(Murray, 2003)
Fission products 28.8 kg
Pu 0.04 Np 0.48 Am 0.14 Cm 0.04 Reprocessing chemicals 68.5 Reprocessing wastes
The weight of reprocessing waste is about one-tenth of the weight of spent nuclear fuel. Sr-90 and Cs-137 are the major problems during the first few centuries of waste storage. Can they be eliminated from high- level waste? This will be discussed later. For now by definition, reprocessing wastes are high-level waste.
Reprocessing wastes include aqueous/nitric acid solutions that contain fission products such as Cs, Sr, Zr, Ni, La and others which are derived from spent nuclear fuel from military applications in the US. Because the US does not reprocess spent nuclear fuel, high-level waste treatment research has not been a major priority in the US. In general, these are high-level liquid wastes that are stored in underground tanks.
https://wiki.engr.illinois.edu/download/attachments/194283148/Waste+treatment.pdf?version=1&modificat ionDate=1330551702000 is a colorfully illustrated primer on radioactive waste treatment. It includes more on calcination, immobilizing, vitrifying and synthetic rocks. Once high-level waste is fixed into some type of wasteform, it may still leach into water of various temperatures, acidity or alkalinity, and with enough time.
Basic concepts of transmutation
Transmutation is defined as transformation of one isotope into another by neutron absorption. The products are either the next heavier isotope or two or more fission products.
Fissile is defined as fissionable by thermal neutrons. 235U is fissile whereas 238U is not. Energy production results in transmutation
235U + η → 236U* → fission products + η + β + γ
The fission products include 90Sr with a half-life of 28.8 years and 137Cs with a half-life of 30.1 years. And by neutron capture
238U + η → 239
U* → 239Np + β- → 239Pu + β- ( 23.5 min) (2.35 d) (24,400 y)
Transmutation as a curse and cure?
Transmutation creates waste management issues with respect to either once-through spent nuclear fuel or in reprocessing spent nuclear fuel. Can transmutation be applied to spent nuclear fuel to reduce its
radiotoxicity by converting radionuclides with long half-lives to ones that decay more quickly?
Some people think so. Several transmutation processes have been proposed. Take for example “The Roy Process.” In 1979, the late Dr. Radha Roy announced he “had invented a new method to render all radioactive waste elements, including plutonium, into non-radioactive elements.”
“With the Roy Process, high-level nuclear waste can be neutralized and totally eliminated at each reactor site, where the waste is now stored in cooling ponds. When treated with the Roy Process, these unstable radioactive isotopes rapidly decay into stable, non-radioactive elements . . .”
From: http://www.lightparty.com/Energy/RoyProcess.html http://www.youtube.com/watch?v=vEyMUBBGePQ
Realities of Transmutation as a Waste-Treatment Technology
Transmutation of persistent fission products: 99Tc + η → 100
Tc → 100Ru (2.12 x 105 y) (16 sec) (Stable) 129I + η → 130m
I → 130I → 130Xe (1.6 x 107 y) (9 min) (12 hours) (Stable)
These are examples of desirable reactions.
The process of transmutation can also initiate undesirable side reactions that produce new radionuclides with long half-lives. For example,
133Cs + η → 135 Cs (stable) (2.3 x 106 y) 241Pu + η → 242 Pu (13.2 y) (389,000 y) 35Cl + η → 36 Cl (stable) (3.1 x 105 y)
Some fission and activation products do not transmute significantly because their cross section for capturing thermal neutrons is too small. The term “cross section” is the probability of a nuclear reaction resulting in transmutation. Some of these products include 79Se, 126Sn, 36Cl, and 14C. This also
includes 90Sr (1.34 barns) and 137Cs (0.176 barns).
Transmutation cannot be applied to solid spent nuclear fuel. Because spent nuclear fuel contains 235U and 238
U, the addition of thermal or fast neutrons would produce more Pu which is not the goal. Transmutation must be coupled with chemical separation of the radionuclides into different wastes streams.
Separation and Transmutation
Under study:
Aqueous chemical separation (PUREX, UREX, TRUEX, etc.) followed by transmutation in light water reactors or fast breeder reactors.
Pyroprocessing separation followed by transmutation in light water reactors of fast breeder reactors.
Current research results
“SNF is placed into a cathode basket that is then immersed in a pool of molten LiCl-Li2O. When a sufficiently high electrical potential is applied, oxygen gas bubbles are evolved at the anode, and actinide oxides are reduced to metals at the cathode. Rare earth fission products appear to remain unreduced in the basket. Alkali and alkaline earth fission products (Cs, Sr, Rb, and Ba) partition into the salt, presumably as chlorides.” (Simpson, 2006)
Still have waste issues . . .
Pyroprocessing
“The accumulation of these alkali and alkaline earth fission products in the salt will require periodic disposal of the salt into a waste form that can be safely stored for approximately 200 years to allow decay of the 137Cs and 90Sr. Salt can be simply removed from the process once it reaches a contamination limit, blended with zeolite, and formed into a ceramic waste.” (Simpson, 2006).
Barriers to Separation and Transmutation
Separation requirements for transmutation:
U and Pu must be separated (PUREX).
Cs and Sr must be separated (under study).
Methods for separating Am, Cm, Np, and turning them into targets for transmutation are still at the experimental stage.
All extractions need to be optimized to extract nearly all of each radionuclide.
What is the best source of neutrons for separation and transmutation? Light-water reactors? Fast reactors? Breeder reactors? Coupled with accelerators? Accelerator Transmutation of Waste? Generation IV
reactors?
Source:
https://wiki.engr.illinois.edu/download/attachments/194283148/Waste+treatment.pdf?version=1&modificat ionDate=1330551702000 is a colorfully illustrated primer on radioactive waste treatment. Its topics
include:
Composition of spent nuclear fuel and reprocessed nuclear waste High-level liquid radioactive waste
French vitrification program
Ceramic wasteforms – ‘synthetic rock’
Realities of transmutation of radioactive waste