Accelerator Driven Subcritical Assemblies
Report to :
Energy Environment and Economy Committee
U.S. Global Strategy Council
June 20th 1998
Recently three groups in Gatchina, Russia,
Los Alamos Scientific
Laboratory USA, and CERN, Switzerland have proposed to use accelerator
subcritical assemblies as sources of electricity as an alternate to
reactors. By this means the proposers hope to avoid some of the
that presently plague these reactors and prevent universal acceptance
expansion of the technology. These proposals are discussed and it is
there is no appreciable improvement in any real safety parameter, and
there may be an improvement in public acceptance this is very
alternate proposal, to use these assemblies to transmute long lived
actinides into other material is also discussed. It is pointed out that
transmutation may well be unnecessary. Nonetheless a modest research
along these lines may well be advisable.
Very early in the studies of nuclear
physics accelerators have been
considered as a source of neutrons. A typical source of neutrons in
1940 was an
accelerator of deuterons where the protons were "stripped" by a target.
nuclear reactors had been successfully built accelerators were
considered to be
useful neutron sources. In 1948, there was a fear that the USA had a
uranium and no access to uranium (at that time for military purposes)
parts of the world. The MTA accelerator project was started to produce
material from U238. It produced an average current of 1/4 ampere of
deuterons. Goldanskii and other in the Soviet Union wrote several
papers on the subject. More recently, about 1975 a large scale project
was proposed by Dr Bennett
Lewis, of Atomic Energy Of Canada Ltd. He proposed to make a high
proton accelerator driving a subcritical assembly of U 238 as a
producer of Pu 239
for use in CANDU reactor as a fuel when uranium supplies run out. This
be an alternate to a fast neutron breeder reactor.
The recent reintroduction of this idea are to produce the heat, and
electricity, directly. Three separate groups have considered this in
the last 10
years - and it appears that each started from a different direction. A
reactor designer, Yuri Petrov of Petersburg Nuclear Physics Institute
proposed a subcritical assembly for energy production to avoid the
criticality that were so evident at Chernobyl (Petrov 1991, 1992,
Petrov 1993). Below I call this the GATCHINA group. This idea was
independently proposed by Bowman of Los Alamos Scientific Laboratory
(LANL) in 1990 (Bowman 1992, Arthur 1995, Krakowski 1995,1996) in 1990
and Rubbia of the European Centre for Nuclear Research (CERN) in 1991,
from the perspective of an elementary particle physicist. (Carminati et
Rubbia et al. 1994, 1995a, 1995b, 1995c, d, 1996) (hereinafter called
group). It was brought once again to the attention of some government
France by Rocherolles (1998) which article stimulated this particular
questions have been asked:
(1) has the approach any advantage
over light water moderated fission
reactors? or over other reactor systems?
(2) if the answer to question (1) is
yes, what is the advantage? and should
the approach be pursued? and if so by whom? by an individual government
as the French or an international group?
This paper is a preliminary answer to
these questions. Since below I
make comparisons with other suggested technology (fusion etc.) that
been shown to work and may never work, it is important to recognize at
that few people doubt that accelerator driven subcritical assemblies
can be made
to work, either for producing energy or for transmutation of waste. The
that have been expressed are of the economic viability of the system.
The discussions of GATCHINA, LANL and CERN do not make a clear
comparison with what is now being done or could be done with existing
water reactors or fast neutron reactors that exist or could be
developed such as
Superphenix (Butler 1993, Lanais 1993, Castaing 1996). The advantages
possessed by nuclear power with the present light water reactors are
advantages claimed for the proposed approach. It is not apparent, what
if any are
the additional advantages (and disadvantages) of using an accelerator.
tended to assume that other possibilities for overcoming the present
nuclear power in western (OECD) countries do not exist. This applies in
particular to the paper by Rocherolles (1998).
All three groups consider an assembly large enough to be an neutron
multiplier but too small to be "critical" and sustain a nuclear chain
reaction. I try
to put the subject (which has been raised from outside the traditional
"industry" as an important approach) into perspective. I ask to what
extent, if at
all, it resolves any of the problems that presently plague nuclear
electric power. I
do this by making some comparisons to the existing reactors. None of
intended to be conclusory, but to ask questions which I would like the
of the technologies to answer. I note that each of the three groups
above came at
the subject from a different direction.
The Neutron Multiplication Process
When neutrons fall upon uranium or other
fissile material, they can
produce fission of the atomic nucleus which releases more neutrons.
described by the neutron multiplication coefficient, k the number of
released from the fission of uranium minus the number of neutrons
the reactor construction and shielding materials. If k > 1 the rate
of the fission
reaction increases with time; In a stable reactor, the fuel assembly
must be have k
= 1. This value is called the "critical" value. One of the important
reactor design is to limit the value of k. In a normal reactor this is
negative feedback mechanisms or insertion of neutron-absorbing control
the value is uncontrolled, the value is limited only by disassembly
the reactor. A release of even a small fraction of the energy stored in
can cause a Chernobyl-scale accident.
Control and Delayed Neutrons
The control of a subcritical assembly is
appreciably different from the
control of a reactor. A subcritical assembly would be controlled by
accelerator power to achieve the desired output heat. Since it is only
multiplicative assembly (indeed the CERN group call it an energy
is little chance of a runaway. For a critical reactor any random
increase in power
generation must be controlled by a rapid feedback mechanism through
control of neutron absorbing rods (it is an energy amplifier with
infinite gain). Fortunately there is a scientific fact that aids in
this control. About (1/2)% of all
neutrons in fission are delayed by periods varying from 20 milliseconds
seconds. These arise because some fission products are produced after
of a parent, in state sufficiently excited that they will promptly
and emit neutrons. The apparent half life is that of the parent and is
the weak beta decay interaction. These neutrons are sufficiently
Fermi declared that "without delayed neutrons we could not have a
program". I would modify this today to read "without delayed neutrons
have to have an accelerator driven subcritical assembly". But delayed
exist - so the necessity is unclear.
If there were no delayed neutrons, the time constant for power changes
nuclear reactor would be of the order of the slowing down time of the
which is 100 microseconds in a light water reactor. No mechanical
can work that fast. However, since delayed neutrons exist, the time
much longer IF changes in reactivity are limited to less than the
delayed neutrons. Then, as demonstrated repeatedly in the many
reactors, a power reactor can be easily controlled. But if a larger
change is made
then the reactor can become "prompt critical" and a rapid power
take place as happened at Chernobyl with disastrous results.
Does reactivity (k) change with time?
I pose the question, implicit in the work
of the above three groups: "can
one have a design where we can state with absolute assurance that the
never become critical yet have a large value of k?" If so there might
be a safety
advantage over a reactor. Then the sub-critical assembly can act (as
by the CERN group) as an energy AMPLIFIER. The energy contained in an
external source of neutrons will be multiplied, or amplified by the
factor of 1/(1-k).
At first sight it might be thought that a subcritical system has the
considerable advantage that if anything goes wrong, the initial source
the accelerator, stops. A large number of possible accident situations
eliminated from consideration. For Petrov, living in a country that
RBMK reactor which is very susceptible to accidents where the reactor
prompt critical (as happened at Chernobyl) there is both a strong
technical and a
strong psychological advantage to avoid criticality. While the
stop in a power failure it is possible to imagine other failure
scenarios where the
accelerator keeps going. For those who live in western countries or
the Light Water Reactor (LWR) is dominant, the advantage is less clear.
very hard for the light water reactor to go prompt critical and only
psychological advantage remains.
Moreover the system would be much less attractive if we cannot be sure
that k is less than unity under ALL conditions; as the fuel moves for
as the uranium (or thorium) is burned up and some of it is transmuted
plutonium (or uranium 233). Rapid reactivity excursions that can occur
critical reactor can also occur in a subcritical assembly and must be
almost as carefully. Since we are here addressing the perceptions of
rather than those of experts, this must be done in a credible manner.
did increase with time in the first design of Carminati et al, this was
later designs. It is important to be sure that k does NOT change over
the burn up
of the fuel; otherwise it would be impossible to have a large
amplification and at
the same time have stability under all conditions. Clearly, these
should be explored further.
