Misunderstood Uranium – Boogeyman and Small Boxes

If we are to get serious about reducing greenhouse gas emissions, we must put down the mud balls, enjoy some sacred-cow burgers, burn the little boxes in which we confine ourselves, and maybe have a little counseling about that boogeyman under the bed.

This post first addresses the little boxes and may assuage fear of the boogeyman. A major step in the right direction is a rebirth of nuclear power as discussed in a recent post, The Nuclear Option.

One thing is for sure, if you find yourself in any sort of reader comment and chat session, there is apparently no shortage of PhD nuclear physicists. Readers can find every combination of nuclear fuel and coolant there is – from uranium to thorium, heavy water, light water, and liquid salt, which I would guess is what we used to call liquid sodium.

I have been out of the nuclear business for 20 years, and I see that the nuclear plants under construction for Georgia Power are using the same fuel and coolant as we used back then: Uranium 235 and light water (the stuff you drink).

Over 99% of natural uranium comes in the form of U238. The 238 is simply the atomic weight, and if you remember from chemistry or maybe physics, the atomic weight is essentially the sum of protons and neutrons an atom possesses. The other 0.7% of natural uranium is U235. Uranium 238 is stable and won’t fission (split) to produce heat and power. Uranium 235 will, once it absorbs a neutron. In the manufacture of nuclear fuel, it is “enriched” to contain something like 5% U235 for commercial use.

As mentioned, U235 must absorb a neutron to fission. Consider neutrons to be like billiard balls, which upon a fission, may be akin to a high speed projectile. Like a bullet, such a neutron can penetrate human tissue. However, we have these lovely molecules called H2O, light water. Neutrons collide with water molecules to quickly lose their energy and thus all we need for neutron protection is water.

For the fissile U235 to fission and produce heat/power, it must absorb a “thermal” neutron. A thermal neutron is one that has slowed by colliding with a bunch of water molecules. This is (pun alert) critical[1]. A nuclear reactor for power generation is self-regulating. More fissions produce more heat. More heat decreases the density of water. Lower water density results in fewer thermal neutrons. Fewer thermal neutrons leads to less fission and heat. To sum: a nuclear reactor automatically adjusts power (nuclear chain reaction) to maintain water temperature.

Beautiful. Extremely stable.

There are obviously many other design features and issues to consider. One is control rods. Control rods are essentially neutron sponges. Simply, when the fuel (U235) is new, the rods are furthest into the core to maintain an appropriate chain reaction and desired water temperature. As U235 is consumed over a period of months/years[2], it’s slowly extracted – like over months/years. There are other ways to control reactivity over the life of the fuel, but I’m not going there for now at least.

Developed countries have a decades-long, safe track record of operating under nuclear power. The only boondoggle in the US was of course Three Mile Island (TMI), which occurred before a large share of you reading this were born. What happened there? Refer to the chart, courtesy Nuclear Regulatory Commission, and follow along below.

At TMI, the primary pumps that move water/heat from the reactor core where heat is produced by fission, inadvertently shut down. The reactor scrammed[3] as designed. The pressurizer that maintains steady high pressure on the reactor side experienced a relief valve lifting – which allowed primary coolant to discharge from the system. The operators apparently had indications that the valve had closed. The water level in the pressurizer had stabilized. However, what was happening is a steam bubble had developed and uncovered the fuel on the top portion of the reactor. Water from the reactor was maintaining level in the pressurizer as it leaked – not good.

With no water (except vapor), there was no fuel fissioning, but decay heat resulted in fuel (metal) melting, and thus the infamous “meltdown”. This worst accident in the country’s history resulted in zero deaths and zero quantifiable health hazards. Since that time, roughly 1.4 million people have been killed in the ubiquitous automobile, in the US alone.

Tell me – Is fear of nuclear power hyper insanity and the mother of all irrational thought, or what?

Per my read of the publicly available Georgia Power brochure, it appears the fuel technology has hardly budged, if at all in the past twenty years and probably going back to the 1970s. The thing that has changed is the plant design – to be simpler and still safer. The design includes fewer moving parts and less piping and cable.

The other thing is passive cooling systems are being added. Fuel damage at Fukushima was due to loss of all available driving forces for coolant (water). This included backup power from offsite as well as on-site generators, which were all wiped out. Passive cooling systems simply use the wonders of gravity and density to move coolant with no external driving force at all. Decay heat from a scrammed reactor heats the water making it less dense. The water rises and circulates to a giant heat exchanger where it is cooled, becomes denser and sinks – round and round it goes. I don’t know the specifics of the giant heat exchanger, but something like a swimming pool would work just fine.

[1] Critical is the term for a self-sustaining, nuclear chain reaction required to generate power.

[2] Months for low enrichment; years for high enrichment.

[3] Control rods dropped to shut down reactor.