User:David MacQuigg/Sandbox/Reactor Safety

Discussions on nuclear reactor safety are often confused by general statements about all reactors. To avoid misleading information, such discussions are best done in the safety sections of the article on each reactor. That said, there are features of many modern reactor designs requiring some technical understanding. This article will provide those details.

Criticality and the Role of Delayed Neutrons

Can it go BOOM ???
SL-1 - Don't yank the rods !!
Chernobyl - Soviet era incompetence, never again !!
Margin of Safety

Decay Heat

Show plot of heat generation (MegaWatts) before and after shutdown.

Engineered Safety Systems

Defense in Depth
Many layers of redundant systems
Plan for every possible human or machine failure

Passive Safety Features

Decay Heat Tolerance

Pool type reactors can absorb it all.

Convection Cooling

Removes decay heat even when the pumps fail

Melt Plugs

Very effective with liquid fueled reactors

Continuous Draining

Fuel drains out without continuous pumping to replenish.
Even safer than melt plugs that could get clogged or not melt quick enough.

Intrinsic Safety - Negative Temperature Feedback

Doppler Broadening of Neutron Absorption Lines

Fig.1 Higher temperature (red) broadens the absorption lines (solid curves). Broader lines punch bigger holes in the neutron spectrum (dashed lines).

from Jack Devanney's Nuclear Magic: Where's the Throttle?

There is a widespread belief that nuclear reactors cannot loadfollow. This is nonsense. In most countries, loadfollowing is a grid requirement. In fact, stable reactors are inherently loadfollowers. A stable reactor is one whose power decreases as the reactor temperature increases. This is the property of any well-designed nuclear reactor, and a strongly enforced, legal requirement.

A few early designs could be put in a state where the opposite was true, an increase in core temperature increased power which increased temperature which increased power, which ... and the whole thing goes boom. One such design was the Russian RBMK at Chernobyl. You had to work pretty hard to put the RBMK in such an unstable state; but human ingenuity was up to it, and we got the Chernobyl explosion.

All temperature stable nuclear reactors have a remarkable ability. They automatically adjust their power output to the load. If the electric load increases, the turbine control system will speed the steam flow up. The increased steam flow will extract more heat from the steam generator. This will decrease the temperature of the reactor coolant. The reactor core temperature will drop, and the reactor will increase power, without us doing anything. It's built into the physics.

Conversely, if the load drops, temperature increases, and reactor power decreases.1 If the loads drops completely, say due to a station black out, the reactor temperature will shoot up, and the reactor will shut itself down. The shutdown does not depend on a system that senses a problem and then tries to react to it. There is nothing a confused operator or malfunctioning control system can do to prevent this process.

Further Reading

Till & Chang, Chapter 7: Safety
Jack Devanney's Substack: Nuclear Magic: Where's the Throttle?