physics

Nuclear reactor coolant

Nuclear reactor coolant is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nuclear reactor coolant rather than just read about it. In short: A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.

Key takeaways

Nuclear reactor coolant belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nuclear reactor coolant to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nuclear reactor coolant from memory before moving on to harder problems.

Reference excerpt

A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment. Frequently, a chain of two coolant loops are used because the primary coolant loop takes on short-term radioactivity from the reactor.

Water Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase transition to steam inside the reactor. About 2/3 are pressurized water reactors at even higher pressure. Current reactors stay under the critical point at around 374 °C and 218 bar where the distinction between liquid and gas disappears, which limits thermal efficiency, but the proposed supercritical water reactor would operate above this point. Heavy water reactors use deuterium oxide which has identical properties to ordinary water but much lower neutron capture, allowing more thorough moderation.

Disadvantages

Tritium leak As the hydrogen atoms in water coolants are bombarded with neutrons, some absorb a neutron to become deuterium, and then some become radioactive tritium. Water contaminated with tritium sometimes leaks to groundwater by accident or by official approval.

Hydrogen explosion Fuel rods create high temperatures which boil water into steam. During a power outage, diesel power generators which provide emergency power to water pumps may be damaged by a tsunami, earthquake or both; if no fresh water is being pumped to cool the fuel rods then the fuel rods continue to heat up. Once the fuel rods reach more than 1200°C, the zirconium tubes that contain the nuclear fuel will interact with the steam and split hydrogen from water molecules. This hydrogen may leak from breaches in the reactor core and containment vessel. If hydrogen accumulates in sufficient quantities – concentrations of 4% or more in the air – then it can explode, as has apparently occurred at Fukushima Daiichi reactors No. 1, 3, and 4. Such an explosion was avoided at Reactor No. 2, which opened its vent to let out hydrogen, decreasing pressure by releasing radioactive hydrogen gas.

Borated water Borated water is used as a coolant during normal operation of pressurized water reactors (PWRs) as well as in Emergency Core Cooling Systems (ECCS) of both PWRs and boiling water reactors (BWRs).

Advantages Boron, often in the form of boric acid or sodium borate, is combined with water — a cheap and plentiful resource — where it acts as a coolant to remove heat from the reactor core and transfers the heat to a secondary circuit. Part of the secondary circuit is the steam generator that is used to turn turbines and generate electricity. Borated water also provides the additional benefits of acting as a neutron poison due to its large neutron absorption cross-section, where it absorbs excess neutrons to help control the fission rate of the reactor. Thus, the reactivity of the nuclear reactor can be easily adjusted by changing the boron concentration in the coolant. That is, when the boron concentration is increased (boration) by dissolving more boric acid into the coolant, the reactivity of the reactor is decreased. Conversely, when the boron concentration is decreased (dilution) by adding more water, the reactivity of the reactor is increased.

Disadvantages Approximately 90% of the tritium in PWR coolants is produced by reactions of boron-10 with neutrons. Since tritium itself is a radioactive isotope of hydrogen, the coolant becomes contaminated with radioactive isotopes and must be kept from leaking into the environment. Additionally, this effect must be taken into account for longer cycles of nuclear reactor operation and thus requires higher initial concentration of boron in the coolant.

Molten metal Fast reactors have a high power density and do not need, and must avoid, neutron moderation. Most have been liquid metal cooled reactors using molten sodium. Lead, lead-bismuth eutectic, and other metals have also been proposed and occasionally used. Mercury was used in the first fast reactor.

Molten salt Molten salts share with metals the advantage of low vapor pressure even at high temperatures, and are less chemically reactive than sodium. Salts containing light elements like FLiBe can also provide moderation. In the Molten-Salt Reactor Experiment it even served as a solvent carrying the nuclear fuel.

Gas

Gases have also been used as coolant. Helium is extremely inert both chemically and with respect to nuclear reactions but has a low heat capacity.

Hydrocarbons Organically moderated and cooled reactors were an early concept studied, using hydrocarbons as coolant. They were not successful.

References

Sodium as a Fast Reactor Coolant, Thomas Fanning, ANL Compares sodium favorably to lead and helium.

External links Media related to Nuclear reactor coolants at Wikimedia Commons

Worked examples

Example 1 — a first encounter with Nuclear reactor coolant

Start with the simplest possible case. Write down what Nuclear reactor coolant claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nuclear reactor coolant before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nuclear reactor coolant ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nuclear reactor coolant

In research: Nuclear reactor coolant appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nuclear reactor coolant in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nuclear reactor coolant is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nuclear reactor coolants, so understanding it makes those chapters shorter.
In everyday life: Look for Nuclear reactor coolant outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Nuclear reactor coolant in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nuclear reactor coolant means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nuclear reactor coolant out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nuclear reactor coolant in simple terms?

Why does Nuclear reactor coolant matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nuclear reactor coolant?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nuclear reactor coolant.