Reactivity initiated accidents are accidents which are induced by postulated faults in the CPS. In accordance with regulatory requirements  the following reactivity initiated accidents should be analyzed for nuclear power plants with RBMK-type reactors:
In the SAR the following cases were analyzed:
Initial conditions have been defined to account for most unfavorable operational conditions. Perturbed axial and radial power distributions have been defined which maximize the effect of the reactivity insertion. For dynamic simulations, power setback signal, the first neutronic trip signal and any trip signal based on process parameters were neglected.
For the single rod or group of three rods withdrawal accidents both at full power and during start-up no safety problems arise because the absolute power remains low and the maximum values of key safety parameters are maintained well below their limiting values. The analysis covers reactivity insertion for high and low rod worth’s. The consequences of high reactivity insertion are limited due to the generation of early trip signals, which terminate the transient earlier than in cases with low reactivity insertion. Also, neglecting the first shutdown signal does not create problems concerning the safety limits.
Total voiding of the CPS channels in the reactor at operational conditions can cause a reactivity insertion of up to 4-5 b . The highest reactivity insertion is obtained for low values of the operational reactivity margin, i.e. when most of the rods are withdrawn from the reactor. The worst case of CPS voiding is a loss of coolant above the reactor core, producing a draining of all CPS channels. The water level in the channels decrease by gravitational forces, thus the process is not very fast. Due to different types of control rods and different control rod insertion depths, the flow velocities differ significantly in the different channels. Thus, the reactivity insertion is non-uniform in the CPS channels. In addition, the reactivity insertion is not very fast. The fastest possible complete voiding of CPS channels in the core occurs in about 10 seconds, while the slowest voiding occurs in about 50 seconds. Multiple scram signal are generated and if the reactor shutdown function is available on demand, no safety limits are exceeded.
Assessments of reactivity initiated accidents show that the Ignalina NPP is adequately protected against this type of accidents. The fuel channels remain adequately cooled both in cases where all systems operate as designed, and when additional equipment or component failures are postulated to coincide with the initiating event. Multiple signals are available either to reduce the reactor power or to shut down the reactor. The main issue is detector coverage, which is shown to be adequate for central and peripheral, single and multiple control rod withdrawals as well as CPS voiding accidents. The single failure criterion is applied through the loss of signals due to the loss of one detector group of six. The loss of a group of detectors does not significantly impact detector coverage because there are many redundant signals based on the remaining detectors, i.e. acceptable consequences are obtained whether or not these signals are available.