[DennisP]

Not much. This is actually a common reactor type, and you don't have to worry about things like cooling pumps going bad or high-pressure pipe breaks.

> The water acts as neutron moderator, cooling agent and radiation shield. The layer of water directly above the reactor core shields the radiation so completely that operators may work above the reactor safely. This design has two major advantages: the reactor is easily accessible and the whole primary cooling system, i.e. the pool water, is under normal pressure. This avoids the high temperatures and great pressures of nuclear power plants. Pool reactors are used as a source of neutrons and for training, and in rare instances for processing heat but not for electrical generation....Most research reactors are of the pool type.

https://en.wikipedia.org/wiki/Pool-type_reactor