A Review of Criticality Accidents A Review of Criticality Accidents

III. POWER EXCURSIONS AND QUENCHING MECHANISMS
The study and understanding of initiating events and
shutdown mechanisms associated with criticality
accidents offer potential for limiting the frequency and
consequences of such untoward events. Although more
fatalities have been associated with critical experiment
and small reactor accidents (12) than with processing
of fissile material (9), greater visibility seems to be
associated with the safety of processing activities.
Perhaps this is because of the larger number of
individuals exposed in processing plants, the larger
economic impact of facility shutdown, and the recognition
of a degree of assumed risk for systems operated
at or near the critical state.
The most obvious and significant characteristic of
the criticality accidents that have occurred in process
operations is that all but one of them involved solutions
or slurries. This is attributable to several factors: the
relatively small quantity of fissile material required for
criticality when well moderated; the high mobility of
solutions; the ease with which they adapt to changes in
vessel shape; the potential for changes in concentration;
and, in several cases, the exchange of fissile
material between aqueous and organic phases. Fortunately,
along with the frequency of solution accidents,
there is a good understanding of quenching mechanisms
and of an inherent limitation to the fission power
density in solutions.
While there must be no implication of neglecting
concern with solid fissile materials, safety interests
may well concentrate on the behavior of solutions for
which criticality control is more subtle. While today’s
practices strongly encourage reliance on criticality
safety features that are built into the process equipment,
complete independence from administrative
controls is extremely difficult to achieve. Studies of
real and simulated accident mechanisms offer insight
into features that can mitigate the consequences of the
unlikely accident, should it occur. One such feature
might involve the inclusion of an appropriately strong
neutron source internal to a necessary, unfavorable
geometry vessel that is to receive solution normally too
lean to support criticality and not having a significant
inherent neutron source. The CRAC 5 experiments
clearly demonstrate the efficacy of such a source for
limiting the size of first peaks of power excursions.
In addition to the understanding gained from
studying the process accidents and the reactor and
critical assembly excursions involving solution
systems, we derive much information from several
series of experiments with controlled excursions in
solutions. In the U.S. the KEWB 85, 86,87 (Kinetic
Experiments on Water Boilers) series is of interest,
while the CRAC experiments performed by the Service
d’Études de Criticité of the Commissariat a l’Énergie
Atomique have direct applicability to estimates of
accident consequences. These programs, which
involved solutions of highly enriched uranium, are
supplemented by a series of measurements at Los
Alamos using the SHEBA assembly. This assembly is
fueled with a 5% enriched uranium solution, that
provides information on dose rates near excursions in
systems of lower enrichment. 88 Analysis of data from
KEWB and CRAC has led to relatively simple computer
codes that follow the early transient behavior
well and rely on thermal expansion and the formation
of radiolytic gas for the shutdown mechanisms.
Transient behavior in moderated solid cores has
been studied in the SPERT 89,90,91 and TRIGA 92,93
experimental programs, while the very fast transients
in simple, unmoderated metal systems are well
understood as a result of studies of Godiva and similar
fast burst reactors.
The quenching actions manifest in the above
mentioned experimental studies and which have
terminated many of the accidental excursions, include
thermal expansion, boiling, 238 U Doppler effect 95 , and
radiolytic gas bubble formation. The order here is of no
importance, and not all are independent. In addition, in
some situations, more than one mechanism contributes
to quench or shutdown the excursion; in many cases,
additional quenching actions set in when energy
densities or temperatures reach some threshold value.
The ramifications of this subject are varied and
numerous, but the simplest and most generally
applicable case is that of the energy model 95,96,97 in
which the change of reactivity is proportional to the
release of fission energy.
For the special case of a step increase in reactivity,
∆k 0 , we can write
∆k(t) = ∆k 0 - bE(t) (1)
where E(t) is the fission energy released to time t and b
is a constant characteristic of the system. With this
assumption, the reactor kinetic equations have been
coded for numerical solution by use of digital computers.
Such codes exist in many laboratories; the results
quoted here are from the Los Alamos RTS Code. 98,99
Figure 63 illustrates a series of computations for
hypothetical systems in which the step increase of ∆k
is 1.20 $ relative to delayed criticality, the value of b is
constant, and the neutron lifetime, l, is varied from 10 -8
to 10 -4 seconds. The power and reactivity traces for the
short neutron lifetime cases are characteristic of
prompt excursions in fast reactors. The very sharp rise
and fall in power is called the spike, and the relatively
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