Frequently Asked Questions

Frequently Asked Questions

Nuclear Physics

What is fission?

Fission is the nuclear process that involves the splitting of a nucleus. At the MIT Reactor Lab uranium-235 fissions in the core to produce heat (which we don’t use) and neutrons (which we use for research and experiments). Some isotopes such as californium-252 can spontaneously fission, though most isotopes that are can undergo fission need some stimulation or disruption, such as the absorption of a neutron, in order to cause fission.

What is a chain reaction?

For example, when a nucleus such as uranium-235 fissions, it emits neutrons. Those can hit other nearby uranium-235 atoms and cause those to fission, emitting more neutrons. This process is the fission chain reaction. This chain reaction can continue if there are enough fissile nuclei in a small enough space, and the neutrons don’t get absorbed by other materials or leak out from that space.

Where do the neutrons come from?

While the reactor is running, the vast majority of neutrons are produced by the fission of U-235 in our fuel.

What are isotopes?

Isotopes are different versions of elements. They are named using the letter abbreviation of the element and the total number of protons and neutrons in the nucleus. For example, the most common isotope of natural hydrogen has just one proton a no neutron, so it is referred to as H-1. A small amount of hydrogen (about 0.01%) has one proton one neutron, so it is referred to as H-2. Because different isotopes of the same element have the same number of protons and electrons, they behave similarly in their chemistry (they may behave slightly differently physically due to their different masses). However, different isotopes of the same element can have very different nuclear properties.

What is heavy water?

Heavy water is mostly just like ordinary water (H2O) but with the regular hydrogen atoms (H-1) replaced by atoms with an extra neutron (H-2). Unlike a lot of isotopes, H-2 has a special name we can call it besides hydrogen: Deuterium. Deuterium is sometimes given the symbol ‘D’ so heavy water can be referred to as D2O. The name ‘heavy water’ is used because deuterium is twice as heavy as regular hydrogen.

What do neutrons look like?

Neutrons are invisible just like all forms of electromagnetic and particle radiation (except for the visible portion of the light spectrum).

How can you have neutrons without protons and electrons?

Isolated neutrons have a half-life of 10.4 minutes and decay to stable hydrogen (H-1) – the neutron essentially splits into a proton and an electron. The neutrons produced in the reactor core get absorbed by fuel or core materials in fractions of a second.

What is radioactive decay?

Radioactive decay is the process through which some atoms revert to a more stable nuclear configuration by emitting energy (e.g., gamma rays, electrons, alpha particles). Most elements have both radioactive and stable (non-radioactive) isotopes. When an atom decays it may become a different element, or the same element with a different number of neutrons (a different isotope of the same element). Sometimes an atom will decay into an atom that is also radioactive producing a radioactive decay chain. The rate at which an isotope undergoes radioactive decay is described by the isotope’s half-life, which is the amount of time it takes for half of any amount of that radioactive isotope to decay.

Reactor Engineering

What does it mean to go ‘critical’?

’Critical‘ is the term used to describe a reactor state in which the number of neutrons being produced equals the number being absorbed, which in turn produces the same number of neutrons. In other words, when the reactor is critical, it is operating at steady-state.

What is a ‘scram’?

A scram is an automatic reactor shutdown. There are many instruments that are designed to automatically shut down the reactor if a parameter is outside its nominal range (for example, if the water temperature becomes too high). When an instrument sends a scram signal, the control blades (which are held up by electromagnets) drop down by gravity, shutting down the reactor in less than one second.

What’s the difference between radiation and contamination?

Radiation is energy that is travelling, and it can be absorbed (for example, your skin absorbs UV radiation from the sun). Contamination is material that emits radiation that gets stuck to surfaces and can be transferred and spread by physical contact. Contamination can behave just like dust or glitter.

How do you measure radiation?

