Reactivity Coefficients in Nuclear Reactors

europhysics BULLETIN OF THE EUROPEAN PHYSICAL SOCIETY n e w s J.A. Volume 18 Number 11/12 November/December 1987 Notice to Members Reactivity Coefficients in As required by the statutes of EPS, members are informed (at least three Nuclear Reactors months in advance) of the intention to ask Council, at its meeting at the end of March 1988 in Dresden, to approve a rise in the unit fee from the current level of M. Hyland, Risley Sw.Fr. 11.- to Sw.Fr. 12.-. This would (United Kingdom Atomic Energy Authority) take effect from 1 January 1989 and would mean that independent Individual Ordinary Members would pay an annual In the months following the accident at the Chernobyl Nuclear Power Station it became ap membership fee of Sw.Fr. 144,- while parent that one of the contributary causes was the 'positive void coefficient of reactivity'. lOMs who are members of an EPS na Reactivity coefficients provide a measure of the way in which the neutron multiplication, or tional society would pay Sw.Fr. 48.-. The reactivity, of a reactor core changes as a function of other reactor variables, such as tempe national society fees would rise in the rature and pressure, and hence indicate the reactor's inherent stability. same proportion. The system for changing the unit fee is deliberately cumbersome to encourage The source of the energy produced in tem is said to be critical. If, however, stability and the last rise became effec all present day nuclear reactors Is the fis more than one neutron causes further tive from 1 January 1986. In effect, com sioning or splitting up of certain massive fissions, the system is said to be super pensation for three years inflation is pro nuclei. When such a nucleus is hit by a critical and the rate of energy production posed at a rate of less than 3% per year. All costs to the Society have not gone up stray neutron it can absorb it momen increases, while if less than one neutron equally and we have been making hard tarily to form a compound nucleus goes on to cause further fission, the sys currency savings through the operations which then splits up or fissions into two tem is said to be sub-critical and the rate of the Budapest Supplementary Secreta parts, releasing in the process two or of energy production decreases and riat. Nevertheless we have seen a comfor three neutrons and a large amount of eventually stops. The state of such sys table operating margin dwindle to a defi cit and the Society needs to counteract energy. A nuclear fission reaction typi tems can be expressed in terms of a mul the trend before it becomes too big. cally releases about 200 MeV of energy, tiplication factor, K, which is defined as In the current year the deficit at first associated with a net loss of mass rela the ratio of the number of neutrons in sight appears alarming — about Sw.Fr. tive to the initial excited compound nu one generation to the number in the pre- 80000.- but a significant part of this ceeding generation. So arises in connexion with extra staff char cleus. This energy appears mostly in the ges necessitated by the unexpectedly form of the kinetic energy of the two fis K < 1 sub-critical power falling high response of authors to the launching sion fragments. A typical fission reac K = 1 critical power constant of Europhysics Letters. This is not a conti tion takes the form : K > 1 super-critical power rising nuing charge on the Society so the situa n + 235U → 236U The reactivity of a system is defined tion can be contained if the rise in the unit - 141 Ba + 92Kr + 3n + as : fee is agreed. beta particles + neutrinos ρ = (K-1)/K IOM Delegates + gamma rays. With this issue of Europhysics News is Whilst this is only one of many possible Contents the membership renewal notice. Please 235U fission reactions, each leading to a check that it has not been left in the different pair of fission products, all fis Notice to EPS Members 133 despatch envelope and then deal with it Reactivity Coefficients in straight away rather than put it to one side sion reactions lead to the production of Nuclear Reactors 133 where it might get forgotten. The fewer energy and two or three fission neu Astronomers in Europe, Letter, the reminders that have to be sent, the trons. A & A Division Replies 137 lower the administrative charges on the It is the production of these neutrons, Dark Matter in the Universe 138 Society. which in turn can cause further fissions No invitation to propose the names of Deep Level Point Imperfections new IOM delegates to Council is included and more neutrons, that leads to the in Semiconductors 142 because no vacancies fall due next year. possibility of a self-sustaining or