NRC Briefing: Severe Accidents and Level 3 PRA

The staff of the US Nuclear Regulatory Commission today held a briefing for the Commissioners on Severe Accidents, and Options for proceeding with Probabilistic Risk Asessment - Level 3 (PRA Level 3).

Traditionally, PRA/PSA Level 3 has not been a strong regulatory requirement because the results of Level 1 (usually the core damage frequency, CDF) and the results of Level 2 (large early release fraction/frequency, LERF) can be used as surrogate proxies for the types of Risk Metrics that a Level 3 PSA/PRA might generate, which could include the following: the number of early fatalities; the number of early injuries, the number of latent cancer fatalities, or the total population dose at different locations; as well as the individual early and latent fatality risk, and the economic cost of mitigation actions taken following a severe accident. The CDF, for example, can proxy for the latent cancer risk, while the LERF could proxy for the prompt fatality risk.

However, a number of potential benefits are foreseen for a full PRA Level 3 analysis, including feedback into risk-informed regulatory guidance for new reactors and use of risk insights in forthcoming SMR design reviews. In addition, capabilities such as modeling of radionuclide aqueous dispersion modes, and multi-unit risk assessment could also be addressed. The modeling of radionuclide dispersal in the event of a severe accident which is initiated by an external event such as an earthquake, tsunami or hurricane also calls for additional modeling efforts, since meteorological variables such as windspeed & direction, ambient precipitation and humidity may not correspond to what is normally expected for that site at that time of year (for example).

NRC Staff plan to use an existing SPAR (Standardized Plant Assessment Risk) model as the basis for proceeding to PSA Level 3. The SPAR model is essentially a plant-specific PSA/PRA Level 1 designed to incorporate both external and internal initiating events, recent modifications of which include capabilities to yield LERFs. Since external hazards are site-specific, much greater value can be expected to be derived if the SPAR model selected for development to PRA Level 3 is for a NPP site that is either representative of the entire population of NPPs, or, has a larger than average number and type of external hazards.

Small and Modular Reactors

Small and modular nuclear reactors (those with a thermal power output below 200 MWTh) have become of strong interest, both in Canada and worldwide for a number of reasons. In Canada, the interest arises from the following sources:
(i) The need to replace the NRU (National Research Universal) reactor with another multipurpose research reactor, as recommended by the NRCan Expert Review Panel on Medical Isotope Production last year [Recommendation I, p. xi Executive Summary; also on p. 73 of the main body of the report]
(ii) The interest expressed by energy providers (as well as industrial users such as in mining and tar sands extraction) in off-grid electric and/or thermal process power in modular and scalable units - partly from remote siting considerations and also from emissions reduction considerations
(iii) University nuclear reactors for training and research
(iv) Reactors for dedicated medical radioisotope production.

Separately of the off-grid power reactor interest from resource extractive industries, there is also interest in small reactors as a possible solution for developing countries and first-time nuclear countries who have small or under-developed electric grids. They are also an attractive option for small gas- or coal- fired generating units as a direct replacement, where grid and transmission infrastructure already exist, as in rural areas of developed nations.

As well, the lower up-front capital cost of the smaller power reactors is a motivating consideration for the increased interest, as is the potential for upward scalability in total power output by addition of more units in a more graded manner. Given the lower radionuclide inventory as well as some passive safety features in some of the small reactor designs, they become of additional interest from both the safety and the proliferation-resistance standpoints.

Although some reactors have a thermal output as low as 20 KWTh, the 200 MWTh threshold is chosen to define the upper limit of 'small reactors' from the point of view of accumulation of the radionuclide inventory - which is much smaller below a threshold of 200 MWTh. As well, given that some reactors may have passive safety features, the balance of engineered safety requirements that are imposed could be different for smaller reactors than for large reactors. Consequently, it is possible to justify what has come to be called a 'graded approach' in the safety assessment of small reactors - a smaller reactor will have safety requirements commensurate to the relative risk, compared to a larger reactor, and not necessarily identical ones. This graded approach could reflect itself, for example, in the containment structure requirement, or in extent of the exclusion zone, where the regulatory requirement may not necessarily be identical to that for large power reactors.

