Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 210

solved technically as a requirement of the KHE Safety Concept by appropriate
routing of the steam pipes, use of the proper valves, which can close during such
accidental situations, and by accident management measures such that it will no
longer be of importance in modern PWRs, e.g. KWU PWR-1,300 and EPR
(Chap. 3 ) as it was the case in the early risk studies e.g. the German Risk Studies
Phase A and B [ 10 , 64 ].

10.3.5 Core Meltdown Under High Primary Coolant
Pressure

WASH-1400 [ 9 ] and the German Risk Study Phase A [ 10 ] had assumed core
meltdown under high primary coolant pressure to lead to failure of the outer
containment, followed by a major early radioactivity release, as a result of the
reactor pressure vessel acting as a bullet. Core meltdown under high primary
pressure could occur in an uncontrolled emergency power case (station blackout)
or uncontrolled failure of the main feedwater supply [ 9 , 10 , 64 ]. In both cases, the
ultimate consequence is heating of the primary coolant plus primary pressure
increase, thus causing the pressurizer relief valves to open. The reactor pressure
vessel will be voided. The water level in the reactor core will drop, the fuel rods will
heat up, and there will be a zirconium—steam reaction at the fuel rod claddings.
This heats the reactor core still further, causing it to start melting. After roughly 1 h,
the core will have molten more than 80 %. After 3 h, the core will melt through the
grid-plate in the reactor pressure vessel.
Molten fuel flows into the water contained in the bottom hemispherical head of
the reactor pressure vessel. This water evaporates quickly. However, the coolant
cannot be removed fast enough through the pressurizer relief valves, which causes
the coolant pressure to rise. After approximately 3.5 h, the core will melt through
the bottom of the reactor pressure vessel at a high primary pressure.
Very high buoyancy forces will arise which can accelerate the reactor pressure
vessel upward.
In the German Risk Study Phase B [ 64 ] there had already been estimates of
mechanical resistance offered by the anchorage of the reactor pressure vessel and
the primary piping. The outcome had been that a primary internal pressure of
> 3 MPa during melt-through would cause the anchorage of the reactor pressure

vessel to fail. However, the integrity of the outer reactor containment would be
jeopardized by the reactor pressure vessel accelerated upward only above a primary
internal pressure of 8-10 MPa.
As a conclusion drawn from all these findings, it was proposed for existing
PWRs to reduce the primary pressure in the reactor pressure vessel by timely
pressure relief by opening of a pressurizer relief valve (accident management
measure), thus allowing the core to melt through at a low pressure as in a loss-of-
coolant accident (LOCA). Failure of this accident management measure results in a