Breeder reactor

A breeder reactor is a nuclear reactor that is used for energy production with simultaneous generation of further fissile material. A non-fissile nuclide is transformed into a fissile, which can then be subsequently used ( after processing and incorporation into new fuel elements) as a nuclear fuel. Although this transformation ( as conversion, sometimes referred to as brooding, see conversion rate ) will be held in each nuclear reactor, but from a " breeder reactor " or " breeder " is called only when it produces more fuel than he at the same time consumed.

The first breeder reactor was the Experimental Breeder Reactor I, who in 1951 was the first nuclear reactor in the world, the generated electricity.

Types of breeder reactors

There are two types of breeder reactors and referred them to the energy spectrum of the neutrons used:

Fast breeder reactors work with uranium -238 as fertile material and with fast neutrons as they are released in nuclear fissions. As nuclear fuel is uranium -plutonium mixed oxide ( MOX). The breeding zone (see below) containing natural uranium or depleted uranium oxide, which consists predominantly of 238U. The fast breeder reactor thus makes it possible to exploit the occurrence of natural uranium more efficiently, but this requires the construction of a plutonium economy.

Thermal breeder working with thorium as a fertile material and with predominantly thermal neutrons. After an initial fill with enriched uranium oxide, plutonium oxide, or MOX is from 232Th by Neutronenanlagerung and beta decay fissile 233U. This technology is interesting because of the large thorium deposits, since these are larger than the uranium deposits.

To be mentioned are concepts for so-called "Advanced Pressurized Water Reactors " (Advanced Pressurized Water Reactors ) or boiling water reactors " with reduced Moderation". You would be working with conventional fuels and coolants, but by their design high conversion rates of 0.7 to 1.0 to achieve (hence sometimes referred to as up-converter ), so would be " almost" breeder reactors.

Fast Breeder

Construction of the reactor

The reactor core is composed of many upright, with, for example uranium -plutonium mixed oxide -filled stainless steel tubes ( fuel rods ). The rods are bundled into fuel assemblies and fill an approximately cylindrical region of eg 3 m and 5 m in diameter from. The control of the chain reaction (see also criticality ) is effected by control rods made ​​of boron steel or other neutron-absorbing material.

The reactor core is divided into an inner and an outer gap - hatching zone. The coolant - that may act at these reactors, not, as in light-water reactor, as a presenter - is a liquid metal such as sodium or potassium. Until about 1970, concepts for gas-cooled breeder reactors were investigated, but were not utilized.

Fuel - breeding process

Natural uranium consists of 99.3 % from the non-fissile isotope 238U and only 0.7 % of the fissile isotope 235U. For the operation of most fission reactors (eg light water reactor ), it must be technically complex enriched to about 3 to 4% 235U before production of the fuel.

In operation, each uranium reactor, a portion of the existing 238U is converted by neutron capture in 239U. This goes without saying by two consecutive β - decays in the fissile 239Pu over, which is still partially cleaved parallel to 235U in the reactor again, but some can also be processed later after reprocessing of the spent fuel to new mixed-oxide fuel.

The " brooding " in the strict sense, ie, an excess of produced so the spent fuel at the same time, but only succeeds in a reactor operating without moderator, a fast breeder reactor, because only in the fission by fast neutrons, the average number is new released neutrons per fission for high enough ( see nuclear fission process in the breeder reactor ). The surplus is expressed therein from that the breeding ratio ( sometimes also called breeding rate or conversion rate ), the number of newly created fuel atoms per consumed fuel atom through 1.0.

The fast breeder is to say not so, because he " quickly hatches", but because he used to fast instead of nuclear fission thermal ( decelerated ) neutrons.

Better utilization of nuclear fuel supplies

For the 238U there are few other useful applications in addition to their use in breeder reactor (among uranium munitions ). Through a network economy from breeder reactors, reprocessing and light water reactors the uranium supply earth could about 60 times provide as much energy as if only the 235U would split. In theory, the complete utilization of 238U would even one over 100 times higher utilization factor, which, however, is technically not feasible at present.

The use of the metal thorium 232Th, which was used as a fertile material from 1983 to 1989 already in the reactor THTR -300 and the fuel 233U results would improve the resource situation of the nuclear power again significant, since the natural thorium deposits that of uranium to a exceed many times over.

Gap zone

In the gap zone of the reactor is obtained by using fast neutrons, the problem is that this compared with thermal neutrons, are far less likely (see cross section ) trigger new fissions. Therefore, the fission product concentration must be increased in the gap zone compared to moderated reactor types. The fissile material is mixed oxide of plutonium oxide and 15 to 20% 80 to 85% uranium oxide. Thus, the enrichment of fissile isotopes is about ten times higher than for the light-water reactors. As coolant - that may not have any moderator effect in the fast reactor, so it must have a sufficiently high mass number - use the previous breeder reactors, liquid sodium; were also examined with concepts gas cooling. The first breeder reactors in the United States and the former Soviet Union still used mercury as a coolant, but this led to problems, inter alia, because of corrosion.

Blanket

The blanket (English breeding blanket ) is disposed around the reactor core and surrounding it completely. The upper and lower parts of a fuel rod, the gap area is not, as the central part of fuel mixed oxide, but depleted uranium oxide filled as fertile material; the radially outer rods containing this over its entire length. Depleted uranium is inevitably produced during the uranium enrichment process residue.

