A report on Neutron temperature, Neutron moderator and Pressurized heavy-water reactor

A chart displaying the speed probability density functions of the speeds of a few noble gases at a temperature of 298.15 K (25 C). An explanation of the vertical axis label appears on the image page (click to see). Similar speed distributions are obtained for neutrons upon moderation.

In a system at thermal equilibrium, neutrons (red) are elastically scattered by a hypothetical moderator of free hydrogen nuclei (blue), undergoing thermally activated motion. Kinetic energy is transferred between particles. As the neutrons have essentially the same mass as protons and there is no absorption, the velocity distributions of both particles types would be well-described by a single Maxwell–Boltzmann distribution.

fission cross section - while a nonlinear relationship is apparent, it is clear that in most cases lower neutron temperature will increase the likelihood of fission, thus explaining the need for a neutron moderator and the desirability of keeping its temperature as low as feasible.

Fission cross section, measured in barns (a unit equal to 10−28 m2), is a function of the energy (so-called excitation function) of the neutron colliding with a 235U nucleus. Fission probability decreases as neutron energy (and speed) increases. This explains why most reactors fueled with 235U need a moderator to sustain a chain reaction and why removing a moderator can shut down a reactor.

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy.

- Neutron moderator

A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator.

- Pressurized heavy-water reactor

The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature.

- Neutron temperature

The mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons have lower energies (neutron temperature after successive passes through a moderator roughly equals the temperature of the moderator) than in traditional designs, where the moderator normally is much hotter.

- Pressurized heavy-water reactor

Heavy water reactors and graphite-moderated reactors can even use natural uranium as these moderators have much lower neutron capture cross sections than light water.

- Neutron temperature

Deuterium, in the form of heavy water, in heavy water reactors, e.g. CANDU. Reactors moderated with heavy water can use unenriched natural uranium.

- Neutron moderator

5 related topics with Alpha

Nuclear reactor

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Atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions.

An example of an induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and nuclear weapons rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.

The Chicago Pile, the first nuclear reactor, built in secrecy at the University of Chicago in 1942 during World War II as part of the US's Manhattan project

Lise Meitner and Otto Hahn in their laboratory

Some of the Chicago Pile Team, including Enrico Fermi and Leó Szilárd

Primary coolant system showing reactor pressure vessel (red), steam generators (purple), pressurizer (blue), and pumps (green) in the three coolant loop Hualong One pressurized water reactor design

NC State's PULSTAR Reactor is a 1 MW pool-type research reactor with 4% enriched, pin-type fuel consisting of UO2 pellets in zircaloy cladding.

Treatment of the interior part of a VVER-1000 reactor frame at Atommash

In thermal nuclear reactors (LWRs in specific), the coolant acts as a moderator that must slow down the neutrons before they can be efficiently absorbed by the fuel.

The Magnox Sizewell A nuclear power station

The Torness nuclear power station – an AGR

Scaled-down model of TOPAZ nuclear reactor

The Superphénix, closed in 1998, was one of the few FBRs.

Three of the reactors at Fukushima I overheated, causing the coolant water to dissociate and led to the hydrogen explosions. This along with fuel meltdowns released large amounts of radioactive material into the air.

To control such a nuclear chain reaction, control rods containing neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission.

Thermal neutrons are more likely than fast neutrons to cause fission.

Heavy-water reactors (Used in Canada, India, Argentina, China, Pakistan, Romania and South Korea).

Uranium-235

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Isotope of uranium making up about 0.72% of natural uranium.

Nuclear fission seen with a uranium-235 nucleus

Its fission cross section for slow thermal neutrons is about 584.3±1 barns.

Heavy water reactors and some graphite moderated reactors can use natural uranium, but light water reactors must use low enriched uranium because of the higher neutron absorption of light water.

A critical chain reaction can be achieved at low concentrations of 235U if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater.

Light-water reactor

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The Koeberg nuclear power station, consisting of two pressurized water reactors fueled with uranium

A pressurized water reactor head, with the control rods visible on the top

Nuclear fuel pellets that are ready for fuel assembly completion

Animated diagram of a boiling water reactor

Animated diagram of a pressurized water reactor

The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel.

A neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.

Heavy water

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Form of water that contains only deuterium ( or D, also known as heavy hydrogen) rather than the common hydrogen-1 isotope ( or H, also called protium) that makes up most of the hydrogen in normal water.

With the discovery of nuclear fission in late 1938, and the need for a neutron moderator that captured few neutrons, heavy water became a component of early nuclear energy research.

These heavy water reactors have the advantage of being able to run on natural uranium without using graphite moderators that pose radiological and dust explosion hazards in the decommissioning phase.

Deuterium's absorption cross section for thermal neutrons is 0.52 millibarns (5.2 × 10−32 m2; 1 barn = 10−28 m2), while those of oxygen-16 and oxygen-17 are 0.19 and 0.24 millibarns, respectively.

Neutron cross section

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Used to express the likelihood of interaction between an incident neutron and a target nucleus.

Interpretation of the reaction rate with the help of the cross section

Scattering (full line) and absorption (dotted) crossections of light element commonly used as neutron moderators, reflectors and absorbers, the data was obtained from database NEA N ENDF/B-VII.1 using JANIS software and plotted using mathplotlib.

Isotopes which have a large scatter cross section and a low mass are good neutron moderators (see chart below).

the incident particle energy, also called speed or temperature (thermal, fast…),

This is the reason why some reactors use heavy water (in which most of the hydrogen is deuterium) instead of ordinary light water as moderator: fewer neutrons are lost by capture inside the medium, hence enabling the use of natural uranium instead of enriched uranium.