The Thorium Cycle
In all three of the proposals discussed
above a Thorium Cycle was
proposed. Although the existing light water reactors are designed so
that they can
use thorium (Lanais 1965), and was used in the first Shippingport
reactor, all of
them at the present time operate on a uranium fuel chain. (It is
sometimes called a
fuel cycle but closure has not been achieved in practice so that it is
not presently a
fuel cycle) The reactor burns up U 235, and transmuting some of the U
238 to Pu
239. Eventually Fermi and others conceived of using the Pu 239 in a
reactor. But plutonium 239 is often used as the fuel in an atomic bomb,
so that has raised
both a legitimate fear, and a larger psychological fear, of the
of this plutonium for proliferation of nuclear weapons. A thorium
cycle, it is
sometimes argued can avoid this fear by avoiding any production of
239. But the thorium cycle does produce U 233, and bomb experts assure
a bomb can easily be made from U 233, and since there are no neutrons
spontaneous fission of U 233, U 233 can also be used easily in a
bomb. The advantages of a thorium cycle are not therefore as large as
However a thorium cycle could, if one wished, be used in a light water
reactor system about as easily as in a subcritical assembly. The number
neutrons per fission is smaller for thorium than for plutonium 239 or
and a thorium loaded reactor must therefore be larger than the minimum
uranium reactor. The reasons that thorium is not presently used as a
fuel is that
initial fuel fabrication and chemical processing are somewhat more
for a uranium cycle, and until the fuel chain is closed into a cycle
there seems no
advantage. Thus many of the advantages claimed for the subcritical
raised by the above three groups are really advantages of a thorium
has been considered before for a light water reactor and never found
enough to be implemented.
Before leaving the discussion of the
thorium cycle, I note that a crucial
issue is WHICH thorium cycle. If the start of the process is a fuel
plutonium and thorium (as suggested by some to destroy weapons
uranium 233 will be produced which can be separated from the spent fuel
easily as plutonium can be separated. Although U 233 has a nasty gamma
ray, it is
only a little more difficult to use in a bomb than plutonium. Thus this
almost as proliferation prone as the plutonium cycle.
A good discussion of the various fuel
cycle possibilities, written in the
context of a heavy water moderated (CANDU) reactor may be found in
and Didsbury (1985).
A more sophisticated thorium cycle would include a little U 238 -
to make the resultant U 233/U 238 mixture less than 20% and therefor
for a bomb without (expensive and tedious) isotope separation. But then
would be produced from the U 238 and the problems of the plutonium
would reappear. But the LANL group argues that although the problems of
plutonium would reappear, they would be less serious because the mix
include a large fraction of the isotope Pu 238 (produced from the
generates a lot of heat and makes the mixture impossible to use in
and difficult to use in other designs. This was raised with
by Coops (1995) and was discussed at an IAEA meeting (Altshuler,
Wilson 1997), but some scientists who are knowledgeable about bomb
insist that a bomb can be made with any amount of Pu 238. But to the
it is more difficult, this may be a non-proliferation advantage.
In passing I note a proposal to use thorium in a fuel chain that is
deliberately never closed but that the whole reactor stays buried
(Teller et al
It is clear also that the distribution of
radioactive isotopes in fission cannot
and does not vary fast with the regeneration constant k. The difference
= 0.95 and k = 1.0 is not and cannot be important. A small residual
remain. If a higher burn up can be achieved, fewer short lived
would be around at the (hopefully less frequent) time of fuel change.
In a Russian RBMK reactor a major problem
is that under some
circumstances, such as those at Chernobyl unit 4, and perhaps an
Transient Without Scram (ATWS) can lead to prompt criticality which
controlled. A subcritical assembly can avoid this if indeed the
configuration makes k = 1 impossible. Although prompt criticality was a
in some early designs of fast neutron reactors cooled with liquid
sodium or lead,
this does NOT apply to the more modern designs, especially those with
metal fuel. The major cause of potential accidents in a light water
reactor, and probably in
modern designs of liquid metal reactors remains in a subcritical
is a failure of Post Accident Heat Removal (PAHR). Although a
assembly cannot explode like a Chernobyl reactor did, accidents such as
Three Mile Island, could still occur as the amplifying assembly
overheats during a
Loss Of Coolant Accident (LOCA). The overheating is dependent ONLY upon
the energy production in the system and not on the value of k, and
therefore is the
same whether the system is a reactor or a subcritical assembly.