The Canadian Nuclear Safety Commission (CNSC) is currently in the process of developing regulatory & licensing guides and related requirements for small nuclear reactors based on these considerations, and will be holding appropriate stakeholder consultations, information sessions and technical workshops during the course of this year to disseminate information and solicit feedback before finalizing the requirements.

Postscript
US Secretary of Energy Steven Chu outlined the interest in small and modular reactors in his Wall Street Journal op-ed on March 23, 2010, a summary of his Congressional testimony of 3-3-2010.

In his 2011 budget request, President Obama requested $39 million for a new program specifically for small modular reactors. Although the Department of Energy has supported advanced reactor technologies for years, this is the first time funding has been requested to help get SMR designs licensed for widespread commercial use.

Right now we are exploring a partnership with industry to obtain design certification from the Nuclear Regulatory Commission for one or two designs. These SMRs are based on proven light-water reactor technologies and could be deployed in about 10 years.

Expanding on the likely advantages of small modular reactors, he said:

Small modular reactors would be less than one-third the size of current plants. They have compact designs and could be made in factories and transported to sites by truck or rail. SMRs would be ready to "plug and play" upon arrival.

If commercially successful, SMRs would significantly expand the options for nuclear power and its applications. Their small size makes them suitable to small electric grids so they are a good option for locations that cannot accommodate large-scale plants. The modular construction process would make them more affordable by reducing capital costs and construction times.

Their size would also increase flexibility for utilities since they could add units as demand changes, or use them for on-site replacement of aging fossil fuel plants. Some of the designs for SMRs use little or no water for cooling, which would reduce their environmental impact. Finally, some advanced concepts could potentially burn used fuel or nuclear waste, eliminating the plutonium that critics say could be used for nuclear weapons.
[...]
To achieve this potential, we are bringing together some of our nation's brightest minds to work under one roof in a new research center called the Nuclear Energy Modeling and Simulation Hub.

Update The Consortium for Advanced Simulation of Light Water Reactors (CASL), the Energy Innovation Hub specific to Nuclear Energy, has been formed, with Dr. Douglas B. Kothe as Director. The Core partners are Oak Ridge National Lab (ORNL), Electric Power Research Institute (EPRI), Idaho National Lab (INL), Los Alamos National Lab (LANL), Massachusetts Institute of Technology (MIT), North Carolina State University (NCSU), Sandia National Labs, Tennessee Valley Authority (TVA), University of Michigan, and Westinghouse Electric Company.

The operational structure and mission statement of CASL explicitly incorporates the vision Prof. Chu has articulated, for example, of 'Bell Labs-like institutions which are mission-driven but solve fundamental problems as well'. See here.

In CASL Director Dr. Kothe's words, (CASL):

• Focuses on a single topic, with work spanning the gamut, from basic research through engineering development to partnering with industry in commercialization
• (creates) Large, highly integrated and collaborative creative teams working to solve priority
technology challenges
• Embraces both the goals of understanding and use, without erecting barriers between
basic and applied research (emphasis added).

A second Energy Innovation Hub also announced is the Joint Center for Artificial Photosynthesis (JCAP), a partnership between Caltech and Lawrence Berkeley Laboratory:

JCAP research will be directed at the discovery of the functional components necessary to assemble a complete artificial photosynthetic system: light absorbers, catalysts, molecular linkers, and separation membranes. The Hub will then integrate those components into an operational solar fuel system and develop scale-up strategies to move from the laboratory toward commercial viability. The ultimate objective is to drive the field of solar fuels from fundamental research, where it has resided for decades, into applied research and technology development, thereby setting the stage for the creation of a direct solar fuels industry.