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EIR-411.txt
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EIR-Bericht Nr. 411
EIR-Bericht Nr. 411
Eidg. Institut fur Reaktorforschung Wurenlingen
Schweiz
Reactor with very low fission
product inventory
M. Taube, W. Heer
Sy
Wirenlingen, Juli 1980
Reactor with very low fission
product inventory
M. Taube, W. Heer,
in co - operation with A. Indrefjord
Swiss Federal Institute
of Reactor Technology
July 1980, Wurenlingen
FEIR - Report 411
Abstract.
A fast converter with one zone and an infernal breeding
ratio of 1.00, with liquid fuel in the form of molten plutonium -
- uranium - and sodium chloride, with a thermal power of 3 GW (th)
allows continuous extraction of the veolatile fission products
(Br, I, Kr, Xe, Te) by means of helium purging in the core. The
non-volatile fission products e.g. Sr and Cs can continuously
be extracted in a chemical reprocessing plant at the reactor
site. The impact on an accidental release of fission products
is rather significant; the amounts released are 50-100 times
smaller than those in a reference reactor (LWR with oxide fuel).
Because the heat sink 1s relatively large and after heat reduced,
the temperature of the fuel does not exceed BOOOC after an
accident, which greatly reduces the consequences of an accident.
Zusammenfassung.
Ein schneller konverter Reaktor mit einer Zone und einem
Brutfaktor von 1.00, mit flissigem Brennstoff{ in Form von
geschmolzenem Plutonium -, Uranium - und Natriumchlorid, mit elner
Leistung von % GW (th) erlaubt die kontinulerliche Enfernung von
fliichtigen Spaltprodukten (Br, I, Kr, Xe, Te) durch Durchleiten
von Helium im Core. Die nicht-flichtigen Spaltprodukte wlie Sr
und Cs werden kontinuierlich in der angegliederten Aufbereitungs-
anlage ausgeschieden.
Die Folgen bei einer unfallbedingten Freigabe von Spaltprodukten
sind recht signifikant; die frelgegebenen Mengeh sind 50-100 mal
kleiner als die beim Referenzreaktor (einem LWR mit Oxiden als
Brennstoff). Well dile Wirmesenke relativ gross 1st, und die
Nachwidrme reduziert ist, Ubersteigt die Temperatur des
Brennstoffs nach einem Unfall 500°C nicht, was zu einer bedeutenden
Reduktion der Unfallfolgen Tfihrt.
INIS DESCRIPTORS
MOLTEN SALT REACTORS REACTOR SAFETY
FISSION PRODUCT RELEASE LOSS OF COOLAN
AFTER HEAT REMOVAL MELT DOWN
Contents.
1.
Introduction
1.1 Present position
1.2 Safety of Fission Reactor: State of the art
1.3 Saflety of Fission Reactor+ desired improvement
1.4 The present State of Reactor Development and the
Criteria for a Safer Reactor.
Accidental release of fission products and decay
heat removed.
2.1 The Rasmussen Scenario
2.2 The problem
Principles of the "SOFT" Reactor
3.1 A Schematic View
3.2 System description
3.3 Mass flows and containments
Nuclear calculations
4.1 The Method
4.2 Nuclear properties of SOFT
Continuous extraction of fissilion products.
5.1 General scheme
5.2Chemical state of fission products in molten
chlorides
5.% Fission product volatility and gaseous extraction
5.4 Problem of delayed neutron emitters
5.5 Gas extraction rate
Page
5.6 Reprocessing of non-volatile fission products
5.7 Some technological problenms
5.8 Possible reprocessing technologies
5.9 The problem of external storage of the fission
products
Heat removal
6.1 Primary circuit: core and heat exchangers
6.2 Secondary circult
6.3 The tertliary circuilt
6.4 Heat removal from the sotred fission products
Accidents - problems and solutions
7.1 Decay heat: Power and energy
7.2 Worst accident scenario
7.3 Spontaneous cooling processes in the containment
7.4 The prompt critical scenario
Conclusions
Literature
Page
4o
L
44
47
b
4T
L3
48
51
51
51
55
61
63
o7
68
1. INTRODUCTION
1.1 Present position
The llight water reactor, the most common power reactor today
and for the next two decades 1s safe enough to be the basic
energy source for society.