The above is a general statement and it is well recognized that safety
depends upon details. Some of the proponents of sub-critical assemblies
that the cooling of the subcritical assemblies can be safer than in a
assembly. The drawings in some of the reports include "passive" safety
that are attractive. But these features could also, probably at much
be added to a critical reactor system. The important question,
unanswered by the
proposals of the three groups is "will the avoidance of control rod
enable a simpler, and more effective heat removal system, or will the
the accelerator beam tube make such a system more complicated and less
I therefore reiterate that the ONLY advantage of a subcritical assembly
producing power using either a uranium or thorium cycle seems to me to
possible psychological advantage of avoiding prompt criticality. This
psychological advantage might manifest itself in different ways. It
example result in less draconian regulation than is now the case in the
therefore may lead to a cost advantage. Since the main reason that
in the USA is not now being pursued is that the cost has risen in real,
corrected terms, this advantage could, in principle be very
The cost estimates in the reports of the different groups are on
bases, but this basis differs from the cost estimates for anyone now
commercial nuclear power reactor. I would like to see a realistic
comparison. My suspicion (which I would be delighted to have proved
wrong) is that there is
NO cost advantage other than the possible one of less regulation. The
that the costs of a nuclear reactor are NOT connected with the
but with the heat removal - and the heat removal problems are almost
the same for
a reactor and a subcritical assembly.
If regulation were logical, which of course it rarely is, I would see
advantage there also. But if accelerators are regulated sensibly and
regulated in a draconian manner, it might be necessary to use
There seems a clear cost disadvantage to the subcritical systems. The
control rods which are relatively simple devices, are replaced by an
which is expensive. And an accelerator is not merely expensive in its
cost, but it will take 20-25% of the power output of the assembly.
I thus come to the conclusion - a conclusion shared by some advocates
using accelerators and subcritical assemblies - that there is no reason
to use them
solely for producing power.
Transmutation of Waste (ATW)
But there may be an advantage not
mentioned in the first of these papers -
in the transmutation of waste. This is the present focus of the LANL
(Venneri et al. 1998). Again there are two possible advantages: a
advantage, and a psychological advantage. Again however, one must be
not to overstate; a large part of the claimed advantage can be gained
reprocessing and use of MOX or similar fuel, a bit more by the use of a
neutron reactor. But there remains some advantage in accelerator
and it remains to be seen whether that is important enough to justify
are closely intertwined with the technical, political and
psychological issues connected with nuclear waste disposal (McCracken
Fenn 1991, McKinley 1992). It is therefore necessary to understand
these to a
considerable extent. They are also very complex and there is a
divergence of views. I endeavor below to discuss two alternate views
neither is at the extreme of expert opinion.
The Waste Problems
The waste products which dominate both the
heat generation and the toxic
hazard in the 10 - 100 year time frame are Cs 137 and Sr 90. (Cohen
et al. 1978, Bodansky 1997, Rasmussen et al. 1997) There seems no
option but to
wait for the decay of these isotopes. The problem MAY be mitigated in
use of the isotopes in a major way for other purposes: sterilization of
supplies and irradiation of foodstuffs. But it is hard to see how
important though they may be, could use more than a small fraction of
inventory of radionuclides. Therefore one must plan to allow these
decay in a secure storage facility.
At 100 years after cessation of fission the transuranics cause two
the decay heat and the decay radioactivity. They last for periods of
1000 to 1,000,000 years. But opinion varies about whether the heat or
radioactivity are large enough to be a problem or whether it is worth
them by fission or spallation. Even if it is accepted that
ultimately desirable, opinion varies on whether this should be done at
whether, for example, it might be done AFTER the cesium and strontium
I maintain that there are three very questionable assumptions in most
discussions of nuclear waste (Jakimo et al. 1978). It is important not
these assumptions, or at least to be aware that one is making them for
political reason and not for a technical reason.
(1) A common misconception is that nuclear waste poses an unusually
dangerous hazard in society. I agree that it is certainly unusual. I
disagree that it
is unusually hazardous. Indeed a former Chairman of the United States
Energy Commission, the honorable Dixy Lee Ray declared that it was the
non problem they had. I sometimes say that it is the only major waste
society for which we have a good technical solution. Normally waste is
but still released to the environment - and now there are arguments and
that low dose linearity is usual rather than unusual in society
Wilson 1995) dilution does not completely avoid the problem. Nuclear
be kept concentrated and kept out of the environment. This is an
NOT a disadvantage.
(2) A common assumption is that we must bury waste in such a way
that it does not come into contact with people for a million years -
and do so
without any human attention. Moreover that this should be done with
of almost complete certainty. This demand is made for no other waste in
society. An alternate demand is that modest human attention - not much
more than a night
watchman be allowed. Since the time constant for something going wrong
repository is of the order of years (compared to microseconds for a
for an RBMK reactor and hours for a LWR) this should be possible.
