Japan’s High Temperature Test Reactor (HTTR) achieved criticality a year ago, in November 1998, and low power physics tests were finished in January. After that, the plant was shut down to modify the standpipe cooling system in order to reduce concrete temperature above the reactor.

On 28 September the reactor was started up with the intention of carrying out a series of power raising tests. The plan was to increase power to 9 MW, with a gas outlet temperature of about 320°C, during October and the first half of November. Later, in February and March next year, power was due to be increased to 20 MW with a gas outlet temperature of 590°C, and in May the full rated output of 30 MW at 850°C would have been reached. Unfortunately, the reactor scrammed on 1 October due to a low primary coolant flow signal. When the trip occurred, the power was 30 kW and the helium temperature was about 150°C.

The trip was caused by a malfunction of the control circuit of one gas circulator. At the time, three circulators were operating and the malfunction of one circulator was sufficient to trip the reactor. This kind of event is commonplace during the start up of any new reactor.

Technically, the problem was easy to fix and the reactor can go back to power at any time. However fate intervened, in the form of the JCO criticality accident, and the Science and Technology Agency, which is the regulator for both JCO and the HTTR, became preoccupied with the criticality accident, and very cautious about all the projects in its jurisdiction.

HTTR personnel were due to explain the problem to the STA with the aim of resuming testing in the week starting 15 November. Even if this happens, the interruptions caused by the year-end holidays may make it impossible to achieve power levels above about 3 MW this year.

The HTTR is a project of the Japan Atomic Energy Research Institute and is located at JAERI’s Oarai site. It has a thermal rating of 30 MW, with a maximum gas outlet temperature of 950°C, and is of the “prismatic block” design (as opposed to the pebble bed). It does not produce any electricity but is intended to research the possibilities of using nuclear power for chemical processes which require high temperature heat supplies. (Reactors of this type, however, could be used for electricity generation using direct cycle gas turbines – see NEI, October 1998.)