ML19308C798
| ML19308C798 | |
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| Site: | Crane  |
| Issue date: | 08/20/1955 |
| From: | Mccullough C, Mills M, Teller E CALIFORNIA, UNIV. OF, LOS ANGELES, CA, LAWRENCE LIVERMORE NATIONAL LABORATORY, Advisory Committee on Reactor Safeguards |
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Text
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O Proceedings of the International Conference 4
on the Peaceful Uses of Atomic Energy Held in Geneva 8 August-20 August 1955 Volume 13 Legal, Administrative, Health and Safety Aspects of Large-Scale Use of Nuclear Energy
-,x- -
UNITED NATIONS New York 1956 Y-12 '1ECHS"' AL LIBRARY Y-140.3 3 8002070 E30
i 1
+9 The Safety of Nuclear Reactors By C. Rogers McCullough,* Mark M. Mills,t and Edward Teller,t USA REACTOR TECHNOLOGY
. dents. So far, there have been essentially no reactor accidents Icading to serious consequences. For this in anv new deld of technology, it is important to
'in'vettig$te, quantitatively if possible, as many fea-reason, statistical intormation about reactor accidents, although all favorable, does not suffice to give useful tures of the netd as seem pertinent for human we!-
statistical information of the type needed by insur-fare. Nuclear reactor techno!cgy is such a field, and ance c mPames, for example, in evaluatmg the nature no one looks to it with hope for. many material f hazards. In other words, to determme what is an bene 6ts for mankind. Among these benefits are the
. possibility of electric power generation, propulsion acceptable risk, a certain amount of judgment, de-tailed techmcal evaluation of a given reactor, and bv nuclear energy, and the utilization of reactors as thearch tools in many branches of science and e uti n must be employed.
\\Vith all the m, herent safeguards that can be put medicine.
into a reactor, there is still no fool-proof system. Any Along with a long list of possibic attractive fea-tures of reactors, there are, unfortunately, certain system can be deteated by a great enough fool. The y real danger occurs when a false sense of security dangerous characteristics. The Advisory Committee causes a relaxation of caution >
on Reactor Safeguards (see Appendix) has the re-Problems of reliability, adequate control, adequate sponsibility of looking at the hazards connected with nuclear reactors. The members of this committee are superdsion, must all be mcluded. It is convement to I k upon the concepts of reactor safety m the fo!-
. exceedingly anxious to see rapid and fruitful devel-
-l opment of reactor technology, but because of the N*'"E '.vays :
One nnportant concept is the division of safety
' nature of the hazards involved, and because they have been specifically requested to look at hazard P,roblems m, to on-site and off-site problems. The on-problems, they feel it important that no undue risks site problems have to do with the protection of be taken in the development of nuclear reactors.
reactor operating personnel and other people who may be at the reactor site in order to make use of it, REACTOR SAFETY and the protection of the economic investment in the immediately, when one attempts to evaluate re-reactor facility. Off-site problems have to do with actor hazards, there is encountered the necessity for the protection of the general public, or persons who attempting to dedne the ' notion of rector safety, and are not more or less directly connected with the op-uhat this notion shall include. Of course, absolute cration of the reactor. One way to minimize off-site safetv is not possible and what is really meant in hazards is simply to locate the reactor at a remote conn'ection with reactor hazards is the minimization and unpopulated place. In terms of reactor utiliza-tion and economics, this solution is often unsatisfac-of hazards until one has an acceptable calculated risk.
The operation of nuclear reactors appears safe tory. The economic utilization of ciectric power and it is, in fact. deceptively safe. A nuclear reactor generated by reactors, for example, nearly always will not. rim away unless a number of serious mis-regtures that the reactor he located reasonably close takes,,i planningituil operation shou!d be committed.
to potential u ers of this power. This means that for ec n mic reasons the reactor shonid be located near It is. however, impossible to conduct extensive op-rations over a long time without occasional occur-poput us, mdustrial areas.
e rences of iuch mistakesi We have been exceedingly Substantial moral and ethical problems are in.
v Ived in connection with reactor hazards. On-site lucky so inr that nobody has as vet been killed by a rtma' war reactor.' It is itot possiijle to count on in...
personnel, like persons working in other industries, definite contiimation of such good luck.
knowingly and willingly submit themselves to what.
~
One of the entrent diinculties in evaluating reactor ever hazards are associated with working near a hazards is this lack of experience with reactor acci-
[eactor because of saiary requirements, special work-mg conditions, or personal interest.
- Chairman A.hbery Committee on Reactor Safeguards, For off-site people, on the other hand, who have US.}EC...
no knowledge or interest in the operation of the re-n)r, a!NN..
'" ~ - actor, it seems that prevention of danger to their.
- Dtpartment of physics, University of California persons or damage to their property is a mandatory
-t.