This however does not mean that the search for improved
safety 1s a waste of time. The continued search for still
safer systems 1is common in all the technologlies making up our
civilization (transportation, chemical technology, domestic
fire harzards etc.).
Fach type of nuclear reactor also has potential for increasing
its safe operation. It must however be remembered that the most
important safety aspect of the current reactor sysfems 1is in
the real practical experience bullt up over many years. Here
the light water reactor is in a privileged position being able
to demonstrate an excellent safety record.
Tn spite of this crucial fact the search nust continue for reac-
types which promise improved inherent safety on the baslis of
their different system design. Such a search seems to be gene-
rally desirable.
The best reason for such a reactor type has been given by
Alvin Weinberg (1979, 1980).
"For the 15 billion curies contained in a 1000
megawatt reactor we could never say that the
chance of a serious accidental releasec was zero.
Thus a most important technical fix for
nuclear energy would be a means of minimising the
amount of land that conceivably could be
contaminated in the worst possible accident.
For nuclear energy to survive we must
reduce the probability of any serious malfunction
much below the 1 in 20 000 per reactor-year
estimated in the Rasmussen report as well as
reducing any possible consequences."
"But 1f the world energy system involved as many
as 5000 reactors - that 1s, 10 times as many
as are now either in operation or under
construction - one might expect an accident that
released sizeable amounts of radiocactivity every
four years. Considering that a nuclear accident
anywhere 1s a nuclear accident everywhere I belileve
this accident probability is unacceptable. If a
man is to live with fission over the long term he
must reduce the a priori probability of accident
by a large factor - say 100."
(The Bulletin of Atomic Scientists.
March 1980)
The aim of this paper 1s to try and present a concept for a
fission power reactor being approximately 100 times less
hazardous than existing reactor types, and with much less than
15 billion curies activity.
Tt must be stressed that this study 1s concerned with a
'paper reactor' (which is of course the safest type of alll)
a concept only. The route from the concept to reallsing an
actual power reactor in service, and contributing to our
energy problems 1s a long and tortuous road and with financial
commitments of tens of billions of dollars. Even this should
not prevent the search for such a reactor. The search 1tself
is of benefit even for a better understanding and possible
improvement to the existing power reactor systems.
How is 1t possible to meet the regquirement for a 100-fold
improvement in reactfor safety?
The following proposals are made below:
a) the amount of fission products in the core during
a normal operation must be reduced by a factor 100
which significantly reduces the effects of a large
accident where the fission products would be
released into the environment,
b) the decay heat level must be reduced and the
internal inherent heat sink, such as the total heat
capacity of the fuel and other components must be
significantly increased. Additionally the maximum
temperature reached by the fuel 1n the case of
failure of all emergency ccoling systems must be
low enough to allow the fuel to be trapped in a
core catcher,
Table 1.1
Characteristics
1)
3)
Fission product
inventory
Decay heat
removal
Pressure in fuel
and coolant
(in containment)
Coolant with low
boiling point
(Explosion
possibility)
Reactor characteristics affecting safety
(data for 3 GW(th) )
Existing Reactor
(e.g. LWR)
~15 Geurie. Possibility
of releasing into the
environment:
a) volatile Fission
Products (F.P.)
b} non-volatile F, P.
Immediately after
shutdown “180 MW(th).
The integrated decay
heat over some hours
is of the order of
1000 Gigajoules
ajyvl50 bar in a PWR
~80 bar in a BWR
in the coolant.
b) internal pressure
in the fuel pins
The presence cof water
in the primary circuit
gives rise to the
hazard of uncontrolled
boiling and production
of large volumes of
steam, pressurizing
the contalnment
Desired Reactor
Minimum in F.P.
inventory but at
least two orders
of magnitude
smaller than in
existing reactors
for both:
a) volatile F.P.
b) non-volatile F,P.
