Reactor Safety

The Nuclear Regulatory Commission (NRC) Office of Nuclear Reactor Regulation (NRR) has the responsibility of overseeing all university research reactors (URRs) including the MITR. Program management, inspection, and operator licensing are the three major oversight responsibilities of NRR. In addition, MIT established a very effective means of ensuring safe operation of the reactor by appointing independent experts to the MIT Reactor Safeguards Committee. This committee, whose members are from MIT as well as from industry, is ultimately responsible for overseeing all nuclear safety issues related to the reactor and ensuring that reactor operation is consistent with MIT policy, rules, operating procedures, and licensing requirements. However, all members of the NRL organization are keenly aware that safe operation of the nuclear reactor at MIT is their top priority. This level of awareness is achieved through the commitment and continuous training provided by the NRL’s management team. An environment of cooperation and attentiveness to detail among reactor employees and experimenters regarding all reactor safety matters is essential. As a result of this approach to safety, each and every individual employed at the reactor can be proud of NRL’s outstanding safety and operating record, which is evidenced by the results of NRC inspections.

Safety Features of the MIT Research Reactor

The MIT Research Reactor (MITR) is currently licensed to operate at 6 MW. As such, its power level is 500 times smaller than that of a typical commercial power plant that produces electricity. The MITR also operates at atmospheric pressure and at low temperature (about 50°C/122°F). The low power level means that the MITR has far less radioactivity in its core than does a power plant. The low pressure and low temperature mean that there is no driving force to push out what radioactivity there is, in the unlikely event of an accident.

The MITR is equipped with both engineered and passive safety features that ensure its safe shutdown. ‘Engineered’ features require some human action and, in general, the use of electricity. An example is the operation of a pump or closure of a hydraulic damper. ‘Passive’ features operate as the result of a physical law and require neither human action nor electricity. An example would be the establishment of natural circulation cooling as the result of density differences in the coolant itself. The essential safety features of the MITR – shutdown and building isolation – are achieved passively.

A summary of major MIT Reactor safety features includes:

Reactor Shutdown: Reactor operation requires electricity. In particular, electromagnets are used to hold the control elements that must be withdrawn from the core in order to create the nuclear chain reaction. Therefore, on loss of electricity, these magnets de-energize and the control elements drop, under gravity, into the core thereby shutting down the reactor. Any loss of electricity results in a shutdown. If there is an external event such as a hurricane, flood, earthquake, or car crash into a utility pole that disrupts the electricity supply, the reactor shuts down.

Building Isolation: The MITR is located inside a containment building (made of two feet of reinforced concrete within a steel shell) that serves to isolate the reactor from the environment. If there were a radiation release from the core, it could be entirely contained within that building thereby protecting the general public. The containment building is also kept at a slightly lower pressure than the outside air. If there is a leak in the containment, air comes in from the outside rather that going out from the containment building. The air in the building is automatically isolated from the outside air if any one of four redundant sets of effluent radiation monitors senses an abnormal radiation level. Isolation can be achieved by the forced closure of hydraulically-operated dampers (engineered feature) or by the closure of gravity-operated backup dampers (passive feature). Either of these system will seal the building.

Building Pressure Relief: In the event that pressure builds within the isolated building to levels that are excessive, venting via filters (charcoal to remove iodine, coarse and fine filters to remove particulates) is possible. The driving force for the venting is the difference between the building pressure and that of the atmosphere (i.e., the outside air); it is a passive system. To place this system on line, two valves, both located outside the reactor building and hence both accessible even if the building is isolated, need to be opened.

Spent Fuel: We ship out our spent fuel on a regular basis and thus do not have a significant inventory. Spent fuel that is awaiting shipment is kept in a special section of our core tank until its residual energy (heat from the decay of fission products) is reduced to a level where air cooling is sufficient. It then goes to our spent fuel pool.

Boston Area Earthquake Summary: The Boston area has one of the longest records of earthquake activity going back to the 16th century – about 450 years of data. About 250 seismic events over this period had epicenters within a 200 km radius and a Richter intensity of 3.0 or greater. The largest event affecting this area was an earthquake off Cape Ann in 1755 (probably around Richter 5-6). That event produced accelerations of about 0.1 g-force in Cambridge. Structures in this area are generally designed, therefore, to withstand a force of 0.225 g, with the expectation that such an event could occur once every 10,000 years based on the existing history. Analysis of the strength of our core tank shows that it can withstand much higher forces (5 g horizontal and 3 g vertical simultaneously).

Reactor Radiation Protection Program

Radiation protection at the MIT reactor coverage is provided by the Reactor Radiation Protection Program (RRPO) of the Environment, Health, and Safety Office (EHS). RRPO personnel include a deputy director for EHS serving as the reactor radiation protection officer, two EHS officers, one technician, and a part-time administrative support staff member. Routine activities include but are not limited to radiation and contamination surveillance, experimental review and approval, training, effluent and environmental monitoring, internal and external dosimetry programs, radioactive waste management, emergency preparedness, and ensuring that all exposures at the NRL are maintained as low as reasonably achievable (ALARA) in accordance with applicable regulations and Institute committees. An EHS officer serves as EHS lead contact to the NRL under EHS management system organizational structure.