586:. The boric acid in the coolant decreases the thermal utilization factor, causing a decrease in reactivity. By varying the concentration of boric acid in the coolant, a process referred to as boration and dilution, the reactivity of the core can be easily varied. If the boron concentration is increased (boration), the coolant/moderator absorbs more neutrons, adding negative reactivity. If the boron concentration is reduced (dilution), positive reactivity is added. The changing of boron concentration in a PWR is a slow process and is used primarily to compensate for fuel burnout or poison buildup.
268:. Since Tritium has a half-life of 12.3 years, normally this decay does not significantly affect reactor operations because the rate of decay of Tritium is so slow. However, if tritium is produced in a reactor and then allowed to remain in the reactor during a prolonged shutdown of several months, a sufficient amount of tritium may decay to helium-3 to add a significant amount of negative reactivity. Any helium-3 produced in the reactor during a shutdown period will be removed during subsequent operation by a neutron-proton reaction.
590:
inserted control rods. This system is not in widespread use because the chemicals make the moderator temperature reactivity coefficient less negative. All commercial PWR types operating in the US (Westinghouse, Combustion
Engineering, and Babcock & Wilcox) employ soluble boron to control excess reactivity. US Navy reactors and Boiling Water Reactors do not. One known issue of boric acid is that it increases corrosion risks, as illustrated in a 2002 incident at
952:
139:
change of concentration during the initial 4 to 6 hour period following the power change is dependent upon the initial power level and on the amount of change in power level; the xenon-135 concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed.
589:
The variation in boron concentration allows control rod use to be minimized, which results in a flatter flux profile over the core than can be produced by rod insertion. The flatter flux profile occurs because there are no regions of depressed flux like those that would be produced in the vicinity of
155:
There are numerous other fission products that, as a result of their concentration and thermal neutron absorption cross section, have a poisoning effect on reactor operation. Individually, they are of little consequence, but taken together they have a significant effect. These are often characterized
168:
eventually leads to loss of efficiency, and in some cases to instability. In practice, buildup of reactor poisons in nuclear fuel is what determines the lifetime of nuclear fuel in a reactor: long before all possible fissions have taken place, buildup of long-lived neutron-absorbing fission products
52:
is normally an undesirable effect. However, neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load. Some of these poisons deplete as they absorb neutrons during reactor operation,
464:
that are shaped into separate lattice pins or plates, or introduced as additives to the fuel. Since they can usually be distributed more uniformly than control rods, these poisons are less disruptive to the core's power distribution. Fixed burnable poisons may also be discretely loaded in specific
452:
To control large amounts of excess fuel reactivity without control rods, burnable poisons are loaded into the core. Burnable poisons are materials that have a high neutron absorption cross section that are converted into materials of relatively low absorption cross section as the result of neutron
138:
decay, which has a 6- to 7-hour half-life, the production of xenon-135 remains constant; at this point, the xenon-135 concentration reaches a minimum. The concentration then increases to the equilibrium for the new power level in the same time, roughly 40 to 50 hours. The magnitude and the rate of
142:
Because samarium-149 is not radioactive and is not removed by decay, it presents problems somewhat different from those encountered with xenon-135. The equilibrium concentration (and thus the poisoning effect) builds to an equilibrium value during reactor operation in about 500 hours (about three
130:
value for that reactor power in about 40 to 50 hours. When the reactor power is increased, xenon-135 concentration initially decreases because the burn up is increased at the new, higher power level. Thus, the dynamics of xenon poisoning are important for the stability of the flux pattern and
443:
containing neutron-absorbing material is one method, but control rods alone to balance the excess reactivity may be impractical for a particular core design as there may be insufficient room for the rods or their mechanisms, namely in submarines, where space is particularly at a premium.
188:). These ease the problem of fission product accumulation in the fuel, but pose the additional problem of safely removing and storing the fission products. Some fission products are themselves stable or quickly decay to stable nuclides. Of the (roughly half a dozen each) medium lived and
214:
isotopes have large absorption cross sections, allowing one nucleus to serially absorb multiple neutrons. Fission of heavier actinides produces more of the heavier fission products in the lanthanide range, so the total neutron absorption cross section of fission products is higher.
