel function. Since η~1/E1/2, the main dependence on energy is concentrated in the exp(-πη) factor so that whereS(E) is the astrophysical factor.

16. Nuclear reaction induced by gamma rays. Photodisintegration (also called phototransmutation) is a physical process in which an extremely high energy gamma ray is absorbed by atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron or an alpha particle is effectively knocked out of the nucleus by the incoming gamma ray. This process is essentially the reverse of nuclear fusion, where lighter elements at high temperatures combine together forming heavier elements and releasing energy. Photodisintegration isendothermic (energy absorbing) for atomic nuclei lighter than iron and sometimes exothermic (energy releasing) for atomic nuclei heavier than iron. Photodisintegration is responsible for thenucleosynthesis of at least some heavy, proton rich elements via p-process which takes place in supernovae. A photodisintegration reaction

. was used by James Chadwick and Maurice Goldhaber to measure the proton-neutron mass difference. This experiment proves that a neutron is not a bound state of a proton and an electron,[3]as had been proposed by Ernest Rutherford. In explosions of very large stars (250 or more times the mass of earth's Sun), photodisintegration is a major factor in the supernova event. As the star reaches the end of its life, it reaches temperatures and pressures where photodisintegration's energy-absorbing effects temporarily reduce pressure and temperature within the star's core. This causes the core to start to collapse as energy is taken away by photodisintegration, and the collapsing core leads to the formation of a black hole. Photofission is a similar but distinct process, in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission .Very high energy gamma rays have been shown to induce fission in elements as light as tin.