The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

[http://nextgenerationnuclearplant.com/index.shtml] by the U.S. Department of Energy
uses helium coolant. The technology for gas-cooled reactors is available today at temperatures
up to ~850°C. Recently there has been work on using low-pressure liquid salts as coolants—the
Advanced High-Temperature Reactor. Recent studies of the liquid-salt cooled
AHTR (Appendix B) indicate the potential for lower costs (Peterson, Griveau), but the
technology is not as fully developed. Most HTR R&D supports the use of either coolant.
The coated-particle fuel has four potential unique characteristics that may provide major
societal benefits and thus the incentives for the federal government to encourage the development
of such a reactor. These benefits have not been fully quantified or proven.
Safety
The high-temperature capability of the fuel enables a different approach to reactor safety
that may offer major benefits. In traditional LWRs, if a large reactor is shut down and cooling
to the reactor core stops, the reactor core will heat up and melt. This is what occurred
during the Three Mile Island accident. HTRs can be designed such that if the reactor cooling
fails, the heat can be transferred by conduction and convection processes to the environment.
This is possible because (1) the fuel can go to very high temperatures without failing
and provide a very large temperature drop to enable heat transport to the environment
and (2) the low power density [dilute fuel] that results in the very slow heatup of the reactor
core after shutdown. In effect, many safety functions are transferred from the reactor
(emergency safety systems) and the reactor operator to the fuel fabricator who is required
to make a fuel that can withstand extreme conditions.
Safeguards and Nonproliferation
The two nonproliferation concerns associated with the nuclear fuel cycle are: (1) uranium
enrichment on the front end that could provide a route to producing weapons-useable
high-enriched uranium and (2) plutonium separation from SNF on the back end. HTRs
require low-enriched uranium and thus have most of the same front-end concerns as associated
with LWRs. However, HTR SNF is different from SNF of other types of power
reactors. The plutonium content of HTR SNF from currently proposed reactors would be
so low as to approach the International Atomic Energy Agency limits for the termination of
safeguards; that is, the difficulty of fissile material recovery is so great that anyone wanting
to obtain weapons-useable materials would likely choose an alternative route to obtain such
materials (Durst 2009). There are several reasons for this (Moses, 2010).
p Burnup. HTR SNF burnup is typically 50% higher than LWR SNF resulting in less attractive
plutonium isotopic mix for weapons.
p Plutonium content. The SNF plutonium concentration is low (~570 ppm for some designs)
and because the fissile plutonium is diluted with carbon, silicon carbide, and
graphite. One must divert almost 20 m 3 of SNF to have a significant quantity of plutonium—the
quantity of concern in terms of building a nuclear weapon.
p Chemical form. The technical and economic difficulties in recovering fissile materials
from HTR SNF are significantly greater than for other types of SNF because (1) the fissile
210 MIT STudy on The FuTure oF nuclear Fuel cycle