There are many devices used to measure radiation. Among them are:

  • Geiger-Mueller counters (also Geiger or G-M counter) which are simple, often handheld devices that can measure radiation when the gas in the tube becomes ionized from the radiation (i.e. the radiation knocks electrons around). The ionized gas allows a current spike to occur across the high voltage field in the tube, and is frequently used to produce an audible click.
  • Film badges are small radiation detectors that radiation workers generally wear to measure their long-term (e.g., weekly or monthly) radiation exposure. After the exposure period of interest is over, the film can be developed and the total dose determined.
  • High-Purity Germanium (HPGe) detectors that measure gamma rays and provide excellent information about the gammas’ energies. This enables the identification of what isotope is emitting the radiation.

What do you do if something or someone gets contaminated?

Because contamination is material stuck to a surface it can usually be removed by wiping or washing it off. When a small piece of equipment becomes contaminated it might be cleaned using a wet rag. A typical personal item that might get contaminated is the bottom of a shoe, which can have a piece of radioactive material stuck to it. This can often be removed using sticky tape, or by washing it with a brush.

The MIT Reactor

How big is the core?

The core is a hexagonal prism about 1.5 ft from side to side, and about 2 ft tall.

How is the nuclear reaction controlled?

The rate of nuclear fission in the core of the reactor is controlled by ’control elements‘ that absorb neutrons. The MIT Reactor uses 6 control blades that hug each vertical face of the core, and one regulating rod on one of the corners. The control blades are stainless steel with approximately 1% boron, and they are used for large power changes (e.g., startup and shutdown). The regulating rod is aluminium with a cadmium wrap that absorbs fewer neutrons and is therefore good for small power manipulations (small adjustments to keep power steady). When the control elements are fully inserted in the core, they absorb too many neutrons to allow the reaction to continue. In order to start up the reactor, the control elements are withdrawn one-by-one very slowly, allowing the reaction to progress, until the desired reactor power is reached.

If you don’t generate electricity then how do you know how much power you are generating?

We have two ways to measure the power we generate. The heat that is generated in the core is removed by the reactor’s cooling systems. The amount of heat leaving the reactor via the cooling systems (essentially the temperature drop across the cooling towers) corresponds to the amount of thermal energy being generated. Secondly, there are detectors around the core that measure radiation, and the signal intensity (amount of radiation absorbed by the detectors) correlates to the power being generated.

What do you use neutrons for?

Neutrons have many uses in physics, materials science, chemistry and medicine. Students and researchers use neutrons for materials testing in the fields of crystallography, radiation damage, instrumentation, radiation resistance, and neutron scattering and attenuation characterization. We also use neutrons for Neutron Activation Analysis (NAA) which is an analytical chemistry technique for determining concentrations of elements in materials, Neutrons have been used for external beam radiation therapy (the MIT-NRL previously participated in Boron-Neutron Capture Therapy (BNCT), and are still used for creating seeds for radiotherapy.

Can the reactor explode?

Fortunately, the reactor cannot explode. A nuclear explosion cannot occur because the fuel is not compact enough to allow an uncontrolled chain reaction. The MIT reactor has a lot of water and core structural materials that slow the neutrons down before they reach other fissile atoms. Even an uncontrolled reaction would happen too slowly to cause an explosion. A thermal explosion cannot occur because our reactor is designed to want to shut down on its own as temperature increases (i.e., it has a negative temperature coefficient), and also the system is not under pressure.

Is the reactor safe?

Safety is our highest priority at the MIT Reactor Lab. We pride ourselves on our safety culture and our safety track record. Nuclear safety is provided by thorough analyses of our reactor’s ability to cope with extremes, our internal procedures that ensure that we always operate the reactor with significant safety margins, and comprehensive inspections of our facility by the U.S. Nuclear Regulatory Commission. Personnel safety is provided by the programs we have in place to perform environmental radiation monitoring, track radiation dose to staff and visitors, and ensure that the appropriate protective equipment is used by all staff performing work. The nuclear industry promotes a very strong safety culture which we embrace at our facility.

