Prompt criticality

Equivalence notwithstanding, the differences between the Heisenberg and Schrodinger approaches prompted criticism from each about the other s ideas. On the one hand, Schrodinger re-... 

The subsequent events led to the generation of an increasing number of steam voids in the reactor core, which enhanced the positive reactivity. The beginning of an increasingly rapid rise in power was detected, and a manual attempt was made to stop the chain reaction (the automatic trip, which the test would have triggered earlier, had been blocked). However, there was little possibility of rapidly shutting down the reactor as almost all the control rods had been completely withdrawn from the core. The continuous reactivity addition by void formation led to a prompt critical excursion. It was calculated that the first power peak reached 1(X) times the nominal power within four seconds. Energy released in the fuel by the power excursion suddenly ruptured part of the fuel into minute pieces. Small, hot fuel particles (possibly also evaporated fuel) caused a steam explosion. 

It is essential for the safety of the reactor to exclude the possibility of super prompt critical state at all times. This requires that the inserted reactivity at potential events should be below 1 under conservative conditions, neglecting reactivity feedback coefficients. 

The basic dynamic characteristics of the core under various reactivity insertion conditions are shown in Fig. 8. The power transient reflects the super prompt critical condition when a large reactivity insertion occurs. On flie oflier hand, the power transient is small for the 4S during potential reactivity insertion at the plant start up phase. 

During the course of the rapid power surge, the reactor became prompt critical . A nuclear chain reaction is maintained within a nuclear reactor because the neutrons produced by fission at one time go on to produce further fissions later on. If the same numbers of fissions are produced, the power remains constant and the reactor is said to be critical if more fissions are produced the power increases and if less are produced the power falls. However, not all the neutrons are produced immediately the fission occurs. The prompt neutrons (over 99%) do appear immediately but the delayed neutrons appear up to a minute later. If the reactor is critical only when the delayed neutrons are taken into account, then the power can change only fairly slowly (over a timescale of a few seconds to a minute) and so is easy to control. However, if enough reactivity is added to make the chain reaction self-sustaining on the prompt neutrons alone, then the power can change much more quickly (over thousandths of seconds). This requires a lot of reactivity to be added, but this is what happened at Chernobyl. 

On Saturday 26 April 1986 at 01.23 local time. Reactor 4 and the surrounding plant and building were destroyed by an explosion caused by a prompt critical power excursion which resulted in a massive release of active debris from the reactor core into the atmosphere. The first effects of this active release felt outside the Soviet Union were measured at the Swedish... 

The xenon-135 isotope is itself radioactive and decays at a slower rate with a 9.2 h half-life. While the reactor is operating at power, the xenon isotope is also removed by neutron capture the resulting xenon-136 isotope has an insignificant cross-section. The first effect of this phenomenon, therefore, is to require some 2-3% additional reactivity to maintain criticality at power as the xenon builds up. This amount is appreciably larger than the reactivity corresponding to prompt criticality. 

It is possible that this peculiarity of rod insertion was the final mechanism for the prompt critical accident. The calculation of the size of the effect depends upon assumptions about the previous operating history of the reactor, but estimates made at the Berkeley Nuclear Laboratory suggest that this final water displacement may have added between Vi% and Vi% reactivity, enough indeed to bring about the prompt critical excursion. 

The situation in the core was one in which a small change in the proportion of steam could lead to an increase in core reactivity which generated enough neutrons for the chain reaction to expand without waiting for the delayed neutrons—a prompt critical situation. 

In general, it is possible to ensure that the additional reactivity due to a control rod expulsion is of the order of 0.15 per cent (but, in any case, well below 0.6 per cent, which would originate a prompt criticality ). The accident reactivity excursion is mitigated by the Doppler coefficient and is terminated by the reactor scram. Roughly 10 per cent of the fuel can be damaged (DNBR < 1) and the effective whole-body doses outside the plant may reach 10-20 mSv in two hours at the edge of the exclusion area. 

