Remix Fuel

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Remix Fuel was developed in Russia to make use of mixed recycled uranium and plutonium from spent nuclear fuel to manufacture fresh fuel suitable for widespread use in Russian reactor designs. [1]

Contents

Compared to "conventional" MOX-fuel

MOX or Mixed Oxide Fuel [2] as deployed in some western European and East Asian nations generally consists of depleted uranium mixed with between 4% and 7% reactor grade plutonium. Only a few Generation II and about half of Generation III reactor designs are MOX fuel compliant allowing them to use a 100% MOX fuel load with no safety concerns.

Nuclear physics background

However all moderated reactors using lightly enriched uranium fuel produce plutonium in the course of normal operation as Uranium-238 (typically 94% to 97% of the uranium content in lightly enriched uranium [3] ) captures neutrons and undergoes successive beta decays until it is transmuted to plutonium-239. This internally produced plutonium increases in percentage until it is common enough that a growing percentage of fission reactions within the fuel are actually within the plutonium generated during the fuel cycle. Approximately half of the plutonium-239 "bred" during the fuel cycle is fissioned and another 25% is transmuted through additional neutron capture into other plutonium isotopes, primarily Pu-240. Virtually all of the minor actinides present in spent nuclear fuel are produced by successive neutron capture of the plutonium produced and as decay products of the more short lived isotopes. As a consequence of these factors the fresh uranium oxide fuel initially generates all of its fission reactions from U-235 but at the end of the cycle this has shifted to 50% U-235/50% Pu-239 fission reactions. In total about 33% of the energy generated by uranium fuel at the end of its life cycle actually comes from the bred and consumed Pu-239. Because the thermal neutron spectrum is not very good for fissioning Pu-239 the fuel shifts from 100% uranium at start of cycle to 96% uranium, 1% plutonium and 3% mixture of transuranic minor actinides and fission products. The longer the fuel remains in the reactor undergoing fission the more the uranium percentage decreases while the other materials increase. In effect all power reactors have been long known to be capable of operating with a mixed fissionable core containing 1% reactor grade plutonium without issues arising like those caused by the more highly concentrated MOX fuel used in western reactors. [4] [5] Ultimately, the spent fuel is removed from power reactors long before all available "fuel" is actually consumed, as neutron poisons and minor actinides with undesirable properties build up to unacceptable levels and alter the reaction parameters too much. Nuclear reprocessing is primarily done to remove undesirable parts of the spent fuel and either re-use the other parts or store them as waste. Reprocessed uranium for example, which is derived from spent fuel, usually has a higher uranium-235 content than natural uranium.

Process

Russia spent nearly a decade developing techniques similar to nuclear pyroprocessing that allows them to reprocess spent nuclear fuel without separating the recycled uranium and plutonium from the other metals as is done in the PUREX chemical reprocessing system used to manufacture MOX fuel. The recovered mixture of uranium and reactor grade plutonium is then converted to oxide and blended with medium enriched fresh uranium oxide fuel in a carefully measured proportion to create a mixture with 4% U-235 and 1% reactor grade plutonium. After extensive testing in a reactor starting in 2016 [6] Russia is now deploying Remix Fuel as replacement fuel for their VVER pressurized water reactors as of February 2020.

Related Research Articles

<span class="mw-page-title-main">Nuclear fuel cycle</span> Process of manufacturing and consuming nuclear fuel

The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

<span class="mw-page-title-main">Nuclear reprocessing</span> Chemical operations that separate fissile material from spent fuel to be recycled as new fuel

Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the reprocessed plutonium was recycled back into MOX nuclear fuel for thermal reactors. The reprocessed uranium, also known as the spent fuel material, can in principle also be re-used as fuel, but that is only economical when uranium supply is low and prices are high. Nuclear reprocessing may extend beyond fuel and include the reprocessing of other nuclear reactor material, such as Zircaloy cladding.

Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation.