79
' 80 l VOL XIll P/853 USA C. R. McCULLOUGH et al.
moral obligation in the operation of a reactor. This of these 6ssion produets from the machine The po-problem is more severe than in the cre of dangerous tential ability of a reactor to run1away makes it
. chemical or explosives plants, becate the radioactiv-possible for this radioactive material to escape to the ity contained in a reactor can constitute a hazard to a surrounding areas. The hazard is crudely analogous wide area if it escapes from a machine and becomes to conducting both explaire and virulent poison
. dispersed.: This public hazard has been one of the production tmder the same roof.'
main concerns of the Advisory Committee on Re-Until really safe nuclear machines of the future actor Safeguards.
become available, we have to construct our reactors From anoth' r point of view, the saiety of a nuclear with extreme circumspection and we must continue e
reactor can be said to depend upon two things: The to operate them with the same caution after ten years intrinsic built-in stability and reliability of the ma-of safe running as on the very first day when they chine, and admiristrative co' trol of the machine and were started up.*
n its operatio,. For example, the reactivity may de-In order to emphasize the characteristic of the crease rapidly with increasing temperature. In this special hazard due to radioactive materials in the case. it may be practically impossible to exceed some reactor, a list of tolerances is presented in Table I.5 safe limit in temperature. This intrinsic stability is Although there has been a substantial effort in the very desirable. In fact, one may say that a machine assessment of the effects of radiation ca biologicat with large intrinsic stability can be so stable, be-systems, particularly systems resembling people, cause of fundamental physical characteristics, that there is still a great deal to be learned.8 However, only a Maxwell demon can make it misbehave. An even allowing for considerable error in the quantita-ordinary machine, which depends on the operation of tive assessment of this problem,it is still evident from the control system to set its power level, can be upset Table I that radioactive poisons are more hazardous by a mere gremlin! One would like to minimize the than chemical poisons by a factor of something like 108,to 10}. This is such an enormous factor that radio-d'ependence: upon administrative control for safe operation of a reactor. However, as a matter of prac-actwe poisons essentially must be considered a quali-tical fact most reactors will nearly always require tative new kind of problem. Furthermore, this implies
/ a certain dependence upon administrative control for Table 1.5 Comparison of Toxic Substances in Air
- safe and reliable operation. This means thst prob-IC "'*"" 'I " I" * @*D lems arise connected with the loading and unloading of fuel, the startup and shutdown of the reactor, 3,3
,fgg proper manipulation of controls, and, adequate ac-
),,g,,,,,,
7,,,,,,
g,y,,.,,
counting for all matenals made radioactive by the re-cwaawi rot...
actor, including both intentionally irradiated' material Chlorine -
19 f
_90:
100 and any radioactive effluent associated with the oper.
',]lium ation. Thus the normal, as well as the abnormal oper-g 1.
04 ation and behavior of the reactor must be carefully considered. It is clear that a reactor which m norma!
operation is well run and under complete and precise
- Ih I
b."' O I
X o
I x
control is much less likely to behave m an abnormal Sr" 1.3 x 104 1.3 x 104 10,000 fashion leading to a serious accident.
' It should be remembered that industrial poisons are THE CONTAINED RADIOACTIVITY usua!!y in many ton quantities, whereas radioactive poisons are m 100-kilogram quantities.
The most serious continuing hazard associated with t" Tolerance" for ehemical poisons is defined as the max-nuclear reactors is due to the large amount of radio.
imum tolcrable level for 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> per day exposure. In the
- *
- I '"di "'ti P i5 "' t lerance is the maximum leve!
activity which they contain. Large reactors maY which can be tolerated every day for 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> equivalent to contain htmdreds of pounds of radioactive fission 0.043 rem per day.
products which correspond to many tons of radium
" Fatal Dose" in the case of chemical poisons is defmed as the," rapidly fatar' dose when the given concentration in in conventional radioactive measure. Not all of these air is mhaled for 30 mmutes to one hour. In the case of ra-fiss. ion products are as hazardous as rad.ium, but dioactive material this means about 507c survival if the nearly all~ of them contribute substantially to the dose is acquired quite rapidly, for example, over a minute or hazar'd.8 There are two ways in which th'e hazard
' @,*P',during an 8-hour day. This is equivalent to about
,e
' of contained fission products may be minimized: One
! Adopted at meeting of the American Conference of is to remove fission products during the operation of Governmental and Industrial Hygienists in Atlantic City, the reactor in such a way as to maintain a minimum
.% L in Apri! 1951.
concentration of such m'aterial in the machine. This iInduurial Fysiene and Toxicolocy, Frank H. Patty, Editor. Interscience Publishers. Inc 1949.
continuous removal of fiss. ion products reqmres some
" llaximum Permissible Amounts of Radioisotopes in
- type of fluid fuel, either liquid.or gaseous in order the 11uman Body and 11aximum Permissible Con (entratic.ns
["difj"[Qs;9,nad 51 National Bunau of N"4 to continue cleanup operations on the fuel during the n,
2 h.e Weapons. t*S Govemment
- operation of the machine. The other }vay to ininimize
' the hazard is to mimmize the possibihty of the escape Printing Orrice, Revised September 1950.
w w
THE SAFETY OF NUCLEAR REACTORS BI that the problem of keeping radioactive materials Toble 11. Delayed Heat Power and Radioactivity" within the reactor and preventing the spread of radio.
offer Normal Shutdown
- active materials over populous areas is very serious.
In Table 11 there is a summary of delayed heat f,g.,,$,Tg7,'f;'"J, on l
production and the corresponding radioactivity from.
6ssion products. For a machine of 250,000-kw heat
'n,,,,,,,,
,e manar sw=
F"'3 88:
ein'3 *8:
power K0,000-kw electric power),- something -like
<<c
>=
arus 6e mi<<
300 inillion etities of activity remains at the end of 10 10 see 12.9 2.1 X 10' 11.000 1.8 x 10' one day after shutdown. This corresponds to 300 tons 1 08 1.7 min 8.0 1.3 x 10' 6800 1.1 x 10' of radium in terms of radioactivity. The sheer quan.