The editors are thankful to the numerous reviewers. The reviewers of individual chapters contributed prompt critical reviews that have helped improve... 

A prompt criticality with core disassembly in the far distant future would involve very little radioactivity compared to the present radionuclide inventory in these cores. By the year 2700, nearly all of the current fission product inventory in the cores would have decayed. Also, the amount of fission products produced in a prompt critical excursion is relatively small. For example, the amount of Cs generated in a 10" fission criticality excursion (about the same as the SL-1 accident in the USA) would be 0.044 GBq [31]. 

The risk of a disturbance causing a change in the orientation of the SNF in the RPVs sufficient to displace the OCRs and initiate criticality, to the extent that prompt criticality may occur. The resulting power excursion from prompt criticality could cause significant and immediate radionuclide release to the environment and prejudice the safety of personnel at the scene. Even if slow criticality occurs, the increase in temperature may accelerate the corrosion rates and cause an early breakdown of the containment barriers. If this occurs with the SNF in sealed containers, they may be ruptured by an accompanying rise in pressure, leading to premature breakdown of the barriers to radionuclide release. 

The risk of a disturbance causing displacement of the SNF in the RPVs due to the corrosion-weakened or damaged condition of the fuel supporting structures. In this case, the possibility of a critical mass being formed at the bottom of an RPV, beneath the CCRs, cannot be ruled out. This could result in either slow or prompt criticality, similar to the situation discussed in the previous paragraph for displacement of the CCRs. 

J. Ernest Wilkins, Jr., The behavior of a reactor at prompt critical when the reactivity is a linear function of time, NDA 14-128. 

At this point, it was postulated that the sodium was expelled from the core in the worst possible manner, with an expulsion time of 0.1 sec, thereby producing a ramp insertion of 16/sec. This caused a prompt critical excursion that was terminated by disassembly. A center fuel temperature of 6500°F was chosen as the point at which vapor generation was sufficient to breech the cladding. The time required to reach this temperature was in the order of 1 sec. Either flow blockage of several... 

When there is a strong negative Doppler effect, the situation is quite different (2,3, 9). The accident begins in the same manner with continually shortening period and beyond prompt critical, and the maximum... 

If one assumes that there is an unlimited supply of reactivity available to be added by whatever has caused the accident, then the accident will proceed to disassembly of the core whether or not there is a strong negative Doppler effect. However, having brought the reactivity below prompt critical, the Doppler effect has succeeded in markedly increasing the time scale of the accident with the result that disassembly can occur at much lower pressure (P). 

If the Doppler effect is not strong enough to bring the reactor below prompt critical, then the behavior is, of course, intermediate between these extremes, but is of a character more like the no-Doppler effect accident than the strong Doppler effect accident. 

Doppler coefficient. Since the Doppler coefficient T dkjdT) is assumed to vary as 1/J, the parameter TidkjdT) is a constant (referred to as A op throughout this chapter). The results on Fig. 1 are consistent with previous analyses (5-9), which have shown that a large Doppler coefficient can reduce the energy release by more than a factor of 10. For calibration, it should be noted that the calculated Doppler coefficient for the 1000-MWe reactor (72) is oop = —0.008. Also, for calibration, it is worth noting that the reactivity steps (above prompt critical) of 0.001, 0.0005, and 0.00025 Ak/k would be approximately equivalent to ramps of 45, 15, and 6/sec in a reactor core having a zero Doppler coefficient. 

Under these conditions, an increase of 10% in reactor power takes 19 s. Reactors must always be built in such a way that the construction makes it impossible to reach a criticality of A > 1 -I-( prompt criticality ). This is a basic law of reactor construction since Fermi s first test reactor. In fact, this is how western reactors are designed. In the reactor at Chernobyl, which was destroyed in an explosion, prompt criticality was not made physically impossible. It was only forbidden by regulations that were not followed on the day of the accident. As a result, the reactor exploded. 

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