A thorium-based fuel cycle for VVERs & PWRs – a nonproliferative solution to renew nuclear power

The growth of commercial nuclear power in much of the world has effectively been stalled since the early 1970s as a result of a number of concerns, including waste disposal, cost, proliferation and safety. Recent concerns about global warming, and the Kyoto accords limiting CO2, however, suggest that future energy demands cannot be met solely through the burning of fossil fuels, and a return to some reliance on the nuclear option may be required; this transition would be significantly aided if the earlier concerns could be successfully addressed.

A novel reactor fuel-cycle concept based on the utilisation of thorium has been proposed that addresses the proliferation and waste issues that currently contribute to limiting the expansion of nuclear power. The concept, known as Radkowsky Thorium Fuel (RTF), was proposed by Alvin Radkowsky (former chief scientist of the Naval Reactors programme under Admiral H Rickover from the 1950s to the early 1970s) and is based in part on the ideas and experiences of the Bettis Atomic Power Laboratory’s Light Water Breeder Reactor (LWBR) programme as implemented and successfully demonstrated at the Shippingport reactor in the late 1970s.

THE CONCEPT

The RTF is a new fuel concept, not a new reactor design, that builds on the successful LWBR experience. In addition to reducing the proliferation potential of the standard nuclear fuel cycle and reducing the requirements for spent fuel storage and disposal, the design is subject to the following constraints:

• Be retrofittable into existing pressurised water reactors, and the Russian variant, the VVER, with minimum changes to existing systems/hardware.

• Be competitive economically.

• Have comparable Environmental, Safety and Health (ES&H) characteristics to those of existing PWRs/VVERs (ie within the current “safety envelope”).

Retrofittability is achieved via the key element of the RTF, the seed-blanket-unit (SBU), which is a one-for-one replacement for a conventional PWR/VVER fuel assembly. The fuel-to-moderator ratios in the seed and blanket regions are different, and optimised to reduce Pu production in the seed, and enhance U-233 production and burning in the blanket. Two additional items enhance the nonproliferative characteristics of the RTF fuel:

• In addition to reducing the production of Pu by a factor of ~5 - 7 relative to a standard PWR/VVER, the plutonium that is produced has a high content of Pu-238, Pu-240, and Pu-242 which makes it impractical for use in a weapon. And,

• It employs a once-through fuel cycle with no reprocessing, with the bred U-233 burnt in situ; in addition, the U-233 that is produced is denatured by admixed uranium isotopes in order to force isotopic separation should extraction and use of the bred U-233 be attempted.

The high residence time of the blankets (~9-10 years, burnups on the order of 100 GWd/t), coupled with the superior neutronic performance of U-233, results in improved fuel utilisation, and hence reduced fuel costs; coupled with the absence of higher-level actinides due to the Th cycle, the concept also has benefits for waste storage with reduced volume, activity and decay heat relative to a conventional cycle. While these latter improvements are relatively modest, additional studies currently underway suggest much more dramatic improvements with respect to toxicity of the waste.

INITIAL STUDIES

Initial development of the concept was performed by the Radkowsky Thorium Power Corp (RTPC). That effort resulted in very preliminary versions of PWR and VVER variants, and suggested reductions in the production of plutonium by a factor of ~5 - 7 relative to conventional fuel cycles. In addition, reductions in waste volume by a factor of ~2, as well as reductions in decay heat and activity were realised, along with fuel-cycle cost savings of more than ~10%.

In 1996 a programme was initiated under the US DOE Industrial Partnering programme (IPP) (now the Initiatives for Proliferation Prevention programme). This R&D programme was aimed at developing and demonstrating key elements of the RTF concept. The project was carried out in co-operation with the Russian Research Centre - “Kurchatov Institute” (RRC-KI) under a subcontract with Brookhaven National Laboratory (BNL), and RTPC (through a Cooperative Research and Development Agreement with BNL). RRC-KI led a team that included participants from approximately 10 Russian institutes, as well as Gosatomnadzor (GAN), the regulatory organisation. To-date, the main thrust of the programme has been focused on VVERs and Russian Federation fuel technology. Initial mechanical design of a demountable SBU for use in VVER-1000s and all required mechanical interfaces has been developed, and satisfactory neutronic and thermal-hydraulic performance over ~10 cycles has been confirmed by calculations. The calculations also essentially confirmed the initial estimate of reduction in overall plutonium production, obtaining a factor ~5 from a more refined modelling. Initial safety assessments show that the RTF version of a VVER, referred to as a VVERT, falls within the existing safety envelope. Experimental programmes to verify predicted neutronic and thermal-hydraulic performance, and qualify the fuel, have been defined, and key issues that need to be addressed have been identified.

A cross section of the currently proposed VVER SBU design is shown in the following figure. As illustrated, the seed zone of the SBU consists of fuel elements with a 3-petal shape. These elements are based on Russian naval reactor technology, and are twisted along the longitudinal axis to provide for their self-spacing in a triangular lattice. All the guide tubes for the absorber elements of the control rod clusters in this design are located in the seed region, and the seed is assembled into a removable module by the use of bands as shown. The blanket rods are spaced by axial spacer grids which are similar to those used in a conventional VVER-1000 assembly; the grids are anchored to a hexagonal tube. The resultant design permits independent movement of the seed and blanket portions of the SBU.

CURRENT ACTIVITIES

A second phase of the project, also funded under the DOE/IPP programme, has recently begun, and builds on the initial work described above. The main objectives of this continuation of the project are development of an optimised VVERT “final design” (Preliminary Safety Analysis Report (PSAR)-level) building off the work performed during Phase-1, and preparation for testing of prototype VVERT subassembly units. These tests are planned as a “lead-test-assembly” (LTA) type programme aimed at inserting 1 - 6 prototypical VVERT SBUs in an operating VVER-1000 in ~2005. In order to support this programme, implementation of required pre-testing and licensing activities has been initiated, as well as design and safety assessments to support the programme.

Recently, a Russian-based subsidiary of RTPC and RRC-KI have formed a joint venture with MSZ Electrostal to provide fuel fabrication capabilities and enhance commercialisation. The joint venture company will produce the LTAs at Electrostal.

Work on an equivalent design of a PWR variant that will allow more definitive comparisons with conventional reactor fuel-cycles, and enhance the commercial potential of the RTF is scheduled to begin soon. This effort will benefit significantly from the ongoing VVER-related work and a special effort has been made in planning the experiments to be performed by RRC-KI to ensure maximum value for the PWR programme. For example, in parallel to thermal-hydraulic measurements performed for a hexagonal (VVER) lattice, additional experiments are planned for a square (PWR) lattice. These experiments will produce the necessary data on power distributions, heat-transfer, and DNB correlations for both VVER and PWR SBUs; these data are required for operational and safety analyses that are accepted as adequate for licensing purposes.