The roles of water in a light water moderated nuclear reactor

In March 2011 there was a great urgency around the world in getting cooling water into the nuclear reactors at Fukushima Diiachi.

At the time, there was concern when it was revealed that TEPCO apparently ordered a delay in the use of seawater as a coolant. TEPCO staff on site apparently disregarded the order and used seawater as coolant urgently.

Two contradictory roles of water in light water moderated nuclear reactors.

1. The water acts as a coolant.
2. The water as a neutron moderator.

The term coolant is self explanatory. The water removes heat from the reactor fuel and pressure vessel.
The term moderator is not so simple. Applied to nuclear reactors, the word moderator means “Neutron moderator”:

“In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.
Commonly used moderators include regular (light) water (roughly 75% of the world’s reactors)…”

It can be seen that if the sources of heat in a nuclear reactor come from 1. sustained fission chain reaction 2. Decay heat emitted by fission products created by the fission chain reaction and that if the water is the neutron moderator which allows fission chain reaction to occur, then the very medium which allows the sustained chain reaction to occur and which also cools the reactor, has two contradictory or paradoxical jobs to do. And that the balance between the two outcomes of its roles – the creation of heat and the removal of heat – results is complex and changing balances within the nuclear reactor.

So in the case of the Fukushima Diiachi reactors, the decision to add cooling water was also a decision to add a neutron moderator. That is, the decision to add water was a decision which took into account the additional heat adding the water would cause.

Despite adding cooling water (and thus increasing fission in the nuclear fuel, creating more immediate heat and creating more fission products which would in turn add more decay heat – the enemy being fought in order to stave off meltdown and containment breach (both of which occurred, despite denials which lasted many weeks in a deception by the Japanese government, TEPCO and the world nuclear industry) was actually not defeated by adding coolant or by getting coolant pumps working.

This balance of heat generation vs heat removal in nuclear reactors is a critical one. The TEPCO operators lost control of this fine balance, the cores overheated and melted. In an effort to keep the cores and reactors cool, water has been pumped in ever since. The many tons of it used has been pumped into the reactors and it has drained out through the damaged bases of the reactors into the basements of the reactor buildings.

In addition to water, which is used to both cool and control the nuclear reactor, three other control options are plainly incorporated into reactors – the use of neutron poisons within the fuel, the use of control rods which absorb neutrons in the same way as neutron poisons built into the fuel, and the use of neutron poisons in the cooling water/moderator medium (water). A common neutron poison is boric acid.

At the start of the emergency at the Fukushima Diiachi nuclear reactor, South Korea offered to send some boric acid to Japan. (Yonhap News Agency Seoul March 16 2011 “(LEAD) S. Korea to ship boric acids to help stabilize Japan’s nuclear reactors
SEOUL, March 16 (Yonhap) — South Korea plans to transfer its reserve of boric acids to Japan to help the country stabilize quake-damaged nuclear reactors that have started to release radioactive material, the government said Wednesday.

The Ministry of Knowledge Economy said that Tokyo requested assistance of the key material vital for stopping fission nuclear reactions after its own stockpile was largely used up at the Fukushima nuclear power plant.

The state-run Korea Hydro & Nuclear Power Co., which operates South Korea’s 21 commercial reactors, said it has 309 tons of the material in powder form and could transfer 52.6 tons in the next few days, the ministry said. It said the remainder is adequate for six months of use.

“Seoul has decided to provide whatever reserves it can spare without jeopardizing local nuclear operations,” a ministry official said. “Depleted reserves will be restocked once the emergency situation has passed.”)

In the event, fuel melted through the pressure vessels of multiple reactors and even though no authority knows the exact disposition of the fuel, nor of the fuel’s actual temperature, Japan declared “Cold Shut Down” in 2011. The declared “Shutdown” in March 2011 must have been hot. Anyway, TEPCO has no way of directly observing the water level in the reactors, which runs out of the reactors, still, very quickly. If the reactor vessels are cool, its only because the molten is not present in them, but rather exists at their bases or at some other point below the reactor pressure vessels.

contaminated water storage containers continue to sprout at the Fukushima Diiachi site. March 11 2012 photo from
Reuters/Kyodo News.

If fresh fuel rods are installed into a brand new nuclear reactor prior to water being added, no sustained chain reaction would occur. The reactor would not overheat as there would be no fresh fission products present sufficient to cause decay heat.

It is only after water is added that neutrons are moderated enabling fission chain reaction to occur. It is only after water is added that heat is generated in a reactor.

A loss of coolant accident is a complex accident.

The following Wikipedia quotes help explain why it was that nuclear industry experts claimed the reactor explosions and containment breaches were proof of safe reactor design (they actually prove that what happened at the Fukushima reactors actually happened. That is, loss of containment, with core material delivered into the living space. That is the nuclear industry best case. They say it proves the safety of nuclear reactions. The reverse is true of course.) Anyway here’s the quotes which explain the propaganda: “The final, deliberately destructive test in 1954 produced an unexpectedly large power excursion that “instead of the melting of a few fuel plates, the test melted a major fraction of the entire core.” However, this core meltdown and release of nuclear fuel and fission products provided additional useful data to improve mathematical models. The tests proved key safety principles of the design of modern nuclear power reactors. Design power of BORAX-I was 1.4 megawatts thermal. The BORAX-I design was a precursor to the SL-1 plant, which was sited nearby and began operations in 1958. The principles discovered in the BORAX-I experiments helped scientists understand the issues which contributed to the fatal accident at SL-1 in 1961.”

