590:, in contrast, achieving prompt criticality is essential. Indeed, one of the design problems to overcome in constructing a bomb is to compress the fissile materials enough to achieve prompt criticality before the chain reaction has a chance to produce enough energy to cause the core to expand too much. A good bomb design must therefore win the race to a dense, prompt critical core before a less-powerful chain reaction disassembles the core without allowing a significant amount of fuel to fission (known as a
387:
just a few milliseconds), the 26,000-pound (12,000 kg) reactor vessel jumped 9 feet 1 inch (2.77 m), leaving impressions in the ceiling above. All three men performing the maintenance procedure died from injuries. 1,100 curies of fission products were released as parts of the core were expelled. It took 2 years to investigate the accident and clean up the site. The excess prompt reactivity of the SL-1 core was calculated in a 1962 report:
334:) occurs, e.g., following failure of their control and safety systems. The rapid uncontrollable increase in reactor power in prompt-critical conditions is likely to irreparably damage the reactor and in extreme cases, may breach the containment of the reactor. Nuclear reactors' safety systems are designed to prevent prompt criticality and, for
391:
The delayed neutron fraction of the SL-1 is 0.70%... Conclusive evidence revealed that the SL-1 excursion was caused by the partial withdrawal of the central control rod. The reactivity associated with the 20-inch withdrawal of this one rod has been estimated to be 2.4% Îīk/k, which was sufficient to
308:
A steady-state (constant power) reactor is operated so that it is critical due to the delayed neutrons, but would not be so without their contribution. During a gradual and deliberate increase in reactor power level, the reactor is delayed-supercritical. The exponential increase of reactor activity
386:
was a prototype reactor intended for use by the US Army in remote polar locations. At the SL-1 plant in 1961, the reactor was brought from shutdown to prompt critical state by manually extracting the central control rod too far. As the water in the core quickly converted to steam and expanded (in
304:
must be greater than 1 (supercritical) without crossing the prompt-critical threshold. In nuclear reactors this is possible due to delayed neutrons. Because it takes some time before these neutrons are emitted following a fission event, it is possible to control the nuclear reaction using control
222:
among them release additional neutrons after a long delay of up to several minutes after the initial fission event. These neutrons, which on average account for less than one percent of the total neutrons released by fission, are called delayed neutrons. The relatively slow timescale on which
291:
The difference between a prompt neutron and a delayed neutron has to do with the source from which the neutron has been released into the reactor. The neutrons, once released, have no difference except the energy or speed that have been imparted to them. A nuclear weapon relies heavily on
375:, and a graphite fire. Estimated power levels prior to the incident suggest that it operated in excess of 30 GW, ten times its 3 GW maximum thermal output. The reactor chamber's 2000-ton lid was lifted by the steam explosion. Since the reactor was not designed with a
223:
delayed neutrons appear is an important aspect for the design of nuclear reactors, as it allows the reactor power level to be controlled via the gradual, mechanical movement of control rods. Typically, control rods contain neutron poisons (substances, for example
404:
explosion destroyed the adjacent machinery rooms and ruptured the submarine's hull. In these two catastrophes, the reactor plants went from complete shutdown to extremely high power levels in a fraction of a second, damaging the reactor plants beyond repair.
250:, is dominated by the time it takes for the delayed neutrons to be released, of the order of seconds or minutes. Therefore, the reaction will increase slowly, with a long time constant. This is slow enough to allow the reaction to be controlled with
777:, Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt; apparently excerpted from notes from the University of Washington Department of Mechanical Engineering; themselves apparently summarized from Bodansky, D. (1996),
284:, is limited only by the fission rate from the prompt neutrons, and the increase in the reaction will be extremely rapid, causing a rapid release of energy within a few milliseconds. Prompt-critical assemblies are created by design in
235:. With the exception of experimental pulsed reactors, nuclear reactors are designed to operate in a delayed-critical mode and are provided with safety systems to prevent them from ever achieving prompt criticality.
313:, by inserting or withdrawing rods of neutron absorbing material. Using careful control rod movements, it is thus possible to achieve a supercritical reactor core without reaching an unsafe prompt-critical state.
