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. 

Sunday, 13 March 2011

How should events in Japan effect Britain's new builds?

The recent events at Japans nuclear power facilities has once again thrust nuclear power to the forefront of world attention. If that should have been the case given the extreme natural disaster at work in Japan at the time is a different discussion.

How should the events in Japan effect Britain's new nuclear builds?

Firstly there is a big difference between how something should affect a situation and how it will.

The events in Japan shouldn't really have any great effect on British new builds. Some of the immediate lessons to be learned is that in the event of an accident communication is key. 

However I believe what will actually happen is as we are seeing in Germany, the ever-present  anti-nuclear factions will seize and exploit recent events for all they are worth.

The handling of the reactor issues in Japan, far from being a catastrophe and a 'death blow' to nuclear power actually demonstrate that in the face of the largest natural disaster the country has ever experienced problems can be overcome.

So let's do whatever we can to ensure than the events of Fukushima over the past few days will be remembered for the right reasons rather than wrong ones because the coming wave of baseless, unnecessary anti-nuclear bile will pose a direct threat to Britain's (and the worlds) future energy security.

Saturday, 12 March 2011

Fukushima 1 (Daiichi) and Fukushima 2 (Daini) Layouts

Given the various news reports on the two power stations I've been having a hard time keeping track of the layout of both. I'm putting these images here mainly for my own orientation but maybe they will help you too. You can click the images to enlarge.
Overhead view of Daiichi (Fukushima 1)

Fukushima 1 Daiichi (Credit: Unknown)
Plan view of Daiichi (Fukushima 1) 

Fukushima 1 Daiichi (Credit: Google)

Overhead view of Daini (Fukushima 2)

Fukushima 2 Daini (Credit: TEPCO)

 Plan view of Daini (Fukushima 2)

Fukushima 2 Daini (Credit: Google , notes by me.)

Sunday, 9 January 2011

It's not waste it's used fuel!

We all know that nuclear power often suffers with a public perception problem. The industry has done itself few favors over the years by permitting and even acknowledging certain to be regarded materials as waste.  The main material I want to focus on for the next few blog entries is uranium fuel.

Values used in my writing will relate to typical numbers for PWR type reactors but the principles apply to any thermal neutron reactor that uses Uranium fuel.

Natural uranium is mined and processed, enriched and fabricated into fuel. Reactor fuel is typically enriched to around 3.6% to 4.1%. That means the U-235 (which is needed for is fissile properties) is increased to that percentage over the more abundant U-238. In contrast natural uranium as mined contains  around 0.7% U-235.

The diagram to the right illustrates the concept of enrichment. I have left the weapons grade enrichment of uranium in place to illustrate just how much enrichment is required to obtain something that could be used in a nuclear weapon verses what is used in reactors. It is important to realise that low enriched Uranium (LEU/Reactor grade) is VERY different to high enriched uranium (HEU/Weapons Grade).

So this reactor grade uranium in the form of fuel assemblies goes into a power reactor. In current reactors it will generally remain there for at least 18 months where the reactor will use it to generate electricity. The fuel assemblies are eventually removed and replaced with new fuel. However what is not obvious is the "spent" fuel that is removed has had less than 1% of the available energy it contains used.

This "spent" fuel, which from now on I will call used fuel is as it is in no way spent is currently generally labeled as waste and treated as such.

The majority of the fuel will still be uranium except it will now have other radioactive elements present known as fission products and actinides. We want to recover most of what is found in the used fuel as it can be used for other purposes mainly producing more fuel.

Currently used fuel will be stored for a number of years often on site at the reactor as it will continue to produce radioactive decay heat. This cooling usually happens in fuel pools which are as exactly what they sound like, pools of water.

Currently the intended destination for this used fuel following several years of cooling is geological storage underground. However to date no undeground geological repository has been brought into operation.

Why are we looking to store used fuel that has 99% of it's energy remaining underground? In the past nobody was quite sure if this used fuel was an asset or a liability. This has led to the used fuel being poorly branded as waste along with all the perception problems that generates.

People rightly ask, "if this isn't waste why must we lock it underground?". Right now we need to shake of the idea that used fuel is waste. Lets not even brand it "spent fuel". Spent implies that it's of no further use and that is energy is depleted.

This is not the case. What comes out from a nuclear reactor is not and should not be viewed as waste. With a combination of reprocessing and recycling this used fuel can be used again and again to provide ultra-low/carbon free power for at least several thousand years.

So the next time somebody asks "Well what about the waste?" at least explain that the fuel isn't really waste at all and can be reused if we want to and we are not simply leaving burdensome 'waste' behind for the next generation. 

More to come on the how of recycling next time.