Minimum decay
heat. At least
by more than one
order of magnitude
No pressure:
2) in the coolant
b) in the fuel
No low boiling
point media
allowed in the
reactor circuits
Characteristics
Chemically active
medium (coclant}
Hydrogen
Evolution
'China Syndrom!
Criticality control
External (away from
reactor) movement
of plutonium
Existing Reactor
{(e.g. LWR)
In a liquid metal
cooled reactor an
exothermic reaction
is possible
4 Na + UO_+2Na,_.0+U
™ 2 2 m
et et
In Light Water Reactors
during an accident the
following reaction may
occur s
Zrmet+H2O+ZrO+H2gaS
The evolved hydrogen
results in an
uncontrolled increase
of pressure in the core
vessel or containment
because of burning in air
For the LWR system the
report WASH-1400
discussed the probability
of the'China-syndrom'
A strongly negative
criticality,for the case
of loss of coolant
control rods
The LWE system being
a producer of plutonium
results in transporting
plutonium away from
reactor (e.g. in
irradiated fuel)
Desired Reactor
No chemically
active agent
is allowed
Ne¢ chemical
agent including
hydrogen is
allowed
Elimination of
this kind of
accident
A self
regulating system
is desired
A self sufficient
reactor with a
breeding ratio of
~1 has no external
circulatiocn of Pu
Both of fthese criteria seem to be possible in a reactor having
a molten fuel with continuous extraction of fission products
in the fuel during normal operation. (Ref . 19).
In this paper one sclution to this problem 1s discussed in
detail: the molten salt reactor. It must be said that such
reactor types have been consldered for many years having molten
salt as a fuel and with continuous extraction of fission products.
Some examples of molten salt reactors: (Ref. 5, 14, 16, 17, 18).
a) thermal breeder with molten fluorides developed decades
ago by Oak Ridge National Laboratory, existing as an
experimental reactor with a power of 8 MW(th), (Ref. 13)
b} fast breeder reactor with molten chlorides discussed
for tens of years, but still a "paper reactor”.
A reactor of the second type is discussed here.
1.2 Safety of a Fission Reactor: state ot the art.
Of course the hazard of a fission reactor is not only connected
with 1ts fission product inventory. There are other important
safety aspects. Table 1.1 shows the 1mpact of other reactor
parameters on its safety and ways of resolving these. The
question arising out of these criteria 1s can all the necessary
improvements be made in a single reactor type? The answer
assumed here 1s yes.
1.5% Safetv of a Fission Reactor+ possible improvements.
Table 1.1 summarises very briefly the properties of such a 'super
safe' reactor. The reactcr proposed in this paper is called SOFT.
alt reactor
rn site reprocessing
ast converter
Floo O e
ask
The properties are discussed below in the order given in table 1.1.
Table 1.2
10
3afety of a Fission Reactor:
possible improvements
Parameter
1) Fission product
inventory
2) Decay heat
removal
%) Pressure
4) Low boiling
point media
(Explosion
possibility)
Properties of the
desired reactor
Minimum F.P.
Two orders of magnitude
less volatile and non-~
volatile F.P.
Decay heat at least
one order of magnitude
less
Works at ambient
presssure
a) in coolant
b) in fuel
Absernce of agents
having low boiling
point reducing also the
explosion hazard
The solution formed
for the SOFT reactor
Continuous extractlon of:
ajvolatile F.P. by means
of gas pumping
blnon-volatile F.P. by
means of chemical
ftreatment and storage
in another contalnment
Decrease of decay heat
energy by:
a)continuous extraction
of F.P.
b) continuous extraction
oszng and storage
in another containment
Use of molten salt as
both:
a) the coolant
b) the fuel in a
pressureless system
Both fuel and coolant
have a bolling temperature
of approx lBOOOC thus
no explosion is possible
Parameter
5) Chemically
active media
6) Hydrcgen
evolution
7} 'China Syndrome!