10'-
16.7 min 5.2 8.4 x 10' 4300 7.0 x 10*
~
10' 2.8 hr J.3 5.3 X 10' 2700 4.4 x 10'
, tity of radioactivity is enormous.-
1 08 28hr 2.0 3.3 X 10' 1700 2.8 X 10' Operanon or this reactor for one year produces about 100 kilograms of fission products. On the basis
- The radioactivity figures are for fission products only mgicm this can contaminate 10' cubic kilo-(do not include radioactive fuels or components). It is as-
- of 104 2
meters of air to tolerance. Said another way, a layer sumed that the mean decay event corresponds to 1.0 liev in c nverting from kw to curies.
of air rme km deep covering an area 1000 km on a
- side could be brought to tolerance level.
of human reaction times and conventional external Another seature or radioactive poisons is that a emergency human actions, nevertheless, a nuclear letha! level is not detectable by human senses. Fur-thennore. very serious injurv may not be detected for react r is a very slugg. h dev. ice and does not pro-is duce a nuclear explosion even remotely approximat-wme years after exposure.:
ing that of an atomic bomb. Indeed, for the large ESCAPE OF RADIOACTh/ITY thermal reactors, nothing like an explosion really.
The wav in which reactors can malfunction and occurs. For very fast reactors with a non-thermal lead to the e> cape of nssion products may be classi-neutron spectrum and heav,ly loaded with ennched i
"T*"I*n, it does appear possible to have an accident
- 5ed as follown (1) a super-critical nuclear excursion which is fast enough so that portions of the machme or nelear runaway; (2) melt-down of reactor com-inay be propelled with velocities of a few meters per
~
ponents, even with the chain reaction shut down, second. This agam does not resemble an atomic bomb because of the delayed heat produced by the radio-exP osion, or even the explosion of ordinary chemical l
active 6ssion proditets; and (3) possibic exothermic
- P osives; rather it is similar to the events that l
chemical reactions among the components of the re-might occur m an automobile accident. Therefore a actor itself. The latter, although it is clearly not
"" **' '"""*"I' *" 'lS*If' does not represent a sen-present if the machine is operating normally, may be us hazard to off-site people.
initiated by a runaway nuclear chain reaction or by II wever, as twinted out above, a nuclear runaway delayed heat melting.
can serve t do two thm, gs j It may disrupt the struc-These problems will be discussed in more detail below. The 6rst two are unique to nuclear reactors ture of the reactor suf6ciently so that radioactn;e Poisons may escape, or it may lead to exothermic as compared to other power sources, and have no true ch mical reacti ns between different components of analogue in other areas of technology. They are dis-the reactor core. and a chemical explosion of con-cussed m. s.me detail, for research reactors,, and siderable violence. Indeed, for certain types of reactor nuclear power plants' elsewhere.
structures, it would appear that the chemical reaction THE PROBLEM OF NUCLEAR RUNAWAY that might follow a nuclear runaway would produce substantiallv greater energy and violence than the
,mg charactenstic of nuclear reactors runaway which preceded it.'
An outstand.
. 25 their potential abihty to actueve extremely high In order to make seme of these notions more quan-power level > in a short time if adequate control of titative, it is convenient to talk about the rising period the machme is lost. A typical nuclear runaway acci-of a nrelear reactor. A nuclear reactor which is super-dent may start and be over in times appreciably less criticai increases in power level by a factor of e at each d
~
than a second. In this respect they are different from interval of time corresponding to the so-called e-fold-any other large-scale machmes, and it is this ex-ing time. In turn, the c-folding time is related to the tremely short time that makes it qmte important that intrinsic neutron generation time of the reactor, and
. automatic control and safety systems be available, be the degree of supercriticalitv. bv the so-called in-hour reliable. and be relatively rapid m their operation.
equation. In Fig.1. a nuniber' of curves are shown Another itature of a possible nuclear runaway is connecting the rising period of the reactor with its that it does not seem to be very violent. A comparison excess reactivity,. i.e., the fraction of excess neu-between a nuclear reactor and an atomic bomb is very trens produced' in one generation. The c-folding
. mi leading and certainly not to the point. From a times shown in the figure are relativelv long. This -
number of studies of possible reactor accidents of is due to the delaved neutrons. As you all know.
this type, it must be' concluded that even though re- ' the 6* ion event pro luces cermin 6hion : products actor accidents could happen quite rapidly in terms which,in turn.after periods ram:ing up to 80 seconds, J
q
- 82' VOL Xtli
'P/853' USA C. R. McCULLOUGH st of.
the e-folding times are correspondingly reduced. Here 3
'10 j
u om may sav that nuclear re: ctors represent a gemune I
departu're from conventional power sources in that
\\
I enanmus lwer lesel increases are pos.ihle in the 2
,o 4
b I I I
event 'i mal-operation in remarkably short times.
l The Saimunrd Committee has urced the develon-meEntoihatic fuses wi.ich would mee a m-it t
I citar runaway m case the reactor cet< not at cetml k
y
'Ihese m-c3 are expected to have two characteristics.
e First of all, ti.ey should be self-contained and wholly ci!
automatic so that they are not subject to error of 10 g'
adjustment or maintenance and are tmt subject to 2
intentional tampering. Second, these fuses are to be g
p activated hv changes in the power level, essentially
)
16,'
nev' oa cen o'.ca Sine,c,v.eoueoa*
- n the flux level of the nuclear reactor. and a
o 5
\\
have rapid enough response so that they will intro-2 2
\\
duce a substantial negative reactivity in the reactor in a time of the order of one second or less. One of bY Eb l
b a[ 0 \\\\
the continuing difficulties in the development of these
\\\\
fuses s this latter requirement for short-time opera-1C. 3 COOOI 0001 O.Ot O.1 to 80 tion. It appears that successful development of such Encess Reactivity,k-I a fuse will soon be achieved, but because of the short-Figure 1. Varionion of rising period with excess reactivity from time requirement this development is neither easy the inhour equationi nor simple.
