597:. 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.
279:. 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.
601:
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
963:
150:
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.
600:
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
166:
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
179:
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
63:
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,
475:
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
463:
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
149:
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
153:
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
141:
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
454:
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.
199:). 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
225:
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.
120:
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
283:
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
464:
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.
484:
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
476:
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.
109:(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
188:
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.
450:
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
914:
292:
to the moderator/coolant) which is commonly employed in pressurized light water reactors also produces non-negligible amounts of tritium via the successive reactions
664:
154:
weeks), and since samarium-149 is stable, the concentration remains essentially constant during reactor operation. Another problematic isotope that builds up is
271:
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
191:
Other potential approaches to fission product removal include solid but porous fuel which allows escape of fission products and liquid or gaseous fuel (
938:
972:
971:, Wisnyi, L. G. and Taylor, K. M., in "ASTM Special Technical Publication No. 276: Materials in Nuclear Applications", Committee E-10 Staff,
777:
736:
1029:
855:
821:
612:
the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various aqueous solutions, including
697:
602:
531:, which can all absorb neutrons, so the first four are chemically unchanged by absorbing neutrons. (A final absorption produces
1080:
911:
848:
280:
221:
Other fission products with relatively high absorption cross sections include Kr, Mo, Nd, Pm. Above this mass, even many even-
960:
875:
660:
652:
1070:
200:
196:
805:
569:.) This absorption chain results in a long-lived burnable poison which approximates non-burnable characteristics.
1038:
586:
1075:
252:
803:
Table B-3: Thermal neutron capture cross sections and resonance integrals – Fission product nuclear data
249:
237:
215:
56:
930:
125:. The period of time in which the reactor is unable to override the effects of xenon-135 is called the
961:
Fabrication and
Evaluation of Urania-Alumina Fuel Elements and Boron Carbide Burnable Poison Elements
260:
181:
110:
192:
138:
33:
446:. If the reactor is to operate for a long period of time, fuel in excess of that needed for exact
894:
617:
234:
185:
781:
728:
259:
more than 5% of total fission products capture are, in order, Cs, Ru, Rh, Tc, Pd and Pd in the
703:
693:
294:
687:
1042:
884:
578:
94:
90:
825:
967:
918:
809:
431:
256:
241:
40:
311:
155:
114:
1064:
447:
245:
898:
889:
870:
869:
Pearson, Richard J.; Antoniazzi, Armando B.; Nuttall, William J. (1 November 2018).
355:
230:
176:
134:
106:
871:"Tritium supply and use: a key issue for the development of nuclear fusion energy"
442:
During operation of a reactor the amount of fuel contained in the core decreases
451:
222:
175:
per fission event in the reactor. The buildup of fission product poisons in the
172:
102:
590:
472:
289:
84:
17:
581:, produce a spatially uniform neutron absorption when dissolved in the water
707:
443:
316:
98:
1009:
802:
430:. All nuclear fission reactors produce a certain quantity of Tritium via
276:
142:
geometrical power distribution, especially in physically large reactors.
67:
The capture of neutrons by short half-life fission products is known as
27:
Substance that can absorb large quantities of neutrons in a reactor core
582:
490:
486:
342:
307:
272:
60:
467:
Fixed burnable poisons are generally used in the form of compounds of
233:
the fission product poison situation may differ significantly because
1031:
DOE Fundamentals
Handbook: Nuclear Physics and Reactor Theory, Vol. 2
692:. Trans. by Andrei Lokhov. London: Taylor & Francis. p. 57.
551:
495:
146:
71:; neutron capture by long-lived or stable fission products is called
608:
Soluble poisons are also used in emergency shutdown systems. During
849:"RBEC-M Lead-Bismuth Cooled Fast Reactor Benchmarking Calculations"
613:
609:
594:
468:
285:
158:, with microscopic cross-section of σ = 200,000 b.
218:
precisely because of their non-negligible capture cross section.
288:
reactors and sold at a profit. Water boration (the addition of
263:, with Sm replacing Pd for 6th place in the breeding blanket.
406:. Fast neutrons also produce Tritium directly from boron via
133:. During periods of steady state operation, at a constant
113:
by these fission products may become so serious that the
32:"Nuclear poison" redirects here. Not to be confused with
101:(microscopic cross-section σ = 2,000,000
1008:
United States
Government Accountability Office (2006).
180:
damps out the chain reaction. This is the reason that
105:(b); up to 3 million barns in reactor conditions) and
729:""Xenon Poisoning" or Neutron Absorption in Reactors"
137:
level, the xenon-135 concentration builds up to its
97:have a high neutron absorption capacity, such as
145:Because 95% of the xenon-135 production is from
780:. Space Nuclear Conference 2007. Archived from
585:. The most common soluble poison in commercial
822:"Evolution of Fission Product Cross Sections"
8:
661:United States Nuclear Regulatory Commission
931:"Ternary Fission | nuclear-power.com"
778:"The advantages of the poisons free fuels"
985:
983:
981:
888:
689:The History of the Soviet Atomic Industry
64:while others remain relatively constant.
