The 6-th International Conference

“Safety Assurance of NPP with WWER”

EDO “GIDROPRESS, Podolsk, Russia

26-29 May, 2009

PRESSURE TUBE REACTORS – SUSTAINABLE AND SECURE ENERGY NOW AND FOR THE FUTURE

Romney B Duffey

Atomic Energy of Canada Limited, Chalk River, ON K0J 1J0 Canada

Expectations of significant expansion in nuclear power programs worldwide are driven by concerns about energy security, sustainability and supply. Not only are all major economies dependent on competitively priced energy supply, there is a growing desire to improve resource utilization and diversity, without increasing atmospheric emissions.

Water-cooled pressure tube reactors (PTRs) provide a major answer. They are competitively priced with proven excellent operation, and fuel cycle options that are more energy efficient, can significantly reduce waste streams and ensure sustainable energy futures, and have fully replaceable major components. The developments in fuel cycles usually focus on fast spectrum reactors for the distant future. There are also many compelling reasons to continue to utilize advanced fuel cycles in thermal spectrum PTRs, including “closable” and sustainable cycles. Hence the PTR development path utilizes complementary designs that are directed not only at specific customers and markets, both now for meeting present energy needs and for addressing future environmental and sustainability requirements. Therefore, PTRs address the multiple requirements of energy security, competitive cost, excellence in operation, sustainable fuel cycles, reduced waste storage and streams, and assured licensability, with an assured smooth development pathway.

Using Atomic Energy of Canada’s (AECL’s) CANDU reactor as an example, the D2O/D2O (CANDU 6) system was optimised for natural and low enriched uranium (LEU) use to provide independence from uranium enrichment sources and hence supply surety, as a reliable and proven introductory unit, and which as a result has a slightly positive CVR and an extremely simple fuel design permitting many “burner” cycle options. The D2O/H2O Advanced CANDU Reactor (ACR) system is optimised for competitive power markets with lower capital cost and the levelized unit energy costs (LUEC), using LEU to provide a slightly negative CVR and higher efficiency, and is also able to use alternate fuel cycles (e.g. thorium and MOX) as resources shift in supply and cost. Finally, as the advanced “Generation IV” concept, CANDU “Ultra”, D2O/H2O variant (SCWR), optimises the development pathway for mass global deployment, with higher efficiency (50%), no core melt, size flexibility, cogeneration options and includes an alternate new fuel cycle (thorium), reduced licensing uncertainty.

By implementing in a smooth development pathway the PTR avoids switching the basic water-cooled nuclear technology but capitalizes on the extensive advances made in the thermal power industry. Experience of building each builds towards building the next. This is not standardization of design as pronounced by some to reduce costs and uncertainty: it is learning from experience as an essential element of the “learning curve”.

Introduction: Sustainable Energy

World wide, there is renewed interest in nuclear power as a result of concerns about climate change, pollution, energy security, and cost and availability of fossil fuels. Nuclear power program decisions will be increasingly based on political, strategic and economic considerations involving the complete nuclear fuel cycle, including resource utilization, radioactive waste disposal, proliferation resistance, and supply assurances. Long-term growth will depend on following a path that addresses these issues by focusing on advanced fuel cycles and reactor designs optimized for such fuel cycles. The overall direction of power reactor development programs should be refocused to provide a greater emphasis on integrated and complementary reactor and advanced fuel cycle development, including the optimized development and deployment of fuel enrichment, and recycling technology and services, while at the same time enhancing reactor safety.

Many nations are interested in attaining and obtaining the technology and know how sufficient to sustain a domestic nuclear power capability into the indefinite future that is independent of current technology and restrictions, while still respecting non-proliferation and sensitive technology issues. This requires new and novel international collaborations on R&D to extend the fuel resource, optimize the fuel cycle and transition to truly voluntary international technology cooperation.

We have conducted a series of analyses that address global energy sustainability while also reducing emissions at reasonable cost.

The CANDU reactor development philosophy

CANDU technology uses an evolutionary development strategy. This strategy ensures that innovations are based firmly on current experience and keeps development programs focused on one reactor concept, reducing risks, costs, and cycle times. Thus, the continuous line of water reactor development is supported into the future. This approach produced the D2O cooled NU-fuelled CANDU 6, and its successor the Enhanced CANDU 6, (EC6) a mid-sized reactor with the capability to also use LEU fuel. With continuous improvements, and the use of H2O cooling, the Advanced CANDU Reactor (ACR-1000) concept will likely remain highly competitive for a number of years. The ACR leads naturally to the next phase of the Generation IV CANDU–SCWR concept. This is also conventional water technology, using supercritical boilers and turbines that have been operating for some time in coal-fired power plants. The advance and challenge lies in adapting the nuclear reactor materials and cycle to take advantage of the much improved thermal conditions. The CANDU-SCWR concept has flexibility of a range of plant sizes suitable for both small and large electric grids, and the ability for co-generation of electric power, process heat, and hydrogen, and also be exploited for advanced fuel cycles.

