Burn Up the Nuclear Waste

by John G. Cramer

Alternate View Column AV-79
Keywords: radioactive nuclear waste long halflife fission product proton induced fission
Published in the July-1996 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 12/5/95 and is copyrighted ©1995 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

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Disposing of nuclear waste, the massively radioactive leftovers from nuclear power reactors and from W.W.II and Cold War era nuclear weapons programs, is widely considered in the USA to be an insoluble problem. It is basically a problem of time scales and politics. Many of the radioisotopes in nuclear waste require many thousands, even millions of years to decay naturally to an acceptable level of radioactivity and appears to need a disposal solution that covers such a geological time scale, while our political processes are geared to a time frame set by the date of the next election.

Nevertheless, the problem of nuclear waste disposal is not insoluble. Good technical solutions have existed for at least 20 years, but for political reasons none has ever been implemented. This could be changing. New accelerator-based technical solutions are perhaps more politically acceptable. This column is about this emerging technology.

Before discussing solutions, however, let's consider the problem . The intense radioactivity of the waste produced in a nuclear reactor comes from two sources: (1) light nucleus fission products consisting mainly of elements 38 (strontium) through 67 (holmium) and (2) transmuted heavy nuclei consisting mainly of actinide elements 90 (thorium) through 96 (curium).

The light nucleus component of nuclear waste is made when uranium or plutonium reactor fuel splits (or fissions) with considerable energy release into two lighter nuclei. These fission products typically have too many neutrons for stability, making them highly radioactive as their excess neutrons are converted to protons through beta decay (emission of an electron and an anti-neutrino).

The heavy nucleus component of nuclear waste is made when the uranium or plutonium reactor fuel is transmuted to heavier elements by neutron-induced reactions that do not lead to fission. The resulting heavy nuclei tend to be radioactive through the process of alpha-decay (emission of a helium-4 nucleus). They decay toward stability in a linked sequence of alpha and beta decays called a "decay chain", the completion of which requires about 100 million years.

The decay rate of a radioactive nucleus is specified by its half life, the time required for half of a sample of radioactive nuclei to decay. It is a remarkable accident of nature that nuclear waste contains no important fission product isotopes with half lives between 90 years and 70,000 years. Therefore nuclear waste can be considered to consist of a heavy nucleus actinide component A, a short-lived light nucleus component B with half-lives less than 90 years, and a long-lived light nucleus component C with half-lives greater than 70,000 years. The radioactivity of component A is dominated initially by the actinides ²⁴⁴Cm (half-life 18 years), ²⁴³Am (7,380 years), ²³⁸Pu (87 years), and ²⁴¹Am (432 years). Component B is dominated initially by the fission fragment isotopes ¹³⁷Cs (half-life 30 years), ⁹⁰Sr (29 years), ¹⁰⁶Ru (1.0 years), and ¹³⁴Cs (2.1 years). Component C is dominated by the fission fragment isotopes ⁹⁹Tc (half-life 214,000 years), ⁹³Zr (1,500,000 years), ¹³⁵Cs (2,300,000 years), and ¹²⁹I (15,700,000 years).

When the nuclear waste is first produced, the radioactivity from component B produces about 99% of the overall activity level, the radioactivity from component A is about 1% of the overall activity level, and the radioactivity from component C is only about one millionth of the overall activity level. After about 300 years the short-lived isotopes of component B decay away. After this the radioactivity from component A is the dominant source of radioactivity and is always greater than that of component C by a factor of 10 to 100.

The 20 year old prescription for disposing of nuclear waste is straightforward.
(1) Perform a chemical separation on the waste which extracts the actinides of component A, all of which have similar chemistry and can be readily separated from the rest of the nuclear waste.
(2) Glassify the remaining B and C components by fusing them into a ceramic material and store them in an underground storage site for several hundred years. And finally
(3) concentrate the A component in reactor fuel pellets and "burn" the actinides in a breeder reactor or other intense source of fast neutrons.

With this strategy the problem of nuclear waste disposal is simplified, since in about 300 years the B waste component will be "cold", leaving only the C component which has an activity level that is a million times lower than the initial level of the waste. Perhaps more relevant, the cooled waste is far less radioactive than the rock from which the uranium reactor fuel was originally mined or the mill tailings generated when the uranium ore was processed and refined. Therefore, political concerns about the integrity of a storage site can be limited to a time scale of a few hundred years.

The A component of the waste was separated out because the actinide decay chains produce a level of radioactivity that is ten or more times larger than that of component C, that continues to decay for millions of years, and that is a severe health hazard if ingested or inhaled. In a high flux of fast neutrons, however, the actinides have a high probability of transmutation leading to fission, and thus can be "burned" to eliminate the A waste, producing energy in the process.

This scheme, unfortunately, is not currently feasible in the USA. The barriers to implementing it were erected in the 1970's by Cold War politics. During the Carter Administration (1976-80) the chemical reprocessing of spent nuclear fuel was considered a technology that encouraged nuclear weapons proliferation (by extracting from spent fuel rods plutonium that might be used to make bombs). President Carter therefore ordered the U. S. program for chemical reprocessing of nuclear fuels terminated "to set an example for the rest of the world". At about the same time, the U. S. program to develop a high flux fast-neutron breeder reactor at Savannah River was terminated for similar reasons. The net result was that the two key technologies needed for effective disposal of nuclear waste were eliminated by political decisions. Neither technology has ever been restarted in this country, although France and Japan now have active programs of both nuclear fuel reprocessing and breeder reactor development.

The breeder reactor is so named because its high flux of fast neutrons transmutes ²³⁸U to ²³⁹Pu reactor fuel faster than it consumes its ²³⁹Pu fuel, thereby making more fuel than it uses. It has proven to be a difficult technology. To achieve the needed neutron flux, a breeder must operate at a very high temperature, so high that it cannot be cooled by boiling water. The French and Japanese breeder reactor designs both use liquid sodium metal as a coolant, a solution which looks good on paper but has proved troublesome in practice because sodium is a very reactive metal that ignites on contact with air or water. France and Japan have both had problems with the liquid sodium plumbing in their breeder reactors. Just last month, for example, a Japanese breeder prototype spilled several tons of non-radioactive but chemically hyperactive sodium inside a reactor containment building.

Since the USA has neither the chemical technology for separating out the actinide A component of nuclear waste nor a high flux fast neutron breeder reactor in which to burn it, the only remaining alternative is to store the unprocessed nuclear waste underground for many thousands of years. This alternative has met with massive opposition, particularly from states in which the nuclear waste repository might be located and from individuals who live near the selected storage sites. The result is a political stalemate in which no progress or decisions are being made, while spent fuel rods pile up in temporary storage areas at about 100 operating reactor power stations.

This brings us to the new approach to the problem of nuclear waste disposal. It does not require a breeder reactor, and instead uses a particle beam from an accelerator. The idea is to direct accelerated particles, for example 1 GeV protons, into a small specially designed sub-critical nuclear reactor in order to produce a "hot spot" supplying a high flux of fast neutrons that makes the reactor super-critical. Neutrons produced by spallation of heavy nuclei by the particle beam are "amplified" in surrounding uranium or thorium, leading to a "hot " region near the particle beam where the density of neutrons is high enough to burn actinides. While a breeder reactor generates heat and fast neutron flux that is broadly distributed over the whole reactor core, the particle-beam activated reactor produces heat and a very high flux of fast neutrons in a localized region. This makes it easier to cool, greatly simplifying the design.

The advantages of this accelerator-based approach over a breeder reactor are:
(1) it is cheaper to design and construct,
(2) the heat is localized and more easily cooled,
(3) the process is very stable and immediately ceases when the particle beam is interrupted,
(4) localized neutron fluxes can be considerably larger than those present in a breeder reactor, and
(5) it can be fueled with thorium and other materials that cannot be used ot build nuclear weapons.

The design of such systems has been a "hot item" at conferences. At a recent meeting in Susono, Japan (see reference below), independent groups from Russia, Japan, and Brookhaven National Laboratory, and Los Alamos National Laboratory all described investigations of variants of this scheme, usually involving high-current proton beams with energies around 1 GeV. Production of such a beam would require construction of a fast-cycling synchrotron accelerator costing perhaps a hundred million dollars, quite cheap compared to the multi-billion dollar cost of a breeder reactor. Published simulations have demonstrated the feasibility of nuclear waste transmutation using this scheme. The process could even achieve "energy payback" by generating enough electrical energy to operate the accelerator, with a surplus to sell to commercial power companies.

This technological development does not solve the whole problem, of course. Chemical processing of nuclear waste would have to be re-established in this country so that the actinide component of the waste could be concentrated for burnup. In the present post-Cold-War political climate, however, there is less concern about setting an example and more concern about disposal of the great surplus of plutonium left over from Cold War Era bomb production, material which could also be burned in an actinide disposal facility. This climate change should make it easier to reestablish a reprocessing facility in the USA.

There remains, of course, the overall problem of the public perception of nuclear energy. Any potential exposure to radiation can produce a strong public reaction, even an exposure that is many time weaker than the natural background radiation in which we all live. The association of nuclear power generation with nuclear weapons production and with the devastating Chernobyl accident has been firmly implanted in contemporary thinking. And the inability of the news media, the U. S. Congress, and a large segment of the general public to deal rationally with issues involving probability and risk assessment is a very serious human psychological limitation. A practical consensus solution to the problem of nuclear waste disposal through burnup might go some distance toward rehabilitating nuclear energy as a viable energy source, but it would not solve these fundamental problems.

Perhaps this will diminish only with the dawning realization that the carbon dioxide produced in burning massive quantities of coal, oil, and natural gas for power generation is a major contributor to the progressive rise in the CO₂ fraction in the atmosphere, now generally acknowledged to produce global warming. Solar energy, geothermal energy, and wind energy, after two decades of subsidy and major DOE investment in research and development, have proved to have negligible potential for large-scale power generation. Hydroelectric power generation has few new sites available and brings the unwanted side effects of dam silting, damage to the natural environment, and devastation of wild rivers and salmon and trout spawning runs. Nuclear fusion is still many decades away from any practical application.

Nuclear power generation technology stands out as the only presently viable technology for large scale power generation that does not contribute to the problem of global warming, but it is only viable if the waste disposal problem can be solved. The emerging technology of accelerator-controlled waste burnup is perhaps a significant step in that direction.

References:

High Level Nuclear Waste:
"High-level radioactive waste from light-water reactors", B. L. Cohen, Reviews of Modern Physics 49, 1 (1977).

Particle-Beam Nuclear Waste Burning:
Proceedings of the International Symposium on Global Environment and Nuclear Energy Systems, Susono, Japan (October 24-27, 1994), published in Progress in Nuclear Energy 29, (1995).

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This page was created by John G. Cramer on 7/12/96 and revised on 2/5/97.