3 __ i = *f_ + a 3**A
As a matter of practical fact, one must consider how a large excess reactivity might be achieved in a s
nuclear reactor.8 First of all, it is clear that it can emit delayed neutrons. The fraction of these delayed not really be achieved instantaneously ahhough some-neutrons produced in Um fission amounts to some-thing analogous to instantaneous excess reactivity thing like M7c of all neutrons produced. If one makes can be obtained on startup of a reactor if only a the excess reactivity of the reactor so great that the weak source of neutrons is used during the startup chain can proceed without the delayed neutrons, then procedure. It is then conceivable that throuch some the reactor is said to be in a prompt critical condition, error, rapid removal of control rods would ailow the that is it is critical or even supercritical on prompt reactor to be highly supercritical before the power neutrons alone. In this condition the e-folding time level had risen to something approaching the normal becomes short and one may estimate it by means of power range. For this reason, startup accidents are the equation:
particularly to be avoided.
J. = h,,
In any event, one must consider not only the possi-ble degree of excess reactivity but also the rate at re to which reactivity may be added to the machine. For In this equation, re is the c-folding time, to is the in, this reason, one would like safety rods and shim rods trinsic neutron generation time which depends on the that move out rather slowly but which could be re-type of reactor, and k,,is the excess reactivity above mserted rapidly at any point during withdrawal.1 p'rompt criticalitv.
One would also like the degree of control residing Tvpicallv nedtron generation times are about one millisecomi,for large thermal reactors, something like in the control and safety system to have a graded Q millisecond for water boilers and small thermal weight so that as the reactor becomes nearly critical.
reactors, and may be as short as a microsecond or less only smaller amounts of reactivity are introduced by for fast-spectrum epithermal machines. The value the withdrawal of control rods. Safety rods vhich that one may assign to k,, depends on the type of must be completely withdrawn and cocked before machine and its requirements for excess reactivity in they may be re-inserted are particularly undesirable.
order to conduct experiments, overcome temperature There is another point which is quite pertinent in eficcts, allow for burnup of the fissionable material, or the serious consideration of how rapidly excess re-override tission-product poisons. However it seems activity might really be added to a given nuclear reasonable to assume an excess k-value of a x>ut 0.01 reactor. Extremely fast reactors. for example, look fraction for terms of discussion. If this is done, then
. I I"
- d" *
""I (I'C*"hutim. a!! remarks concern-an c-foldinE, time for the farEe thermal machines'of mg control and safety rystems will be made as though these 1,j o second is obtamed. Only seven c-fold.ng times, were comentional asorber synems. of course it is entirety
' that is' Pto second, are required in order to increase I.wspiMe to increase the reactiyity of a reactor by insening by chanm? *^he characterat:es of a reticetor. Our d.se the 1 lower level of the machine by a factor of 1000.
gt
-For machines w,th shorter neutron generation times, will auume that all controls are of an absordire tym i
.x E
83 THE SAFETY OF NUCLEAR REACTORS particularly dangerous because of the very short Since the negative reactivity coef6cients can lead
.'c.iolding time that one can achiese with modest ex-to shutting oti the imclear accident without ik+true-eess reactivity. Ilowever, part of this danger is spuri-tive effects, a few words about these coefticients niay ous' becau-c the reactor will become supercritical - be desirable. First of all. large negative reactivity L
enough to run through a complete runaway accident coef6cients are clearly wanted. Ilowever, these co-before very much excess reactivity can be added by ef6cients must be quick geting, abic to take effect and ordinary n'iethods of operation of controls. Only very shut down the reactor during the transient con litions sudden motion of the control rods, motion so rapid of a runaway. Primary changes in temperature are that it would have to be induced by special pneumatic caused by the generation of 6ssion heat in the fuel svetems could lead to a rapid, explosive type of acci.
elements. IIeating of the fuel elements may ch:mge d'ent with these inst reactors. For this reason, a care.
the reactivity of the machine negatively if the fuel 4
fut study of the possihte rate of reactivity increase, elements contain large quantities of U
- 5. This nega-rather than the total potential excess reactivity avail, tive change is due to the merensed absorption of resonance energy neutrons by Doppler broadenmg or able,.shouhl be carried out when the nuclear runaway the U"$ absorptmn resonances. For large lumped problem is considered.
thermal reactors, this enect amouats to about 10~
It'seems that the prevention of nuclear runaway n
f reacuv, y per degree C temperature rise.
fraco n accidents is very closely associated with the problent A sec ndary mason for the temperature coefficient of excess reactivity and the rate at which excess
'5 the heat, g of the moderator. In many reactors.
m reactivity might he'added to a given machine. This in tin.s will benefcially reduce reactivity. However, the turn depends on the tecimical details of any given tune for heat to flow from the hot fuel elements to 4
machine, both in its neutronic behavior and in the the nioderated portion may be suf6ciently long, sev-4 operation of co'arol devices and possible other ways eral smonds to a minute in sotue cams, so that af of changing the reactivity, perhaps because of the
. presence of experimenta1' irradiation facilities. This though the moderated temperature coef6c,ent i
is famaMe, it does not have time to come into play is not a problem that can be generally solved for all during a nuclear runaway. For example, the thermal machines, but each machine must be studied on its difrusion time across a four mch thickness of graph-own merits.
te-moderator m a large lumped thermal machme is We will now turn to the characteristics of a nuclear runaway, assuming that it is actually underwav. As nearly a minute. Tlus ume is so long that the coefn-pointed out above, a nuclear runawa'v is not pa'rticu-CI'".t associated with moderator heatmg plays no part 1
durmg the runaway.
