Neutron reflector

A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, tungsten carbide, or other materials. A neutron reflector can make an otherwise subcritical mass of fissile material critical, or increase the amount of nuclear fission that a critical or supercritical mass will undergo. Such an effect was exhibited twice in accidents involving the Demon Core, a subcritical plutonium pit that went critical in two separate fatal incidents when the pit's surface was momentarily surrounded by too much neutron reflective material.

Nuclear reactors

In a uranium graphite chain reacting pile, the critical size may be considerably reduced by surrounding the pile with a layer of graphite, since such an envelope reflects many neutrons back into the pile.

To obtain a 30-year life span, the SSTAR nuclear reactor design calls for a moveable neutron reflector to be placed over the column of fuel. The reflector's slow downward travel over the column would cause the fuel to be burned from the top of the column to the bottom.

A reflector made of a light material like graphite or beryllium will also serve as a neutron moderator reducing neutron kinetic energy, while a heavy material like lead or lead-bismuth eutectic will have less effect on neutron velocity.

In power reactors, a neutron reflector reduces the non-uniformity of the power distribution in the peripheral fuel assemblies, reduces neutron leakage and reduces a coolant flow bypass of the core. By reducing neutron leakage, the reflector increases reactivity of the core and reduces the amount of fuel necessary to maintain the reactor critical for a long period. In light-water reactors, the neutron reflector is installed for following purposes:

  • The neutron flux distribution is “flattened“, i.e., the ratio of the average flux to the maximum flux is increased. Therefore reflectors reduce the non-uniformity of the power distribution.
  • Because of the higher flux at the edge of the core, there is much better utilization in the peripheral fuel assemblies. This fuel, in the outer regions of the core, now contributes much more to the total power production.
  • The neutron reflector scatters back (or reflects) into the core many neutrons that would otherwise escape. The neutrons reflected back into the core are available for chain reaction. This means that the minimum critical size of the reactor is reduced. Alternatively, if the core size is maintained, the reflector makes additional reactivity available for higher fuel burnup. The decrease in the critical size of core required is known as the reflector savings.
  • Neutron reflectors reduce neutron leakage i.e. to reduce the neutron fluence on a reactor pressure vessel.
  • Neutron reflectors reduce a coolant flow bypass of a core.
  • Neutron reflectors serve as a thermal and radiation shield of a reactor core.

Nuclear weapons

A similar envelope can be used to reduce the critical size of a nuclear weapon, but here the envelope has an additional role: its inertia delays the expansion of the reacting material. For this reason such an envelope is often called a tamper. The weapon tends to disintegrate as the reaction proceeds and this tends to stop the reaction, so the use of a tamper makes for a longer-lasting, more energetic, and more efficient explosion. The most effective tamper is the one having the highest density; high tensile strength is irrelevant because no material remains intact under the extreme pressures of a nuclear weapon. Coincidentally, high-density materials are excellent neutron reflectors. This makes them doubly suitable for nuclear weapons. The first nuclear weapons used heavy uranium or tungsten carbide tamper-reflectors.

On the other hand, a heavy tamper necessitates a larger high-explosive implosion system. The primary stage of a modern thermonuclear weapon may use a lightweight beryllium reflector, which is also transparent to X-rays when ionized, allowing the primary's energy output to escape quickly to be used in compressing the secondary stage.

While the effect of a tamper is to increase efficiency, both by reflecting neutrons and by delaying the expansion of the bomb, the effect on the critical mass is not as great. The reason for this is that the process of reflection is time-consuming. By the time reflected neutrons make it back into the core, several generations of the chain reaction have passed, meaning the contribution from the older generation is a tiny fraction of the neutron population.

See also

External links

Advanced Test Reactor

The Advanced Test Reactor (ATR) is a research reactor at the Idaho National Laboratory, located east of Arco, Idaho. This reactor was designed and is used to test nuclear fuels and materials to be used in power plants, naval propulsion, research and advanced reactors. It can operate at a maximum power of 250 MW thermal power and has a "Four Leaf Clover" core design (similar to the Camunian rose) that allows for a variety of testing locations. The unique design allows for different neutron flux (number of neutrons impacting one square centimeter every second) conditions in various locations. Six of the test locations allow an experiment to be isolated from the primary cooling system, providing its own environment for temperature, pressure, flow and chemistry, replicating the physical environment while accelerating the nuclear conditions.

The ATR is a pressurized light water reactor (LWR), using water as both coolant and moderator. The core is surrounded by a beryllium neutron reflector to concentrate neutrons on experiments, and houses multiple experiment positions as well. It operates at low temperature and pressure 71°C (160°F) and up to 2.69 MPa water pressure. The ATR reactor vessel is solid stainless steel, 35 feet tall by 12 feet across. The core is approximately 4 feet tall by 4 feet across.

In addition to its role in nuclear fuels and materials irradiation, the ATR is the United States' only domestic source of high specific activity (HSA) cobalt-60 (60Co) for medical applications. HSA 60Co is used primarily in gamma knife treatment of brain cancer. Other medical and industrial isotopes have also been produced, and could be again, including plutonium-238 (238Pu), which is useful for powering spacecraft.


Atmea is a joint venture between Mitsubishi Heavy Industries (MHI) and EDF Group that develops, markets, licenses and sells the ATMEA1 reactor, a new generation III+, medium-power pressurized water reactor (PWR). The company is headquartered in Paris.


CROCUS is a research reactor at École Polytechnique Fédérale de Lausanne, a research institute and university in Lausanne, Switzerland.

The uranium nuclear reactor core is in an aluminum container that measures 130 centimetres (51 in) across with 1.2-centimetre (0.47 in)-thick walls. The aluminum vessel is filled with demineralized light water to both serve as a neutron moderator and neutron reflector.Power output is controlled either by adjusting the water level in the reactor—with a ±0.1-millimetre (0.0039 in) level of control, or with the adjustment of two boron carbide (B4C) control rods—with a ±1-millimetre (0.039 in) level of finesse. The reactor has six separate safety systems: two cadmium shields and four storage tanks, any of which can shut down the reaction in less than second.CROCUS has a license to produce 100 watts (0.13 hp) or a neutron flux of ~2.5 × 109 cm-2s-1 at the core's center.

Critical mass

A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. The concept is important in nuclear weapon design.

DIDO (nuclear reactor)

DIDO was a materials testing nuclear reactor at the Atomic Energy Research Establishment at Harwell, Oxfordshire in the United Kingdom. It used enriched uranium metal fuel, and heavy water as both neutron moderator and primary coolant. There was also a graphite neutron reflector surrounding the core. In the design phase, DIDO was known as AE334 after its engineering design number.

DIDO was designed to have a high neutron flux, largely to reduce the time required for testing of materials intended for use in nuclear power reactors. This also allowed for the production of intense beams of neutrons for use in neutron diffraction.

DIDO was shut down in 1990. The primary facilities decommissioning is expected to be complete in 2023 with the reactor decommissioning completed in 2031 and final site clearance achieved in 2064 In all, six DIDO class reactors were constructed based on this design:


PLUTO, also at Harwell, first criticality 1957.

HIFAR (Australia), first criticality January 1958.

Dounreay Materials Testing Reactor (DMTR) at Dounreay Nuclear Power Development Establishment in Scotland, first criticality May 1958.

DR-3 at Risø National Laboratory (Denmark), first criticality January 1960.

FRJ-II at Jülich Research Centre (Germany), first criticality 1962.HIFAR was the last to shut down, in 2007.

Demon core

The demon core was a 6.2-kilogram (14 lb) subcritical mass of plutonium measuring 89 millimetres (3.5 in) in diameter, which was involved in two criticality accidents, on August 21, 1945 and May 21, 1946. The core was intended for use in a third World War II nuclear bomb, but remained in use for testing after Japan's surrender. It was designed with a small safety margin to ensure a successful explosion of the bomb. The device briefly went supercritical when it was accidentally placed in supercritical configurations during two separate experiments intended to guarantee the core was indeed close to the critical point. The incidents happened at the Los Alamos laboratory in 1945 and 1946, and resulted in the acute radiation poisoning and subsequent deaths of scientists Harry Daghlian and Louis Slotin. After these incidents the spherical plutonium core was referred to as the "demon core".

Energy Multiplier Module

The Energy Multiplier Module (EM2 or EM squared) is a nuclear fission power reactor under development by General Atomics. It is a fast-neutron version of the Gas Turbine Modular Helium Reactor (GT-MHR) and is capable of converting spent nuclear fuel into electricity and industrial process heat.

Harry Daghlian

Haroutune Krikor "Harry" Daghlian Jr. (May 4, 1921 – September 15, 1945) was a physicist with the Manhattan Project which designed and produced the atomic bombs that were used in World War II. He accidentally irradiated himself on August 21, 1945, during a critical mass experiment at the remote Omega Site of the Los Alamos Laboratory in New Mexico, resulting in his death 25 days later.

Daghlian was irradiated as a result of a criticality accident that occurred when he accidentally dropped a tungsten carbide brick onto a 6.2 kg plutonium–gallium alloy bomb core. This core, subsequently nicknamed the "demon core", was later involved in the death of another physicist, Louis Slotin.

Ivy King

Ivy King was the largest pure-fission nuclear bomb ever tested by the United States. The bomb was tested during the Truman administration as part of Operation Ivy. This series of tests involved the development of very powerful nuclear weapons in response to the nuclear weapons program of the Soviet Union.

The production of Ivy King was hurried to be ready in case its sister project, Ivy Mike, failed in its attempt to achieve a thermonuclear reaction. The Ivy King test actually took place two weeks after Mike. Unlike the Mike bomb, the Ivy King device could theoretically have been added to United States' nuclear arsenal because it was designed to be air-deliverable.

On November 16, 1952 at 11:30 local time (23:30 GMT) a B-36H bomber dropped the bomb over a point 2,000 feet (610 m) north of Runit Island in the Enewetak atoll, resulting in a 500 kiloton explosion at 1,480 feet (450 m). The tropopause height at the time of the detonation was about 58,000 feet (18 km). The top of the King cloud reached about 74,000 feet (23 km) with the mushroom base at about 40,000 feet (12 km).The Ivy King bomb, designated as a Mk-18 bomb and named the "Super Oralloy Bomb", was a modified version of the Mk-6D bomb. Instead of using an implosion system similar to the Mk-6D, it used a 92-point implosion system initially developed for the Mk-13. Its uranium-plutonium core was replaced by 60 kg of highly enriched uranium (HEU) fashioned into a thin-walled sphere equivalent to approximately four critical masses. The thin-walled sphere was a commonly used design, which ensured that the fissile material remained sub-critical until imploded. The HEU sphere was then enclosed in a natural-uranium neutron reflector. To physically prevent the HEU sphere collapsing into a critical condition if the surrounding explosives were detonated accidentally, or if the sphere was crushed following an aircraft accident, the hollow center was filled with a chain made from aluminum and boron, which was pulled out to arm the bomb. The boron-coated chain also absorbed the neutrons needed to drive the nuclear reaction.The primary designer of the Super Oralloy Bomb, physicist Ted Taylor, later became a vocal proponent of nuclear disarmament.

Lead-cooled fast reactor

The lead-cooled fast reactor is a nuclear reactor design that features a fast neutron spectrum and molten lead or lead-bismuth eutectic coolant.

Molten lead or lead-bismuth eutectic can be used as the primary coolant because lead and bismuth have low neutron absorption and relatively low melting points.

Neutrons are slowed less by interaction with these heavy nuclei (thus not being neutron moderators) and therefore help make this type of reactor a fast-neutron reactor.

The coolant does however serve as a neutron reflector, returning some escaping neutrons to the core.

Fuel designs being explored for this reactor scheme include fertile uranium as a metal, metal oxide or metal nitride.

Smaller capacity LFR (such as SSTAR) can be cooled by natural convection, while larger designs (such as ELSY) use forced circulation in normal power operation, but with natural circulation emergency cooling.

The reactor outlet coolant temperature is typically in the range of 500 to 600 °C, possibly ranging over 800 °C with advanced materials for later designs.

Temperatures higher than 800 °C are high enough to support thermochemical production of hydrogen.

The concept is generally very similar to sodium-cooled fast reactor, and most liquid-metal reactors have used sodium instead of lead.

Few lead-cooled reactors have been constructed, except for some Soviet nuclear submarine reactors in the 1970s, but a number of proposed new nuclear reactor designs are lead-cooled.

The lead-cooled reactor design has been proposed as a generation IV reactor.

Plans for future implementation of this type of reactor include modular arrangements rated at 300 to 400 MWe, and a large monolithic plant rated at 1,200 MWe.

Mitsubishi APWR

This article is about the Mitsubishi Heavy Industry's design. For the Westinghouse AP series, see AP1000.The Mitsubishi advanced pressurized water reactor (APWR) is a generation III nuclear reactor design developed by Mitsubishi Heavy Industries (MHI) based on pressurized water reactor technology. It features several design enhancements including a neutron reflector, improved efficiency and improved safety systems. It has safety features advanced over the last generation, including a combination of passive and active systems. None are currently under construction.

Pit (nuclear weapon)

The pit, named after the hard core found in fruits such as peaches and apricots, is the core of an implosion nuclear weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.

Reactor pressure vessel

A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the reactor core.

Small, sealed, transportable, autonomous reactor

Small, sealed, transportable, autonomous reactor (SSTAR) is a proposed lead-cooled nuclear reactor being primarily researched and developed in the United States by Lawrence Livermore National Laboratory.

It is designed as a fast breeder reactor that is passively safe.

It has a self-contained fuel source of uranium-235 and uranium-238 which will be partly consumed by fast-neutron fission and, more importantly, converted into more fissile material ("breeding" plutonium).

It should have an operative life of 30 years, providing a constant power source between 10 and 100 megawatts.

The 100 megawatt version is expected to be 15 meters high by 3 meters wide, and weigh 500 tonnes. A 10 megawatt version is expected to weigh less than 200 tonnes. To obtain the desired 30 year life span, the design calls for a movable neutron reflector to be placed surrounding part of a column of fuel. The reflector's slow downward travel through the entire length of the column would cause the fuel to be burned from the top of the column to the bottom. Because the unit will be sealed, it is expected that a breeder reaction will be used to further extend the life of the fuel.

SSTAR is meant to be tamper resistant, which would prevent the leasing country from opening the reactor to use the generated plutonium for nuclear weapons. The tamper-resistant features will include radio monitoring and remote deactivation. The leasing country will therefore have to accept the capability for remote foreign intervention in the facility. The feature might, however, interfere with possible recovery work during an accident.

They are being researched as a possible replacement for today's light water reactors and as a possible design for use in developing countries (which would use the reactor for several decades and then return the entire unit to the manufacturing country).

A prototype was scheduled for manufacture in 2015. However its development seems to have ended.

TOPAZ nuclear reactor

The TOPAZ nuclear reactor is a lightweight nuclear reactor developed for long term space use by the Soviet Union. Cooled by liquid metal, it uses a high-temperature moderator containing hydrogen and highly enriched fuel and produces electricity using a thermionic converter.


Tamper may refer to:

Tamper, to use a tamp, a tool for material compaction

Tamper, a pipe tool component

Tamper, a neutron reflector used in nuclear weapons

Tamper, to interfere with, falsify, or sabotage

Ballast tamper, a machine that tamps railroad track ballast

Toshiba 4S

The Toshiba 4S (Ultra super safe, Small and Simple) is a micro sodium reactor design.

Training Reactor VR-1

The VR-1 training reactor is a pool-type, light water reactor based on enriched uranium, at the Czech Technical University in Prague. The neutron moderator is light demineralized water, which is also used as a neutron reflector, as biological shielding, and as a coolant. Heat is removed from the active core by natural convection.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.