418:
292:
prompt-supercriticality (to produce a high peak power in a fraction of a second), whereas nuclear power reactors use delayed-criticality to produce controllable power levels for months or years.
194:
Most of the neutrons released by a fission event are the ones released in the fission itself. These are called prompt neutrons, and strike other nuclei and cause additional fissions within
189:
382:
In the other two incidents, the reactor plants failed due to errors during a maintenance shutdown that was caused by the rapid and uncontrolled removal of at least one control rod. The
765:, Los Alamos National Laboratory, LA-13638, May 2000. Thomas P. McLaughlin, Shean P. Monahan, Norman L. Pruvost, Vladimir V. Frolov, Boris G. Ryazanov, and Victor I. Sviridov.
55:
An assembly is critical if each fission event causes, on average, exactly one additional such event in a continual chain. Such a chain is a self-sustaining fission
436:
The following list of prompt critical power excursions is adapted from a report submitted in 2000 by a team of
American and Russian nuclear scientists who studied
433:
contributed to this research. Many accidents have also occurred, however, primarily during research and processing of nuclear fuel. SL-1 is the notable exception.
348:
With the exception of research and experimental reactors, only a small number of reactor accidents are thought to have achieved prompt criticality, for example
514:
714:
316:
Once a reactor plant is operating at its target or design power level, it can be operated to maintain its critical condition for long periods of time.
440:, published by the Los Alamos Scientific Laboratory, the location of many of the excursions. A typical power excursion is about 1 x 10 fissions.
672:
371:
effect resulted in an overheated reactor core. This led to the rupturing of the fuel elements and water pipes, vaporization of water, a
43:. As a result, prompt supercriticality causes a much more rapid growth in the rate of energy release than other forms of criticality.
379:
capable of containing this catastrophic explosion, the accident released large amounts of radioactive material into the environment.
360:. In all these examples the uncontrolled surge in power was sufficient to cause an explosion that destroyed each reactor and released
335:
47:
are based on prompt criticality, while nuclear reactors rely on delayed neutrons or external neutrons to achieve criticality.
774:
708:
662:
791:
741:
280:) without any contribution from delayed neutrons. In this case the time between successive generations of the reaction,
607:
130:, for the neutrons released in a fission event to cause another fission. The growth rate of the reaction is given by:
594:). This generally means that nuclear bombs need special attention paid to the way the core is assembled, such as the
493:
75:= 0.42 = 42 % probability of causing another fission event as opposed to either being absorbed by a non-fission
338:, reactor structures also provide multiple layers of containment as a precaution against any accidental releases of
413:
A number of research reactors and tests have purposely examined the operation of a prompt critical reactor plant.
136:
505:
819:
761:
486:
689:
595:
300:
In order to start up a controllable fission reaction, the assembly must be delayed-critical. In other words,
480:
697:
Proving the
Principle: A History of The Idaho National Engineering and Environmental Laboratory, 1949â1999
330:
Nuclear reactors can be susceptible to prompt-criticality accidents if a large increase in reactivity (or
809:
568:
71:(with an average of 2.4). In this situation, an assembly is critical if every released neutron has a /
700:
670:
520:
444:
814:
653:
Atomic
America: How a Deadly Explosion and a Feared Admiral Changed the Course of Nuclear History
535:
450:
553:
357:
83:
56:
749:
Additional
Analysis of the SL-1 Excursion, Final Report of Progress July through October 1962
591:
581:
231:, that easily capture neutrons without producing any additional ones) as a means of altering
35:(the threshold for an exponentially growing nuclear fission chain reaction) is achieved with
437:
376:
325:
8:
619:
544:
276:
and prompt-supercritical if it is supercritical (the fission rate growing exponentially,
239:
211:
20:
559:
349:
123:
704:
658:
651:
599:
430:
251:
199:
629:
342:
219:
268:
In contrast, a critical assembly is said to be prompt-critical if it is critical (
745:
676:
624:
372:
273:
262:
246:
greater than one. Thus the time between successive generations of the reaction,
207:
76:
64:
40:
28:
738:
587:
529:
285:
254:
44:
36:
550:
Idaho
Chemical Processing Plant, 17 October 1978 (very nearly prompt critical)
803:
603:
541:
Mayak
Production Association, 10 December 1968 (2 prompt critical excursions)
426:
32:
203:
82:
The average number of neutrons that cause new fission events is called the
392:
induce prompt criticality and place the reactor on a 4 millisecond period.