In Generation IV CANDU-SCWR has novel primary circuit layout and channel design. The R&D in Canada is integrated with the NRCan National Program and the Generation IV International Forum (GIF) plans.

Utilizing the CANDU thermal spectrum provides Pu and actinide burning capability, inherent in the EC6 and the ACR-1000 designs.

In the global market, CANDU and AECL competes with multinational corporations, who also offer integrated full fuel cycle and waste management services. These also include enriched fuel supply, recycling, fuel reloads, and increased uranium utilization. Canada is a major supplier of yellowcake to many of the competing entities, who have nationally supplied or endorsed enriched or recycled fuel processing, enrichment and manufacturing capabilities at commercial scale.

The CANDU options

There are now more than 40 CANDU–type reactors operating in the world, including those in India, Pakistan, Korea, Argentina, Romania, Canada and China that have accumulated more than 400 operating reactor years. The forward development path chosen represents the optimum strategy for improvements in water-cooled reactor technology, without requiring major shifts in concept, but still allowing advances in design, licensing, safety and performance. This optimal development path is illustrated uses the widely adopted reactor “Generation” terminology.

The options are fairly simple and logical, and imply several hybrid outcomes or paths, which then lead to a logical technical and business decision. The implications are that redirection of the effort in technology development is needed, in order to position correctly in the future and astutely present the correct vision. The developments envisaged present significant challenges to and development of current CANDU technology:

  1. Integrated optimization of external “waste to energy” and spent fuel minimization cycles with interfaces to LWRs, FBRs and fuel from separation streams, with the automated estimations of isotopic composition and heat loading for spent fuel;
  2. Integrated optimization of internal (core) fuel cycles, for U, Pu and Th, including bundle recycling, heterogeneous core loadings, differentially loaded fuel pins, with fuel loading and handling;
  3. Optimization of CANDU core design, including lattice pitch, driver fuel enrichment, fuel reload strategy, reactivity control and ROP margin determination;
  4. Demonstration of the performance of long life, high burn up, multiple recycle fuel designs, that are demonstrated to perform with mixtures of actinides, Pu and Th, and matrix concepts;
  5. Establishment of fuel separations technology, isotopic and actinide optimization, to enable the recycling, reuse and remanufacture of spent fuel bundles;
  6. Development of an integrated spent fuel interim storage, on site storage and handling, fuel waste management, and adaptive phased management of waste streams and spent fuel, including recycling; and
  7. Develop the concept of a closable CANDU reactor fuel cycle, for EC6, ACR and SCWR

Technology for Today and Tomorrow

The extent of nuclear technology deployment will depend also on emissions and carbon constraints to limit and manage climate change. The IPCC marker scenarios are available, and we have accessed them through the MAGICC/SCENGEN package [1], which as a surrogate for a global climate model allows one to modify the assumptions on energy usage by source (coal, oil, gas, nuclear, renewables) at five-year intervals for the entire 21st Century.

We have modified the UN IPCC’s assumptions replacing: 1) 80% of whatever coal consumption had been projected is replaced with nuclear or another non-CO2-emitting technology over the period from 2010 to 2030; and 2) 80% of whatever transport consumption had been projected with the same non-CO2-emitting energy between 2020 and 2040. We assume coal is predominantly used to generate electricity. Now, the IPCC figures are for hydrocarbon fuels at their input values but for nuclear and renewables as electricity. Since coal is converted to electricity with about 40% efficiency, 2.5 times as much coal will be displaced as is introduced as nuclear. Similarly, since oil-based fuels are typically used with about 15% efficiency in transport applications and the conversion of electricity to hydrogen by conventional electrolysis followed by a fuel cell to reconvert hydrogen to electricity is about 35%, a similar factor of 2.5 was applied.

Results show that our modifications to the IPCC’s B1 scenario, which envisages a nine-fold expansion of nuclear deployment, necessitates slightly more than a 20-fold expansion. Much of the world’s existing nuclear fleet (440 units) was committed in roughly 15 years from the late 1960s to the early 1980s. Today’s unit size is larger and we would be contemplating about 5000 new units committed over twice as long a period. This is formidable but it is a plausible and workable solution and energy demand for Scenario B1 peaks about 2040 at rather more than twice the current usage.

In developed economies, 25 to 30% of all energy is used for transportation and the proportion is growing. No route to atmospheric CO2 stabilization can hope to achieve the required 60% reduction in emissions without addressing transportation. To some extent, improved efficiency would help but it seems clear that gains from this approach in the developed economies will be offset by rapid expansion of transportation in emerging economies.

