The Fast Reactor

The fast reactor appeared very attractive to British scientists and engineers in the 1950s, partly because it seemed to offer something for nothing it could, in theory, produce more fuel than it consumed. To understand why, we must revisit the meaning of three words fissile, fissionable and fertile. 

A fissile material is one whose nuclei are easily split by neutrons and can thus be used as part of a chain reaction. A hssionable material is one whose nuclei can be split only by very energetic neutrons, and it is difficult to make the material take part in a chain reaction. A fertile material is one that can be easily converted into a fissile material. 

A fast reactor is so called since the neutrons within the reactor have not been slowed down — thus they are fast neutrons. A fast reactor does not need a moderator. The purpose of a moderator is to slow down the neutrons, and a reactor which uses a moderator is often called a thermal reactor . In addition, the way in which the nucleus breaks apart varies with the speed of the neutrons, and more neutrons per fission are produced from fast neutrons compared with slow neutrons. More neutrons mean that fertile material can be converted to fissile material faster. 

There are problems with the design of a fast reactor. The first is that the nuclear cross-section of uranium 235 — that is, the apparent size of the nucleus — appears to be smaller than a fast neutron compared with a slow one, and hence there needs to be more fissile material in the reactor. Unlike a thermal reactor, such as Calder Hall or the magnox reactors, a fast reactor cannot be built using natural uranium, but needs highly emiched uranium 235 or plutonium as fuels. 

Since there is no moderator, the size of the core is very much reduced. A paper of 1953 made the following estimates for the core of a fast reactor  

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In addition it may be helpful to set up, or at least approve, a number of worid specialized manufacturing facilities for example, quality facilities to manufacture the hundred pressure vessels needed each year. Laboratories should be available to expeditiously solve reactor operating problems which may arise worldwide, and to improve reactor performance. And there should be an international program to develop the Fast Reactor which generates some sixty to a hundred times as much energy from a pound of uranium than do our present commercial reactors. 

With the expansion of nuclear power, the Fast Reactor is likely to be needed in the next half century as supplies of uranium for the present reactor types become scarce. Because of its efficiency in utilizing uranium the Fast Reactor can supply the world s energy needs indefinitely. 

Perhaps there should be centralized areas for storage of spent fuel from present reactor types. These storage areas should be adjacent to reprocessing plants which would be built to process the spent fuel to provide the new fuel for the Fast Reactors going on line. 

The fast reactor high level waste is accumulated at the reprocessing plants and retains its toxicity for only a few hundred years, rather than the tens of thousands of years ofthe spent fuel wastes from our present reactors,. Thus, the nuclear waste disposal problems are minimal and arrangements for disposal could be made on a global basis. 

Fig. 4. Schematic illustration of a symbiotic fiiel cycle that utilizes both thermal and fast reactors. The fast reactors are used to hum long-lived actinides (DOE 2002).

Fig. 7.1. Schematic presentations of (a) the once-through cycle (b) the thermal reactor cycle and (c) the fast reactor cycle. UjO, = yellowcake, UF = uranium hexafluoride, MOX = mixed oxide fuel (uranium/...

Flowers, R. H., Johnson, K. D. B., Miles, J. H., and Webster, R. K., "Possible Long Term Options for the Fast Reactor Plutonium Fuel Cycle," Paper presented at the 5th Energy Technology Conf., Washington, D. C., 1978. 

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

Items 1, 2, and 3 could be considered as the main components released if fuel were badly overheated up to but not greatly exceeding the melting point. Items 4, 5, and 6 are the additional components of activity which are here assumed to be released, mainly in fine particulate form, if the fast reactor fuel is rapidly dispersed as indicated in the text. 

In 1968, faced with expanding computational demands and limited funding for the fast reactor physics program, the USAEC Reactor Physics Branch, upon the recommendation of the Advisory Committee on Reactor Physics (ACRP), initiated a cooperative code development policy 152). W. H. Hannum, Branch Chief, endorsed the published ANS standard on computer... 

These events, which are not, generally, specific to the Fast Reactors technology, could be reduced in frequency thank to the progress made in matters of man-machine interface and equipment testability. 

The following 2 incidents characteristic of the fast reactors tire classified level 2 ... 