. larly violent but it does take place in a remarkably i -
short time. The runaway will proceed r.ccording to pne f the important techm,eal areas associated with understandmg r aclear runaway behavior of a the following steps. Firs't of all, excess reactivity is
3ctor is that of he4t transfer under transient con-inserted, the reactor then rises exponentially in power
' level with a nearly constant c-folding ' time until d tions. Relauvely little knowledge m tius area has enough energy is ficemuulated in the structure to been avadable becayse most heat transfer studies are affect the behavior of neutrons. These early effects.
c nducted under steady-state conditions.
are characterized by the term, temperature coeSicients It nPpears likely that a nuclear runaway will cause of the reactivity. These may be either positive, that is
'". ugh disruption of the reactor structure so that 6ssion products wdl start to leak out of the reactor making the reactor more reactive, or negative, mak-into the surrounding area. How fast this escape of ing the reactor less reactive and tending to shut it 6ssien products wdl be depends upon the type of down. If an increase in power level tends to make the mactor and type of mactor accident. It may be possi-reactor more rea;tive and increase the power level ble to show tiiat there w,l be very little mechanical d
not oniv further but make the further increase more vi lei 1ce outade the reactor shield, so that if the rapid. one sometimes savs that this is an autocata-buikh,ng which houses the reactor can be made gas-lytic reactor. Such a rea'clor appears to be particu-ught, then escape ot fissior. products to areas outside U
larly dangerous, and can possibly achieve really short the reactor building can be greatly reduced. We wish c-foIding times. A few strongly autocatalytic reactors to emphasize that in most cases the buildiIEllG e
are known. I,or most reactors, negative reactivity co-g, ge gas-bght and not explosion-proof.
D etlicients wdl take eff ect and lead to a lengthenmg ps p in the c-fohling time.
DELAYED ENERGY PRODUCTION
'L
Finally, a third pha>c of the runaway will occur Nuclear reactors have another somewhat unfavora-
.when.enough of the reactor structure is actually hie characteristic. Because of the accumulated fiaion
. melted, vaporized or otherwise affected (in most products, and the accompanving exothermic radio-cases reactivity coefficients will not be adequate to active transformation of these fission products, a
~
31 ut the. reactor down without destructive effects, nuclear reactor will continue to produce heat even t
l ahhough for some reactors this will indeed he the when the nuclear chain reaction is shut down. The i
ca e), and these destructive effects will shut down energy produced by the fssion products has leen
.the nuclear chain reaction and stop the runaway.
studied, and the result for power production in the l
l i
84 VOL. XIll P/853 USA C[ R. McCULLOUGH ct cl.
~
reactor which has been operating for a long time may w
&mw%
" "g,'yl,' g "cu' be summarized in the following equation:
j i
I%oa = 0.07 P.,,,,,, [t(sec)]-", t > 1 sec io.oca
/
/ j IIere 1%,,,,,,, is the normal operating inwer level of i
the reactor, I%.,a is the delayed heat power level of 8,co c the reactor in the same units as the normal power i
level, t is the time in seconds, and the 0.07 is an ex-ioc /
o perimentally _ determined coefficient. Although the i
65sion prmhicts individually decay exponentially, the t
ya'ja,g; lg,c'l,'
,e result of their statistical production is to make th,s l
i delayed heat decay with the relatively weak power e
l 3,,,,,,
c,,,,,
law indicated. For about one second after the reactor j
j / g a gigoco u i
i is shut down the delayed power levelis approximately 1
f 77c of the normal power level.
j 7
o' The delaved heat _ problem is clearly a_ serious one- _
i fiiome failure m the coohng system %ould occur, ooi breakdown of pumps, loss of pumping power, me-
[
i
{
chanical failure of cooling piping, then e.ven if the c oa, '[ '
l nuclear chain reaction is immediately shut down by
'o
'o
'O'
'o*
'o'
""^""5"*'""'"*
inserting control or safety rods, there wih still be F3 "'. 2. calcolot.d e.mp rotor. rise oft., shutdown of the choi.
left a substantial heat load which must somehow be 8
reaction. (Heat copocity assumptions or. Indicot.d.) (Courtesy of disposed of.
- w. H. zina, 4, senn. Nationo! Laboratory)
For example, if the fuel elements from the Mate-rials Testing Reactor were suddenly removed from gency arrangements of some sort which might be the reactor and left standing in the open air, they rather analogous to tire-fightmg equipment. The would melt down by themselves by delayed heat pro-
fire-fighters" would then approach the reactor and duction. If they were suddenly immersed in water, make suitable connections and force through emer-probably this melting would not take place.
gency cooling. A third possibility is to have standbv
~
One may consider the delaved heat nrnhbm from force'd convection cooling similar' to the main cooling
_the,cinit of view of suddenly stopping forced conhne system but connected to a special power supply and in t ie reactor ami abo suuocmv ermme the ch'"'
with special separate piping.
