Nuclear power excursions

More recent evaluations have shown that an essential contribution to the nuclear power excursion was due to the particular design of the RBMK control rods. Under certain circumstances, insertion of these rods into the reactor core (in order to initiate a reactor scram) will result in an increase in power rather than in a reduction, i. e. the nuclear chain reaction will not be terminated, but accelerated (so-called positive scram effect). 

Very high releases on the first day, caused by the nuclear power excursion and its direct consequences. 

The accident at the Three Mile Island (TMI) plant in Pennsylvania in 1979 led to many safety and environmental improvements (4—6). No harm from radiation resulted to TMI workers, to the pubHc, or to the environment (7,8), although the accident caused the loss of a 2 x 10 investment. The accident at the Chernobyl plant in the Ukraine in 1986, on the other hand, caused the deaths of 31 workers from high doses of radiation, increased the chance of cancer later in life for thousands of people, and led to radioactive contamination of large areas. This latter accident was unique to Soviet-sponsored nuclear power. The Soviet-designed Chemobyl-type reactors did not have the intrinsic protection against a mnaway power excursion that is requited in the test of the world, not was there a containment building (9—11). 

In the early years of nuclear power development, the cores of several research reactors in the US were deliberatedly damaged by experimental power excursions in order to study the behavior of the fission products under such conditions. To be sure, the characteristic data of these reactors, such as the nature of the nuclear fuel, the design of the safety installations, the construction of the buildings etc., showed great differences from that of modem power reactors so that the results are of only limited value for the assessment of severe accidents. However, certain qualitative impressions can be derived from these results, as can be seen from the summary paper of Smith (1981). 

Th potentiml for power axcurilons in %hf NFR, their consequences and the effectiveness of remedial measures are discussed in this section Power excursions are directly related to nuclear excur-sions They are induced by an upset in the reactivity balance In the reactor ich can be brought about by either esnrors or accidents involving the control systems or possibly by other accidents ich can alter the neutron balance. 

H. B. Smets, Unlimited Power Excursions in Nuclear Reactors, Proc. Symp. Dynamics Nucl. Syst., Tucson, Arizona, 1970 Univ. of Arizona Press. 

The Doppler effect in the fuel would be expected to stabilise the void coefficient with a negative effect. But the Doppler coefficient only operates if the fuel temperature is high. Operated below 30% power, the normally high temperature rise in the ceramic fuel is much decreased and the Doppler effect inoperative until, during the course of the accident, the fuel temperature rose markedly. Indeed it is possible that this was the mechanism that terminated the nuclear excursion before break-up. 

The rapid escalation in heat generation in the reactor core reached about 100 times full power in a few seconds, leading to fuel melting and a rapid rise in pressure, shattering pressure tubes. This was the first explosion. A second explosion was reported which may have been another nuclear excursion or other parts of the pressure circuit failing, perhaps combined with a chemical explosion. 

The Chernobyl accident has been analysed in the Soviet Union. Two explosions were reported, although there is some doubt as to whether the second occurred. The first was a nuclear excursion leading to an enormous increase in heat production in the core. The Soviet modelling also supports the occurrence of a second nuclear excursion. The reported maximum rate of heat release has been given as about 100 times normal full power. At the very high temperatures produced by the nuclear excursions, chemical reactions between Zircaloy and steam/water would be rapid and chemical reactions are likely to have contributed to the second explosion and the subsequent course of the accident. 

Excursion (of power) Fast and uncontrolled increase of the power produced in a nuclear reactor following an accident. 

A reactor is operating at 2 Mw (ioox full power) and the reactor scram is set for 1255 fuli power. What will be the peak power if a nuclear excursion creates a 100 millisecond period and the scram delay time is O.l seconds after i25X power is reached (Assume no temperature or void effects and that the period scram is inoperative)... 

Natural-circulation flows around loops and flows in parallel channels are both susceptible to departures from steady operation and excursions into oscillatory and, potentially, unstable states. Thus Gen IV nuclear reactor power systems combine the type of fluid flow and geometries that are known to potentially lead to undesirable states. In particular, undesirable oscillatory states under steady-state operation should be avoided. The complete system and associated operational envelope are designed to avoid unstable states. 

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