PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00078/1485
Reactor Transmutation of Waste
Rick J. Migilore
Technetium-99 and iodine-129 are long-lived fission products generated during the normal operation of a nuclear reactor.
Methods were studied on ways to transmute these radionuclides into nonhazardous forms and thereby eliminate the longterm
risks associated with their disposal in a repository.
Project Description
The purpose of this project is to determine the amounts of
technetium-99 (Tc-99) and iodine-129 (I-129) that may be
transmuted into stable isotopes using existing commercial
nuclear reactors. It would be advantageous to destroy
these long-lived isotopes, and to mitigate the associated
long-term risks of storing and containing them in the
Yucca Mountain repository. The computer program
Monte Carlo N-Particle (MCNP) transport code was used
for the transmutation calculations. We estimated that
approximately 63% of the projected Tc-99 inventory and
35% of the projected I-129 inventory could be transmuted
during the years 2010 through 2030 using existing light
water reactors. This concept appears to be feasible, as
commercial nuclear reactors can be designed to burn more
of these isotopes than they produce.
Introduction
Reactor transmutation of waste has been studied
extensively over the past 25 years using a variety of
calculational methods and reactor types (Wachter and
Croff 1980; Binney et al. 1990; Wootan et al. 1992;
Abrahams et al. 1995). The goal of waste transmutation
studies is to demonstrate that reactor waste can be
destroyed, thereby circumventing the risks associated
with disposing of the waste in an underground repository.
The spent fuel repository slated to be built at Yucca
Mountain must comply with federal regulations for a
period of 10,000 years, although the risks to the public do
not end after that time. Reactor transmutation of waste
would eliminate the long-lived fission products in the
waste stream, leaving only shorter-lived fission products
to be disposed. Reactor transmutation of waste would
reduce the required repository confinement lifespan to
500 years or less, because after that time the waste would
be no more radioactive than the original uranium ore.
Most transmutation studies include transmutation of
actinides (heavy metals such as plutonium, neptunium,
etc.) in addition to transmutation of fission products such
as Tc-99 and I-129. The current study focused only on
the transmutation of long half-life fission products Tc-99
and I-129 and assumed that all actinides are recycled.
Tc-99 and I-129 are transmuted to stable isotopes by the
following reactions:
99 Tc + n ➝ 100 Tc ➝ 100 Ru (stable) 16s (β - )
129I + n ➝ 130 I ➝ 130 Xe (stable) 12.4h (β - )
Further irradiation of Ru-100 and Xe-130 leads only to
stable isotopes. Xenon-130 is a gas and that the target
must be designed to accommodate the increase in pressure
from buildup of radioxenon.
Tc-99 is metallic and may be formed into slugs for
irradiation without difficulty. I-129, conversely, is a gas
and must be bonded with another material prior to
irradiation. For the purposes of this study, solid
vanadium iodide (VI2) was selected as the form to be
irradiated. Vanadium iodide was selected for both its
high density (5.44 g/cm 3 ) and high melting temperature
(1431°F). The stability of this compound in a true
radiation environment is unknown. Iodine compounds
may be corrosive to cladding materials.
Approach
The MCNP reactor physics code was used to calculate the
instantaneous transmutation rate for each of the nuclides.
A simplified 3000 MWt pressurized water reactor core
was modeled using 17x17 fuel assemblies. Each fuel
assembly has 24 locations that may be used for inserting
fission product target rods. To maximize the quantity of
fission product targets that may be inserted into the
reactor, all of the target locations are filled with target
rods. In reality, this would not be possible, as locations
must be retained for control rods. Six different cases
were executed.
Nuclear Science and Engineering 369