chain AMP Divisional Board 144 In part, this arises from the change in the reaction, continuously producing large Uppsala Conference of standard length of term of office (from amounts of energy. If, on average, one of HEPP Division 145 three to four years), but more importantly, the fission neutrons goes on to cause a two serving delegates have been elected 1987 Nobel Prize Winners 147 to the Executive Committee and so their further fission, the rate of energy pro Europhys. News Index for 1987 148 term is automatically extended. duction remains constant and the sys Published by the European Physical Society : 10 issues per year. © 1987. Reproduction rights reserved. ISSN 0531-7479 133 and it is thus a measure of the deviation from the condition of criticality (ρ = 0). An important factor in controlling a NEUTRON CAPTURE nuclear reactor is the rate at which the FISSION power level changes when the reactivity is non-zero and in particular when it is positive. The time between successive generations of neutrons, that is the average time between the appearance of a fission neutron and the appearance of any further fission neutrons, is typi cally a millisecond or less. If all neutrons were created at the time of fission, this very short time interval between suc cessive neutron generations would mean that the neutron population and hence the reactor power would change extremely rapidly and would be difficult Fig. 1 — Neutron fission and capture cross-sections for 235U. to control. However, not all neutrons are 238U are shown in Figs. 1 and 2 respecti Typical moderating materials used in emitted at the instant of fission. A small vely. It can be seen that 235U has a signi reactors are water (both H2O and D2O) fraction — less than 1% — are emitted ficant fission cross-section at all neutron and carbon, in the form of graphite. by certain fission products which decay energies and particularly at low ener Increasing the relative amount of fis by neutron emission rather than by β- gies, whereas 238U will only undergo sile isotopes in the reactor fuel is another decay. These delayed neutrons appear fission when struck by a neutron with an means of achieving a chain reaction and from a fraction of a second to a few energy greater than about 1 MeV. For this process is referred to as enrichment minutes after the fission event and they this reason 235U is referred to a fissile of the fuel. If there is a suitably high frac play a vital role in the control of nuclear isotope and 238U is referred to a fissio tion of fissile isotopes it is possible to reactors. A reactor is designed so that nable isotope. sustain a chain reaction without the the delayed neutrons are required to The cross-sections for neutron cap need for a moderating material. Reac keep the chain reaction going and the ture exhibit rapid changes with energy in tors operating on this principle are refer fact that they do not appear until on the 10-1000 eV region, particularly so red to as fast reactors, as the bulk of fis average some 10-20 seconds after fis for 238U. These changes are due to reso sions in them take place with fast neu sion means that changes in the power nance effects and represent the very trons, that is neutrons with energies level do not occur very rapidly. high neutron absorption probability higher than the resonance region. If, however, enough reactivity is ad when the energy of the incident neu ded, such that the delayed neutrons are tron, together with its binding energy in Factors Affecting Reactivity no longer necessary to maintain the the compound nucleus, coincide with an The proper and detailed analysis of chain reaction and the prompt neutrons energy level of the compound nucleus. the possible changes in reactivity, dur alone can sustain it, then the power level Natural uranium contains 99.3% 238U ing normal operations and under all fore will rise very rapidly and will be extre and only 0.7% of the fissile isotope seeable accident conditions, is a vital mely difficult to control. This is what 235U. The high level of resonance cap part of the overall safety assessment happened at Chernobyl. ture in 238U means that it is impossible carried out for every nuclear reactor. to sustain a chain reaction in natural Nowadays these calculations are ac Controlling the Chain Reaction uranium alone. If, however, some means complished with the aid of large com Fission neutrons are emitted with a is found of slowing down the fast neu puter models of the reactor system, broad range of energies but, typically, trons, while keeping them out of contact which are able to deal with the complex they are in the range of 2-3 MeV.

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