This system has caused the largest number of the plant generation losses this being accounted for by incomplete applicability of the typical designs used by the designer of the fast reactor power unit. This system had been remarkably modified before it stopped to be a source of the troubles. 

The main conclusion is that owing to the comprehensive work, CRC availability has been restored and also experimental data of great importance for the fast reactors development and operation have been obtained. 

In the current structure of nuclear power, light water reactors (LWRs) are predominant over a small number of heavy water reactors (HWRs), and even smaller number of fast breeder reactors (FBRs). However, an increase of FBR share can be predicted for the future, taking into account their unique properties. First of all, there is the capability of nuclear fuel breeding by involving into the fuel cycle. Secondly, there is the fast reactor s flexibility permitting its use as plutonium incinerators and minor actinides transmutation. Thus, unless new sources of energy are found, the development of nuclear power will be necessarily based on fast breeder reactors. 

Now, because of the delay in the fast reactors commercial introduction, there is an opportunity to investigate alternative technical solutions. The objective is to obtain complete knowledge of their characteristics allowing to make, when the time has come, the best choice... 

Absence of poisoning effects in the fast reactor (FR), low value of negative temperature reactivity coefficient, compensation of fuel bum-up and slagging processes by plutonium generation as well as partial reloadings enable to ensure the operative reactivity margin to be less than delayed neutron share and to diminish or eliminate the probability of runaway by prompt neutrons in the reactor under operation conditions. 

The fast reactor operation under that regime is not in conformity with the existing view points on the FR role in NP, as in this case built-up plutonium is neither extracted nor reused, but mainly utilized directly inside the reactor. This is the reason for reconsideration of traditional FRs. 

One of the attractive features of the fast reactor is its hard neutron spectrum. To expand this feature, a metallic fuel core is employed in the 4S. However, it is more difficult to reduce void reactivity for a core with a harder spectrum. It is very important to design the void reactivity to be negative in order to prevent a severe nuclear accident in the event of sudden loss of coolant, sudden loss of coolant flow or a large gas bubble entrainment in the core. 

India is in the initial stages of the commercialization of the fast reactors and the efforts on knowledge preservation goes in parallel with the design and operation. Care is taken to ensure that all design and operation data are documented and archived with proper identification. In FBTR all design, drawings and operation related documents are stored separately in an air-conditioned record room for immediate and future reference. However, for PFBR, from... 

JNC considers it extremely important to reflect the lessons learnt from previous experience in the fast reactor field to the operation and maintenance of Monju and the design of future reactors. 

The Experimental Breeder Reactor-II (EBR-II) was designed as a 62.5 MWt, metal fueled, pool reactor with a conventional 19 MWe power plant. The productive life of the EBR-II began with first operations in 1964. Demonstration of the fast reactor fuel cycle, serving as an irradiation facility, demonstration of fast reactor passive safety and lastly, was well on its way to close the fast breeder fuel cycle for the second time when the Integral Fast Reactor program was prematurely ended in October 1994 with the shutdown of the EBR-II. 

A major benefit to the fast reactor community would be a reactor whose cost to decommission was equal or near the cost for a water-cooled reactor (PWR or BWR). Clearly, there will be significant disagreement from both within and outside of the fast reactor community. However, until a common goal is established, this or another one, and is embraced by the fast reactor community, no integrated progress will be made. 

Over recent years major cutbacks in overseas projects have been witnessed. It is regrettable to see the reduced presence of some countries at international meetings. Many of the staff with whom we used to have contact have now left the fast reactor field to go into other areas or have retired. And this fact would also make it difficult for our partners to locate information. 

JNC has two fundamental issues to address in the future of our international co-operation knowledge preservation and communication. However, while the fast reactor project has slowed down, progress in computer technology has accelerated. There are opportunities provided by cheap digital storage and by the Internet technology, and it is these that form the basis of JNC s proposals. 

The fast reactor, which can generate electricity and breed additional fissile material for future fuel stocks, is a resource that will be needed when economic uranium supplies for the advanced water cooled reactors or other thermal-spectrum options diminish. 

The fast reactor has been the subject of research and development programmes in a number of countries for more than 50 years. Now, despite early sharing and innovative worldwide research and development, ongoing work is confined to China, France, India, Japan, the Republic of Korea, and the Russian Federation. Information generated worldwide will be needed in the future. Presently, it is in danger of being lost — even in those countries continuing the work. 

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