Why don't control rods stop meltdown?

braxx20

I thought that control rods were raised and lowered to control nuclear reactions. If so, why wouldn't they simply be lowered in a crises to stop the reaction?

Pentiger

Even after the nuclear reaction has been stopped by insertion of the control rods, the reactor core still is extremely hot and must be cooled. If the cooling system fails, as it did at Fukushima Daiichi, the core will melt.

Astronuc

In Pressurized Water Reactors (PWRs), the control rod drives are located on the upper head of the reactor vessel, and control rods are withdrawn during operation (except for certain reactors using 'grey' power shaping rods) into the region above the core.

In BWRs, the control rods are inserted hydraulically or by fine motion screws from beneath the core. Some BWR control rods are inserted for reactivity control during operation of the core.

When a PWR or BWR is shutdown, it is normal to fully insert all control rods. However, fission products, those radionuclei produced from fission, are still decaying and mostly by beta emission, and some with gamma emission.

Also, in LWRs, transuranic elements, which decay by alpha or beta emission, are formed, and they contribute to the decay heat that must be removed by the residual heat removal system during shutdown of the reactor.

OmCheeto

It's been over 30 years since I studied nuclear reactors, but as I recall, the daughter products of Uranium are pretty much all radioactive, and their decay produces energy. This energy of decay is what causes the temperature of the reactor to go up even after the control rods have been fully inserted. Without forced cooling, the reactor will melt.

Check out the first image in wiki's article on Nuclear fission as an example. It shows how a Uranium 236 atom fragments into a Krypton 92 atom, a Barium 141 atom, and 3 neutrons. The control rods will effectively absorb the 3 neutrons, but they cannot stop the Krypton and Barium atoms from decaying.

Eek! The chart of the nuclides site I just linked to for the Krypton isotope just went down.

Oh well, I suppose you can use wiki: Isotopes of Barium

Just look for ¹⁴¹Ba

Bandit127

It is called decay heat. It is caused by the radioactive elements decaying, that process adds heat to the reactor. As others have said, enough heat to cause damage if sufficient cooling is not kept up. This process continues to create sufficient heat for possible damage for many years.

What I didn't know was that a significant part of the heat generated in the Earth's interior is also due to decay heat.

A Wiki article here:
http://en.wikipedia.org/wiki/Decay_heat

blazzano

Indeed. Lord Kelvin tried to estimate the age of the earth by calculating how long it would take for it to cool from some primordial melted state to its current temperature (i.e. blackbody radiation etc.). He got a few different age ranges over the years, from the 10^8 range to the 10^7 range, and I think he eventually settled for something around 40 or 50 million years. Big difference between that and the actual age - all that is radioisotope decay.

Caniche

So you can insert as many control rods as you like ,but if you lose control of cooling functions those rods will end up as part of corium minestrone,cos control rods melt. but I may be wrong,then again ,whatever happened to "controlled shutdown"?

Astronuc

There are two functions - control and coolability - and both are required/mandatory for a nuclear reactor per the General Design Criteria, 10 CFR 50, App. A.

The control rods stop the fission reaction. The cooling system removes the heat. Decay heat, i.e., thermal energy vis-a-vis decay of fission products, must be removed in a closed system such that the core/fuel is not damaged. The decay heat removal is accomplished with a closed system.

http://www.nrc.gov/reading-rm/doc-co...t050-appa.html

Part 50 of Title 10 CFR, DOMESTIC LICENSING OF PRODUCTION AND UTILIZATION FACILITIES - http://www.nrc.gov/reading-rm/doc-co...s/cfr/part050/

If one wants the entire Title 10 of the Code of Federal Regulations
http://www.nrc.gov/reading-rm/doc-collections/cfr/

Appendix A:
I. Overall Requirements
II. Protection by Multiple Fission Product Barriers
III. Protection and Reactivity Control Systems
IV. Fluid Systems
V. Reactor Containment
VI. Fuel and Radioactivity Control

Caniche

When cooling is lost and the molten control rods dissolve within the molten fuel rods and their molten casements , is it possible that that the molten control rod material acts as an extremely efficient moderator?

Astronuc

No. Water is the only moderator in an LWR. If the temperatures got to the melting point of cladding, there is no water, only superheated steam. Stainless steel melts at a lower temperature than Zircaloy-2 (BWR) or Zircaloy-4/Zr-1Sn-1Nb/Zr-1Nb (PWR), but Zr and Fe do form eutectics.

