Time for another thread on nuclear power, this time on how the reactor core is protected from major casualties, and what the law says about it. So, we'll start with 10CFR50.46. It lays out 5 criteria the Emergency Core Cooling Systems at all US reactors must meet. 1/

1 - Peak cladding temperature shall not exceed 2200F. This prevents a self-sustaining reaction between the steam and Zirconium alloy tubes that make up the outside (cladding) of the fuel rods. This reaction weakens or destroys the cladding and produces hydrogen. 2/

2 - Cladding oxidation shall not exceed .17 times the clad thickness. Oxidation is bad, it weakens the cladding. This requirement says that in the worst possible accident the cladding will be oxidized no more than 17% (83% still good) through the wall. 3/

3 - Total hydrogen production shall be less than 1% of the amount that would be produced if all the cladding reacted and made H2. Minimizing H2 will prevent an explosive atmosphere from forming, possibly damaging the containment building and letting out the fission products. 4/

4 - Coolable geometry. Even in the worst possible accident the geometry of the reactor core will remain amenable to cooling. Even TMI, with half the core melted, allowed flow once the pumps were turned back on. 5/

5 - Long term cooling. After the initial phases of the accident the ability to provide long term cooling to the core must exist. This is generally taken to mean 1 year using just the systems installed at the plant. We do this by recirculating the water we pumped into the core. 6/

So, how does a NPP meet these requirements? We have an ECCS system. It is composed of two identical, but completely separate systems. This is called redundancy, and it is required by other parts of the law. I'm going to explain one train, know there is another just like it. 7/

The accident of concern in this case is called a LOCA, Loss of Coolant Accident. The coolant is leaving the core, and we need to replace it. We have 3 sub-systems to do that.
Safety Injection, Residual Heat Removal, and ECCS Accumulators. 8/

SI - A high pressure pump designed to supply sufficient water to make up for small to medium sized breaks in the reactor coolant system. It can push against 2000 psig of pressure and supply ~ 700 gpm. 9/

RHR - A low pressure pump designed to supply large amounts of water against lower pressure for a large break that depressurizes the RCS. It can push against 200 psig and supply ~ 3500 gpm. 10/

Accumulators - Big pressurized tanks that hold enough water to refill the core to the half way point. They are completely passive, no power required, just pressurized with nitrogen to push the water into the core. 11/

All 3 sub-systems work together to protect the core and meet the ECCS Acceptance Criteria in the law. The electrical bus that supplies them power has 2 grid sources and a back up diesel generator. It is all designed so that no one failure can stop it. - Done -