Why dont we use pius process inherent ultimate safety nuclear reactors?

Camilla

I was thinking of this lately due to the disaster in Japan and their Nuclear power problems. PIUS works the Sweetish guy came up with the idea in the 70's and no one has wanted to use this method because of the lack of welding power it hypothetically can produce.



Tortolita

I'm not too familiar with PIUS. As I understand the major safety feature was a "passive" or natural circulation cooling system that didn't require pumps in event of an emergency shutdown. Anyway, as you're probably aware, the problem with all water cooled reactors is the core continues to produce large amounts of heat after it's shut down. Even when there is no active fission taking place. This is due to the decay of short-lived fission products. The fuel will continue to produce significant heat for months, and will produce SOME heat for as long as it remains heavily radioactive. For this reason, the "scrammed" core needs to be actively cooled with thousands of gal./ min, or else it can still easily melt down. Pressurized light water reactors and boiling light water reactors are in a sense evolutionary generation 2 designs, They are descended from reactors designed expressly to produce plutonium. (In fact, spent fuel from most boiling water reactors contains maybe 1% Pu, as I understand it.) We use these kinds because they're supposedly "proven technology." (In this case, I believe "proven" means we've demonstrated their fatal flaws through application of Murphy's Law.)(Side note: it's an interesting fact that the majority of energy from current reactors comes from the creation, then fission, of Plutonium, and not from the uranium itself. The U-235 acts as a "seed' that gets the reaction going, and in a typical fuel cycle 5% or less of the U-235 is burned. Plutonium has a significantly larger "fission cross section" for neutrons. The reason fuel needs to be removed is some of the fission products that build up over time are neutron absorbing "poisons" that gradually halt the chain reaction. This is why fuel-reprocessing is a useful activity. Without the buildup of fission products the uranium fuel would actually get MORE reactive over time.)The amount of design, concept testing, and legal/regulatory red tape a new rector design is currently required to go through is monstrous, gigantic, very prohibitive. Most proposed reactor designs have never and will never see light for this reason. This is the same reason almost no new reactors have been built in the US in almost 30 years. That, and popular fears which may not be as paranoid as they seem. Even today the cost of fuel reflects less than 1/10 the cost per kWh of nuclear electricity, over the life of a plant. "natural circulation reactors" would be something of an improvement over the most popular designs. There's still a possible drawback. The direct reactor coolant would need to be in a closed-loop system since it would be heavily contaminated with radioactive substances from the core. The primary coolant itself would still need to be cooled continuously to avoid a boil-over, and subsequent loss of coolant as seen in both the TMI and Fukushima reactors. I'm not aware if PIUS accomplishes secondary cooling through natural circulation as well. PUIS describes itself as not being a major redesign of the inner workings of the core itself, only of the ancillary cooling systems. I feel that the current "gen 2 plus" canadian CANDU designs are safer than PIUS, so far they have an outstanding, though limited safety record. CANDU places individual fuel elements in their own separate pressurized tubes. In a way it's like having almost a hundred separate containment pressure vessels. the array of pressurized tubes is placed in a large pool of heavy water which acts as both a neutron moderator and a passive safety device. In this design, damage or even complete melting of one or two fuel elements within it's individual pressure tube is unlikely to affect the safety of the entire core, and can easily be repaired. Total core loss is rather unlikely, neither a major release of contaminated steam and/or hydrogen. this is due to the relatively low density of fuel in the rector compared with the volume of water surrounding it. Oak Ridge Laboratories operated an interesting experimental reactor called the "molten salt reactor." This would be conceivably much safer than a water reactor since the fuel itself is already liquid. This makes passive safety measures a lot simpler. For example a "freeze plug" that melts and dumps the fuel into a large tank filled with sodium borate.