109:
Xenon-135 in particular tremendously affects the operation of a nuclear reactor because it is the most powerful known neutron poison. The inability of a reactor to be restarted due to the buildup of xenon-135 (reaches a maximum after about 10 hours) is sometimes referred to as
272:
will produce small but notable amounts of tritium through neutron capture in the heavy water moderator, which will likewise decay to helium-3. Given the high market value of both tritium and helium-3, tritium is periodically removed from the moderator/coolant of some
453:
absorption. Due to the burn-up of the poison material, the negative reactivity of the burnable poison decreases over core life. Ideally, these poisons should decrease their negative reactivity at the same rate that the fuel's excess positive reactivity is depleted.
473:
A non-burnable poison is one that maintains a constant negative reactivity worth over the life of the core. While no neutron poison is strictly non-burnable, certain materials can be treated as non-burnable poisons under certain conditions. One example is
465:
locations in the core in order to shape or control flux profiles to prevent excessive flux and power peaking near certain regions of the reactor. Current practice however is to use fixed non-burnable poisons in this service.
98:(σ = 74,500 b). Because these two fission product poisons remove neutrons from the reactor, they will affect the thermal utilization factor and thus the reactivity. The poisoning of a
177:
contains about 97% of the original fissionable material present in newly manufactured nuclear fuel. Chemical separation of the fission products restores the fuel so that it can be used again.
439:
must be added when the reactor is fueled. The positive reactivity due to the excess fuel must be balanced with negative reactivity from neutron-absorbing material. Movable
903:
281:
to the moderator/coolant) which is commonly employed in pressurized light water reactors also produces non-negligible amounts of tritium via the successive reactions
653:
143:
weeks), and since samarium-149 is stable, the concentration remains essentially constant during reactor operation. Another problematic isotope that builds up is
260:
In addition to fission product poisons, other materials in the reactor decay to materials that act as neutron poisons. An example of this is the decay of
180:
Other potential approaches to fission product removal include solid but porous fuel which allows escape of fission products and liquid or gaseous fuel (
927:
960:, Wisnyi, L. G. and Taylor, K. M., in "ASTM Special Technical Publication No. 276: Materials in Nuclear Applications", Committee E-10 Staff,
766:
725:
1018:
810:
601:
the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various aqueous solutions, including
520:, which can all absorb neutrons, so the first four are chemically unchanged by absorbing neutrons. (A final absorption produces
961:
900:
837:
210:
Other fission products with relatively high absorption cross sections include Kr, Mo, Nd, Pm. Above this mass, even many even-
949:
641:
844:
686:
591:
794:
558:.) This absorption chain results in a long-lived burnable poison which approximates non-burnable characteristics.
1069:
269:
864:
649:
1059:
189:
185:
792:
Table B-3: Thermal neutron capture cross sections and resonance integrals – Fission product nuclear data
919:
114:. The period of time in which the reactor is unable to override the effects of xenon-135 is called the
1027:
950:
Fabrication and
Evaluation of Urania-Alumina Fuel Elements and Boron Carbide Burnable Poison Elements
575:
1064:
241:
435:. If the reactor is to operate for a long period of time, fuel in excess of that needed for exact
770:
717:
248:
more than 5% of total fission products capture are, in order, Cs, Ru, Rh, Tc, Pd and Pd in the
676:
238:
226:
204:
45:
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814:
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170:
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8:
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878:
859:
858:
Pearson, Richard J.; Antoniazzi, Armando B.; Nuttall, William J. (1 November 2018).
873:
567:
83:
79:
860:"Tritium supply and use: a key issue for the development of nuclear fusion energy"
956:
907:
798:
431:
During operation of a reactor the amount of fuel contained in the core decreases
420:
245:
230:
29:
164:
per fission event in the reactor. The buildup of fission product poisons in the
300:
144:
103:
996:
1053:
570:, produce a spatially uniform neutron absorption when dissolved in the water
436:
234:
696:
998:
344:
219:
165:
123:
95:
791:
440:
419:. All nuclear fission reactors produce a certain quantity of Tritium via
211:
161:
131:
geometrical power distribution, especially in physically large reactors.