Nuclear fission process in the breeder reactor

The " breeding " requires that the cleavage of the nucleus to release an average of more than two neutrons for a neutron is to trigger the next cleavage requires ( criticality of the chain reaction ), and another neutron needs to create a new fissile core to replace the divided core, ie to achieve a breeding ratio of 1.0. In addition, neutrons but inevitable losses through leakage to the outside and by absorption processes that lead neither to divide nor to Pu production, namely absorption in the structural material in fission products in the coolant and in the control rods.

With some simplifications can be the situation well by the neutron yield factor (eta ) describe the number of newly liberated neutrons per absorbed in the fissile neutron. This number is a little smaller than that of the free neutrons per fission, as also in the fissile material, not every absorption results in the cleavage. In fission by thermal neutrons is for the easily fissionable nuclides 233U, 235U and 239Pu just over 2.0. In fission by fast neutrons of energy 1 MeV, however, is 239Pu free about 2.8 neutrons. This can also be generated in losses of about 0.5 neutrons per neutron absorbed in the fuel significantly more than 1 new fissile core per core split.

Energy

Problems arising from the division of a core usually two fragments ( " fission fragments " ) carry the energy gain of the reaction, a total of about 200 MeV, as kinetic energy. You are decelerated in the surrounding fuel material and heating it. The primary sodium cooling loop absorbs the heat and releases it through a heat exchanger to a secondary cooling circuit Atrium on. In this secondary sodium cooling loop steam is produced by a steam generator, which - applied to the turbine - as in a conventional coal - or oil-fired power plant. The blades of the turbine convert the heat into rotational energy that is used to drive a generator. The exiting from the turbine exhaust steam is condensed in a condenser and supplied again to the boiler circulation. The condenser is cooled by an external cooling circuit, which transfers heat to a flow of water, for example.

Thermodynamic cycle

The breeder reactor technology is based in some areas on the basis of the light-water reactor technology, but has some significant differences. The heat carrier sodium is characterized by high thermal conductivity and a large usable temperature range. It melts at 98 ° C and boils at 883 ° C. Because of this high boiling point, a pressure of only 10 bar is necessary in the sodium loop, which represents a certain security advantage ..

In contrast to the light-water reactor is between the sodium loop, the fuel cools (primary circuit ), and the water - steam cycle, a second sodium circuit (secondary circuit) switched on. While this reduces the efficiency, but is necessary for safety reasons so even in the case of a steam generator leakage reacts only non-radioactive sodium with water. One or more intermediate heat exchangers transferring heat from the primary to the secondary refrigerant. In German breeder reactor designs, the so-called loop system is used in which all of the pumps and heat exchangers are spatially separated from the reactor and the reactor tank is filled above the sodium with nitrogen. The pool system, which is used in other countries more often, is the primary circuit including primary pumps and intermediate heat exchangers in the reactor tank itself, in which case argon is used as a protective gas in the tank.

Threats and countermeasures

Compared to about light water reactors, the operation of a breeder reactor requires additional safety devices. Physical reasons for this are mainly the not "automatically" negative void coefficient, moreover, the opposite uranium lower proportion of delayed neutrons from fission.

Sodium vapor formation or loss makes the reactor does not automatically subcritical. The subcriticality must instead be made ​​by technical means sufficiently rapidly and reliably in such a case. These breeder reactors have except the normal control rods further, independent sets of safety or shutdown rods which fall into it in case of need in the reactor core or inside can be "shot" ( SCRAM ). Is triggered such a shutdown by sensitive systems for the detection of excessive temperatures and boiling processes.

The smaller the uranium - plutonium mixed oxide fuel - delayed neutron fraction means a smaller distance between the operating points " delayed critical" and " prompt critical" (see criticality). This is taken by suitably sensitive, accurate measurement of the neutron flux and fast response of the control rod system account.

General risks of breeder technology lie in the large-scale use of the sodium coolant, which can be triggered in contact with air or water fires, as well as plutonium, which is compared with uranium significantly hazardous to health.

Use

Currently breeder reactors in the United States, Russia, China and India are operated. In Japan (as of 2007 ) is currently developing a new commercial breeder reactor. The regular operation of this type is provided for 2050.

The first German sodium- cooled experimental reactor KNK -I ( Compact Sodium -cooled nuclear reactor plant in Karlsruhe ) was built in the years 1971 to 1974 at the Nuclear Research Center Karlsruhe. The plant was converted to a fast breeder called KNK - II in 1977 and until 1991 was in operation.

On the Lower Rhine at Kalkar an industrial prototype breeder reactor power plant was built, called SNR -300 from 1973. After numerous protests and the nuclear accident at Chernobyl in 1986, it never came to commissioning or even power generation, which was planned for 1987.

Some breeder reactor experiment or demonstration plants, such as nuclear power plant Creys - Malville in France and the nuclear power plant Monju in Japan, were due to accidents ( largely caused by sodium- induced corrosion problems, leaks due to the high coolant temperatures, and others) permanently or for several off years. This is, however, as well as the abandonment of the German - Belgian-Dutch breeder reactor project Kalkar, due to the fact that in the current uranium supply situation, there is still no economic pressure to introduce these more expensive variant of the nuclear energy production.

Examples of breeder reactors

Thermal breeder

See thorium nuclear power plant THTR -300 and Rudolf Schulten