361:
339:
258:
195:
309:
is slow enough to make it possible to control the criticality factor,
456:
Los Alamos
Scientific Laboratory, December 1949, 3 or 4 x 10 fissions
215:
126:
with time. How fast it grows depends on the average time it takes,
16:
Sustained nuclear fission achieved solely by prompt neutron emission
400:
reactor accident, 10 were killed during a refueling operation. The
118:
In a supercritical assembly, the number of fissions per unit time,
68:
502:, 3 January 1961, 4 x 10 fissions or 130 megajoules (36 kWh)
422:
414:
228:
60:
408:
206:, or 10 ns). A small additional source of neutrons is the
106:
is less than 1 the assembly is said to be subcritical, and if
368:
224:
499:
383:
353:
265:
are designed to operate in the delayed-criticality regime.
110:
is greater than 1 the assembly is called supercritical.
793:
DOE Fundamentals
Handbook, Instrumentation and Control
210:. Some of the nuclei resulting from the fission are
139:
779:
Nuclear Energy: Principles, Practices, and
Prospects
198:(an average time interval used by scientists in the
367:At Chernobyl in 1986, a poorly understood positive
102:is equal to 1, the assembly is called critical, if
650:
511:Los Alamos Scientific Laboratory, 11 December 1962
477:Los Alamos Scientific Laboratory, 12 February 1957
288:and some specially designed research experiments.
242:assembly, the delayed neutrons are needed to make
183:
113:
459:Los Alamos Scientific Laboratory, 1 February 1951
801:
795:, Volume 2 of 2. DOE-HDBK-1013/2-92 (June 1992).
681:
462:Los Alamos Scientific Laboratory, 18 April 1952
526:Oak Ridge National Laboratory, 30 January 1968
471:Oak Ridge National Laboratory, 1 February 1956
67:, it typically releases between one and seven
703:, Idaho Operations Office. pp. 138â149.
474:Los Alamos Scientific Laboratory, 3 July 1956
409:List of accidental prompt critical excursions
122:, along with the power production, increases
319:
184:{\displaystyle N(t)=N_{0}k^{\frac {t}{T}}\,}
468:Oak Ridge National Laboratory, 26 May 1954
180:
465:Argonne National Laboratory, 2 June 1952
635:
84:effective neutron multiplication factor
802:
648:
364:fission products into the atmosphere.
720:from the original on 29 December 2016
687:
642:
295:
79:or escaping from the fissile core.
13:
785:
608:University of California, Berkeley
575:
492:Los Alamos Scientific Laboratory,
14:
831:
762:A Review of Criticality Accidents
494:Cecil Kelley criticality accident
86:, usually denoted by the symbols
565:Sarov (Arzamas-16), 17 June 1997
451:Los Alamos Scientific Laboratory
445:Los Alamos Scientific Laboratory
272:) without any contribution from
690:"Chapter 15: The SL-1 Incident"
506:Idaho Chemical Processing Plant
114:Critical versus prompt-critical
754:
732:
606:, and other scientists at the
149:
143:
50:
1:
775:"Nuclear Energy: Principles"
481:Mayak Production Association
63:-235 (U-235) atom undergoes
7:
613:
39:alone and does not rely on
10:
836:
579:
569:JCO Fuel Fabrication Plant
323:
744:27 September 2011 at the
701:U.S. Department of Energy
521:White Sands Missile Range
489:, 16 June 1958 (possible)
320:Prompt critical accidents
688:Stacy, Susan M. (2000).
657:. New York: Free Press.
536:Aberdeen Proving Ground
554:Soviet submarine K-431
394:
358:Soviet submarine K-431
261:, and accordingly all
185:
820:Nuclear weapon design
649:Tucker, Todd (2009).
