Securing reactor shutdown

The standard reactor modules and "safety-related" buildings, structures, systems, portions of systems, and components dedicated to assuring reactor shutdown, decay heat removal, fission product retention, and security, including new (unirradiated) fuel. 

The void coefficient of reactivity is an important inherent safety characteristic of reactor core. The calculations performed with continuous energy Monte Carlo code MVP show negative void coefficient of -15% Ak/k (at 40% void, BOL) and temperature coefficient of -2.3E-4% Ak/k/°C for graphite. These coefficients are rated sufficient to secure passive shutdown of the reactor core in accidents. 

The upper, open-ended band represents low-probability severe reactor accidents with consequences beyond the facility boundary. It is not expected that any off-site evacuation of the public would be needed under any circumstances for small power reactors, however, minor releases of radioactivity within the specified regulatory limits may occur. The accident response team would secure the site, restore a safe, contained shutdown state, and perform any required cleanup. 

The EFWS can be used to supply feedwater to the steam generators for normal plant cool downs. To accomplish this, the pumps are manually started and the flow rates manually controlled by positioning the flow modulating valves. When the reactor coolant hot-leg temperature is reduced to 350°F (176.7°C) and the RHRS has been activated, the EFWS is secured. The EFWS is not normally used for plant cool down during shutdown because other design provisions contained in the main feedwater system should be used to supply the steam generators in this case. 

A cost reduction (up to a factor of 1.5 against the first-of-kind plant) may be achieved in the case of a serial production of the ELENA-NTEP plants. The reactor operates at nominal power in an unattended mode that does not require the involvement of operating personnel in the process control. The start-up, shutdown, routine inspection and if required, repair, are performed by personnel from the control centre on a shift basis. The site security may require auxiliary personnel residing in the plant deployment area. 

By mid century, district-level conversion of hydrogen to electricity - as opposed to conversion of heat to electricity at the STAR reactors sited at the city perimeter - is envisioned for several reasons. The first - and the one, which is already driving a transition -is supply reliability. Micro-turbines and (imminently) fuel cells could provide secure electricity at a district level, even if the broader grid suffers a shutdown, because they run on a storable supply - currently natural gas, but eventually hydrogen. Some planners believe that distributed generators will, in fact, eventually drive the grid. 

It is proposed, under the Safestore decommissioning strategy, that some buildings and structures could be retained on the site for up to about 135 years after station shutdown. It is therefore necessary to ensure that these facilities can endure this long period in a safe, secure and weatherproof condition. The plant and structures, eg the reactors, that it is intended to retain are of substantial and robust construction and the bulk of the remaining radioactivity is within activated materials and hence physically locked in and not mobile. Therefore the main requirement is to provide conditions that will minimise degradation mechanisms affecting these materials. 

Complying to the "Safety Design Review Guide" (2), the plant is designed so that it enables safe shutdown the reactor and secures cooling following shutdown in case of loss of total power source for a short duration. 

On the other hand, their operational functions of Adjustment and Control and of Fast Extinction are parts of one of the most important safety systems of the reactor the First Shutdown System (FSS). Therefore a complete experimental program including both experiment-aided design and qualification tests is necessary to secure high reliability performance together with low maintenance requirements. 

Another way of reducing the specific mass of lead bismuth coolant is to increase its average flow rate and to diminish the length of circulation circuit. However, this approach has its own constraints caused by the necessity to meet safety requirements. The first requirement is defined by the necessity to provide the power level of the reactor with naturally circulating lead bismuth coolant at the level not less than 5...7% of its nominal power. This makes it possible to eliminate inadmissible temperature increase under a shutdown of main circulation pumps. The second requirement is conditioned by the necessity to secure conditions for the assured surfacing of steam bubbles from lead bismuth coolant to its free surface level under the rupture of steam generator (SG) tubes. This is important to eliminate steam ingress into the core and inadmissible pressure increase in the mono-block vessel. 

Shutdown would cause a reduction in the turbine inlet temperature of the Brayton unit energy converters. This would reduce their power output, and require that power be supplied from the test facility to motor their alternators for decay heat removal (similar to the beginning of the reactor startup sequence). The Brayton units could be continuously motored to provide decay heat removal, or a separate circulator could be added to the prototype support facility to provide the function and allow the Brayton units to be secured. The prototype support facility would also need to provide a continuous source of power to the 28 Vdc bus for uninterrupted power to the reactor controllers to completely withdraw the sliders and allow for continuous reactor monitoring after the shutdown. 

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