Hybrid vehicles are a development that improves efficiency and points in what is probably the right direction: electric propulsion. Ideally, electricity would be used to displace hydrocarbons for transportation but to do so requires batteries that are (1)lighter; (2)longer-lasting; (3)cheaper; and (4)faster to recharge. Despite much effort and some progress, batteries as a primary energy supply to transportation remain only a small niche where total energy requirements are low and the range is small. This may change but the obvious way ahead would seem to depend on deployment of hydrogen as a fuel [2].

We have also shown [3] how hydrogen could be produced at a competitive price by conventional water electrolysis if electricity production were divided between sales to the grid at times when demand and price were high (on peak) and production of hydrogen when low (or off peak). Variations in the price of oil and natural gas are completely avoided.

Substantial amounts of energy supplied by wind turbines could be incorporated alongside nuclear-produced electricity if electrolysis cells were used with a capacity to handle about a 40% variation in current density. Provided wind turbines can achieve their projected level of output of around one-third of nameplate capacity, our NuWind© concept to harness the nuclear and wind in tandem [3] looks economically viable. Both with and without wind, co-production of electricity and hydrogen is more profitable than selling electricity alone.

The required number of reactors to stabilize GHGs we calculate between 4000 and 6000 1GW(e) units. This requires a major enhancement in safety because of the large numbers.

Safety Assurance: Learning from Experience

With a large required new build to meet global needs we can utilize on the experience with the successful PTR deployment worldwide. We may define safety assurance as follows:

a)Demonstrated licensability in multiple regulatory jurisdictions, meeting national and international regulations covering probabilistic risk levels, core damage frequencies and operational constraints, and all siting and environmental requirements;

b)Well-defined and established formal documentation, safety analysis reports, qualified methods, open reviews, and supporting data, backed by rigorous quality assurance and R&D support;

c)Diverse, redundant and reliable safety systems (both active and passive) with demonstrated performance, and a large impenetrable and secure containment to limit radioactive releases;

d)Established and known safety margins and operational limits, related to known phenomena, for both internal causes (pipe and vessel breaks and plant transients) and external events (large earthquakes, fires and terrorist attacks);

e)Limitations to, and intolerability of excessive radioactive releases to the environment and local eco-systems; and

f)Safe, effective and flaw-free operation, learning from experience and providing relentless preparation, operational excellence and dedicated safety management at all levels.

In addition, there must be scrutable and effective communication of the safety to the public, policy makers, politicians, government representatives and the media. Measures must be in place to assure how close to the ideal and acceptable limits the design, operation and staff are performing and to demonstrate that learning and risk reduction is occurring. Important adjuncts are proven on schedule, on budget project performance, and a demonstrated and robust supply chain.

Now some 50 commercial PTRs are already deployed and operating world-wide, in Argentina, Canada, China, India, Pakistan, S. Korea, and Romania. In addition, Russia had followed its own PTR variant, the RBMK, which importantly, did not have a full containment structure. The learning from the Chernobyl accident, and the parallel event for LWRs at Three Mile island, demonstrated the need for continued vigilance, enhanced safety, revised regulation, robust containment structures, ability to withstand severe accidents (outside the nominal design envelop), and skilled and knowledgable operation.

Thus, for the CANDU PTR, we have met the intent of these safety assurance requirements in each and every country. The exceptions are those countries with LWR vessel reactor technology, where the PTR system has been excluded either by fiat or by implicit trade or regulatory barriers, or because of not having a domestic pedigree.

As an example, a robust safety assurance process for PTRs is followed for the current commercial ACR-1000 design. This process includes not only what is needed for purely compliance purposes but also what is also required and desirable for safe and effective operation. Thus, in the design stage the development of the ACR-1000 safety case is largely based on the ACR-1000 Design Program, which consists of three distinct project phases: the Product Definition phase, Basic Engineering Program, and Project Final Design phase.

Safety Assurance: by Design

A key to safety is also to have a safe design process. The safety design objective requires excellence in safety to protect the general public, plant personnel, and the environment, as well as plant investment. As noted above, the ACR1000 plant is designed based on the “defence-in-depth” safety philosophy applied to all the CANDU plants with enhancements to further improve the overall safety of the plant. This includes the core being designed to have a negative power coefficient of reactivity and a small negative coolant void reactivity (CVR) under nominal design conditions, improved performance of safety systems, and provision for a robust containment design to meet Canadian and international practice for new plants, as applicable.

The ACR-1000 design takes advantage of both passive and engineered safety characteristics, including distinctive features that arise from CANDU design principles. Central to ACR-1000 safety are two fast-acting, fully capable, diverse and separate shutdown systems, physically and functionally independent of each other and also from the reactor regulating system. Based on proven CANDU technology, each shutdown system is designed to cover the whole spectrum of design-basis events and to perform its safety functions with a high degree of reliability. Additional defence-in-depth is derived from the inherent passive-safety features of the CANDU fuel channel core, including extra heat sink redundancy for potential accident conditions.