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Symposium

Next Generation Nuclear Plant

23-24 February 2005

Looking to help improve the performance of nuclear power plants of the future with regards to economics, safety, waste generation, and physical security, the US Department of Energy (DOE) announced its intention to support the building of a Next Generation Nuclear Plant (NGNP) at the site of its Idaho National Laboratory. The nuclear plant is desired to demonstrate the ability to produce hydrogen using heat and/or electricity at a cost attractive to the market. Thus, several vendors and the INL have been developing appropriate designs of high temperature reactors (HTRs) that can meet most if not all the announced goals.

The purpose of holding this symposium was to:

Some 70 experts and 20 students attended the two-day symposium. In his introductory remarks, Prof. Mujid Kazimi of MIT reviewed the incentives for developing a nuclear plant that can produce with high efficiency both electricity and hydrogen, namely the need to shift to power plants and transportation fuels that reduce carbon dioxide emissions, and reduce the reliance on fossil fuels imported from politically volatile regions. The readiness of the market for such plants was reiterated by luncheon speaker Dan Keuter of Entergy, who said that 75% of US hydrogen production occurs in the district served by his company, and that gives a very good size market for any plant that can produce hydrogen.

Many challenges exist in designing a plant with a coolant exit temperature at or above 1000°C, and the majority of the first day speakers addressed both the need for such a high temperature and the implications for the plant. It was remarked by Dr. David Baldwin of GA that the goal of very high reactor temperature was meant to increase the efficiency of hydrogen and electricity production. However, it is more appropriate to try to reduce the cost of hydrogen production, which could end up with a plant limited to 850 °C. In this regards, Dr. Pavel Hejzlar of MIT presented recent results from MIT that showed that decoupling of the reactor exit temperature can be as low as 650 °C while the hydrogen plant can operate at 900°C, if heat recovery is well engineered in the plant.

The benefit to the NGNP of the design studies of the South African pebble bed design PBMR was outlined by Ed Wallace of PBMR and W. He felt that the main engineering issues of the pebble bed design would be solved by that design, which could become a more certain starting point for the NGNP. Mr. Phil MacDonald of INL presented a comparison of point designs at INL for both the graphite block core and the circulating pebble bed core. He pointed out that each has strengths and weaknesses, and that the INL group has not reached a conclusion about which design would be superior.

One of the main choices for a graphite moderated plant is the choice of a gas coolant vs molten salt coolant. While helium cooled reactors were favored by most of the designers, Dr. Charles Forsberg of ORNL showed that a much larger size of the plant could be accommodated if the coolant was molten salt, and consequently its economics would be more favorable. It was pointed out however, that the use of molten fluoride salts would imply a strong requirement of preventing radiolysis in salts in contact with metals, which might complicate the design of the plant vessel.

The choice of the power cycle seemed to vary a lot. Dr. Michele Lecomte of Framatome ANP presented a design for an indirect power cycle that relies on nitrogen as the turbine working fluid and has a bottoming steam Rankine cycle. He said that due to the higher efficiency of power conversion, the overall cost of electricity will be lower than in a direct helium cycle. Dr. Pavel Hejzlar suggested that using an indirect supercritical CO2 power cycle would be more favorable due to compactness of the equipment and the ability to get high efficiencies at lower temperature than helium or nitrogen.

The second day of the symposium addressed specific issues for the designs. In a session on safety issues, it was pointed out by Prof. George Apostolakis of MIT that a question remains as to how to apply risk information in the design process of the plant. He said that if there is any doubt about the risk, the regulator is likely to apply the defense in depth approach to avoid unforeseen risk. Prof. Andrew Kadak of MIT presented a listing of the main safety issues in gas cooled reactors and outlined the efforts at IAEA to draft a regulatory guide that will be technology neutral. He also showed the results of a safety test conducted at the Chinese 10MW pebble bed reactor in which the reactor shut down due to intrinsic features after the coolant flow stopped, with only a small overpower transient.

Regarding the fuel requirements, it was pointed out by Dr. David Petti of INL that the fuel for NGNP would serve under conditions well beyond any earlier experience with TRISO fuel. He outlined the differences of operating conditions and their potential implications. He called for a thorough fuel irradiation testing program well before final commitment to a design is made. He felt that the process control approach would have to be relied on rather than the operation monitoring control in order to avoid fuel failure. While many issues remain to be resolved, Dr. Don McEachern of GA felt that previous experience at Fort St.Vrain and in Germany provide a good guide for further fuel development. He pointed out that both uranium and plutonium oxide fuels were taken to very high burnup at Fort St Vrain with good performance.

Other issues discussed include: the use of NGNP to test deep burning of transuranics, the need for experimental results to verify the proposed hydrogen production methods and the disposition of the spent fuel from this type of reactor.