91:
56:
The capture of neutrons by short half-life fission products is known as
16:
Substance that can absorb large quantities of neutrons in a reactor core
579:
461:
278:
73:
456:
Fixed burnable poisons are generally used in the form of compounds of
222:
the fission product poison situation may differ significantly because
1020:
DOE Fundamentals
Handbook: Nuclear Physics and Reactor Theory, Vol. 2
681:. Trans. by Andrei Lokhov. London: Taylor & Francis. p. 57.
432:
305:
87:
60:; neutron capture by long-lived or stable fission products is called
597:
Soluble poisons are also used in emergency shutdown systems. During
838:"RBEC-M Lead-Bismuth Cooled Fast Reactor Benchmarking Calculations"
265:
147:, with microscopic cross-section of σ = 200,000 b.
571:
479:
475:
331:
296:
261:
207:
precisely because of their non-negligible capture cross section.
49:
540:
484:
277:
reactors and sold at a profit. Water boration (the addition of
135:
252:, with Sm replacing Pd for 6th place in the breeding blanket.
602:
598:
583:
457:
395:. Fast neutrons also produce Tritium directly from boron via
274:
122:. During periods of steady state operation, at a constant
102:
by these fission products may become so serious that the
21:"Nuclear poison" redirects here. Not to be confused with
857:
90:(microscopic cross-section σ = 2,000,000
997:
United States
Government Accountability Office (2006).
835:
150:
169:
damps out the chain reaction. This is the reason that
94:(b); up to 3 million barns in reactor conditions) and
718:""Xenon Poisoning" or Neutron Absorption in Reactors"
126:
level, the xenon-135 concentration builds up to its
67:
86:have a high neutron absorption capacity, such as
1051:
764:
134:Because 95% of the xenon-135 production is from
769:. Space Nuclear Conference 2007. Archived from
574:. The most common soluble poison in commercial
811:"Evolution of Fission Product Cross Sections"
650:United States Nuclear Regulatory Commission
920:"Ternary Fission | nuclear-power.com"
767:"The advantages of the poisons free fuels"
974:
972:
970:
877:
678:The History of the Soviet Atomic Industry
53:while others remain relatively constant.
582:, which is often referred to as soluble
160:and accumulate at an average rate of 50
674:
1052:
981:
967:
962:American Society for Testing Materials
566:Soluble poisons, also called chemical
468:
642:"Nuclear poison (or neutron poison)"
151:Accumulating fission product poisons
447:
13:
845:International Atomic Energy Agency
561:
426:
48:. In such applications, absorbing
14:
1081:
930:from the original on 7 March 2022
728:from the original on 3 April 2018
656:from the original on 14 July 2014
592:Davis-Besse Nuclear Power Station
68:Transient fission product poisons
270:Pressurized heavy water reactors
255:
46:neutron absorption cross-section
1011:
990:
942:
912:
894:
879:10.1016/j.fusengdes.2018.04.090
851:
1030:. January 1993. Archived from
829:
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785:
758:
749:
740:
710:
668:
634:
158:lumped fission product poisons
44:) is a substance with a large
1:
865:Fusion Engineering and Design
836:A. A. Dudnikov, A. A. Sedov.
722:hyperphysics.phy-astr.gsu.edu
628:
244:, the fission products with
173:is a useful activity: solid
7:
190:long-lived fission products
186:aqueous homogeneous reactor
10:
1086:
901:Boron use in PWRs and FHRs
576:pressurized water reactors
71:
20:
1028:U.S. Department of Energy
765:Liviu Popa-Simil (2007).
675:Kruglov, Arkadii (2002).
755:DOE Handbook, pp. 43–47.
746:DOE Handbook, pp. 35–42.
112:xenon precluded start-up
28:In applications such as
906:4 February 2022 at the
538:, which beta-decays to
343:or (in the presence of
106:comes to a standstill.