8) Criticality
control
11
Properties of the
desired reactor
Absence of chemically
active media which
could react with fuel
e.g. sodium and oxide
Absence of substances
which could give rise
to hydrogen. E.g. water
reacting with Zirconium
Elimination of any
formation of a mass of
molten fuel, slumping
through containment
Fuel inherently 'self
contrclling'
The solution formed
for the SOFT reactor
Both fuel and coolant
are in the form cf molten
chlorides. The thermo-
dynamic stability of all
constituents exclude any
exothermic chemical (or
explosive) reaction
Neither fuel nor
coolant contain
hydrogen
The significant decrease
in decay heat and the
presence of large heat
sinks (reflector, core
catcher) eliminates a
'China Syndrome'
Fluid fuel reactors are
known to have a large
negative temperature
cocefficient and
Doppler ccoefficient
12
1.4 The present State of Reactor Development and the
Criteria for a Safer Reactor
The present state of reactor technology results from a 1long
and complicated development and marked by not always the most
logical decisions.
If the following are the four main targets of reactor
development:
2) an economically and technically feaslible source of
electrical energy,
b) economically and technically feasible source of
high temperature heat for chemical processes,
¢) high grade utilisation of uranium resources (and
even thorium resources),
d) satisfactory level of safety,
then it may be possible to show with the aid of a diagram
(Fig. 1.1) the present state of reactor development.
Of course there are other ways of eliminating the characteristics
presenting the greatest hazards - high pressure and high F.P.
inventory. Figure 2 shows some possibilities. It seems
however that only molten salts can solve both these problems.
In this paper the accent is on the safety problem 1.e. the
effort to reduce the consequences of the worst credible accident -
core melting. Paradoxically as this might sound the solutlion to
this problem is the use of a reactor with a molten fuel core.
Tn this reactor:
a) the most important parameter to be optimised is the
reduction in the inventory of volatile fission
products (which will control the consequences of an
accident during the first hours) and fission products
such as Sr-90 and Cs-137 (which determine the
accessibility of the contaminated area for tens of
years) ,
13 14
Fig. 1.1
TRENDS IN THE PRESENT
DEVELOPMENT OF THE
DIFFERENT REACTOR TYPES AND THE
REACTOR TECHNOLOGY
SAFETY PROBLEMS
HIGH
HIGHEST
TEMPERATURE
SAFETY
HEAT
- HIGH Pressure in the
(QTSOFT (G?\‘Eggg‘?) coclant or fuel
HIS PAP T
- No High
;l\ {good +) (bad -
i SOFT/r Ligquad Light
‘\K Metal Water
High Fast Reactor
{pad -) Breeder
(=) (=)
Fission (+) (=)
product
inventory Low Molten Gaseous
. LIQUID ™\ and (good +) Salt Fuel
E@@% FAST}) after- Reactor Reactor
BRE@QEB/ .
heat (thermal-flucrideyd (-)(+)
fast-chloride)
HIGH
(+)(+)
FEASIBLE UTILISATION
POWER OF
URANTUM
Improvement
possible
No possibility
for further
improvement
b) decay heat resulting from the spontaneous radioc-
active decay of fission products and actinides but
alsoc of the structural material is significantly
lower, even 1f it 1is not enough,
¢) the breeding ratio has been set at 1 in order to
minimise the need to transport fissionable material.
this makes the reactor a net zero consumer and zero
producer of fissionable material,
d) the entire system fuel, primary and secondary
cooling systems have been desligned to operate at
amblent pressure,
e) no highly reactive compounds, (e.g. metallic sodium)
are present In the system,
f) no hydrogen containing compounds (e.g. water), which
can evolve free hvdrogen., are present. (see Ref. 14, 15
2
16, 17, 18).
