Reliability of accelerator driven systems

I was the chairman of the committee that recommended to the DOE that the Accelerator Driven Systems (ADS) approach to dealing with the long-lived component of spent nuclear fuel be terminated. It was, indeed, an accelerator issue but not the one implied in your article ("Taking the Heat out of Nuclear Waste," February 2012). Superconducting systems were already in use and everyone believed that they could be scaled up to the many-megawatt beam level required for ADS to work. At Los Alamos, the most difficult part, the low-energy front end, had been built and tested. The issue was reliability.

The ADS proposal as it was back then was to fission all of the actinides including the plutonium, not just the minor actinides, americium, neptunium, and curium. Including the plutonium made the system so expensive that the only way to make it cost effective was by selling the electrical power. That meant that such systems were going to put gigawatts of electricity on the grid. At that level, frequent power trips would be too disruptive to tolerate. The best of the accelerators we have now trip many times per day and it would have been chaos if that much power went off in the blink of an eye at unexpected times.

What has changed is not the accelerator technology but the ADS scheme. Beginning with the Japanese, the emphasis has shifted to only fissioning the minor actinides and using the plutonium either as MOX fuel or in breeder reactors. This reduces the amount of material by about a factor of 10 and makes the system economic without putting large amounts of power out on the external grid.

There are still important safety issues. One of them is related to the frequency of accelerator trips. Frequent starting and stopping of a reactor, even a subcritical facility driven by an accelerator, stress the reactor. The standard fission reactors we use today trip very infrequently and each is investigated to find out why before permission to restart is given. I know of no analysis of allowable trip frequency versus down time that would be acceptable. There are a few early versions, but none has been through the kind of hardnosed peer review that our regulators would require. Clearly an outage of one second does not change temperature much and would not be a problem. Outages of minutes would begin to be.

I also note that Fukushima's problem was not stopping the fission reaction; all the control rods went in and the reactors were off. The problem was heat generated by the decay of fission products. Failure of cooling melted the cores even though the reactors were "off" and would also do so in a subcritical ADS.

Burton Richter, SLAC

Two scientists quoted in the original article—Eric Pitcher from Los Alamos National Laboratory and Stuart Henderson from Fermi National Accelerator Laboratory—respond:

Dr. Richter makes good points about the motivations behind the cessation of US Department of Energy (DOE) funding for Accelerator Driven System (ADS) research a decade ago. While there was already good confidence at that time that multi-megawatt-scale accelerators could be constructed, the advances made in the last decade in building and operating the megawatt-class superconducting linear accelerator and spallation target system at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory has certainly increased confidence that we can build and operate accelerators of sufficient power to meet the ADS application.

In fact, a working group was formed in 2010 at the request of the DOE to assess the readiness of accelerator and spallation target technology for ADS [1]. That working group concluded that the technology has advanced substantially over the last 15 years and is now capable of supporting an ADS demonstration capability in which a megawatt-class proton accelerator is coupled to a subcritical core, such as is being pursued in Belgium for the MYRRHA project. It also concluded that for industrial-scale transmutation requiring tens of megawatts of beam power, many of the key technologies are in hand, but demonstration of some components, improved beam quality control, and demonstration of highly-reliable sub-systems is required.

Regarding the question of accelerator reliability, SNS operation has demonstrated the ability to compensate for lost linac cavities by adjusting the radiofrequency voltage and phase in neighboring cavities, which enhances reliability. Indeed, this capability is one of the key features that motivates worldwide interest in superconducting linear accelerators for ADS. SNS performance has also demonstrated that, at one megawatt, beam losses are well within acceptable limits. While beam losses are difficult to accurately estimate, the experience gained with SNS operation lends confidence that beam losses will remain acceptable at the higher powers in excess of 10 megawatts that are needed for ADS applications.

Dr. Richter is correct that current accelerator reliability is insufficient to meet the requirements needed for reliable delivery of electricity to the grid, and research and development of higher reliability accelerator systems are needed to meet this mission. However, a study assessing the ramifications of beam trips on cyclic thermal stresses in components of the subcritical core and primary coolant loop of an accelerator-driven system concluded that allowable trip rates are within the range of what a new accelerator should be able to meet [2]. The working group considered the accelerator reliability requirements for the range of ADS missions, and concluded that the demanding requirements needed for transmutation could be met with further R&D and with the incorporation of redundancy and modern reliability engineering principles to the design of the accelerator system, something which has never before been done for a high-energy particle accelerator. In this respect, transmutation is a more realistic near-term mission than electricity production.

That ADS is best suited as a minor actinide burner has been recognized by some for a decade now [3]. Indeed, the studies performed by the DOE a decade ago [4] did not include this option. Perhaps it is time for the DOE to reevaluate the contributions that ADS could make to the nuclear fuel cycle, in particular to the destruction of minor actinides.

As Dr. Richter points out, failure to remove the substantial decay heat that precipitated the accident at the Fukushima Daiichi power plant is a concern for both critical (traditional nuclear reactor) and subcritical (ADS) cores. The ability to safely and reliably remove a core's decay heat under all plausible accident scenarios will certainly be a design requirement for ADS.

1. H. Ait Abderrahim et. al., "Accelerator and Target Technology for Accelerator Driven Transmutation and Energy Production."
2. H. Takei et. al., "Comparison of Beam Trip Frequencies Between Estimation from Current Experimental Data of Accelerators and Requirement from ADS Transient Analysis," Proc. OECD/NEA 5th Workshop on Reliability and Utilisation of High Power Proton Accelerators, 2007, Mol, Belgium.
3. "Accelerator Driven Systems (ADS) and Fast Reactors in Advanced Nuclear Fuel Cycles," 2002.
4. G. Van Tuyle and Ph. Finck, "Candidate Approaches for an Integrated Nuclear Waste Management Strategy–Scoping Evaluations," LA-UR-01-5572, September 2001.