Why say the explosions proved safety? because it blew the core apart. “The tests proved key safety principles of the design of modern nuclear power reactors. ” The Fukushima nuclear disaster is supposed to be, in the industry view, proof of the last control mechanism – an explosion which destroyed the cores. (which melted anyway). These dudes think only in terms of big bangies and have spent decades denying the dangers of fallout. When the reactors went up, the nuker voice overs down here on TV said “this is normal”. “One kinetics factor is the tendency of most light-water-moderated reactor (LWR) designs to have negative moderator temperature and void coefficients of reactivity. A negative reactivity coefficient means that as the water moderator heats up, molecules move farther apart (water expands and eventually boils) and neutrons are less likely to be slowed by collisions to energies favorable for inducing fission in the fuel. Because of these negative feedback mechanisms, most LWRs will naturally tend to decrease their rate of fissioning in response to additional heat produced within the reactor core. If enough heat is produced that water boils inside the core, fissions in that vicinity will drastically decrease.”

But both the Sl-1 and the Fukushima reactors did what they did and the accident of March 2011 shows that what happened in Japan did actually happened, even if nuclear industry claims the explosions are proof of safety. Which they aren’t.

The worst outcome of the Fukushima nuclear diaster is what actually occurs. The reactors remain breached and leaking, and the disposition of the cores remain unknown.

And so there are people concerned about an increased rate of emissions of core contents should a combination of events act in conjunction with the volumes of water interacting with the corium in such a way as to produce a sudden increase in the rate of fission.

Conventionally, molten fuel, being less dense than solid, is more removed from sustaining a chain reaction.

But what the hell would I know. Fission occurs spontaneously in various percentages according to the type of fuel.

The contamination which has robustly been documented now in Japan and in food sources of other lands is any indication, the claimed last control input, the destruction of the core, is not a proof of safety as claimed, but rather an expected worst outcome. What has happened has indeed happened. And it is not safe.

As many qualified people have stated, the event is not over and it will not be until 1. The reactors have been dismantled. That won’t happen, according to the Japanese Government, for 30 or 40 years. 2. All farm land and living areas are clean. That will take decades.

In the meantime, some experts worry the situation may worsen.

Given the problems of cracking and crumbling and poor servicing of control rod blades in US Mk1 reactors,
I personally wonder at the prospect of the same issue affecting Japanese Mk1 control rods. How mach earthquake damage did the Fukushima Diiachi and Dani control rods suffer and were all those control rods successfully inserted into the cores in March 2011?

Clearly, as stated by Japanese nuclear authorities, the full facts regarding the past and present disposition
of the reactors and the fuel is not known. This, despite the pat statements regarding the alleged safety which is claimed.

This post is relation to the reactor cores. The situation regarding the spent fuel pools remains.

Evacuate Fukushima.

This is crap: “The final, deliberately destructive test in 1954 produced an unexpectedly large power excursion that “instead of the melting of a few fuel plates, the test melted a major fraction of the entire core.” However, this core meltdown and release of nuclear fuel and fission products provided additional useful data to improve mathematical models. The tests proved key safety principles of the design of modern nuclear power reactors.”

The proof is actually the actual performance and results of the systemic, multiple and predicted failures of the Fukushima reactors. What actually happened and is happening is the actual case.

One Response to “The roles of water in a light water moderated nuclear reactor”

  1. CaptD Says:

    Wate,r water everywhere but it is all radioactively polluted…

    The tank fram looks impressive but what about all the “other” water that has been leaking or dumped into the Pacific Ocean?

    The Japanese Gov’t. is playing a radioactive JOKE on the Planet by allowing ever more polluted water to find its way into the Pacific Ocean.

    More here:
    Fukushima plant “to keep contaminating the Pacific Ocean for the rest of time” if fuel can’t be removed — No good solution, must constantly pump out water from under buildings (VIDEO)

    and here:
    More on Ocean dumping:
    Nuclear waste dumped at “SEA”
    Ocean disposal of radioactive waste:
    Status report:

    Maybe all that is now also part of our Background readings!

    ALL because of this:
    The Trial Of Minoru Tanaka: The high cost of investigative journalism in Japan & “the nuclear mafia

    Why information about nuclear reactors does not appear in MSM, sounds familiar!

    So given the above, why should we expect Japan to start talking about how they are polluting the Pacific except to issue ever more “leak” reports that do nothing to describe the ever growing “TOTAL” amount they are dumping, allowing to leak and every other thing they can do to get rid of radioactive water…

    I suggested building a cofferdam about a year ago and then contain the radioactive water but they have been dragging their feet and all the while it continues to flow into the Pacific Ocean!

Comments are closed.

%d bloggers like this: