factor at the loop type Prototype Fast Breeder Reactor Monju at Tsuruga, Japan. Using lead or molten salt coolants mitigates this problem as they are less reactive and have a high freezing temperature and ambient pressure. Lead has much higher viscosity, much higher density, lower heat capacity, and more radioactive neutron activation products than sodium. Multiple proof of concept Gen IV designs have been built. For example, the reactors at Fort St. Vrain Generating Station and HTR-10 are similar to the proposed Gen IV VHTR designs, and the pool type EBR-II, Phénix, BN-600 and BN-800 reactor are similar to the proposed pool type Gen IV SFR designs. Nuclear engineer David Lochbaum cautions, "the problem with new reactors and accidents is twofold: scenarios arise that are impossible to plan for in simulations; and humans make mistakes". As one director of a U.S. research laboratory put it, "fabrication, construction, operation, and maintenance of new reactors will face a steep learning curve: advanced technologies will have a heightened risk of accidents and mistakes. The technology may be proven, but people are not". Design projects See also List of small modular reactor designs Nuclear reactor Nuclear material Nuclear physics List of reactor types Generation II reactor Generation III reactor Integral Fast Reactor Liquid fluoride thorium reactor Breeder reactor Small modular reactor List of nuclear reactors References External links Article from Idaho National Laboratory detailing some current efforts at developing Gen. IV reactors. Generation IV International Forum (GIF) U.S. Department of Energy Office of Nuclear Energy, Science and Technology Gen IV presentation Science or Fiction - Is there a Future for Nuclear? (Nov. 2007) - A publication from the Austrian Ecology Institute about 'Generation IV' and Fusion reactors. "In the wake of a severe plant accident, advanced reactor designs are getting renewed attention." International Thorium Energy Organisation - www.IThEO.org International Thorium Energy Committee - iThEC Nuclear power reactor types Idaho National Laboratory