Nuclear safety of the reactor
The reactor nuclear safety is, according to the Czech law, defined as the state and capability of the facility and staff to prevent incontrollable expansion of nuclear chain reaction and unallowable escape of radioactive materials or ionizing radiation into the environment and to limit consequences of the accidents.
The great attention is systematically paid to guarantee a high level of the nuclear safety of the school reactor VR-1 operation. The attention covers all aspects which results from the above mentioned definition:
- The capability of nuclear facility: The reactor was designed and manufactured to fully correspond to contemporary requirements on nuclear research facilities. Also the facts that the reactor is to be operated by the university and is to be used for students' education were taken into account. From the technical point of view, the facility corresponds to the requirements coming from the recommendations of IAEA. As examples can be mentioned the number and mutual independence of measuring channels, the number and functionality of reactor control rods, reactor neutron source, core configuration, used nuclear fuel type, ways of manipulation in the core and its environment, the sufficient awareness and the comfort of the staff etc. The nuclear safety related equipment (like reactor vessel, control rods and control system) are governed by the intensive quality assessment and control system.
- The state of nuclear facility: even the very well designed and manufactured facility could not reliably and demonstrably fulfill its function unless being maintained in a desired state on a long term basis. Due to carefully elaborated and strictly observed reactor operation organization, the whole facility is, and particularly its nuclear safety related parts permanently keep in repair. This results from the adequate maintenance and from regularly operation checks as well. The reactor vessel and reactor internals inspections, control rod functionality and condition checks and regular check of reactor control and safety systems (as a part of each reactor start up) relates particularly to the latter.
- The state of the staff: The reactor staff has a very important role in securing the nuclear safety. Thus, such conditions are established, which enable their number (e.g. according to work type; but also the necessary redundancy, requalifications and training), mental condition and health state to correspond with strong requirements. It is essential that the working shifts fulfill their duties in good mental and physical conditions
- The capability of the staff: Great care is given to staff capability. It begins with the staff selection and their entrance health examination. Before the state exams, which are necessary for obtaining the license for the implementations of selected operations on the reactor (shift supervisor, reactor operator, reactor starting-up group supervisor, and supervisory physicist), all workers have to go through corresponding professional training. The training encompasses obtaining of the necessary theoretical knowledge as well as the capability of properly operate the whole facility. In addition the necessary background reading and prescripts are available to all workers. After a given period of time, the staff has to pass, so called, periodical professional training and again the state exams.
Reactor VR-1 possesses also some important technical aspects related to its nuclear safety. Out of them, the most important follows:
- Nuclear fuel used for the reactor operation is designed to be slightly undermoderated. It means in case that the fuel is flood with water then the ratio of hydrogen to U-235 nuclei in a fuel cell is lower than its optimal value. Thus, the reactor has negative thermal and void coefficients of reactivity. So, during a hypothetical emergency condition connected with power escalation, it should strongly tend to stop itself spontaneously.
- Due to low power of the reactor, there are almost no fission products accumulated in the fuel. Thus, the fuel, which is designed for power research reactor as well, remains physically fresh during the whole time of reactor operation. Annually, less than 0.1 g of fissionable U-235 is spent. For comparison, in a typical core configurations, there are some 4.3 – 4.8 kg of U-235 in the core. It means, even during the hypothetical beyond-design accident connected with potential partial melting down of the fuel, the environment would not be significantly endangered. The fission product amount, which could be released outside the reactor hall during such a improbable event, is so small that no additional actions are needed for the minimization of such accident consequences.
© DNR FNSPE CTU in Prague, 2016 www.ReactorVR1.eu