593:, which is often referred to as soluble
171:and accumulate at an average rate of 50
644:
973:American Society for Testing Materials
577:Soluble poisons, also called chemical
7:
653:"Nuclear poison (or neutron poison)"
162:Accumulating fission product poisons
856:International Atomic Energy Agency
59:. In such applications, absorbing
25:
941:from the original on 7 March 2022
739:from the original on 3 April 2018
667:from the original on 14 July 2014
603:Davis-Besse Nuclear Power Station
79:Transient fission product poisons
281:Pressurized heavy water reactors
57:neutron absorption cross-section
890:10.1016/j.fusengdes.2018.04.090
1041:. January 1993. Archived from
169:lumped fission product poisons
55:) is a substance with a large
1:
876:Fusion Engineering and Design
847:A. A. Dudnikov, A. A. Sedov.
733:hyperphysics.phy-astr.gsu.edu
255:, the fission products with
184:is a useful activity: solid
201:long-lived fission products
197:aqueous homogeneous reactor
1097:
912:Boron use in PWRs and FHRs
587:pressurized water reactors
82:
31:
1039:U.S. Department of Energy
776:Liviu Popa-Simil (2007).
686:Kruglov, Arkadii (2002).
766:DOE Handbook, pp. 43–47.
757:DOE Handbook, pp. 35–42.
123:xenon precluded start-up
39:In applications such as
917:4 February 2022 at the
549:, which beta-decays to
354:or (in the presence of
117:comes to a standstill.
1081:Nuclear reactor safety
966:11 March 2023 at the
489:. It has five stable
216:nuclear transmutation
1010:"Report to Congress"
998:DOE Handbook, p. 32.
989:DOE Handbook, p. 31.
182:nuclear reprocessing
480:Non-burnable poison
253:Cooled Fast Reactor
214:, are proposed for
193:molten salt reactor
34:Radiation poisoning
1071:Nuclear technology
1048:on 3 December 2013
808:2011-07-06 at the
618:gadolinium nitrate
235:neutron absorption
186:spent nuclear fuel
828:on 2 January 2009
382:and subsequently
95:nuclear reactions
93:generated during
69:reactor poisoning
16:(Redirected from
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837:
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459:Burnable poisons
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248:. In the RBEC-M
242:thermal neutrons
213:
211:
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91:fission products
73:reactor slagging
49:neutron absorber
41:nuclear reactors
21:
1096:
1095:
1091:
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1086:
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1076:Neutron poisons
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968:Wayback Machine
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573:Soluble poisons
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438:Control poisons
432:ternary fission
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257:neutron capture
240:can differ for
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127:xenon dead time
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47:(also called a
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663:. 7 May 2014.
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636:O), are used.
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238:cross sections
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156:gadolinium-157
115:chain reaction
83:Main article:
80:
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53:nuclear poison
45:neutron poison
26:
24:
18:Nuclear poison
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13:
10:
9:
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3:
2:
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935:Nuclear Power
932:
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883:: 1140–1148.
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769:
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738:
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724:
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709:
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699:0-415-26970-9
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619:
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444:monotonically
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357:
356:fast neutrons
352:
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291:
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267:Decay poisons
266:
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254:
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246:fast neutrons
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203:, some, like
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131:poison outage
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35:
30:
19:
1052:23 September
1050:. Retrieved
1043:the original
1030:
1023:Bibliography
1015:. p. 1.
1003:
994:
959:
955:
943:. Retrieved
934:
925:
907:
880:
874:
864:
842:
830:. Retrieved
826:the original
816:
798:
788:27 September
786:. Retrieved
782:the original
771:
762:
753:
741:. Retrieved
732:
723:
711:. Retrieved
688:
681:
669:. Retrieved
656:
647:
607:
599:
576:
483:
466:
462:
452:control rods
441:
270:
250:Lead-Bismuth
231:fast reactor
228:
220:
190:
168:
165:
152:
144:
135:neutron flux
130:
126:
122:
119:
111:reactor core
107:samarium-149
89:Some of the
88:
72:
68:
66:
52:
48:
44:
38:
29:
448:criticality
223:mass number
139:equilibrium
1065:Categories
640:References
591:boric acid
473:gadolinium
290:boric acid
147:iodine-135
85:Iodine pit
589:(PWR) is
99:xenon-135
964:Archived
939:Archived
915:Archived
899:53560490
832:12 April
806:Archived
743:12 April
737:Archived
708:50952983
665:Archived
657:Glossary
513:through
491:isotopes
277:helium-3
61:neutrons
945:7 March
583:coolant
487:hafnium
340:(n,α n)
273:tritium
975:, 1959
897:
713:4 July
706:
696:
671:4 July
620:(Gd(NO
418:(n,2α)
370:(n,2n)
1046:(PDF)
1035:(PDF)
1013:(PDF)
895:S2CID
852:(PDF)
614:borax
610:SCRAM
595:boron
469:boron
394:(n,α)
286:CANDU
229:In a
173:barns
103:barns
51:or a
1054:2012
947:2022
834:2023
790:2007
745:2018
715:2014
704:OCLC
694:ISBN
673:2014
616:and
579:shim
328:and
261:core
244:and
177:fuel
43:, a
885:doi
881:136
471:or
275:to
167:as
129:or
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540:Hf
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503:Hf
493:,
434:.
392:Li
380:Li
368:Li
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338:Li
325:Li
310:,
212:Tc
195:,
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