Sunday, 3 April 2011

Fukushima isn't going to go boom nuclear bomb style!

Rebuttal to “Remote Possibility of thermo nuclear at Fukushima”

This blog and comments from Robert Alvarez are so wrong I’m driven to put them straight.

“While the spent fuel rods and some rods in #2 reactor at Fukushima are  MOX fuel rods ( 6% plutonium ) And if the control rods were not fully deployed”

The control rods position makes little different to the proposed outcome. Also it is known that as soon as the quake hit the reactors were scrammed (resulting in insertion of shutdown rods). Secondly the reactors have been pumped with thousands of litres of borated water (a neutron absorber). The shut down rods would have brought the reactor subcritical at the time of the quake. Other factors contribute to keeping a reactor critical (or subcritical), shape of the core, water moderation (flow, voids, temperature). Neutron poisons (absorbers) are also present in various forms inside a reactor. One significant neutron poison Samarium-149 actually doubles in the 15-20 days following a reactor shutdown. Removing residual decay heat is the problem that has caused the main difficulty up to this point.

or if the spent MOX rods were damaged in the chemical explosions there is a possibility of a melt downand temps in the 5000 to 10000 degree range.”

Presumably the blog means 5k-10k Deg F? Either way the top end of 10k Deg F is far too hot . For comparison 10k Deg F is 60 Degrees above the average temperature at the surface of the sun.

“This could result in a uncontrolled chain reaction and another chemical explosion, similar to Chernobyl. But at the 10000 degree range is there the possibility of a thermal nuclear explosion?”

This information aside, even if the core of the reactor got this hot this could and would not result in an “uncontrolled chain reaction”. Why not? Because a critical nuclear fission chain reaction has nothing to do with the temperature of the nuclear material. This is one of the most common misconceptions surrounding all things nuclear. Although professionals use terms like “burn-up” and “burning nuclear fuel”, there is no burning in the conventional sense. Temperatures of nuclear fuels have no bearing on their state of criticality. Hypothetically even if nuclear fuel could reach one million degrees it wouldn’t cause a nuclear explosion.

Now to address Robert Alvarez comments. Video here (

“fuel core really goes into a meltdown and the fuel starts to slump, that quarter ton of plutonium can concentrate [inaudible] there'll be too much in one place at one time. And that can cause what they call a major criticality event. “

The notion that a non homogenous mixture of a totally melted reactor core, with various posions, and mixed with cladding, and many other materials will settle at the bottom of the reactor and then become super critical is absurd. Also there seems to be undue focus on the fact that 6% of the material is Plutonium. Criticallity can be acheived with any fissile material for example the fissile uranium has not been mentioned (further highlighting the confusion(?)) of the speaker.

Why is the notion of a nuclear explosion absurd? Nuclear explosions are not easy to achieve. They require highly enriched concentrations of fissile material, machined into specialist shapes, the addition of neutron reflectors (and usually neutron sources). Additionally the required concentrations of fissile material (say Pu239) are compressed into tiny masses many times their critical mass to create the super critical masses required to provide an exponential chain reaction. This all has to happen without the presence of anything to absorb neutrons that would hinder the creation of a runaway chain reaction.

In summary, a nuclear reactor core in operation provides probably the worst environment to create a nuclear explosion, a fully or partially melted one is even worse.