I
_rerti-71he met elements, and other materials in It is clear that a delaved accident, if it could not the reactor in close thermal contact with the fuel be brought under contr'ol, might very well lead to elements wd, l then start to increase in temperature.
suf6cient disruption of the reactor core to allow fis-The rate of temperature rise wdl be proportional t sion pmducts to escape. It is also clear that this again the precedmg steady power level of the machme, and will not be of itself a very violent event, and again the rate of temperature rise wdl be reduced if there as in the case of the nuclear runaway it is probable is a large heat capac ty m mtimate thermal contact that an accident of this kind can be minimized a good with the fuel elements. In fact, since the rule of du deal by providing a gas-tight building around the Long and Petit indicates that the heat capacity of reactor.
solid materials is proportional to the number of atoms they contain, a crude rule of thumb would state that CHEMICAL REACTIONS the rate of temperature rise is proportional to the Either a nuclear runaway or a delayed heat accident power of the reactor per atom of n,aterial in good may cause considerable melting and mixing of re-thernul contact with the fuel elements. The simple actor components and lead to exothermic chemical expre sion we have given for the delayed power indi-reactions between these components. A simple exam-cates that the time rate of temperature rise should be ple of this type is that of an air-cooled graphite re-proportional to the 0.8 power of the time. In Fig. 2 actor. A sudden temperature rise in the uranium are curves showing the rate.of temperature rise fol-fuel may be sufficient to cause it to melt and heat the
- lowing uncooled shutdown from normal operation for adjacent graphite so that both the uranium and a few reactors? One concludes that this temperature graphite can burn in the cooling air. If the air sup-rise, although not so rapid as to constitute a sudden ply is not turned ott, it is likelv that a substantial event in terms of human reaction times, is neverthe -
portion of the reactor could be consumed in this way.
les rapid enough to be quite troublesome. In Table II This would then disperse radioactive fission products are summarized some delayed heat power levels.
into the surrounding area through the exhaust portion l
Three types of preventive designs are suggested.
of the cooling system.
q One is to have a standby emergency cooling system Another example is that of a heavy-water-moder-t which %rks either by gravity flow of coolant or by ated-and-cooled natural uranium reactor. In a ma-1 natural convection. Another is to have standby emer.
chine of this sort a runaway accident could melt the
THE SAFl:TY OF NUCLEAR REACTORS 85 uranium ano allow it to mix intimately with the be taken that a failure cannot put the controh out of water. In this ca e the thermodynamic potential indi-operation.
cates that an exothermic chemical reaction can take In order to prevent a delayed heat accident, it is place. Whether or not such a reaction would be rapid important that enough natural convection heat trans-and violent is not clearly known. In this case one has fer can take place in the overheated core to dispose 1 to deal with the chemical kineties of a heterogene-of the delayed heat, perhaps just into the ground.
ously reacting chemical system (among other things Even if the structure is damaged, one must try to the probable degree of dispersion of the uraniun' into - keep the temperature lower than that temperature the water is not known). Prestunably, the rate will which would start a substantial pressure rise in the depend upon the intrinsic molecular kinetic process reactor structure. In that case, the 6ssion products in the conventional chemical sense, but it will also may be kept inside the reactor shield. This means depend upon the degree of dispersion of the uranium that some coolant contained in the core should have into the water, the rate at which reacting molecules a large surface to which it can transfer heat by natu-can diTuse through the uranium oxide %yer that ral c,onvection or by boiling convection, and that this would be formed between the uranium and the water, degree of cooling should be suf6eient to keep the bulk and the degree of turbulent mixing and scrubbing of of all volatile materials below their boiling points. It the two reactants against each other. This latter ci-may be remarked that boiling heat transfer is known fect might be generated by the reaction itself. This is to be especially ef6cient, so that in any event there clearly a complex problem and a great deal more will tend to be a ceiling put on the temperature rise needs to be learned. However, one can say this: If at about the boiling temperature of the original cool-an exothermic reaction of this type goes to comple-ant employed. This in turn implies that by appro-tion, the resulting energy release will nearly always priate construction one may limit the pressure inside be substantially greater than the energy generated in the shield to a few atmospheres. Thus it may be a preceding nuclear runaway. Thus it is important to rather easy to make sure that the 6ssion nroducts are
- determine the possible chemical reactions. A substan-kept inside the shield.
tial increase in reactor safety can be achieved by the Finally, the problem of chemical reaction among elimination of possible reacting components in the reactor components can often be minimized. For ex-reactor structure.
ample, already-reacted components might be used in some cases. Uranium oxide rather than uranium SAFE DESIGNS metal in an air-or water-cooled reactor may serve it seems worth while to summarize the preceding as an example.
discussion with a few remarks concerning the ap-The other general conclusion that the Safecuard i
proach to safe reactor designs. First of all, it is desir-Cnmmittee has come to is that explosive hazard in able to provide a large negattre reactivity coeflictent.
reactor accidents is minor, at icast for people not at 4
This can usually be actueved by thermal couphng of the reactor site. Indeed, for man e reactors, it appears the fuel elements to those portions of the reactor unlikely that ther,e will be much mechanical violence which give a substantial reducton to the neutron external to the reactor shield. For this reason, a gas-multiph, cation when heated. For example, m, the case ot enriched, water-moderated reactors, close thermal tight building. or a moderately gas-tight imildin$
~
which may con 6ne the fission products during a cool contact between the fuel elements and the moderated ing period and from which the fission products are water can lead to enough heitmg aml vaporization exhausted into scrubbers and out a high stack, may of the water to reduce the water density m the event serve to prevent the spread of 6ssion products fof-of a nuclear rtmaway, and shut down the reactor lewing a reactor accident. For some reactors the d
before sermus damage is done. Successful tests of confining building will have to be a gas-tight pres-tlns sort have bven made.[" A design of this sort must sure vessel. Safe-design procedures represent an im-be thought through carciully m order to make sure portant fiehl of nuclear reactor development.
that enough heat transfer surface is available and l
~ rapid enough heat flow will take place to shut down a ADMINISTRATIVE CONTROL 1
^
machine during an accident.