Basically the molten fuel would exclude water, which would exclude moderation.

The Zr-alloys would react with water in an oxidation reaction of which ZrO₂ and H₂ are products. Actually, Zr-alloys and water react rapidly well below the melting temperatures, so the Zr-alloys would tend to oxidize with available water well before melting. ZrO₂ and UO₂ can also form solutions at high temperatures.

Caniche

Cheers ,How about the control rods in a BWR entering the mix?

Astronuc

Same. BWR and PWR control rods use much the same structural material, SS304L, and in some cases, SS316L, of high purity (i.e., low S and P). PWR control rods typically use more Ag-In-Cd for absorber, but can use B4C. BWR control rods use B4C and perhaps some Hf.

Dmytry

Other potential problem post-melting - older fuel contains fissile materials which have different chemical properties from uranium (e.g. Pu-239), and thus can in principle be concentrated by chemical means, e.g. crystallize first and precipitate from the solution.
I'd guess it would be hard to rule out possibility of their concentration to the level permitting unmoderated criticality, other than by general "that seems unlikely" gut reaction (needs to concentrate from about 1% to something like 5%).

Caniche "is it possible that that the molten control rod material acts as an extremely efficient moderator":

Molten control rod material is no moderator, but neutron absorber. The terminology may be confusing - control rods sure can be said to 'moderate' the reaction by absorbing neutrons, but in nuclear engineering the moderator is specifically the material that slows down the neutrons so that they become more likely to be captured by U235 versus U238, increasing the reactivity - not the neutron absorber. The control rods simply absorb the neutrons, decreasing the reactivity - while they do slow down some neutrons in process, that effect is insignificant.

Caniche

Thank you ,most informative.
My question was probably poorly phrased though. I was aware that the control rod function was to absorb rather than moderate.
I was postulating that perhaps , in molten form ,with fairly uniform dispersion throughout the corium mass ,the control rod material might slow down many more neutrons than it would in it's original configuration.

Dmytry

Ahh. Well, the slowing down of neutron happens by scattering of the neutrons on the nuclei of moderator (exchange of momentum, much like bounce). The control rod material's nuclei (in particular, boron nuclei in boron carbide) instead absorb the neutrons that collide with them, so whenever dispersed or not, boron does not work as moderator. Though, that makes me wonder what happens to boron carbide when it all cooks together. Carbon does not capture neutrons and thus can by itself serve as moderator. But i'd not expect the carbon to become separated and the stuff to form into a shape that'd sustain chain reaction.
Actually only few isotopes of few elements make good moderators. Moderator needs to have very low neutron capture cross section compared to it's scattering cross section. Nuclear reactors only use hydrogen, heavy hydrogen (both as water), carbon, or beryllium as moderators. My impression is that most stuff tends to capture neutrons too much.

jim hardy

The moderator and the control rod act as a team.

Boron is a great absorber of neutrons that have already slowed down but it is nearly transparent to fast ones.
The moderator, which is really the hydrogen atoms in water, slows them down so that they can be absorbed by either fuel or the poison (boron) in control rods.

So academically your answer is yes - the boron in corium is likely to participate in the neutron slowing down process before it absorbs them. Not much hydrogen in corium so moderation gets done by other atoms.
But the likelihood of fission in fuel is smaller for fast neutrons than for slow ones so the two effects pretty much cancel. in fact more than cancel.
There's a better chance of non fission absorbtion in the slowing down process in corium than in water- that's because to slow down neutrons with heavy atoms takes more collisions than it does with light ones. Remember freshman physics and elastic collisions , ....

once the neutrons have got slowed down the boron and fuel compete for the survivors as usual.

A google on "neutron absorbtion cross section" will get you to a lot of articles. Look at graphs of cross section vs energy for various elements. .
then compare to fission cross section.

A real nuke would give you a more erudite answer but perhaps this helps you get a 'feel' in preparation for that. if i'm pestering you please advise.


edit i see Dmyrty already clarified better than i. Indeed when you add to the mix steel from reactor parts you have added mild poisons.

Fast fission is another whole field of study of which i am mighty ignorant. Are you heading that direction?
I was worried in first few days about all that salt but convinced myself it was not a concern.


old jim

Morbius

braxx20,

The control rods control the fission reaction. Meltdowns come from decay heat
due to the radioactivity of the fuel. Control rods don't control radioactivity, hence they don't
prevent meltdowns.

Greg