1070:Nuclear reactor safety
955:11 March 2023 at the
478:. It has five stable
205:nuclear transmutation
999:"Report to Congress"
987:DOE Handbook, p. 32.
978:DOE Handbook, p. 31.
171:nuclear reprocessing
469:Non-burnable poison
242:Cooled Fast Reactor
203:, are proposed for
182:molten salt reactor
23:Radiation poisoning
1060:Nuclear technology
1037:on 3 December 2013
797:2011-07-06 at the
607:gadolinium nitrate
224:neutron absorption
175:spent nuclear fuel
817:on 2 January 2009
371:and subsequently
84:nuclear reactions
82:generated during
58:reactor poisoning
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813:. Archived from
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448:Burnable poisons
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237:. In the RBEC-M
231:thermal neutrons
202:
200:
199:
80:fission products
62:reactor slagging
38:neutron absorber
30:nuclear reactors
1085:
1084:
1080:
1079:
1078:
1076:
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1074:
1065:Neutron poisons
1050:
1049:
1040:
1038:
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799:Wayback Machine
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773:on 2 March 2008
763:
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689:
673:
669:
659:
657:
640:
639:
635:
631:
624:
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616:
612:
564:
562:Soluble poisons
553:
551:
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541:
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531:
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504:
503:
497:
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471:
450:
429:
427:Control poisons
421:ternary fission
414:
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258:
246:neutron capture
229:can differ for
198:
196:
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194:
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116:xenon dead time
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70:
36:(also called a
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652:. 7 May 2014.
632:
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227:cross sections
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152:
149:
145:gadolinium-157
104:chain reaction
72:Main article:
69:
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42:nuclear poison
34:neutron poison
15:
9:
6:
4:
3:
2:
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1021:
1016:
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993:
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951:
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929:
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924:Nuclear Power
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915:
909:
905:
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880:
875:
872:: 1140–1148.
871:
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861:
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812:
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793:
788:
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569:
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481:
477:
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433:monotonically
424:
422:
346:
345:fast neutrons
341:
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302:
298:
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280:
276:
271:
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256:Decay poisons
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235:fast neutrons
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206:
192:, some, like
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120:poison outage
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1041:23 September
1039:. Retrieved
1032:the original
1019:
1012:Bibliography
1004:. p. 1.
992:
983:
948:
944:
932:. Retrieved
923:
914:
896:
869:
863:
853:
831:
819:. Retrieved
815:the original
805:
787:
777:27 September
775:. Retrieved
771:the original
760:
751:
742:
730:. Retrieved
721:
712:
700:. Retrieved
677:
670:
658:. Retrieved
645:
636:
596:
588:
565:
472:
455:
451:
441:control rods
430:
259:
239:Lead-Bismuth
220:fast reactor
217:
209:
179:
157:
154:
141:
133:
124:neutron flux
119:
115:
111:
108:
100:reactor core
96:samarium-149
78:Some of the
77:
61:
57:
55:
41:
37:
33:
27:
18:
437:criticality
212:mass number
128:equilibrium
1054:Categories
629:References
580:boric acid
462:gadolinium
279:boric acid
136:iodine-135
74:Iodine pit
578:(PWR) is
88:xenon-135
953:Archived
928:Archived
904:Archived
888:53560490
821:12 April
795:Archived
732:12 April
726:Archived
697:50952983
654:Archived
646:Glossary
502:through
480:isotopes
266:helium-3
50:neutrons
934:7 March
572:coolant
476:hafnium
329:(n,α n)
262:tritium
964:, 1959
886:
702:4 July
695:
685:
660:4 July
609:(Gd(NO
407:(n,2α)
359:(n,2n)
1035:(PDF)
1024:(PDF)
1002:(PDF)
884:S2CID
841:(PDF)
603:borax
599:SCRAM
584:boron
458:boron
383:(n,α)
275:CANDU
218:In a
162:barns
92:barns
40:or a
1043:2012
936:2022
823:2023
779:2007
734:2018
704:2014
693:OCLC
683:ISBN
662:2014
605:and
568:shim
317:and
250:core
233:and
166:fuel
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264:to
156:as
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492:Hf
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