The control system must be carefully designed so Ahhough good administrative control of the re-that in the event of too high a power level, too high a actor does not lead to the rame degree of confidence rate of rise of power level, a serious reduction in in the good behavior of the machine that intrin ic
~' coolant flow, or any major failure of fuel elements, gremlin free built-in stability does, nevertheless, the reactor will shut down in a time interval small. good administrative em -.a bes enhance the safety enough to minimize damage. All potentially danger.
and reliability of reactor operation. 'ndeed, goc.d ad-ou4 failure 4 shonhl be monitored by instruments and ministrative control is mandatory 1 'r those people control channels' leading to shut-down or " scram."
who have an economic stake in the re; ctor. From the I These monitors and channels should be at least in point of view of public hazard, carefu reactor opera-duplicate, independent of each other, and preferably tion and maintenance makes it very much less likely
[
of different types. In addition, particular care should that there will be a reactor accident.
I i
i.
, ' 8.-6 VOL Xill P/85"l USA C. R. McCULLOUGH et al.
~
3 not radioactive and should not be allowed to accumniate llower-r, this administrative control shot start :just when the reactor is put into operation.
unduly, or to be lost, or to be handled in an irrespon-
~
.Thn>m:hout the design and construction of the re-sible manner.
actor, thorough supervision, careful desiga for relia-CONSEQUENCES OF AN ACCIDENT
~ bility, and thorough testing of all reactor components We believe,the following discussion outlines the including coolant system, ofi-gas systems, shims, main features which can make a nuclear incident safety and control mechanisms, and all control and opera'ing instrunientation should be carried through.
dangerous. In the event of a reactorliccident, there All these components should be given systematic and will probably result a release of radioactive material thorough shakedown testing before the' reactor is put '
from the' reactor. Operating personnel may be seri-ously injured or perhaps even killed. The reactor into operation and before it becomes radioactive so that modification, correction, and maintenance can be itself may be damaged beyond repair or recovery.
The reactor building and its associated equipment done with less ditiiculty. Indeed, it is extremely diffi.
are very likely to be heavily contaminated and indeed, cult to emphasize how important it is to have com-it niay not be possible to clean up the buildmg sum-plete, thorough, systematic shakedown of all portions c ently to put it into operation again. Design of the of reactor control and instrumentation.
building so that possible cleanup operation > are as In the design of the reactor, careful attention easy as possible,s desirable." Smooth, clean surfaces, y i
should be given to the problem of maintenance after perhaps clad,m ttainless steel, would make cleanup it is placed in operation. It should be possible to enter perations easier. Fmally, radioactive matenals can all instrument areas, most of the control areas, and escape k m the reactor site altogether. Fission prod-obviously the central control room, after the machine ucts may be carn,ed m, the wmd and spread over has started up and been operating for some ime.
"diC""* P P"I **d
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' "S*'*"**
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Fuel-element failure, a continuing problem, may hazard. Radioactive material may escape into the
~
allow radioactivity to enter portions of the reactor ground and be carried by the percolatmg ground structure which normally would be expected to be water to adjacent rivers or other water supplies.
radiation free. Tlus should be taken into account in Although a great deal needs to be known about the character of radioactive material that might escape the on,gmal design.
Once the reactor is placed into operation, contmu-from a reactor, whether it is in large or small particles, whether it is indeed gaseous, whether it would rise mg close supervision is essential. Maintenance pro.
cedures should be carefully followed and mamtenance high into the air or seep slowly along and into the checks should l>e scheduled in an appropriate way.
ground, nevertheless, some notion of the possible The period of reactor startup is a particularly en,tical spread u.he hazard could be obtained by studv of one, and should be followed very closely, Reactor the meteorology, and hydrology at tiie rea'ctor loading and unloading are delicate operations, par-site.228 " It is desirable, for example, to have the ticularly the unloading of now-radioactive fuel ele-prevailing wind to blow trom the reactor to uninhab-ments. Startup of a reloaded reactor must be carefully ited are:rs. It is also desirable to have the reactor site considered since the reactivity may have been atTected not be located on a main watershed. From the point by a new fuel loading. The normal, or routine, day-of view of the hazard alone, it is of course desirable i
to-day operation requires close supervision so that to.have the reactor site far from populous or vital i
troubles may be detected at an early date and cor-industrial areas. It will not always be possible to
.obtain this reniote location and still obtain economic i
rective measures taken. Clearly, careless operation of
- utility from the reactor. For this reason, the Safe-the controls may lead to a supercriticality accident and the manipulation of the controls should be carried guard Committee is continuing to emphasize the
.out only by people who are thoroughly familiar with importance of safe reactor designs, the development the characteristics of the reactor and its associated of contained fuses to minimize the possibility of a 7unaway accident, and the use of gas-tight containing equipment.