(3) An American assumption, not shared by all countries and in
particular not shared by France, is that "final" waste disposal must be
(4) The United States Environmental Protection Agency has set a
limiting standard for radionuclides that could be allowed to be
released to the
environment (Standard 40CFR191). While it is clearly desirable to keep
low, some critics (falsely) imply that exceedance os the standard would
disaster comparable, for example to the arsenic problems in Bangladesh
Indeed calculations of the integrated dose for the waste disposal part
end) of the nuclear fuel chain are always less than those for the
milling part (back end). Examples are given in Tables 6-2 and 6-3 of
(1997) based on NCRPM (1987) and Oak Ridge (Michaels 1992) studies. The
change due to transmutation by any method only a small reduction (about
person-Sv or 150 person-Rem per plant year). Therefore even if one
linear dose response at low doses of radiation, the change in the
health risks from the waste disposal is small (0.02 cancers per plant
the 1975 NRC guideline in RM-30-2 that risks should be reduced if they
than $1,000 per man rem (updated to $2,000) the estimated expenditures
transmutation of waste are excessive by over a factor of 100. The risk
even smaller (and the cost exceedance even bigger) if the risks are
economists and public policy analysts suggest that they should be) at
discount rate. But the public preoccupation with these minute health
manifests itself in political opposition to waste disposal facilities(1) is not in accord
with these professionally made calculations. This makes it difficult to
use them as
a basis for public policy.
Nonetheless if one rejects these common assumptions, a simple approach
to waste disposal might be as follows:
(i) keep all fuel rods in a reactor
spent fuel pit under water for 10 years and
then when that pit is full:
(ii) move the fuel rods intact to dry
spent fuel storage (above ground or
easily retrievable aerated storage) after 10 years for a time UP TO 100
This procedure has been demonstrated and
seems safe. Aircraft, meteorite
or atomic bomb hits could disperse the waste, but these have low
subsequent clean up is possible. This storage could be at a reactor
site or central
storage depending upon security requirements and economic and political
(iii) after 100 years (or earlier if
(a) retrieve the waste, extract the
plutonium for use to generate electricity
(and with precautions to discourage use for bombs) and bury the rest.
It would be
easier to meet any requirement to limit the release to the environment.
problems would no longer be important. One of the more imaginative
that of Watters and Chandra (1985).
(b) bury the whole spent fuel rods in
a way that would ensure that they
would not become a "plutonium mine" (Pedersen 1998) for would be bomb
According to this view, the
radioactivity in the transuranic material is
NOT a problem. According to this view therefore transmutation of waste
unnecessary. This view has the merit of being close to the
wisdom" before 1975 (since which date the USA and perhaps the world,
no coherent plans for nuclear power development).
I characterize (or perhaps my detractors
would say - parody) the pre-1975
"conventional wisdom" as follows:
(i) after 10 years in storage extract the plutonium from the fuel for
fast breeder reactors or perhaps in LWRs - the choice between these two
made by economics.
(ii) transmute other transuranics in
fast neutron reactors, used for this
purpose as "burners" (Lanais et al 1993, Castaing et al 1996).
(iii) bury the rest.
I note that this "conventional wisdom"
automatically got rid of most of the
transuranics and might easily get rid of all in the fast neutron
reactors (although so
long as nuclear power were used there would always be an inventory): I
was this pre-1975 way of considering a logical nuclear fuel cycle that
led to the
common misconceptions about nuclear waste that I noted above.
Another extreme view might be that all fissionable material, including
long lived fission products Iodine 129 and Technicium 99, should be
BEFORE burial, and preferably as soon as possible. It seem to be this
drives the suggestion that accelerators be used for transmutation of
various technologies for transmutation is discussed in some detail by
et al. (1996).
Even if the first view has the most likelihood of being "proven" or
eventually accepted as correct, some research on the alternate
possibility of the
second view could well be considered a worthwhile task, roughly in the
most people are willing to accept that the world should spend money on
research even though most of the scientists who support such research
doubts that fusion will actually work in the sense of being
competitive with alternates.
Specific Comments upon the Present Proposals
I will take as a guide the present
proposal of the LANL group (Venneri et
al. 1998). The CERN group has variants of this. They combine the
four technologies (1-4) below:
(1) A high intensity accelerator - LANL chose a superconducting proton
linear accelerator but CERN was at one time considering a sector
cyclotron (Rubbia et al. 1995c)(figure 1). I note that either choice is
most advanced of the technological developments needed for the
Transmutation of Waste and needs the least research.
