Expert: Osman Kemal Kadiroglu - 6/17/2009

Question
what is the experience so far with the Pebble HTGR . Has it been fully commercialized and how many plants so far are in operation worldwide ?is  there another system(design)for HTGR and what is it's name . Wagt are the  advantages and disadvatges of HTGR versu LW nuclear power plants ?

Answer
Hi,

Pebble bed nuclear reactors are not new. There were two important pebble bed nuclear reactors operated in Germany in the early 70's. The first was the AVR reactor in Julich, Germany, which operated very successfully for more than 20 years and later decommissioned. The second one was a prototype power reactor again in Germany. This reactor operated few years but due to political consideration at that time due to Greens was stopped and later decommissioned. Both reactor proved that pebble bed nuclear cores are feasible. Another high temperature gas cooled reactor built and operated in USA, the Fort Saint Vrain reactor. It had prismatic fuel elements. This reactor was a very problematic reactor due to many innovative new systems integrated into the design. Finally this reactor was also decommissioned. Presently, there is one pebble bed test reactor operating in Beijing University, China. also there is one prismatic fuel high temperature test reactor in Japan.

South Africa is trying to design a new pebble bed reactor that is inherently safe and utilizing the gas turbine cycle (Brayton). Few weeks ago when I was in South Africa, I heard that they changed their plans and coming up with a different design based on the German Modular reactor that used steam turbine cycle (Rankine).

The most important advantage of HTGR is that they could be designed as inherently safe reactors. That means there could be no core meltdown probability for this type of reactors. Even if they are not designed as inherently safe they would be much much safer than Light Water Reactors. Also, the gas temperature in the reactor core could reach up to 1000C, which means that not only electricity can be produced at high thermal efficiency but, also the nuclear plant could be used as the heat source for many chemical processes, like coal gasification, oil recovery, etc.. If gas turbine cycle is used the thermal efficiency of the plant could reach around 50 which is around 20 higher than the LWR power plants. The temperature of the rejected heat from a gas turbine pebble bed reactor is more than enough to use it in desalination processes. Maximum temperature in LWR are around 300C and this is only sufficient to produce electricity at lower thermal efficiencies. The temperature of the rejected heat is as low as the atmospheric temperature and it can not be utilized.

For more information on the pebble bed reactors please see: www.pbmr.com or do a Google reach on various types of nuclear reactors.