How often does the fuel have to be changed?

The reactor is refuelled approximately every 3-4 months, but not all the fuel is replaced at once. Usually 2-3 new fuel elements are installed, 2-3 old elements are removed and the remaining elements are shuffled. The shape of our fuel elements allows them to be rotated 180 degrees and flipped upside-down during refuelling in order to get the most out of each element before it is discharged.

How does the fuel get into the core?

We load the fuel into the core by hand using special tools.

What happens to the spent fuel once it leaves the facility?

The fuel is stored indefinitely. The United States does not currently have a final repository or reprocessing program for nuclear fuel. The technology exists, and other countries such as France and Sweden have the capability to reprocess and reuse spent fuel. There are even reactors that have been designed to run entirely on unprocessed spent fuel.

Where does the fuel come from?

Our fuel is uranium-235 enriched to 93%, and it came from the “megatons to megawatts” agreement between the U.S. and Russia to deplete Russia’s nuclear weapons stockpile.

What is shielding, and how much shielding surrounds the core?

Shielding is material that absorbs or blocks radiation. It is usually placed around a source of radiation to limit exposures of nearby workers and the public. Around the reactor core itself there are several feet of shielding material including combinations of water, graphite (a neutron reflector), lead (a good absorber of gamma radiation), and concrete. There is also shielding in the walls of the reactor’s containment building, and around many other radioactive sources at the lab. All the concrete surrounding the core and used in the construction of the containment building is special heavy concrete containing metal slugs in addition to water, sand and cement, making it approximately five times more effective at shielding from radiation than regular concrete.

What is the ‘containment’ building?

The MIT reactor’s containment building is the blue, domed structure that you can see from the street. It is designed to contain radioactive material. It is sealed and the inside air is maintained a slightly lower pressure that the air outside so that if there were a leak in the containment air would leak in, rather than out. To maintain this pressure difference, there are airlocks at all of the entrances to the containment building.

How long does it take to start up the reactor?

It takes an experienced shift operator and supervisor approximately 6 hours to perform all the necessary valving, system checks and scram checks, and another 1-2 hours to withdraw the control blades to the critical bank height and raise power to the desired operating power level. If the reactor was operated within the last 24 hours then it can be restarted in less than 2 hours. It takes less than 1 second to shut down the reactor and another hour to perform the normal shutdown valving and checks.

Do students really operate the reactor?

Yes! We hire MIT undergraduate students and train them to be operators. We teach them everything they need to know from reactor physics to how the reactor’s pumps and valves work. After an intensive training program, and the students pass the NRC’s licensing exam, they can take shifts and operate the reactor. The students who learn to be reactor operators often remark that the practical training and experience helped them build a deeper understanding of how nuclear reactors work (both the mechanical aspects and the physics) than they would have attained from their degree coursework alone.

How much radiation do people working at the reactor typically receive in a year?

We pride ourselves on our radiation protection program and our safety culture. Every activity we perform at the reactor is carefully planned to minimize the amount of radiation dose to personnel. We are constantly working to find ways to

  1. Reduce the time we spend working in radiation fields,
  2. Maximize distance from radiation sources, and
  3. Maximize shielding of radiation sources.

As a result our total facility dose decreases each year. In 2020 the average annual dose was 8 mrem per staff member, well below the NCRP maximum of 5000 mrem for radiation workers. For comparison, the annual background radiation for people living in Boston is approximately 300 mrem (not including medical procedures and air travel).

How does the reactor compare to a power reactor?

The MIT reactor is similar to a power reactor in that heat and neutrons are produced by nuclear fission, the fuel is U-235, the coolant is water and the core is located in a large tank inside a containment dome. Our reactor differs from power reactors in that the core is much smaller, it does not produce electricity so it does not have any steam turbines or generators, it operates at a much lower temperature (50°C compared to 300°C), and the core tank is not pressurized to raise the boiling point of the coolant.

To learn more about different types of power reactors, click here.