If a reactor is employed as an irradiation facility, vessels and buildings.
it is possible for experiniems in give rise to sudden Perhaps it is important again to emphasize the i'
changes of reactivity. Experiments should be planned, degree of public hazard that might follow a reactor the_ plan revicwed'by the administrative staff, and -
accident. Assuming that good luck prevails and no suitable emergency procedures decided upon, before one is killed, it may nevertheless be necessary to
'in3ertinq experiments into the reactor.
evacuate a large city, to abandon a major watershed, Finally, there is one phase of the administratively and very probably it would be necessary to make the
- controlled reactor which is usually taken for granted reacter site itself a forbidden area for some years to but may require a word or so
- The careiul accounting conie.
Despite all these possih!e dire consequences it is for all materials which have been irradiated in the ~
the belici of the Advisory Committee on. Reactor machine. There is usually available a number of test
- holes in which experimentalirradiations may,bc car-Safeguards that nuclear reactors will soon start to ried out. The samples so-irradiated can be highly produce substantially increasing material benefits for
f l
'S THE SAFETY OF NUCLEAR REACTORS
_8_7 l
humanity. We believe that useful c!cctric power in REFERENCES 4'
large quantitics can be generated by nuclear reactors.
- 1. Tci!er, E., testimony. Atomic Power Dettlatment and It is our concern that rapid progress shall be made-Pritate Enter /risc. Hearings before the Joint Commit.
but that enou;;h caution be observed so that no catas.
tee on Atomic Energy,83rd Congress,1st Session. June-July D53 mt. Prinung Office, Washington, D. C.).
trophic event will dclay the fruition of reactor devel-
- 2. Weil, G. L, Ha:ards of Xuclear Power Plants, Science epment.
121, No. 3140, 315 (1933).
APPENDlX. THE ADVISORY COMMITTEE ON REACTOR
- 3. Aforg n, K. Z. and Ford. AI. R., Detclopments in In.
- '"1 Dm Drunn: nations, Nuc!c mes 12, No. 6, 26 SAFEGUARDS TO THE UNITED STATES ATOMIC (334),
ENERGY COMMISSION
- 4. Teller, E., Reactor Hasards Predictabic, Nucleonics The Advisory Committee on Reactor Safeguards 11, No.11, so (1953).
j
- 5. 31ccullough, C. R., Ccncral Critcria for Saft Reactor /
was formed by combining the Reactor Safeguard D' sign and Orcration, AS51E Annual Afecting, Nov. 3 Committee and the Indmtrial CommiNee on Reactor
~
Location Problems. At this time r. embers aret 31.
6.11 ills,11. hi, Huards of 1.= Pown RcmrcIs R u.
f
' Henedict,' Slassachusetts Institute of TechnoloE7;
. tors, Proc. 3rd Annual Oak Ridge Summer Symposium,
- 11. Brooks, Harvard University; W. P. Conner, Jr.,
TID 5031 (Technical Information Services, Oak Ridge, Hercules Powder Company; d. L. Doan, Phillips Tenn.,1951).
Petroleum Company; H. Fri< dell, Western Reserve
- 7. Hurwitz, H, Jr., Safcpuard Considerations for Nucicar [
University; L U. Johns, 3fonsanto Chemical Com-Power Plants, Nucleonics U, No. 11, 80 (1953).
pany: C. R. 3fcCullough, Chairman, ACRS; 31. bl.
- 8. Way, K. and Wigner E. P., Radiation from Fission hiills, University of California Radiation Laboratory; Products, Phys. Rev. 70 115 (1946).
K. R. O.-born. Allied Chemical and Dye Corporation ;
- 9. Zinn, W. H., Private communication.
D. A. Rogers, Allied Chemical and Dye Corporation;
- 10. Dietrich, J., E.rterimental Determination of the Self C. R. Russell, Secretary, ACRS: R. C. Stratton, Regulation, and Safcty of Watcr Reactors, P/481. Vol.
^
Travelers Insurance Company; E. Teller, Depart-13, Internatimal Conference m the Peaceful Uses of At me Energy, Aagust S-20, $55.
ment of Physics, University o'f California; H. Wex-
- 11. Gilbert, F. W., Decontamination of the Canadian Reac.
ler, United States Weather Bureau; and A. Wolman, tor, Chem. Eng. Progr. 50, 267, (1954).
The Johns Hopk.ms l..mversity.
The Reactor SafeEuard Committee was formed in
- 12. Fhzgerald. J. J,1,Iurwitz, H., Jr., and Tonks, L.
Method for Et.aluatsng Radiatson Huards from a hu-1947, and the Industrial Committee on Reactor Loca-ct,ar Incident, KAPL-1045 (Knolls Atomic Power tion Problems was formed in 1949. The following Laboratory, Schenectady, N. Y.,1954). Available from were also associated with these committees for pro.
the Office of Technical Services. U. S. Dept. of Com-merce, Washington, D. C.
' longed periods: Cmdr. J. Dunford, US Atomic Energy Commission; Col. B. Holzman, US Air
- 13. Fitzgerald J. J., Hurwitz, H., Jr. and Tonks. L,
- ' ' [ an['jz" 'h'o. I "78'( 954) ~
- Force; J. Kennedy, Washington University, St.
395 Louis; F. Seit, University of Illinois; G. Weil'
- 14. Afester, R. B., and Widdoes, L C Etaluating Rcartor formerly Davnion of Reactor Development, US Heards from Airborne Fission Products, Nucleonics ACC; and J. A. Wheeler, Princeton University.
12, No. 9, 39 (1954).
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