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How Reactors Work
Published in Geoffrey F. Hewitt, John G. Collier, Introduction to Nuclear Power, 2018
Geoffrey F. Hewitt, John G. Collier
The term positive void coefficient is reactor physicist’s jargon for the fact that reducing coolant density results in an increase in neutron population (light water is a strong absorber of neutrons) and hence in an increase of reactor power. However, as the power increases so too does the fuel temperature, and this has the effect of reducing the neutron population (negative fuel coefficient). The net effect of the positive void coefficient and the negative fuel coefficient clearly depends on the power level.
Water-Cooled Reactors
Published in William J. Nuttall, Nuclear Renaissance, 2022
At the risk of digression, it is interesting to note that the United States was not the only country to seek to pioneer the early development of BWRs. The Soviet Union was also acutely aware of the advantages of such an approach, but the Russian team at Minatom followed a path steered by the fact that they did not have early access to the results of the US Borax experiments. The consequence was that the unique Russian RBMK design is a BWR with a graphite moderator. The use of a graphite moderator had been key to the Obninsk-1 reactor concept described earlier. The use of graphite for the RBMK was intended to circumvent the two-phase difficulty that the Americans originally expected with a boiling moderator. In part, the use of a graphite moderator and a boiling light water-coolant gave rise to a significant safety problem that was eventually to be the undoing of the design. The RBMK reactor design suffers from a positive void coefficient when steam voids form within the reactor core. It is this aspect that lies at the heart of the disaster at the Chernobyl Unit 4 plant in 1986. G F Hewitt and J G Collier provide a thorough account of the Chernobyl accident in their book Introduction to Nuclear Power [69]. The RBMK was an extremely large and capital-intensive design, but it was the accident at Chernobyl that ensured that the Russian graphite-moderated BWR technology will not figure in a global Nuclear Renaissance. That does not mean, however, that Russia has nothing technological to contribute to a renaissance. We earlier considered the success of the VVER-1200 PWR and in the area of fast reactors, Russia also has much useful knowledge and experience with advanced reactor concepts and we shall consider such aspects further in Chapter 8.
Investigative Study of Neutronic Safety Parameters of HPR and EPR Using the MCNP Code
Published in Nuclear Technology, 2020
R G. Abrefah, P. M. Atsu, R. B. M. Sogbadji
In water-moderated reactors, changes in moderator density significantly affect the reactivity. Changes in moderator density can be due to thermal expansion, void formation, or loss of coolant. A change in the moderator void content leads to a change in multiplication factor k and alters the reactivity of the system. The void coefficient of reactivity is therefore defined as the rate of change in the reactivity of a water-moderated reactor resulting from any modification of the moderator/coolant as the power level and temperature change. The principal effect is the loss of moderation that accompanies a decrease in moderator density and causes a corresponding increase in resonance.7 For PWRs, about 80% of neutron moderation occurs in the light water moderator.7
Preliminary Neutronics Design and Analysis of the Fast Modular Reactor
Published in Nuclear Science and Engineering, 2023
Hangbok Choi, Darrin Leer, Matthew Virgen, Oscar Gutierrez, John Bolin
The coolant void reactivity relates the change in neutron multiplication to the loss of coolant. The typical void coefficient varies from 0.33 pcm/% void at BOC to 0.62 pcm/% void at EOC at operating temperatures. The coolant void reactivity is positive due to spectral shift, but is very small when compared with a liquid-metal reactor.