582:Nuclear weapon design
438:criticality accidents
389:
186:
675:21 July 2011 at the
636:References and links
487:Oak Ridge Y-12 Plant
377:containment building
326:Criticality accident
212:radioactive isotopes
137:
620:Subcritical reactor
598:method invented by
571:, 30 September 1999
545:Kurchatov Institute
21:nuclear engineering
810:Nuclear technology
560:Chernobyl disaster
547:, 15 February 1971
538:, 6 September 1968
515:Sarov (Arzamas-16)
496:, 30 December 1958
352:, the U.S. Army's
278:k > 1
181:
25:prompt criticality
710:978-0-16-059185-3
664:978-1-4165-4433-3
600:Richard C. Tolman
586:In the design of
508:, 25 January 1961
431:BORAX experiments
252:electromechanical
220:nuclear reactions
200:Manhattan Project
177:
827:
766:
758:
752:
751:, November 1962.
736:
730:
729:
727:
725:
719:
694:
685:
679:
668:
656:
646:
630:Void coefficient
556:, 10 August 1985
483:, 2 January 1958
447:, 21 August 1945
343:fission products
336:defense in depth
296:Nuclear reactors
274:delayed neutrons
263:nuclear reactors
240:delayed-critical
208:fission products
190:
188:
187:
182:
179:
178:
170:
164:
163:
41:delayed neutrons
835:
834:
830:
829:
828:
826:
825:
824:
815:Nuclear physics
800:
799:
788:
786:Further reading
770:
769:
759:
755:
746:Wayback Machine
737:
733:
723:
721:
717:
711:
692:
686:
682:
677:Wayback Machine
665:
647:
643:
638:
625:Thermal neutron
616:
588:nuclear weapons
584:
578:
576:Nuclear weapons
562:, 26 April 1986
517:, 11 March 1963
411:
373:steam explosion
328:
322:
298:
286:nuclear weapons
270:k = 1
255:control systems
169:
165:
159:
155:
138:
135:
134:
116:
74:
65:nuclear fission
53:
45:Nuclear weapons
37:prompt neutrons
31:event in which
29:nuclear fission
17:
12:
11:
5:
833:
823:
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798:
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731:
709:
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612:
580:Main article:
577:
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573:
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542:
539:
533:
532:, 5 April 1968
530:Chelyabinsk-70
527:
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497:
490:
484:
478:
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324:Main article:
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192:
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176:
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158:
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148:
145:
142:
115:
112:
72:
57:chain reaction
52:
49:
15:
9:
6:
4:
3:
2:
832:
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808:
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772:
771:
764:
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747:
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735:
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712:
706:
702:
698:
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684:
678:
674:
671:
669:See summary:
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641:
631:
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626:
623:
621:
618:
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611:
609:
605:
604:Robert Serber
601:
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523:, 28 May 1965
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453:, 21 May 1946
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427:Godiva device
424:
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124:exponentially
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78:
77:capture event
70:
66:
62:
58:
48:
46:
42:
38:
34:
30:
26:
22:
792:
778:
760:
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748:
734:
722:. Retrieved
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644:
585:
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366:
350:Chernobyl #4
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259:control rods
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119:
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107:
103:
99:
95:
91:
87:
81:
54:
27:describes a
24:
18:
724:8 September
362:radioactive
340:radioactive
332:k-effective
244:k-effective
233:k-effective
214:with short
196:nanoseconds
108:k-effective
104:k-effective
100:k-effective
88:k-effective
51:Criticality
33:criticality
804:Categories
216:half-lives
739:IDO-19313
610:in 1942.
596:implosion
59:. When a
742:Archived
715:Archived
673:Archived
614:See also
257:such as
202:was one
69:neutrons
423:SPERT-I
396:In the
229:hafnium
98:. When
61:uranium
707:
661:
592:fizzle
429:, and
356:, and
305:rods.
218:, and
781:, AIP
718:(PDF)
693:(PDF)
402:K-431
398:K-431
369:scram
238:In a
225:boron
204:shake
92:k-eff
726:2015
705:ISBN
659:ISBN
500:SL-1
419:KEWB
415:CRAC
384:SL-1
354:SL-1
227:or
94:or
73:2.4
19:In
806::
773:*
713:.
699:.
695:.
602:,
425:,
421:,
417:,
345:.
90:,
23:,
728:.
667:.
311:k
302:k
282:T
248:T
175:T
172:t
167:k
161:0
157:N
153:=
150:)
147:t
144:(
141:N
128:T
120:N
96:k
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