2. ACCIDENTAL RELEASE OF FISSTON PRODUCTS AND DECAY HEAT
REMOVAL
2.1 The Rasmussen Scenario (Ref. 12, 2L4),
The scenario of an accident used here is taken from the reporst
WASH-1400 as the most fatal case: the PWR-1 which can be
characterised by a steam explosion on contact of molten fuel
with water in the reactor vessel. This accident category
includes the following fraction of fission product core
inventory release:
16
Rasmussen German risk study
(maximum)
Xe - Kr 0.9 1.0
1 0.7 6.8
Cs=Rb 0.5 0.5
Te-5b 0.4 0.35
Ba-Sr 0.05 0.052
Ru (Rh,Co,Mo,Te) 0.4 0.38
La (Y,Zr,Nb,Ce,Pr,
Nd, Np, Pu, Am, Cm) 3%10"° 2. 6x107°
The release of fission products and actinides to the
environment results in two different scenarios (simplified
here):
a) the impact of volatile short lived fission products
from the passing cloud in the direct neighbourhood
of' the reactor - some hours after the accident,
b) the impact of non-volatile long lived fission products.
2.2 The Problenm
From consideration of this scenario of the worst reactor
accident 1t 1s clear that to improve the safety significantly
the proposed reactor must allow the continuous extraction of
both classes of fission products - the short lived volatile F.P.
and the long lived non-volatile F.P. This 1s the most
important aspect. (Table 2.1)
For the accident described above to occur in its entirety it
1t 1s necessary that the last barrier, the containment building
becomes breached.
17
Table 2.1 The most dangercus Fission Products according to
(WASH 1400)
Release Critical Nuclides Relative Proposed
Counter-
Cat
egory Organ Dese measure
Bg@éfifigfl bone I-132,135,133 w500 continuous
from marrow I-131 extraction
Kr-88,Te-13%2 of volatile
assi
passing F.P. by
- [ mn
cloud lung I-132,135,133 VR4 means of
I-131,Kr-388
from ? He-gas
Te-1%2,3b-129 .
purging
reacton
lower I-132,135,133 240
hours large 1-131,5b-129
after intestine Te-132,131m
1 .
release thyroid I-132,135,13%3% 330
gland I-131,Kr-88
Te-132,3b-129
Long term bone Sr-90,Cs-134 6.7 continucus
_fffect marrow Cs-137 extraction
T f non-volatile
Ce-1Uk Ru-106 ©
(0-10 yrs) F.P. in
of inhaled lung Ru-106,Ce-144 126 the fuel
: reprocessing
radio mineral Cs-134,137 n18
plant
nuclides bone Sr-90,Ce-144
15 /lm
breast Cs-134,137 vl,25
f'rom
Ru-106,Ce-144
reactor
Fig. 3.1 18
1SOFTTREACTOR VERSUS CONVENTIONAI, REACTOR
N
SOLID <
FUEL
REACTOR
FU
‘ RE,
-4 a
CoO ""* : b
L = 3 . * *
ol Al 2 Bl
ol x| x o *
* % - :
U FU
'SORT! CN
REACTOR
o Uranium makeup
ki P
]
U 7
P
A% % F l
S
EZ:a Fuel element
<+ Fission products (F=z=P=)
~—~—+ Decay heat RE = Reactor )
RP = Heprocessing
Power, FP = Fission Product
: : CN = Containment
; Criticality FU = Fuel element
Co =
Cooling of irradiated
elemMents
19
This can occur in the following ways:
a) steam explosion,
b) leakage of pipes,
¢) hydrogen burning,
d) overpressure due to heating,
e) melting of the core systems,
f) loss of fluid giving overpower (for fast reactors).
It seems that In the reactor system propesed here almost all
of these failure mechanisms (excluding the leakage of pipes)
could be fully eliminated. The safety is thus significantly
improved.
3. PRINCIPLES OF THE 'SOFT'" REACTOR
3.1 A Schematic View
Of course there is no possible way to bulld a fission reactor
in which the dangerous fission products are not produced at all,
or in which they could be fully destroyed (by transformation)
in situ. Therefore the only possibility 1s to shift in time
and space: