The Nuclear Fuel Cycle Lecture 5

Transcription

1 The Nuclear Fuel Cycle Lecture 5 David J. Hamilton d.hamilton@physics.gla.ac.uk 7th February 2011

2 1. Overview Limitations of thermal recycling of Pu. Fast critical reactors: core physics; breeders; transmutation. Partitioning and Transmutation technologies for MA disposition. Accelerator-driven Subcritical Systems. Generation IV. A Nuclear Renaissance.

3 2. Recap (Lecture 4) The once-through cycle does not represent a sustainable use of resources. Reprocessing of spent fuel allows extraction of U and Pu components for further reactor utilisation, reducing significantly the volume of HLW for geological disposal. As a result of its adoption in some countries (mainly UK and France) there are growing stockpiles of separated plutonium. MOX fuel is the most mature of the plutonium technologies. It is currently used in moxified LWR, but only with 30 % core loading. Advanced LWR designs allow 100 % MOX loading, in addition to offering increased plant safety, lifetime and enhanced economic competitiveness. IMF and Thorium based fuels can be utilised in ALWR in order to reduce production of MA's. The other mature technology in Gen III+ is the HTR, which could consume significant quantities of Pu in a once-through cycle through the use of coated particle fuels.

4 3. Goals of Advanced Fuel Cycles Sustainable use of resources. Reduction of radiotoxicity of spent fuel. Reduction of volume and lifetime of geological repositories. Extraction and re-use of not just plutonium but also the MA's.

5 4. Limitations of Pu Recycling in Thermal Reactors Single MOX recycling allows for a 12 % increase in energy extracted from the initial ore and a factor of 3 reduction in radiotoxicity c.f. Once-through. However, multi-recycling of MOX in thermal systems is not straightforward: The isotopic composition of the plutonium degrades at each step (meaning higher enrichments are necessary); The radiotoxicity of the resulting HLW actually becomes higher than in the once-through cycle because of MA build-up from radiative capture reactions on U-238 and Pu isotopes. These MA's (predominately americium, neptunium and curium isotopes) can not be utilised/disposed of in thermal critical reactors: They are strong thermal neutron poisons; Require fast neutrons to induce fission (fissionable but not fissile); Have very small delayed neutron fractions. Optimal Pu and MA incineration requires a fast reactor system.

6 5. Fast Critical Reactors Fast critical reactor systems utilise fast neutrons (> 1 kev) to induce fission and sustain a chain reaction. There is no moderator in a fast core. Extremely compact cores (high power densities) means large amounts of heat removal needed. These considerations led to the use of liquid metal coolant. Several candidates have been used: sodium, lead, lead-bismuth eutectic. Advantages include higher plant thermal efficiency, no boiling and atmospheric pressure operation.

7 5. Fast Critical Reactors Fast critical reactor systems utilise fast neutrons (> 1 kev) to induce fission and sustain a chain reaction. There is no moderator in a fast core. Extremely compact cores (high power densities) means large amounts of heat removal needed. These considerations led to the use of liquid metal coolant. Several candidates have been used: sodium, lead, lead-bismuth eutectic. Advantages include higher plant thermal efficiency, no boiling and atmospheric pressure operation. The world's first electricity producing reactor was a fast critical system (1.4 MWth) with sodium coolant (EBR-I, Idaho, 1952).

8 5. Fast Critical Reactors Core Physics Fission cross sections are smaller for fast neutrons than for thermal (for U235 it is 500 b thermal and only 2 b fast). Higher fissile enrichment is needed (typical Pu enrichments of %) due to the fact that: Prompt neutron lifetime in a fast system is 10-7 s (c.f s for thermal), though delayed neutron fractions (β) for fissile isotopes are around the same. Control rods are less effective in fast systems (B-10 absorption cross section is smaller, leading to a mean free path of 43 cm). Fast neutron fluxes are more homogeneous than thermal, due to the smaller cross sections. Typical burn-ups are much larger than even LWR systems, up to around 100 GWd/tHM.

9 5. Fast Critical Reactors Breeding The most important difference between fast and thermal systems for advanced fuel cycles is that there are more neutrons released per fission (larger η). In the 60's and 70's there were concerns about uranium price and resources, leading to development in several countries of a fast-breeder reactor programme. The principle involves utilising excess neutrons to breed more fuel (Pu-239) in the outer region of the core known as the blanket (U-238). This would lead to the much sought after closure of the fuel cycle. Several plants were built: SuperPhenix, Dounreay, BNR, Monju.

10 5. Fast Critical Reactors Breeding Ratio Fuel breeding occurs in all reactors, normally through radiative capture on U-238. It is characterised by the breeding ratio. In current LWR designs operating with burn-ups of GWd/tHM, the breeding ratio is around That means around half the fuel consumed is bred in the core. Advanced LWR designs will achieve breeding ratios of around 0.8. In a fast reactor, where neutron economy is better, a fissile starter of HEU or Pu based fuel is needed but the reactor can then operate with natural uranium fuel at a breeding ratio of 1. Plutonium production for use in other reactors can also be achieved with the addition of a U-238 blanket, leading to breeding ratios greater than 1. The same principle holds for reactors fuelled with Thorium for U-233 breeding.

11 5. Fast Critical Reactors Transmutation In recent years, focus has switched to utilising the excess neutrons in a fast reactor for MA incineration. This serves two purposes: Reduction of radiotoxicity, volume and heat of HLW. Use of energy from MA fission. Am-241 and Np-237 transmutation has been demonstrated. The major problem is that only small fractions of MA's can be incorporated into a fast critical core because of small delayed neutron fractions. Also U-238 is still needed for reactivity control, leading to more MA production.

12 5. Fast Critical Reactors Non-oxide Fuels There are two other fuel types aside from the commonly used oxide form: Metallic fuel Ceramic fuel Neutronic and thermal properties (expansion, lifetime) of these fuel types are typically better than oxides. Fuel which can operate at higher temperatures allow for an increase in overall plant efficiency and increased in-core lifetime due to a reduction in macroscopic damage. Metallic fuels are dense, meaning that you get the highest fissile nuclei content per unit volume, and have a high heat conductivity (e.g. UzrH). They have been successfully used in fast reactor designs (EBRII). Other ceramic fuels, such as nitrides (UN) and carbides (UC), are less technologically mature. Advantages are similar to metallic fuels. Nitride fuels require enrichment in N-15, as N-14 has a high n capture cross section.

13 5. Fast Critical Reactors Economics Several fast reactors were designed and built through the 60s, 70s and 80s. The cost associated with them is higher than for thermal systems, mainly due to new fuel cycle infrastructure. In spite of the fact that these reactors operated well, the anticipated rise in Uranium prices never happened. For this reason, the fast reactors were not able to compete economically and the programmes were abandoned. That situation is now changing, with China and India now investing heavily in fast reactor technology.

14 6. Partitioning and Transmutation If the transmutation of the MA's could be achieved through advanced Partitioning and Transmutation (P&T) fuel cycles, the volume and heat of HLW could be reduced by more than a factor of 100. Two new technologies would be necessary: Advanced reprocessing for MA extraction; Dedicated advanced reactor systems for MA incineration. Pu UOX LWR MOX LWR Fast Reactor (Breeder) MA HLW (FP) MA Transmutation

15 7. Pyro-processing Aqueous reprocessing techniques such as PUREX do not separate the MA's from the FP's. In order to achieve MA extraction with high efficiency (U and Pu extraction in PUREX ~ 99.9 %) a new reprocessing technique known as pyro-processing is being developed. This electro-metallurgical process involves: conversion of spent fuel to metallic form; heating of the spent fuel in a molten salt; use of an electro-refiner to separate the U, Pu and MA's from the FP's which remain in the salt.

16 8. Accelerator-Driven Systems (ADS) MA transmutation in fast critical systems is difficult because of the effects on reactivity margins. One solution that has been proposed is to operate a reactor in a sub-critical state, with an external neutron source coupled to it to sustain the chain reaction. The most advanced such concept is the fast Accelerator-Driven System (ADS). This operates on the principle of proton-induced spallation generating the external neutron flux. It requires a high-power proton accelerator to drive the system.

17 8. Accelerator-Driven Systems (ADS) - Spallation When a high-energy proton beam (~ 1 GeV) interacts with a heavy target, nuclear spallation leads to the production of neutrons (around per incident proton). As the reactor core is sub-critical, a great many spallation neutrons are required to drive the chain reaction. This means: a high energy, high current proton beam is needed (> 10 ma); A heavy (solid or liquid?) target like W, Pb or Pb-Bi near the core is needed. This technique is widely used in material science (SNS at Oak Ridge).

18 8. Accelerator-Driven Systems (ADS) Core An ADS core would typically be designed such that k = It all other respects, it is very similar to a fast critical core, albeit one which can accomodate large fractions of MA's. The neutrons produced in spallation reactions are fast; there is no moderator. Choice of coolant is the same as fast reactors. In order to not only burn MA's but also generate significant amonts of energy it has been proposed to fuel the core with the fertile material thorium (Rubbia). Much work is still needed.

19 9. The Generation IV Initiative

20 9. The Generation IV Initiative The Generation IV International Forum seeks to direct research on the next generation of nuclear reactors, for deployment around In its 2004 RoadMap the six most promising reactor designs (and associated fuel cycle and plant technology) were identified. These include a variety of plant sizes, fast and thermal systems and open and closed fuel cycles. The focus on the development of these systems is improved safety and security, economic competitiveness and sustainability.

21 9. The Generation IV Initiative Fast Reactors There are three fast reactor designs considered by Gen IV: Sodium Fast Reactor (SFR); Lead Fast Reactor (LFR); Gas-cooled Fast Reactor (GFR). Of these, the GFR is by far the most advanced and technologically challenging concept. The intent for all designs is operation of a closed fuel cycle, with the potential of extracting 100 x more energy than the once-through cycle. An important component on the Gen IV fast programme is replacing oxide fuel with high-density carbide or nitride fuels (burn-ups around 200 GWd/tHM).

22 9. The Generation IV Initiative Epi/Thermal Reactors The other three reactor designs operate with thermal or epithermal fluxes: Very High Temperature Reactor (VHTR); Super-Critical Water Reactor (SCWR); Molten Salt Reactor (MSR). The VHTR offers the potential of moving beyond electricity generation to hydrogen production. The SCWR offers much greater thermal efficiency than today's LWR (44 % c.f. 33 %) and simpler plant design. The MSR is very advanced concept, and involves a homogeneous liquid salt mix of fuel and moderator.

23 10. A Nuclear Renaissance A global nuclear renaissance looks increasingly likely. If trends are to be believed, there is insufficient Uranium through current mining strategies to support this renaissance. There are several technological options which could support such a large increase in nuclear energy production. These include: Other Uranium sources (such as extraction from seawater); Thermal MOX multi-recycle; Fast reactor deployment; Thorium Fuel Cycle (4x more abundant than U); ADS. In practice, countries will adopt a mix of these options according to their own particular situation (e.g. India has 300,000 thm of Thorium).

24 10. A Nuclear Renaissance Good, Bad or Necessary Compared to other energy sources, such as coal, gas and renewables, nuclear energy has several advantages and disadvantages. Security-of-supply is one clear advantage It does not depend on the weather, or on fossil fuel supplies from the Middle East, Russia and other potentially unstable regions. Following plant construction, it is a zero-carbon technology. It is a globalised industry and resource: Technological improvements are already being made through international initiatives. It can be deployed in the third-world without the need for fuel cycle infrastructure in those countries. It spite of perceptions, its safety record is good. Its use in a closed fuel cycle with fast breeder reactors represents one of the most sustainable of all energy options.

25 10. A Nuclear Renaissance Good, Bad or Necessary Compared to other energy sources, such as coal, gas and renewables, nuclear energy has several advantages and disadvantages. The public perception of the nuclear industry is not good, leading to a large trust deficit. Associations with nuclear weapons, reactor accidents and a general fear of radiation all contribute. Partially as a result of this deficit, the waste issue is a serious one. Transmutation fuel cycles could help in this regard. It is currently more expensive than gas or coal technologies: Plant construction and licensing can take up to years, leading to high capital costs. Proliferation risks are ever present, not just today but in the very far future. These risks are higher for more advanced fuel cycles, especially those involving fast breeder reactors.

26 11. Summary Although the nuclear industry is dominated by engineers and chemists, there are several interesting areas of research for nuclear and material physicists. These include: Reactor design/operation; Fuel design/characterisation; Advanced fuel cycles and transmutation; ADS. Informal feedback ( more than welcome.