At Los Alamos about $200 million a year is already being spent on
developing such an accelerator to produce tritium for weapons.
note that technically this is unnecessary for 80 years at least because:
(i) the purpose is to keep the US
weapons stockpile at a level 10,000
weapons whereas the 50 odd nuclear weapons available at the Cuban
were demonstrably enough to scare the country and create effective
reduction by a factor of 100 (over 7 half lives) would allow the
stocks to last for 80 years.
(ii) tritium is produced in surplus in
Canadian heavy water reactors. Although Canada will not sell this
tritium for military purposes, this could change
if there develops a proper international regime for control of nuclear
(iii) Russia has a large surplus of
tritium and this could be available for a
price less than that of building another production source.
(iv) An existing reactor such as FFTF
in Hanford might be modified to
produce tritium (Lanais et al 1993)
(v) If the government of the USA can
overcome its distaste of
reactors, another special reactor such as that at Savannah River or
a choice which is more logical than an accelerator.
(vi) A light water reactor could be
modified to produce tritium if there
were no objection to the use of civilian facilities for that military
that tritium is not presently a Strategic Nuclear Material.)
Although if any one of the items (i)
to (vi) were correct the development
of superconducting accelerators for military purposes might cease, this
is far enough advanced that it is still the most advanced of the
needed for a viable subcritical assembly.
It follows that although an accelerator is
the easiest of the technologies to
work on, and might well be the one that actually would show progress,
it is not the
limiting factor in the ATW technology. Accordingly I believe research
accelerators should have a LOW priority for the overall ATW program.
would especially apply in China where there has not yet been enough
power produced to justify a program at all.
(2) A subcritical assembly. This would be a fast neutron facility. The
mechanical designs suggested by both CERN and LOS ALAMOS use a vertical
beam pipe bringing the proton beam down onto the target. (figure 2)
to be based upon the (paper) designs for Liquid Metal fast neutron
(LMR) of Westinghouse and General Electric. This has important inherent
devices as shown in the attached figure 3 from Westinghouse. The
likely to use U 233/Th as fuel. Subcritical assemblies analogous to
reactors an Molten Salt Reactors (Arthur et al. 1995) have also been
but are not in the present designs.
(3) The choice of coolant for a fast neutron assembly is interesting.
The main choices are:
(i) Helium gas
(ii) liquid sodium
(iii Lead-bismuth eutectic (LBE)
LANL are choosing to work on (iii) the
lead bismuth eutectic (LBE). Many starry eyed individuals, of which I
used to be one, like (i) helium gas as a
coolant. One reason is that such a system would have been a natural
from the High Temperature Gas Cooled (graphite moderated) Reactor
(HTGR). But the HTGR has been abandoned by every country that has
started work on it
and Germany(4)) and this would then be a
big extrapolation in
Number (ii), liquid sodium has a lot
of advantages. It is non-corrosive
(except to such comparatively unimportant materials as human skin) and
low neutron capture cross section. It has been widely used in test
reactors for 40
years. Minor problems have developed but the expert engineers assure us
these are soluble. But psychologically the public fear of a runaway
sodium fire is
important. Without this fear the fast neutron reactor at MONJU would
been shut down for 2 years.
Number (iii) - LBE - is attractive
superficially. As a heavy element it is
excellent as a target for the initial production of neutrons. As shown
by the phase
diagram between lead and bismuth shown in figure 4 there is a liquid
moderate temperatures. Neither bismuth nor lead burn as readily as
but it is heavy. It is corrosive at high temperatures - unlike liquid
in the USA failed in the past to overcome this. But Russian work
succeeded. Several of their latest submarines have reactors cooled by
liquid lBE (figure 5). It
is self shielding and this makes a compact assembly, keeps the weight
enables the submarine to go faster. I do not know whether that will be
alternate to liquid sodium, and solve the public perception problems.
it is worth the research. But I note that the Russian BN 600 fast
breeder/burner reactors, designed at the same place, Obninsk, use
and there are no plans for using LED for any but submarine reactors.
However, I believe that understanding in
the west (OECD countries) of
how to use LBE will have implications beyond the ATW program and is
(4) The subcritical assembly does not avoid the need for chemical
separation of the different parts of the spent fuel. It must be
remembered that it
was the concern about chemical separation that led to the end of the
"conventional wisdom" (Keeney et al 1975). In the USA it was felt that
chemical separation plants that isolated pure plutonium was highly
from the point of view of weapons proliferation, and ceased activities
to "set an
example", an example that has NOT been followed by France, Germany or
Japan. LANL propose to use electroprocessing whereby chemicals are
separated by their
electrochemical potential and pure plutonium is not separated (it comes
higher actinides). (figure 6) This technology has been sponsored during
20 years by Argonne National Laboratory. It is attractive from the
point of view
of non-proliferation because:
(i) plutonium is never separated from
the higher actinides nor separated from
all the fission products so that it is unusable except in a fast reactor
(ii) unlike the PUREX (solvent
extraction) chemical separation processes
pyroprocessing is viable in small sizes so that it is feasible to have
facility for each reactor. Then even impure plutonium need never leave
reactor site. Although an independent review (Basolo et al 1995) was
not sure of
all the claims of proliferation resistance, it was generally supportive
research. However I note that the Integral Fast Reactor (IFR) program
Argonne National Laboratory (which was developing this concept) was
by the US administration and congress (using rhetoric that shows that
to understand the important nonproliferation features) in 1994 and only
processing program remains. Whether LANL can successfully reinstate
program is uncertain.
However, I believe that research in
electroprocessing is important. It
shows great potential for being a proliferation resistant technology
therefore overcome one of the legitimate public concerns about nuclear
The Shippingport reactor used thorium as a fuel. The use of thorium has
been abandoned since then. However, use of thorium as a fuel shows
being able to extend the time when light water reactors will be the
workhorses" of the electric utility industry and delay the time when
reactors may be needed. Research on the thorium cycle would seem to be
appropriate but I believe it would best be associated with light water
rather than accelerator driven subcritical assemblies.
Both CERN and LOS ALAMOS groups claim that
it will be faster to use
up all the fissionable material in a subcritical assembly than in a
reactor as shown in figure 6. The overall mass balance for 65 years of
with the principal purpose of getting rid of fissionable material is
shown in figure
7. This depends critically upon details and is in any case only
relevant if it is
desired to get rid of all traces of nuclear fission as soon as possible.
A Possible Program
My opinion should be clear from the above:
Transmutation of Waste (ATW) must be considered in the context of a
nuclear power program. What such a program could or should be in the
becomes more uncertain as time proceeds. But I presently envisage the
(in addition to the monitored waste storage mentioned earlier):
(1) Continue to use light water reactors, and heavy water reactors
(CANDU) in Canada, with uranium fuel of appropriate (including zero
enrichment for the CANDU) enrichment. Elsewhere (Wilson 1997) I have
pointed out that the present impediment is COST, The cost of nuclear
electricity has increased a factor of at least 2.5 over earlier costs
in the 1970-1975
period primarily by overregulation.
(2) Study the use of thorium as a fuel
in light and heavy water reactors
to extend the availability of fuels.
(3) For those countries that wish to
do so, chemically separate
plutonium fuel, with stringent safeguards against
non-proliferation, and burn it as
(4) Endeavor to develop an economic
fast neutron system. Probably a
reactor but perhaps an accelerator driven assembly.
(5) Develop the chemical and target
facilities for either reactor or
accelerator transmutation of waste but build large facilities only if
disposal system demands it (probably only due to public perception).
There is a large field, or set of fields,
of scientific endeavor that use
neutrons for basic scientific studies - including studies in physics,
biology and medicine. Since reactors are now politically difficult to
spallation sources of neutrons are now preferred. Early facilities used
electron accelerators but nowadays pulsed proton accelerators are
preferred. There are basic facilities at the Rutherford laboratory
(ISIS) at Los Alamos
(LANCE) at Argonne National Laboratory (IPNS) and proposed for Oak
(SNS) and a new European facility (ESS).
One set of uses of spallation sources demands accurate timing. This is
separate the neutron energies by time of flight especially in the multi
energy region. But there are others for which timing is not so crucial
maximum number of neutrons is needed. One estimate is that these latter
comprise 75% of all uses (Greene 1998).
Some of the early electron accelerators and the ISIS accelerator used a
subcritical fissionable assembly of uranium as a neutron multiplier to
yield. The multiplication was a factor of 1.6 in ISIS far less than the
factor of 20 -
100 proposed in the ATW and related programs. But the target size had
small to obtain a high brightness, and this would not be necessary if
numbers of fissions were the goal. This is not now done (Appleton and
1996). The reason stated for the abandonment is that the target
lifetime (6 weeks)
was too small due to anisotropic growth of the uranium crystallites -
associated with hydrogen and helium embrittlement. (Taylor 1998). A
keep the pulse structure of the neutron beam short to satisfy those
need the short pulse does not enter the balancing of issues till the
factor is much larger.
Another reason (consistent with what I noted above) is that getting
power in an accelerator is easier than coping with a multiplying target.
Whatever the reason, there is an obvious
inference and an obvious simple
plan for immediate study.
(i) the inference is that my belief
that the multiplying target is difficult is
(ii) research on a multiplying target
(subcritical assembly) could begin
IMMEDIATELY at any one of the spallation neutron facilities. Indeed
on the spallation process itself is under way (Boudard, Leray and
1998)(Rubbia et al. 1994).
The negative approach of this paragraph
can be replaced by a positive
approach if the proponents desire. There exist accelerators where parts
program can be tested NOW at moderate expense. These include ISIS,
and also the Alternating Gradient Synchrotron (AGS) at Brookhaven
Laboratory. It is noteworthy that a liquid lead target for the LANCE
source is being built by the scientists at OBNINSK (Venneri 1998). This
a beginning of Americans learning how to use the Russian technology of
A renewal of the program to understand multiplying targets at these
spallation sources could also allow an immediate start of understanding
potentially limiting part of the technology. A more urgent approach to
Argonne National Laboratory program on electron and pyroprocessing can
this technology too to the stage where the whole ATW approach can be
I can envisage a nuclear power future with
a waste management program
that would satisfy me without a program either for
accelerator transmutation of
waste (ATW) or any fast neutron program for transmutation of waste. It
likely to me that a program without transmutation would be
cheaper than one with
transmutation but maybe not appreciably so.
However, a modest program aimed at opening up possibilities is probably
in order. In my view it should have a higher priority than a fusion
program, but a
lower priority than a nuclear research program on reducing costs of the
light water reactor and related programs to the 1973 levels. Although
much of the
impetus for the present proposals comes from people knowledgeable about
accelerators, I believe that the best approach to such a modest program
enhance research in the problems of targets, coolant, and
existing facilities. Since the purpose is to aid in coping with waste
clean up problems, any funds from the USA should come from these ample
budgets rather than from the nonexistent (in FY 1988) or minuscule (in
nuclear research budget, and certainly not from the basic energy
The major conclusion in the above is consistent with the conclusion in
monumental study of Rasmussen et al (1997). "The committee found no
that applications of advanced Separation and Transmutation (S & T)
sufficient benefit for the U.S. High Level Waste (HLW) program to delay
development of the first permanent repository for commercial spent
"over the next decade the United States should undertake a sustained
and carefully focussed research and development program on selected
and Transmutation (S & T) technologies with emphasis on improved
separating LWR and transmuter fuels..."
I note that some of these conclusions were also reached independently
committee of the European Commission (Pooley 1997), and by Panofsky
The ideas herein were developed during
conversations with many people. I particularly want to thank Dr Yuri
Petrov for his personal hospitality on several
occasions and Los Alamos National Laboratory for being my host for 2
March 1998. I would also like to thank A. Birkhofer, Robert Budnitz and
Panofsky for useful comments upon a draft.
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1. The refusal of the Unites States Senate in
June 1998 to
vote on high level waste storage facilities in Nevada is an example
of this trend.
2. The first electricity producing reactor at
Peach Bottom was
a gas cooled graphite reactor. A larger reactor at Fort St Vrain
in Colorado was built and operated (intermittently) for a while.
However it proved difficult to keep in operation for a variety of
engineering reasons. Several still larger reactors in the 1000 MWe
range were ordered in the 1970s but these orders were canceled by
Gulf General Atomic in 1978 (and $500 million in penalties paid)
and G.A. then got out of the business. Attempts to start again
have always been conditional on large subsidies.
3. England had a number of gas cooled graphite
reactors, the MAGNOX type and the Advanced Gas Cooled Reactors
(AGR). Many still operate. However England made a decision to
switch to the Pressurized Water Reactors around 1982 and also
abandoned the experimental helium cooled reactors at Winfrith
4. Experimental work on a helium cooled reactor
Julich for some time after it was abandoned in USA and UK but has
ceased by now.