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Advancing the interests of Australia and Australians internationally

Australian Government - Department of Foreign Affairs and Trade

Advancing the interests of Australia and Australians internationally

Australian Safeguards and Non-Proliferation Office

Annual Report 1999-2000

Nuclear Waste ManagementPartitioning And Transmutation

Introduction

Naturally occurring radioactive materials such as uranium and thorium are common elements within the earth's crust. Natural background radiation from these elements and other sources has always been present and is a constant feature of life on earth. Spent fuel from a nuclear reactor is highly radioactive, but over time this radiation decreases and becomes less significant. It would take many thousands of years for the radiation from the spent fuel to fade away completely, but once it reaches the same level as natural background radiation it no longer needs to be kept separate from the biosphere.

If spent fuel is directly disposed of without reprocessing it will remain more radioactive than the corresponding quantity of uranium ore for over 30,000 years. The principal objective of spent fuel reprocessing is recovery of plutonium and uranium for recycling as reactor fuel. There are also substantial waste management advantagesremoval of plutonium and uranium reduces the period in which the remaining high level waste will be more radioactive than the corresponding uranium ore to around 2,000 years[11].

2,000 years of course is still a significant period. While studies of natural areas of high radioactivity, such as the Oklo natural reactors in Gabon[12], and ore bodies in the Alligator Rivers Region in the Northern Territory, show that radiotoxic elements can be immobilised and isolated from the biosphere for many hundreds of thousands of years, nevertheless it would be advantageous to reduce the period of high radiotoxicityif for no other reason, to establish public confidence in waste management programs.

Accordingly, a number of countries have been carrying out research into the possibility of partitioning radioactive isotopes from high level waste. Partitioningin the context of spent fuel management, refers to the processes that provide efficient separation of long-lived radioactive isotopes (fission products and minor actinides) from spent fuel and/or high level waste for further treatment and disposal.

If, for example, reprocessing of spent fuel is modified to remove some of the minor actinides, such as neptuniumand americium, then the remaining waste will decay to a radioactivity level similar to uranium ore in 1,000 years. If the process is further refined to also remove certain long-lived fission products, the waste will decay to a radioactivity level similar to uranium ore in about 500 years.

Partitioning of minor actinides and fission products will be more advantageous if there is a further process in place for treating these elements to reduce their half-lives. Hence the concept of transmutationthe return of the materials to reactors for transmutationthrough fission or neutron captureinto elements with shorter half-lives. In other words, transmutationrefers to the process of gaining a substantial reduction in the period over which waste arising from nuclear energy remains highly radiotoxic, by using the neutron flux within a reactor or other intensive source of neutrons to turn (transmute) long-lived radiotoxic elements into short-lived or stable elements. This transmutation step can substantially decrease the time needed to render the partitioned material harmless.

Efficient transmutation requires fast neutrons (neutrons not slowed down by a moderator). As there is only limited availability of fast neutrons in thermal reactors (such as light water reactors), research into partitioning and transmutation arose in the context of expectations of the early deployment of fast breeder or other fast neutron reactors. While the delay in the introduction of fast neutron reactors has led to some diminution of interest in partitioning and transmutation in the short term, nonetheless it is a concept of considerable promise for the futureand for example is the basis of the Russian concept of a transmutational fuel cycle (on page 68).

Neptuniumand americium

Two of the materials of interest for partitioning and transmutation are neptunium and americium. Since these are fissionable materials (i.e. they can be fissioned by fast neutrons), recycle in a fast neutron reactor would have the advantage that they would contribute to the energy production in the reactor, in other words they would be a useful component of the reactor fuel.

Neptunium and americium are produced in very small quantities in irradiated fuel. Typically (depending on the irradiation history) reactor spent fuel would contain about 1 gram of neptunium for every 20 grams of plutonium. Americium is produced in irradiated fuel at a lower rate, roughly one quarter as much as neptunium, and also arises in separated plutonium or spent fuel through decay of the isotope plutonium-241.

Because neither material is fissionable by thermal neutrons, to date there has been limited use for neptunium or americium, and generally they are not separated from fission products: they are either contained within spent fuel or, if reprocessing is undertaken, mostly end up in the waste stream. Both materials have been separated in significant quantities only by the nuclear-weapon States (mainly the US and Russia) for specialised applications. Separated neptunium is used for the production of plutonium-238, which is used in thermo-electrical generating systems for satellites and heart pace-makers. Separated americium is widely used in smoke detectors. Both materials are also used as industrial radioisotopes, e.g. in borehole logging equipment and in instruments for measuring the thickness of processed metals.

Only very small quantities of neptunium and americium have been separated in the non-nuclear-weapon States. Separation in significant quantities would require substantial quantities of spent fuel and a reprocessing programthere are few NNWS in this situation, and there has been no incentive to separate these materials, because the tiny amounts required for research or for the commercial applications mentioned above have been available from NWS. Nonetheless, because these materials are fissionable, and because of ongoing research into their possible separation for transmutation, in recent years interested States and the IAEA have been considering how they should be managed from the safeguards perspective. ASNO identified this issue early on and has played an active part in the ensuing deliberations.

The matter was considered by the IAEAs Board of Governors in September 1999. In the case of neptunium, the Board decided it is of little proliferation risk in current circumstances, where there are only very small quantities of separated neptunium in the NNWS. The Board decided to establish arrangements to monitor international transfers of neptunium and to verify there is no undeclared separation of neptunium in NNWS. If a significant change in the current situation appears likely the Board will consider the matter further, including whether formal extension of safeguards to neptunium is warranted. The Board considered that the proliferation risk posed by americium is even lower than for neptunium. Not only are there very limited quantities of separated americium in NNWS, but major heat and radiation problems would make any attempted explosive use extremely difficult. Accordingly, the Board asked the IAEA Secretariat to keep the situation under review and report to it if appropriate.

Australia agrees with other Board Members that this is a pragmatic approach in current circumstances, considering the limited quantities of these materials in separated form in NNWS and considering also the uncertainty that significant quantities will be separated in the future. Delays in the development of fast neutron reactors obviously impacts on the interest in separating these materialsand if transmutation programs do proceed, it is possible transmutation could be effected without actually separating the materials, e.g. they could be separated from fission products but remain in stream with plutonium and uranium, covered by the safeguards measures on those materials.

Since all spent fuel contains neptunium and americium, clearly a proportion of these materials in spent fuel is derived from AONM. Accordingly Australia has discussed this matter with relevant bilateral partners, i.e. those that reprocess AONMUK, France and Japan. Discussions have also been held with the US and with the IAEA. Through these discussions ASNO has established that no neptunium or americium has been separated from AONM. The situation will be kept under review, and Australia will take an active part in any further IAEA Board consideration of this matter. While extension of our bilateral agreements to include these materials is a possibility if they become safeguardable materials, this is not expected to occur for many years, if at all.

[11]. Time periods taken from Radioactive Waste ManagementAn IAEA Source Book, 1992 (figures 7 and 8).

[12]. The Oklo natural reactors evolved 1.8 billion years ago, at a time when the content of the fissile isotope uranium-235 in natural uranium was much higher than it is todayaround 3%, similar to the level in LEU used in light water reactors. Water saturation of the uranium ore bodies created the conditions for a self-sustained chain reactionthe resulting heat evaporated the water, bringing the chain reaction to a halt. This process repeated itself over many thousands of years, creating natural deposits of fission products and plutonium normally found only in spent reactor fuel. The movement of these radiotoxic materials through the ore bodies has been limited to only a metre or so, providing practical evidence that such materials can be successfully isolated for periods well in excess of that necessary for the protection of the biosphere.

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