<span class="mw-page-title-main">Uranium-238</span> Isotope of uranium

Uranium-238 is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

<span class="mw-page-title-main">Fast-neutron reactor</span> Nuclear reactor where fast neutrons maintain a fission chain reaction

A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain advantages over the existing fleet of water cooled and water moderated reactors. These are:

A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general classes of such devices. One uses neutrons provided by a nuclear fusion machine, a concept known as a fusion–fission hybrid. The other uses neutrons created through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator, a concept known as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor.

<span class="mw-page-title-main">Integral fast reactor</span> Nuclear reactor design

The integral fast reactor is a design for a nuclear reactor using fast neutrons and no neutron moderator. IFR would breed more fuel and is distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site.

<span class="mw-page-title-main">PUREX</span> Spent fuel reprocessing process for plutonium and uranium recovery

PUREX is a chemical method used to purify fuel for nuclear reactors or nuclear weapons. PUREX is the de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid–liquid extraction ion-exchange.

<span class="mw-page-title-main">Nuclear fuel</span> Material fuelling nuclear reactors

Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.

<span class="mw-page-title-main">Fertile material</span>

Fertile material is a material that, although not fissile itself, can be converted into a fissile material by neutron absorption.

<span class="mw-page-title-main">Plutonium-239</span> Isotope of plutonium

Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years.

Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long before being found in nature, the first isotope synthesized being plutonium-238 in 1940. Twenty plutonium radioisotopes have been characterized. The most stable are plutonium-244 with a half-life of 80.8 million years, plutonium-242 with a half-life of 373,300 years, and plutonium-239 with a half-life of 24,110 years. All of the remaining radioactive isotopes have half-lives that are less than 7,000 years. This element also has eight meta states; all have half-lives of less than one second.

<span class="mw-page-title-main">Thorium fuel cycle</span> Nuclear fuel cycle

The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, 232
Th
, as the fertile material. In the reactor, 232
Th
 is transmuted into the fissile artificial uranium isotope 233
U
 which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material, which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, 232
Th
 absorbs neutrons to produce 233
U
. This parallels the process in uranium breeder reactors whereby fertile 238
U
 absorbs neutrons to form fissile 239
Pu
. Depending on the design of the reactor and fuel cycle, the generated 233
U
 either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.

<span class="mw-page-title-main">Spent nuclear fuel</span> Nuclear fuel thats been irradiated in a nuclear reactor

Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.

<span class="mw-page-title-main">Weapons-grade nuclear material</span> Nuclear material pure enough to be used for nuclear weapons

Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon and has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear weapons are the most common examples.

Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Reactor-grade plutonium (RGPu) is the isotopic grade of plutonium that is found in spent nuclear fuel after the uranium-235 primary fuel that a nuclear power reactor uses has burnt up. The uranium-238 from which most of the plutonium isotopes derive by neutron capture is found along with the U-235 in the low enriched uranium fuel of civilian reactors.

Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material separated during reprocessing.

<span class="mw-page-title-main">Nuclear transmutation</span> Conversion of an atom from one element to another

Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.

A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for a pressurized water reactor. While heavy water is very expensive to isolate from ordinary water (often referred to as light water in contrast to heavy water), its low absorption of neutrons greatly increases the neutron economy of the reactor, avoiding the need for enriched fuel. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or alternative fuel cycles. As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors.

References

  1. ↑ "TVEL outlines innovation in nuclear fuel : Uranium & Fuel - World Nuclear News". www.world-nuclear-news.org.
  2. ↑ "MOX, Mixed Oxide Fuel - World Nuclear Association". www.world-nuclear.org.
  3. ↑ "Nuclear Fuel Facts: Uranium".
  4. ↑ "Nuclear Fuel Fabrication - World Nuclear Association". www.world-nuclear.org.
  5. ↑ "REMIX fuel pilot testing starts at Balakovo reactor - World Nuclear News". www.world-nuclear-news.org.
  6. ↑ "Russia loads REMIX fuel into MIR research reactor - World Nuclear News". www.world-nuclear-news.org.
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